For Neptune, or rather knowledge thereof, the early 1970s CE were a simpler time. In fact any time between 1949 and 1989 was a simpler time. Back then, Kuiper having discovered Nereid, a smaller and peculiar moon, at the end of the ’40s, Neptune only seemed to have two moons: Triton and Nereid. This state of affairs continued until the end of the ’80s, which was approximately one Neptunian season. Four decades during which the planet only appeared to have two moons. I’ll start with that.
I’ve already mentioned Triton, the oddball moon of the Neptunian system two hundred times as massive as all its other moons put together, orbiting backwards and at an angle, in an almost perfectly circular trajectory. I haven’t mentioned the equally oddball second moon discovered, Nereid, and I say the early ’70s were a simpler time but in fact its own orbit is very peculiar. Nereid has the most eccentric known orbit of any moon. It sometimes feels like discussing the orbit of a celestial body is a bit tangential to the core of its nature, but orbits have important consequences for the nature of planets, moons and their neighbours, and in this case it’s so odd that it would be strange not to mention it, particularly back in 1971 when that was practically all that was known about it. It sometimes feels like the Solar System “frays at the edges” with all this stuff, because things out here are really quite outré compared to the relatively regular innards of this system we call solar. Nereid’s orbit is entirely outside Triton’s, approaching Neptune by 1 353 600 kilometres at its closest and moving out to a maximum of 9 623 700 kilometres distance from the planet. It takes five days less than a year to go all the way round, which is appealingly similar to Earth’s sidereal period. In fact of all Solar System objects its year seems closest to ours. No other moon is remotely as eccentric. At its closest, Neptune would be a little larger than the Sun is in our own sky, and at its furthest, six months later (so to speak), about the size of a lentil on one’s dinner plate. This is probably the result of Triton’s capture, which to me suggests there are other former moons wandering far beyond Pluto or even in interstellar space, or maybe in the “Gap“.
Nereid is small and grey. There is no good image. The best one is this:
Not very impressive, eh?
Unlike Triton, Nereid orbits in the usual direction, as do two other irregular moons Sao and Laomedea, further out. Another moon, Helimede, is a remarkably similar colour but orbits the other way. It’s considered to be a bit that chipped off of Nereid. Nereid itself is about 360 kilometres across on average and may be somewhat spherical but by no means perfectly so. It’s one of several bodies in the system which are right on the border of being round, and is almost as large as the definitely round (sans Herschel) Mimas, but also rather denser. Its shape is therefore hard to determine. Certainly its gravity would be sufficient to pull Mimas-like material into a spheroid, since it’s higher, but that very density may result in the moon being tougher and more able to support its own weight without collapsing. However, its variation in brightness probably means it’s quite irregular in shape and closer to Hyperion in form. Its colour is markèdly unlike that of most centaurs, and it’s therefore probably a “native” Neptunian moon. There’s water ice on its surface.
Proteus is the one which really surprised me. On the whole, the Voyager probes and others only discovered small moons, although Charles Kowal’s discovery of Leda skews that for the Jovian satellites because it’s unusually small for a telescopic discovery of that time. Proteus is actually the second largest Neptunian moon, being somewhat larger than Nereid, and is shown at the top of this post. It orbits the planet at 117 647 kilometres from the barycentre on average in a fairly round orbit, though nowhere near as round as Triton’s. It can be determined not to be perfectly spherical and is in fact not even particularly rounded, with dimensions of 424 x 390 x 396 kilometres. Its surface consists of a number of planes (or plains) with sharp angles between them at their edges and it’s uniform in colour, being somewhat reddish like many other outer system worlds. It was discovered by Voyager, but two months before the space probe got to Neptune.
Unlike Nereid, Proteus was close enough to Voyager 2 to be mapped. As can be seen above, it’s heavily cratered and its surface is therefore likely to be quite old, meaning that nothing much has happened to it in a long time. NASA also had a very steep “learning curve” with Proteus compared to Nereid as it went from being unknown to being mapped within a few weeks, whereas Nereid’s existence has been established for six dozen years now and still there is no map available except possibly the kind of vague albedo feature map which used to be done for Pluto before a spacecraft got there. It can also be seen through the Hubble Space Telescope. It’s fairly dark, probably because its surface consists of hydrocarbons and cyanides. The only named feature on its surface is the relatively large crater Pharos, 260 kilometres across, but due to its somewhat irregular shape this fails to give it the “Death Star” appearance Mimas has. Proteus is also receding from Neptune due to tidal forces and is now eight thousand kilometres further from it than when it first formed. Unsurprisingly, given that it was undiscovered for so long, it’s a lot darker than Nereid.
The inner moons generally are coated in the same material as Proteus. A couple of them are quite notable. For instance, Larissa, which is 194 kilometres in diameter, was accidentally observed passing in front of a star in 1981, leading to the correct but unwarranted conclusion that Neptune has rings. The chances of a moon of that size being seen to cover a star are very small just anyway, but in Neptune’s case it’s even less likely because it moves against the “fixed” stars so slowly, taking almost three months to cover a distance equivalent to the face of the Sun. Larissa’s period is about twelve hours and it orbits only 73 400 kilometres above the centre of Neptune, putting it close to the Roche Limit, where large bodies are torn apart by gravity. It was, however, given a provisional designation in ’81, namely S/1981 N1, so it was accepted as a moon back then. Like the other inner satellites, it’s likely to be a rubble pile, without enough gravity to pull itself together as a solid object. It may be a future ring.
Another somewhat interesting moon is Hippocamp, which is so dim Voyager failed to notice it and had to wait for the Hubble Space Telescope to discover it, which was done by the combination of a number of images as even then it was too faint to be spotted. It seems to reflect less than ten percent of the light falling on it. It’s only seventeen kilometres across.
The closest moon to Neptune, and in fact to any solar gas giant at all, is Naiad, taking only seven hours to travel round the planet. It’s quite elongated at eight by five dozen kilometres, and will either become a ring or fall into the atmosphere in the relatively near future. Thalassa, the next moon out, is coörbital with it. Their orbits are only eighteen hundred kilometres apart but they never approach that closely because they move north and south of each other as they orbit, putting them a minimum of 2 800 kilometres apart. It’s about the planet’s radius from the cloud tops, making Neptune occupy most of its sky. This would make the surface look deep purple if it has a reddish coating like the others.
Like some other moons, the naming scheme has the prograde moons end in A, the retrograde in E and the highly tilted in O. The two outermost moons, Psamathe and Neso, are relatively close to each other, and stand in contrast to Naiad by being the most distant moons of any known planet at forty-six and fifty million kilometres. Neptune’s lower mass also gives them exceedingly long years of around a quarter of a century.
Triton, along with the similarly-named Titan and also Ganymede, is one of the largest moons of the outer system. Before Voyager 2 reached it, it was considered possibly the largest moon of all. Moreover, apart from our own highly anomalous Cynthia, it’s large in proportion to its planet’s size. Using the largest moons of each planet, the proportions of their masses work out thus:
Just for reference, the ratios for Pluto:Charon are 1.96 for diameter and 8.22 for mass, but Pluto‘s status as a planet is not unquestioned. It can in any case be seen that of all the large moons, Neptune’s Triton is still in proportion and there’s a big gap before our own special case, but it is still unusually big.
A common mechanism for the formation of moons is for the region around a planet to behave like the solar nebula did when the planets themselves were formed, with eddies in the cloud pulling in matter as the planet takes shape. Hamlet’s moons may be an exception to this, as they may result from the trauma that planet underwent. Outer and irregular moons are, however, often the result of captures and this is particularly evident when they orbit the opposite way from most Solar System bodies, and Triton is by far the largest body to do this. This has been known since its orbit was plotted in the nineteenth century. Due to its size and therefore relative brightness, the moon also holds the record for the shortest gap between the discovery of its planet and its own, as it was found in October 1846 CE, only a month after Neptune. This, however, is not as impressive as it sounds because all the planets out to Saturn have been known since ancient times and Pluto is very small and may not be counted as a planet, so it basically means that of the two planets discovered in the telescopic age, one of them has a very large and relatively bright moon which was easy to spot.
Certainly by the ’70s, Triton was, as it still is, considered to be a captured planet, though that would probably generally be qualified as “dwarf” now. Given the controversy of what counts as a planet, Triton of all worlds in the system has surely got to be the closest to that definition, as although it may have undergone the mishap, if that’s an appropriate word, of being grabbed by Neptune, it’s quite large and massive and probably used to dominate its orbit, as the 2006 IAU definition demands. Strictly speaking, and perhaps by being a bit arsy, Earth doesn’t even count as a planet by that definition. Hear ye then: Triton is a planet. I was first introduced to this piece of information in the ’70s, which is how I can make that provisional estimate of its timing, and consequently looked forward to the Voyager missions as including an encounter with a body likely to be very like Pluto. At the time there was little prospect of a mission to that planet, so it was the best I felt I could hope for.
The Voyagers took advantage of a rare planetary alignment which only occurs once every two centuries and started in 1976, dubbed the “Grand Tour”, which would allow probes to visit several planets in a row. This idea dates from at the latest 1971, and there were initially three possibilities. Two involved Jupiter, Saturn and Pluto and the other all the gas giants, but it was impossible to visit both them and Pluto on the same mission, at least efficiently enough to be practical. The ultimate decision was to take the last option, although Voyager 1 is a bit like the first two with the omission of Pluto. Also, although the Voyager 2 mission resembles the final option quite closely, it isn’t actually the same as the initial plan, which involved launching in 1979, visiting Jupiter and Saturn in ’81 and ’82 respectively, Hamlet in ’86 and Neptune in ’88, as the Voyagers were launched in ’77. The earliest option for Pluto inolved a ’76 launch, visits to the two inner gas giants in ’78 and ’79 respectively and Pluto in ’85. Although the final choice was, I think, a good one, it’s interesting to contemplate what might have been. It would be disappointing not to have visited the ice giants but amazing to have got to Pluto so early, and it also seems very likely that if that had happened, Pluto would never have been demoted. However, it was not to be, and this makes Triton a kind of Pluto substitute. It is in fact very likely to be similar to Pluto and it’s worth comparing the two.
Excluding the Sun, Triton is the fifteenth largest body in the system, Pluto the sixteenth. Eris is next on the list, incidentally. In terms of mass, Eris is between Pluto and the more massive Triton. Circling Neptune, Triton takes 165 years to orbit the Sun , Pluto 248, which is close to a 3:2 ratio (lots of ratios in this post for some reason) like the other plutinos. Considering its similarity, it seems likely that Triton was itself a plutino with a 248-year period like Pluto’s (which is what defines them), and right now I’m also wondering whether some of the other moons of Neptune, particularly Nereid with its peculiar orbit, were in fact originally moons of Triton. I expect this has already been researched.
Being retrograde is not the only peculiar feature of Triton’s orbit. It also varies its tilt through a cycle corresponding to only four Neptunian years, and is moreover remarkably round, by contrast with Nereid’s. Its distance from the barycentre (centre of gravity between two bodies) varies by less than six kilometres each way. This may be the roundest orbit in the Solar System and is quite remarkable. Our own orbital eccentricity is a thousand times greater. Hence there are a few combined mysteries here, which are probably related: the moon orbits backwards, shifts rapidly (over a period of about five centuries) in how tilted its orbit is and hardly varies at all from its mean distance of 354 759 kilometres from the barycentre, which is around seventy-five kilometres from the centre of Neptune. The size of the orbit is also only a little less than Cynthia’s around Earth. I have an illustration by Luděk Pešek of the moon in Neptune’s sky, painted in the early ’70s, and at the time it was considered much larger than Cynthia. It’s now been found to be somewhat smaller at 2706 kilometres diameter, and is of course somewhat less dense due to its ice content, although Cynthia, being formed from the Earth’s outer and lighter layers, is only about 50% denser. That said, Triton still averages over twice the density of water, making it one of the densest objects in the system beyond the orbit of Jupiter, and also denser than Pluto. Given the nature of its surface, this is all the more remarkable, and I’ll come to that.
Before its capture, Triton would’ve dominated its region of the system beyond Neptune, and perhaps even have counted as a planet in its own right by the IAU 2006 definition. Neptune is in a peculiar position regarding the Bone-Titius Series, and if that is in fact a law of nature it could be expected to have been somewhere else in the past. This would presumably in turn have meant that the plutinos have fallen into orbital resonance with it since it moved and the presence of small, solid planets beyond its orbit would lend the Solar System a pleasing symmetry, with small rocky planets in the inner system, gas giants in the middle and a further succession of small icy planets beyond them. It is of course highly speculative to suggest that Neptune used to be somewhere else. Olaf Stapledon supposed Neptune to be followed by a further three planets, of which Pluto was extremely dense and made of iron, because only with such a hefty planet would be able to perturb Neptune to the extent it is. It was common at the time for scientists to presume this as they’d predicted Pluto’s existence from these perturbations, but I’ve gone on about this elsewhere.
Pluto and Triton are almost the same in composition, suggesting a common origin. The moon’s surface, however, is somewhat different. It’s unusually flat, with variations in elevation of less than a kilometre. It also has a surprising composition: it’s made of frozen nitrogen. At this distance from the Sun, the gas which makes up most of our atmosphere composes the solid, though also soft, surface of a world. It’s therefore no surprise that the surface temperature is exceedingly low at -235°C. However, there is also a greenhouse effect, in this case considerably more literal than usual. The nitrogen forms a clear surface which traps the sunlight just below it, heating the subterranean nitrogen and causing it to erupt out of the surface like geysers or volcanoes to a height of around eight kilometres. This then drifts downwind by as much as a hundred kilometres, leaving streaks on the landscape. This process also maintains the moon’s nitrogen atmosphere, which is thin by terrestrial standards but not as tenuous as many of the atmospheres of other moons, at fourteen microbars. Although this may not sound like much, it’s enough to be a collisional atmosphere. That is, the molecules in Triton’s atmosphere are near enough to one another to come in contact at least occasionally, which means the air behaves as a fluid like air at sea level on Earth, rather than just bouncing around or orbiting the moon as it does on our own. Even so, Triton’s atmosphere is a lot thinner than expected. The lower the temperature, the easier it is for a body to hold on to gases and perhaps liquids if the atmospheric pressure supports them. Nonetheless, Triton doesn’t seem to be very good at it. Its surface gravity is 0.0794 that of ours, over half that of Titan, whose atmosphere is several times denser than Earth’s and whose temperature is something like two and a half times higher. There’s a small amount of methane in the atmosphere too, making it like a much thinner version of Titan’s, but also colder since it’s below both substance’s freezing points. Just as an aside, it’s been conjectured that of all the substances likely to form oceans on planets or moons somewhat similar to Earth, i.e. oceans on the surface along with land masses or islands, nitrogen would actually be the most common liquid of all, with water only coming in second. Triton is not a world with permanent bodies of liquid on its surface, but like Cynthia, it does have large flat plains of solidified “lava”, in this case frozen nitrogen, which contributes to its general flatness. Unlike water, most liquids freeze “under” rather than “over”, so the frozen nitrogen lava plains of Triton would have done so by cooling on the surface and then precipitating down inside the body of liquid, gradually filling up until the whole lake or sea was frozen solid, except that it would then have melted and vaporised in some places and pushed through once again. The geysers are near the south pole, similar to the Enceladus situation, but this is a much larger and heavier world than that moon. However, there are also claims that the lava is in fact an ammonia-water mixture, so all of this is provisional. The fact remains that most of the atmosphere is nitrogen.
The resolution of the picture at the top of this post is surprisingly large considering it’s a mosaic of images captured by a camera from the mid-’70s. Although it’s diminutive on this page,clicking on it will show it in its full glory. Pixels are only five hundred metres across at the centre, so this is a pretty detailed map of most of the surface and would show medium-sized parks if it were a picture of Earth. It’s like a photo of Earth from the ISS, although of course the whole of our planet wouldn’t be visible from such a distance. A distinctive feature is the so-called “canteloupe terrain” because it looks a bit like this kind of melon:
Triton’s version looks like this:
The winding heights are several hundred metres high and a few hundred kilometres across, and the plains they surround are safely two hundred kilometres wide, which is significant for a moon which, though large, is only about ten times that in diameter. The ridges consist of water ice which has been squeezed upward, and the whole surface of the moon is quite young as it has few craters. It could even be Cenozoic. This is possibly a surface which didn’t exist when T. rex walked the earth, although another surface did. To my mind, this raises the question of whether Triton was actually an independent planet at the time and if this melting can be blamed on the capture.
The similarity of the smooth basins to lunar maria will not have escaped you. The difference is that whereas those are made of basalt, these are nitrogen, as I’ve said. It’s worth bringing up again though, because on different worlds at different temperatures the same kinds of processes and structures exist but are made of different substances. On the whole, most substances which can be solid, liquid or gaseous in a given situation without major changes are, unsurprisingly, broadly subject to the same kinds of physical laws. The exception, more surprisingly, is water, because in the state with which we’re familiar, that is, under enough pressure to give it a liquid phase but only enough to ensure it has the most loosely spaced solid one, it expands and therefore floats when it freezes. This would have consequences such as the canteloupe terrain on Triton, which could be caused by its expansion as it solidified. Ironically, liquid nitrogen and molten rock (a bit of a generalisation) have things in common which they don’t share with water, a highly anomalous substance, due to water’s expansion on cooling and surface tension, among other things.
The solid nitrogen on Triton can be seen as the slightly blue-green streak across the image at the top of this post. It’s actually β nitrogen, which forms hexagonal crystals although they don’t form arrays like graphite or honeycomb. I can’t swear to this, but since the element immediately below nitrogen in the periodic table is phosphorus, whose least derived form is the dangerous but waxy white phosphorus, and I suspect that solid nitrogen fairly close to its triple (“melting”) point is also like this. This is not a thorough scientific appraisal so much as a hunch. White phosphorus slowly combines with oxygen in Earth’s atmosphere, and nitrogen as such is highly reactive, hence its use in explosives, but generally reacts with itself to form a highly inert gas at temperatures compatible with human life. On Triton, whether or not it’s reactive it may not have much to react with and the lower temperature would inhibit many such reactions. The issue here is really that although, as I’ve said, in some circumstances it hardly matters whether the substance in question is silica or nitrogen, as both can form volcanoes, erupt, produce lava flows and the like, such a substance as solid nitrogen or a mass of liquid methane on a lake on Titan is far from our own experience and our expectations can be misleading. However, it does seem highly feasible that the plains of the canteloupe terrain and the general flatness of the landscape is due to the waxy softness of the nitrogen which forms part of them. At this temperature also, water ice is almost a normal solid, expanding with increasing temperature and contracting as it cools, but it has clearly passed through the anomalous phase we think of as normal behaviour for a liquid.
What’s Triton’s interior like? Nitrogen in this solid form is very slightly denser than water at our freezing point, so it unsurprisingly covers the surface and forms a substantial part of the crust. The moon is rockier than the other moons trans the asteroid belt with the exception of Io and Europa, which are basically just balls of rock like the inner planets with a thin coating of other substances. Triton does still have an icy mantle but it will have a rocky core high in metals like a terrestrial planet’s. The brightness of the nitrogen surface cools the moon while simultaneously heating the upper layers of the crust, making it one of the coldest worlds in the known Solar System. The geysers are driven by the heat of the Sun, such as it is, emphasising what looks to us from here, close in to the Sun, to be a thermally delicate state. It might be expected not to last long in its present form when the Sun becomes a red giant, but the same is true of Earth. Solid and liquid matter as such is not the kind of thing which can cope well with the kind of temperatures found near stars. There’s also the “logarithmic” effect of low temperatures. The freezing point of water is about half the temperature of a hot oven and its boiling point at sea level is less than twice the temperature at our South Pole in midwinter. Nitrogen and oxygen have similar melting and boiling points at the rather mind-boggling sea level atmospheric pressure, and to us the fourteen degrees of difference between the boiling and freezing points of nitrogen sounds very narrow, but if centigrade had been standardised with nitrogen instead of water, absolute zero would be -550 degrees below zero. There’s an effectively infinite range of temperature before reaching absolute zero, which is like the speed of light in that respect – effectively inaccessible and some kind of ultimate limit.
Although they have their own smaller moons, Pluto and Charon are effectively a double planet system. It’s been theorised that the same was also true of Triton before its capture. Many other Kuiper belt objects are binary, and modelling of the dynamics of capture show that Triton is more likely to survive if this was so. The other object would be ejected from the system. To my mind, this contrasts with Hamlet’s situation, where a similar collision may have resulted in the “moon”, such as it was, being incorporated with the substance of the planet itself and also disrupting its axial tilt. The question then arises of where Triton’s companion might be now if it survived the encounter, and in my current ignorance I wonder about the similarly-sized Eris.
The name Triton originates from Poseidon’s (i.e. Neptune the god’s Greek counterpart) son, and has been more widely used for other purposes than most other names of major planets and moons. For instance, this is a triton:
This is the animal that first springs to mind for me when I think of newts, but they are nonetheless known as tritons. It’s also used as the name of a sea snail and a species of cockatoo. The list is much longer than for many or most other names also used for celestial bodies, which seems rather anomalous to me and possibly reflects the relative obscurity of the moon compared to some others, though maybe I’m out of touch in saying that.
Neptune’s satellite system as a whole is sparser than the other gas giants’, with only fourteen known moons. Until the ’80s, only two were known. This may be connected to Triton’s presence, either enabling it to remain without disturbance or maybe due to its own disturbance of the system. When Triton first arrived, its surface is likely to have been molten for an æon. In Triton’s case this presumably means a liquid nitrogen ocean over a water ice bed, which makes it seem that it was captured in the late cryptozoic eon, if that estimate is at all accurate. Hence over the period when Earth was almost frozen over itself and had little or no surface liquid, Titan and Triton both had oceans, and the latter would’ve been a possible member of the very large number of worlds with liquid nitrogen bodies of liquid on their surfaces, which is plausible but unknown. It’s also unclear whether it had landmasses. But in any case, the number of moons is surprisingly small. The comparably-sized Hamlet has more than two dozen, but Neptune only has fourteen. All but two of these were unknown before Voyager. Triton’s mass is two hundred times the mass of all the other moons put together.
As a world, Triton is somewhat smaller than Cynthia. Its surface area is 23 million square kilometres, 40% of which has been imaged. This makes it bigger than any country and a little larger than North America, but smaller than Afrika or Eurasia. It seems entirely feasible, probable in fact, that its surface is covered by more nitrogen than is present in our own atmosphere. Triton and Pluto both have irregular pits with cliff edges on their surfaces which are not craters, called “cavi”. Ten of these have been named, all after water spirits. Cavi usually occur in groups. There are only nine named craters. Other features include those found elsewhere on other solid bodies in the system (and probably throughout the Universe): dorsa, sulci, catenæ (chains of craters caused by meteoroids breaking up before impact), maculæ (dark spots), pateræ (irregular craters, not the same as cavi), planitiæ and plana. There are also “regions”.
Tholins are present on Triton, where they are distinctive in containing heterocyclic nitrogen compounds. This makes them chemically similar to alkaloids, which are a family-resemblance defined class of nitrogenous compounds which tend to have rings containing nitrogen in their molecules, a markèd physiological effect on some organisms and originate in plants. However, there are animal alkaloids such as toad poisons and adrenalin, so it’s entirely feasible that there are basically drugs on Triton’s surface. Unlike Titan, there are no persistent solvents on Triton, so in a similar way to moondust being chemically different from matter in a wet or oxygen-rich environment, Tritonian tholins might be quite reactive on Earth, and might in fact be explosive. All this is my speculation, but I stand by it and feel quite confident that it would be so.
To conclude, then, probably less is known about Triton than any other body of comparable size in the system up to and including Pluto. It’s only been visited once, by Voyager 2, and was in fact the last world to be encountered by it before the “void”. Nonetheless, it’s an important world and has probably the best claim to planethood of any moon. The behaviour of objects in the outer Solar System at this point reminds me of snooker.
Next time, the other moons of Neptune, which are also interesting but even less well-known.
Neptune may be the outermost planet. After the torridity of having to refer to the previous planet by a silly name or bear the brunt of using an unofficial name, it’s nice to have the calm of just being able to call it “Neptune” without the irritation of puerile jokes. That said, things could’ve turned out very differently because one of the names considered for the seventh planet was actually Neptune!
The two planets are the most similar pair in the entire system. That said, having fixated on Hamlet for so long, right now the two don’t look that alike to me. Neptune has no obvious rings, spins more upright and is a much clearer and more vivid (livid?) blue than the hazy and almost featureless Hamlet. The further out a gas giant is, the more likely it is, even if bigger than Jupiter, to look like Neptune. If Tyche exists, it will be blue, and outer planets in other star systems whose stars provide less radiation than about a thousandth of solar intensity at our distance from it are also probably going to look like this, although much dimmer. The above image is actually more colourful than it would look to the unaided human eye, at least at first. At Neptune’s distance, the Sun is nearly a thousand times dimmer than at Earth’s. The logarithmic nature of senses means that this wouldn’t seem as dim as that suggests. It’s still about 360 times brighter than Cynthia ever gets. Moonlight is insufficient to make out colour, but I don’t know about sunlight on Neptune. In a way it’s odd even to consider what colour Neptune would look like to human vision as nobody will ever see it in person and it would appear to be coloured to some species who live on this planet, particularly nocturnal ones.
The Titius-Bode series does not apply to Neptune. It’s actually 30.1 AU from the Sun rather than the predicted 38.8, although Pluto is much closer to that distance. That doesn’t mean Pluto is or isn’t a planet by the way, but that astronomers expected there to be one there and therefore called it one. What’s actually happening there is quite interesting, but I’ll leave that for now. Neptune was discovered in 1846, by which time a large number of asteroids had also been found and Ceres was no longer considered a planet, which led to the idea that Bode’s Law was mere coincidence. The revision which was able to include Hamlet’s major satellites could be seen, again, as a form of pareidolia, where an increasingly vague formula is used to fit observed phenomena which actually doesn’t reflect any real process or effect but just corresponds to the various coincidences. The sequence was originally n+4, with n=0 for Mercury, rather than a simple doubling sequence, and the fact that the asteroid belt intervenes and Neptune doesn’t fit makes the idea that it’s an actual law more doubtful because there are then three out of ten exceptions to the rule. A side issue, probably not important, is the surprising convenience of Earth being at a round ten units from the Sun. The question arises, then, of whether there really is something about Neptune which puts it in the “wrong” place or whether it’s just that the spurious correlation was revealed by it. Most astronomers would agree with the latter possibility.
Neptune is not the coldest planet in the system in spite of being further from the Sun than any other known planet, at least consistently. This is because, unlike the seventh planet, it has a significant internal heat source. It takes 165 years to orbit the Sun, and having a moderate axial tilt this gives the temperate regions four-decade-long seasons. The axial tilt is 28° and the day lasts sixteen hours, which is technically close to Hamlet’s but differs in that the poles don’t spend most of their time pointing towards or away from the Sun. It might therefore be expected to have seasons dominated by the Sun, but this isn’t obvious because unlike its twin, Neptune is heated internally. This leads to Neptune being warmer than the other ice giant at cloud top level. Like the other outer planets, this heat is due to contraction of the planet from the part of the solar nebula it formed from, but in Neptune’s case there may be an extra factor in the form of its large moon Triton’s tidal influence. The centre is at around 7000°C compared to the other giant’s 5000, possibly because Neptune wasn’t disrupted, but it could also be that both planets go through warmer and colder phases and we happen to be living at a time when it’s that way round. I don’t actually know how they arrived at these figures considering that there are theories that the clouds are cold due to insulating convection layers, meaning that heat doesn’t leak out and is therefore presumably undetectable, but this is what they say. Neptune’s centre is therefore hotter than the surface of the Sun.
Regardless of the temperature at the core, the cloud tops are still very cold at around -200°C. Before Voyager 2 got there, it was speculated that the low temperature could give rise to fast winds in the atmosphere because the vibration of gas molecules at higher temperatures was absent, leading to a low-friction environment, and this did in fact turn out to be so. The winds are the fastest recorded in the system at over 2000 kph. At the equator, the average wind speed is around 1100 kph, which is about the same as the speed of sound at sea level on Earth. On Earth, the Coriolis Effect is somewhat significant in generating wind but the main driver is the primary or secondary solar heating and cooling. The Sun heats the air on this planet, causing it to expand, or cooler areas have contracting air over them, allowing the warmer air to move in and occupy the space due to the pressure difference, or in a more complicated process, land and water change temperature at different rates, causing air movement. Although the core of Neptune is far hotter than its exterior, this doesn’t seem to drive the extreme high velocity winds near the cloud tops. My guess is that it’s somewhat similar to a perpetual motion machine, which of course cannot exist. The input from whatever source to the weather systems, such as the Coriolis Effect, tidal forces and the hot interior of the planet, puts the atmosphere in motion and due to the lack of friction that energy is only lost very slowly, and consequently the winds accelerate until they reach the speed of sound, which prevents them from moving any faster. This is not a detailed explanation and may well be completely incorrect. It’s just a guess.
Neptune has more visible banding than the other ice giant, and also has rotating storms in its atmosphere which have been observed to last up to six years. This is far less durable than Jupiter’s storms, but the size and energy input are smaller so this might be expected. Neptune’s Great Dark Spot is visible in the lower part of the picture at the start of this post, but here it is again:
The spot was 13 000 kilometres long by 6 000 wide, and is a hole in the cloud deck. The white clouds around it are cirrus made of frozen methane and were instrumental in enabling the wind speed to be measured. It’s thought that the spots disappear as they approach the equator, which can take years. As I may have mentioned before, the Great Dark Spot was at the same latitude as Jupiter’s Great Red Spot, and this suggests it’s recurrent. If it is, it also shares with the GRS a tendency to appear and disappear. I’ve mentioned elsewhere that it seems to be more than coincidence that planets tend to have a fluid-related feature at this latitude, including Hawaiʻi, Olympus Mons, the Great Red Spot and this storm, which is intermittent, and although I have a vague impression of a pyramid superimposed on the bodies in question with the apex at one pole, I can’t put my finger on why this would happen or whether it actually is more than cherrypicking.
Neptune’s blueness can’t be explained simply through Rayleigh scattering and there must actually be something blue in its atmosphere which isn’t in Hamlet’s, but what this is exactly is another question entirely. Even so, it is true that the methane contributes by absorbing red light. The different hydrocarbon content contributes to it being warmer than Hamlet due to a greenhouse effect, although this is only relative as it’s still at the temperature of liquid nitrogen on Earth.
This is a fairly well-known image of clouds on Neptune above the more generally blue cloud deck. These clouds are frozen methane, but the picture also seems to show that not far below them is a blue haze with a definite level top to it. The clouds are about fifty kilometres above the haze and are casting such definite shadows because the Sun is low in the sky at this point, as evinced by the night on the right hand side of the image. Although the widths of the clouds here varies between around fifty and two hundred kilometres, I don’t know how that scale compares to the clouds in our own sky. It does sound rather larger at first consideration. I’m also tempted to see them as having been streamlined by the powerful winds and feel they don’t have much chance to be wispy, unlike Earth’s cirrus clouds. They’re almost like contrails in a way.
One theory about Neptune’s clouds is that the planet’s atmosphere is effectively a giant cloud chamber. A cloud chamber is a delicately balanced humid atmosphere used to detect subatomic particles, whose energy as they move through it leaves wakes in the form of clouds. This can be created using the steam from dry ice. The planet in question is of course very cold at the height the clouds can be seen, and it’s been theorised that galactic cosmic rays stimulate the atmosphere into producing these streaks. The coolness of the atmosphere makes these things much more significant for Neptune than here, so if this is how it happens, the cause is similar to the high winds. Ultraviolet light from the Sun is also probably responsible for features in the atmosphere, but probably the haze more than the clouds.
The rate of rotation has the same features as that of the bigger gas giants, as the planet does not rotate as a solid body would. The magnetosphere can be taken as a guide to the rotation period if you like, but it isn’t necessarily any more “real” than anything else and we only think it is because we’re from a planet with a solid surface and a shallow atmosphere. The magnetosphere takes sixteen hours, the equator eighteen and the poles twelve. All of this also raises the question of whether it even means anything to assert that Neptune has powerful winds. Maybe that’s just the rotation of the planet, which varies, but it doesn’t mean they actually amount to winds just because different parts rotate at different rates. The understanding of fluid movement used with Jupiter, that they’re cylinders rotating independently, actually cancels out the idea that there are such winds, although there could still be slipstream areas where the wind would be felt.
Unsurprisingly, the interior of the planet closely resembles the other ice giant’s. As I mentioned before, Olaf Stapledon described Neptune, important in ‘Last And First Men’, thus: “. . . the great planet bore a gaseous envelope thousands of miles deep. The solid globe was scarcely more than the yolk of a huge egg.” The upper atmosphere is mainly hydrogen and helium with some methane. Deeper inside is a liquid, becoming solid, layer composed of water, ammonia and methane, and at the centre is a core somewhat larger than Earth made of silicate rock and iron. Like Hamlet, it probably rains diamonds and there are likely to be diamond-bergs floating in the ocean. There may even be a whole layer of diamond deep within the planet.
There being two similar planets of this kind in the system might be seen as coincidence, but in a cosmic context seems not to be. In fact, Neptune-like planets are more common in the Galaxy than Jupiter- or Saturn-sized ones, and the fact that only one spacecraft has ever visited either hampers understanding of a disproportionately large number of worlds. There are nearly 1 800 known Neptune-like planets, notably referred to as “Neptune-like” rather than “Uranus-like”, which makes me wonder again about that ridiculous name although Neptune is more “typical” seeming since it isn’t tipped on its side. Even more common, and absent from the known Solar System, is the intermediate-mass type of planet both smaller than Neptune and larger than Earth. Some of these are much closer to their stars than our own ice giants, and can’t therefore really be classified as such. Nonetheless, this size and mass of planet is common in the Universe.
Getting back to our own Neptune, one surprising finding was that like Hamlet’s magnetic field, Neptune’s is off-centre and at a radically different angle to its axis of rotation. This creates another puzzle because the orientation of Hamlet’s magnetosphere was attributed to its peculiar tilt and misadventure with a large body in the distant past, but given that Neptune’s is also like that suggests that this is irrelevant and makes me wonder if that ever happened, although the tilt does need to be explained. It’s offset by 55% of the planet’s radius and the magnetic poles are 47° from the axis of rotation, yet no explanation based on collisions or close encounters with large objects has been offered so far as I know.
That said, Neptune does in fact show some evidence for this. Discounting Pluto and Charon, the planet has the largest proportionate satellite of any planet in the system but Earth, namely Triton, which is also the only large moon to orbit backwards, and appears to be a captured dwarf planet. Also, the moon Nereid has a comet-like orbit with its closest approach to the planet being much greater than its greatest distance, making it elongated and highly elliptical. Hence one catastrophe may have occurred to Hamlet and another radical event to Neptune, and the question then arises of what was happening in the outer solar system early in its history. Neptunian auroræ are not distributed like terrestrial ones due to the different magnetic field and the presence of rings, which reduces the quantity of charged particles trapped in the magnetosphere. Neptune and Triton also interact magnetically in a similar manner to Jupiter and Io, although not so strongly. There are diffuse auroræ close to the equator to just over half way to the poles, and more definite rings of auroræ closer to the poles, and brighter near the south pole at the time of the Voyager 2 encounter. Neptune has the weakest magnetosphere of any gas giants.
As mentioned above, Neptune has rings. Once Jupiter’s rings had been discovered by the Voyagers, Hamlet already having had them detected, it seemed inevitable that it would have them too, and it has. They were discovered from Earth in 1984 CE but had been seen occulting a star in 1981 in a manner compatible with them not being complete. That is, it was established that there were curved objects orbiting the planet but not that they went all the way round. This is probably because their width varies more than the other three planets’. There was an uncomfortable period in the early ’80s when for me it seemed inevitable that Neptune would be ringed but there was no evidence either way on the issue. I wanted the giant planets to be uniform. For some reason its ringedness is less emphasised than the others’, maybe because it had become routine by that time and it would’ve been more surprising if it hadn’t been.
There is no uniform scheme for naming planetary rings, as can be seen with Hamlet’s. Neptune’s are named after astronomers associated with the planet, specifically Galle, Le Verrier, Lassell, Arago and Adams. Adams is the one with the wider arcs, which are named Liberté, Egalité, Fraternité and Courage. Egalité is split into 1 and 2. Three small moons orbit between the rings, and there’s another ring associated with the moon Despina. In a way it’s quite nice that there’s a French theme to the naming contrasted with the English theme for Hamlet, but I don’t know if it’s deliberate. One really surprising thing about them is that the supposèd “discovery” was actually an occultation by the moon Larissa, so although they were correct about them being rings, they were correct by chance and misinterpretation of an unusual astronomical event. Neptune is a harder target for ring detection than Hamlet, although that is itself not easy, because it moves so slowly against the background with its 165-year orbit. The rings are, like the other ice giant’s, very dark and of course even dimmer due to the greater distance from the Sun. There’s a big contrast in the widths, with the three inner rings being only about a hundred kilometres wide (i.e. their height) and the others being several thousand, which is unlike Hamlet’s much thinner ones. An image of them with the contrast turned up to show details of the structure looks like this:
It’s really come to something when a planet invisible to the naked human eye is made so bright that its glare almost bleaches out the view of its even dimmer rings. This is a ten-minute exposure made by Voyager 2, which was right there, and still the rings are hard to see without that kind of technique. Whether the average human eye could see them is another question, as ours are very good at adjusting to low-light conditions. It still isn’t that low though, at least compared to bright moonlight, but I fear I’m repeating myself. In fact, all the conditions that apply to sunlight on Pluto also apply to it on Neptune because their orbits overlap distance-wise (they don’t literally). Hence the Sun at Neptune’s distance is just a star. The minimum visible object to someone with good vision is one minute of arc across. After that, it’s visible if it’s luminous but not as an actual shape. This is equivalent to a hair’s breadth viewed from twenty-five centimetres away. From Earth, the Sun appears as a disc thirty times that diameter and is therefore very obviously a ball of light. Neptune, however, is thirty times as far away and the Sun could therefore not be seen as anything more than a star, which is effectively a point source of light. This is, however, quite misleading as it’s still many thousand times as bright as any other star in the sky, and might therefore not appear as a point due to its glare. Lighting conditions on Pluto have been likened to those on Earth after a sunny day shortly after sunset, so the same kind of thing can be expected on Neptune and its moons. In other words, you’d probably hardly notice it at all after a while and it would look like broad daylight, except that the actual illumination is only a thousandth that of the Sun’s here. Looking at it from the other end of the telescope, as it were, Neptune is the only planet in the system, taking Pluto as a non-planet, which is never bright enough to be seen. Its maximum brightness is something like four times dimmer than it would need to be to become visible. Of course there will, as with other celestial bodies, be other species who can see it and in fact Galileo saw it, through a telescope of course, but didn’t notice it was a planet. Likewise, it was reported that it had rings shortly after it was discovered but in this case it was probably an illusion.
Neptune has a rather odd array of satellites. At one point it was thought that Triton might be the largest in the Solar System, and as I mentioned above it orbits backwards compared to most other moons. Nereid has a very eccentric orbit. Up until the 1980s, these were the only two moons known, but Voyager 2 surprisingly discovered a moon, now called Proteus, which is actually larger than Nereid, making it the largest object discovered by the Voyager probes. Due to the mistake leading to the accidentally correct conclusion that the planet has rings, the moon Larissa was also detected in 1981 but it wasn’t realised that this had happened, rather like Galileo and Neptune itself. Voyager 2 found another five, including Proteus, and a further six were discovered this century. Neptune also holds the record for having the most distant moon and the longest time taken for that moon to orbit, Psamathe, which is fifty million kilometres from it and has a period of almost twenty-five years. There are various interesting things going on with Neptune’s moons but that can wait until my next post.
Probably the most prominent appearance of Neptune in science fiction is in Olaf Stapledon’s ‘Last And First Men’. Published in 1930, the science is well out of date, although the description of a yolk in an enormous egg is valid. In this account, our distant descendants are living on Venus an æon hence when they observe a mass of gas on a collision course with the Sun which will cause the Solar System to be disruptd and the Sun to become what we would probably call a red giant the size of the orbit of Mercury. Humanity decides, though not en masse, to escape to Neptune, where it has to contend with enormous gravity and pressure, and first a very cold climate followed by a very hot one. Humans cease to be intelligent and take four hundred million years to evolve into a sentient form again. This is partly because their lifespan is much longer, as most species live at least one Neptunian year. They ultimately become superhuman beings who notably have ninety-six genders and a life expectancy of a quarter of a million years. I find this section of the novel, if that’s an accurate description, to be a particularly satisfying example of speculative evolution, although one which has been left standing by scientific discoveries about the planets involved.
That’s probably a fairly adequate introduction to Neptune. Next time: Triton.
I’m not quite sure how this post is going to go. It’s not going to be the same as the others. Then again, neither was the one about the name of the seventh planet from the Sun. It is, however, somewhat similar to the one about the space between the Sun and Mercury.
Up until now, I’ve mainly posted about the planets, moons and asteroids of the Solar System in one way or another, but one of the most impressive things about the system is not so much the worlds it contains as the enormity of its scale, which is of course itself dwarfed by interstellar space. In a way, the Solar System is a rather irregularly-shaped space centred on the Sun with a radius of at least a light year in every direction. Beyond those points, the gravity of other bodies, usually stars, becomes more significant than that of the Sun. This is a significant feature of the system because out there in the darkness lurk countless bodies, perhaps future comets, and although the distances between them are huge, it can’t be said that this space is entirely empty. However, that isn’t what most people think of as the Solar System, and that’s what I want to consider here.
Most people probably think of this thing as the Sun plus eight or nine planets with the occasional comet. Whereas popular perception may not be the best way to go with science, the system isn’t just about science. The Apollo astronauts walking on the nearest celestial body, for example, weren’t merely “science”, and the discoveries about the Martian atmosphere which helped end the Cold War were very significant for all of us. Looking at Saturn through a telescope for the first time isn’t about science either, but awe and a sense of beauty. Nonetheless, science comes into this.
Saturn is something over ten times Earth’s distance from the Sun. Its orbit circumscribed the known Universe for thousands of years. Within that ellipse, with a little latitude either side of its orbital plane, is a rather busy and light community of stars, asteroids and moons, the largest of which have been visited more than once by space probes. In fact, the largest object never to have been subject to a mission at all in this region is the asteroid Hygeia, with a diameter of 450 kilometres. Outside this region are the seventh and eighth planets plus Pluto and the distance between Neptune and the next planet in is a minimum of almost eleven times that between Earth and the Sun. In other words, the entire distance between the Sun and Saturn is smaller than the width of this gap. It’s also fair, if technically inaccurate, to consider the orbit of Neptune as the true border of the Solar System. It isn’t of course, but we can think of it in that way if the system is considered the set of official planets orbiting the Sun.
Voyager 2 took eight years to travel from Earth to Uranus (sigh). It then took only three more years to cross this enormous gulf, slowing as it went. This is a bit of a distortion because there’s a huge gap between the sixth and seventh planets too, about the same as that between Earth and Saturn at their closest to each other, so in other words a comparable distance to the next gap. However, beyond Saturn lie most of the centaurs, the relatively huge satellite system of that planet and also its magnetic field, and Saturn is also large and able to exert a considerable gravitational influence on the region. This gap is different. The two planets marking its edges are smaller and less massive and in fact roughly the same size. This means there will be a moving point halfway between them where their gravity is balanced, at around two dozen AU from the Sun but varying and sometimes being dominated by the forces of other bodies. The situation is actually quite like that between Venus and Earth due to their similar masses but on a much larger scale. Although the gravitational forces involved are much lower, there’s also a lot less to perturb these forces in such an uncluttered region.
Many objects orbit on either side of this gap. The centaurs are substantially found further towards the Sun and the realm of the Kuiper Belt is beyond Neptune. This raises a couple of complementary questions. One is whether the gap contains such objects and the other is, if it doesn’t, why? Clearly the further out from the Sun one goes, the less stuff there is. However, in this case there’s a difference between the objects on either side, with small, comet-like objects within Hamlet’s orbit and often relatively large, planet-like objects beyond Neptune’s. What, then, is present between them, if anything?
One very definite and obvious object spending time in the gap is of course Pluto. It was there between 1979 and 1999 CE. Pluto does not, however, cross Neptune’s orbit, at least most of the time. It’s inclined at seventeen degrees to the ecliptic compared to Neptune’s one degree and four dozen minutes. Like all planets, however, both Pluto’s and Neptune’s orbits gradually “spirograph” around the Sun over a long period of time, and therefore theoretically they could collide at some point. They never have, or Pluto would no longer exist, or perhaps it’d be a moon of Neptune like Triton is, along with its own companions. Pluto ventures twenty-three million kilometres closer to the Sun than Neptune does, although that doesn’t mean it ever gets as close as that to Neptune due to the angle of the orbit. In fact, even at their closest approach the two can be as far apart as Earth and Saturn, which gives a sense of the scale involved at that distance. Pluto’s tilt also doesn’t really need an explanation. It’s simply that the gravitational influence of the Sun is weaker out there, and Pluto is not very massive and therefore has less momentum than the likes of Neptune.
It’s been calculated that the region from twenty-four to twenty-seven AU from the Sun is a haven for objects with stable orbits over the age of the Solar System to date, so there are some reasons for supposing there will be something there. This is actually quite exceptional and doesn’t apply, for instance, to centaurs, whose orbits are unstable over a period of a few million years. An object around that distance in a roughly circular orbit will never be captured by a planet, dragged into the inner system like a comet or ejected far out of it. In fact, this is the largest region where there are potentially stable orbits all the way from the Sun to Pluto’s aphelion. This belt is divided into two particularly stable regions around 24.6 and 25.6 AU. These calculations also predict that there are around three hundred objects at least fifty kilometres in diameter in this belt, which can be compared to the Kuiper Belt, some of whose occupants are much larger than that. The reason they haven’t been detected is likely due to their darkness and the dimness of solar illumination at that distance. They would be hard to spot. If they do turn out not to be there, it suggests that an event such as a planet shifting its position through that region has cleared them from it. That planet might actually be Neptune because it’s in the wrong place. Neptune alone among the planets does not obey Bode’s Law. Another possibility, and I say this as a naïve amateur, since to my knowledge this has never been suggested, is that this apparently clear space is the result of a “mini-Neptune” moving through it before being ejected from the system. The most common type of planet detected orbiting other suns is a planet about midway between Earth and Neptune in mass, so maybe this is where it used to be. It’s unlikely because it would again violate the Titius-Bode Series.
Suppose, then, that a typical cis Neptunian object has a diameter of fifty kilometres and has a roughly circular orbit near the ecliptic plane at a mean distance of 25.6 AU. What features would it be likely to have? Well, it would take 129 years to orbit the Sun, which at that distance would be six hundred times dimmer than from Earth. It would also have a fairly dark reddish surface, moderately cratered, and probably hasn’t been geologically active for a very long time if at all. Surface temperature would be around -200°C and it would be composed of an undifferentiated mixture of ice and rock. In fact, it would probably very closely resemble some of the outer moons of the two ice giants and that may be where those came from in the first place.
Another thing about this region is that the centaurs and Kuiper Belt each impinge on it, from opposite sides. There are a considerable number of trans-Uranian centaurs. One of these sounds very much like the predicted type of object: 2005RL43. This is 24.6 AU from the Sun, has negligible eccentricity and has a diameter of 143 kilometres. Another is Nessus, although that has a very elliptical orbit. On the other side, clearly at least one plutino moves into the gap so the question arises of whether there are any others. The definition of a plutino is that it orbits twice for three orbits of Neptune, which is what Pluto itself does. This does put their mean orbit at a distance of 39 AU, about the same as Pluto’s and also in accordance with the Titius-Bode series, but there’s no reason a body beyond Neptune wouldn’t be within its orbit for part of its year, and this would put them in the gap. I don’t want to spend too long on this because at some point I want to talk about Kuiper Belt objects in their own right, but it would be remiss of me not to mention them here too. Some of them dip into the stable region at their perihelia. A few of them even approach the Sun more closely than Saturn and there’s one which not only does this but also recedes to a maximum of more than thrice Pluto’s distance. Plutinos are not the only class of trans-Neptunian object. Cubewanos stay beyond it and twotinos orbit once for every two orbits of Neptune. There are two known twotinos whose perihelia are within Neptune’s orbit, just barely. In other words, their orbits are less eccentric than those of the plutinos.
There are some comets whose aphelia are lower than Neptune’s. Each gas giant has an associated cometary family, whose aphelia are close to those of the planets. Jupiter’s is largest, followed by Neptune’s. That of the seventh planet is particularly small. There isn’t too much more to say about it than that, except that as well as these two families, other comets move through this region on their way in and out of the inner Solar System. The gas giants attract and steer comets into these orbits, and this happens with the two ice giants. Neptune is closest to the Oort cloud, so it’s particularly significant in this respect.
If the orbits of the planets marking the edges of the gap are projected onto the ecliptic, the area of this region is just over 1500 square AU, which is almost five hundred times the area of Earth’s orbit and 175 times the area of the inner system. Space has more than two dimensions of course, and the bodies occupying and defining this region don’t orbit in the same plane, particularly the moons of Hamlet. Features characterising it are quietness, coldness and dimness.
I just thought I should mention it because it’s easy to ignore the space between things.
I’m not sure how much to make of the idiosyncratic naming scheme for the moons of the seventh planet from the Sun. As a fan of language and word play, they appeal more to me than they perhaps should if I’m just going to be talking about them in a scientific way, but the fact is, there’s the Universe and there’s the person observing the Universe, and you can’t entirely step outside yourself. The rest of the Universe is, in a sense, your mind reaching out to it and placing it within your own private world. It’s part of you. That said, science tries hard to be objective. However, it’s significant to many of us that twelve Americans walked on Cynthia and that people do romantic things “by the light of the silvery Moon”. Cynthia is culturally significant to us.
With regard to the twenty-seven known moons of the planet I’ve been calling Hamlet, it might be a little hard to imagine how such a small system so far away from us could have any consequences for us Earthians. They don’t figure prominently even in the realms of science fiction and astronomy. If we had sent more than one probe to the system, maybe it would be more significant to us all. If it turned out to be the only other abode of life in the system, it would be considered hugely important. There is in fact at least one aspect to the planet which makes it relevant to life here. There is only a weak internal heat source and the Sun makes little contribution to its temperature, leading to computer models of the atmosphere being dominated by the Coriolis Effect. Due to the abstraction of the model from observed conditions, which of course confirm its accuracy, this constitutes yet another refutation of the hypothesis that Earth is flat, because of how the effect operates in our own atmosphere and attempts by flat Earthers to explain this in terms of solar heating (and perhaps lunar cooling!). Even this, though, is something of a niche explanation.
The moons concerned, taken together, don’t add up to much, which is why I’m dealing with them all in one go. Their total mass is less than half that of Titan, and also of Neptune’s giant moon Triton, but this isn’t the same as saying they’re small for two reasons. Firstly, Titan itself is 96% of the mass of everything orbiting Saturn including the rings, so the seventh planet’s moons are actually bigger en masse than all of Saturn’s except for Titan. Secondly, volume, surface area and diameter are counter-intuitive. Our own moon has only 1/81 Earth’s mass but has a diameter a quarter of our planet’s. By the time you get this far out from the Sun, even many compounds gaseous on Earth are frozen solid. Umbriel is probably the warmest moon, because it’s dark and absorbs more light, and has a maximum temperature of -188°C, barely warmer than the boiling point of air. One consequence of this is that the densities of the moons are very low, which means they’re larger than their masses suggest. It’s also interesting to compare the situation here with that in Neptune’s vicinity.
I’m going to reiterate this yet again in case you’re coming across this post without having read any of the others: the moons of the seventh planet don’t take their names from any mythological tradition, but from works of literature, mainly Shakespeare’s plays. I find this refreshing but there is an element of cultural imperialism to this. Then again, the same is true of the dominant Greco-Roman tradition for the other planets, moons and asteroids in the system, but what’s done is done I suppose. There were two widely separated phases of discovery, which is also true to an extent of the other gas giants but in the cases of Jupiter and Saturn the rate of discovery is rather different. Jupiter’s Galilean moons were all discovered in 1610 CE, then nine moons were found between 1892 and 1975, followed by three via the Voyager probes and a spate of discoveries from 2000 on. Saturn’s show a more regular distribution between the seventeenth and nineteenth centuries, a rush associated with the Voyager missions and a further sequence of discoveries from 2000s on as with Jupiter’s. My experience of Hamlet’s moons is that five were known when I was a child, and because one’s childhood experience is just how things are, and one hasn’t yet gotten used to change, that was just how things were. I wasn’t aware of the peculiar naming scheme because at the time they seemed just to be kind of Latinate, for instance Ariel and Miranda, although one is much more likely to come across a human Miranda in everyday life than, say, a Phœbe, and way more likely than meeting someone called Ganymede. The first four were discovered in pairs in the eighteenth and nineteenth centuries, then Miranda in 1948, then we had to wait until Voyager for any more discoveries. After that, Caliban and Sycorax were found in the ’90s, Perdita was discovered using old Voyager data and the rest come from between 1999 and 2003. Since then, no more discoveries have been made but this might be because Hamlet is a neglected planet compared to the others, so maybe nobody’s looking. It is also very dim and distant, so it might be that.
Titania is the largest. This is quite possibly the poorest decision ever in naming a moon. Titan was already known by the time it was discovered and there are different ways of pronouncing it. And how do you refer to something to do with Titania without people thinking you’re talking about Titan? However, we can talk about the place. It’s the largest and most massive of the moons in a system which isn’t particularly large or massive. Here it is:
That slight blurring is probable due to the impossibility of correcting entirely for Voyager 2’s motion blur. About forty percent of its surface has been seen. Like the other moons, Titania doesn’t orbit near the plane of the Solar System due to its planet rotating on its side, meaning that that illuminated surface in the picture remains in daylight for decades at a time, just as the other side stays in night. This means that one pole is somewhere near the middle of the lit portion of that image, in this case the south, because like all such images of the moons, this was captured in 1986. All the large moons are about half rock and half ice, so they’re actually denser than many of Saturn’s, and Titania is both the largest and densest of all of them. All the moons also have largely grey surfaces, Umbriel being darker than the others, hence its name. Titania is half Cynthia’s width and has icy and dry ice patches on its surface. It’s considered likely that it’s differentiated into distinct layers with a rocky core and icy outer layers. There may be a little liquid water inside at some level. There could also be a very thin non-collisional atmosphere of carbon dioxide.
Oberon was discovered with Titania and is slightly smaller, orbiting outside Titania’s path. It’s more heavily cratered. Both are at comparable distances from their planet as Cynthia from Earth. For some time after the pair was discovered, it was thought that there were six moons overall but after many years the others came to be considered spurious, although of course there are other moons. A significant difference between it and Titania is that the latter orbits entirely within the magnetosphere whereas Oberon passes in and out of it. Again, only forty percent of the surface has been mapped. It’s also the outermost large moon. Oberon’s features are named as follows:
The surface has a sheen to it and is slightly red except where newer craters have yet to acquire that: those are slightly blue. This reddening is due to space weathering, where electrically charged particles hit the surface. Unlike all the other large moons, the trailing hemisphere has more water ice than the trailing one. It’s almost exactly the same size as Rhea, which makes me wonder if there’s a peak in moon sizes at about this diameter across the Universe as it’s also quite close to Titania in size. There are apparent rift valleys, such as Mommur Chasma. In the distant past, when the moon was young, processes within it had an influence, namely its slight expansion by about half a percent of its diameter. Mommur Chasma is apparently named after the original French version of the tale of Oberon’s home, «Huon de Bordeaux».
Miranda and Umbriel are probably the most distinctive of the large moons. “Miranda” the word is a gerund meaning “worth seeing”, hence the “-anda” names Amanda – “worthy of love” and Miranda. Samuel Johnson once said of the Giant’s Causeway that it was “worth seeing, but not worth going to see”. Well, Miranda seems to fall into the same category. It is indeed worth seeing but given that only one spacecraft has ever been there, possibly not worth going to see. However, it’s still remarkable. Here it is:
As you can see, it looks rather rough. It has a diameter of 370 kilometres and is therefore on the lower edge of worlds whose gravity is able to smooth them into an approximate sphere. At some point in the past, it was hit by something and shattered into small pieces which then all fell back together haphazardly. There are enormous cliffs all over the moon, including the highest cliff in the System, Verona. Twenty kilometres high, if an object falls off Verona cliff it would take ten minutes to fall to its foot. Although it’s tempting to believe that these cliffs are the result of the shattering, they’re more likely to be due to the same kind of expansion as Oberon’s chasms. The number of craters suggests Miranda was only formed during the Mesozoic, or at least that whatever happened to it took place then.
Umbriel is the only major moon not at least ambiguously named after a Shakespeare character. Instead, the name is taken from Alexander Pope’s ‘The Rape Of The Lock’, where it refers to a “dusky melancholy Spright”, also referred to as a gnome. Clearly the name is related to the Italian and Latin “umbra” – shadow. As well as being particularly dark, Umbriel has a crater outlined in bright white material where a pole would’ve been if it orbited normally, but it so happens not to be situated there because of its primary’s odd axial tilt:
The mere fact that the light ring is at the top of this picture shouldn’t be taken to indicate that it’s at any kind of pole, because the moon rolls round as it orbits in a manner typical of such bodies, but its orientation here makes it look like a polar feature. Its name is Wunda and the feature is ten kilometres wide. Its origin is unknown. The surface is generally dark bluish, although that’s a relative way of describing it along the lines of “black” often being tinged with a cast of a particular hue rather than it being pure black. However, it also seems odd to me because most dark objects in the outer system are red-tinged rather than blue, suggesting that it isn’t the usual tholins that are coating the surface. Nothing other than craters are known on the surface unless you count the ring.
Ariel is the other major moon with an ambiguous name, as it could be named after either Ariel from Shakespeare or Ariel from Pope. Its mass is about the same as all the water on Earth’s surface. It’s somewhat bigger than Miranda and slightly larger than Ceres. It’s half ice and half rock, and despite its name has no washing powder on its surface. That comment isn’t quite as flippant as it sounds because other bodies in the Solar System do have washing soda in and on them, including Ceres.
Not the same thing
What the heck is it about this planet and its system which leads to it having such peculiar names‽
Right, so Ariel is the second closest major moon to its planet. It’s also the brightest per area at around four times as bright as Cynthia, although being twenty times as far from the Sun it only has a four hundredth of the sunlight falling on each square metre in the first place and is well under half the size. Its surface is more varied than the likes of Umbriel, as far as has been seen anyway, with canyons, ridges, craters and plains all present. The chasms are often bowed in the middle rather than flat or tapering, and seem to result from freezing water and ammonia altering the dimensions of the moon. Chasms often become ridges, suggesting that they are a similar response to the freezing of liquids, so the moon’s surface could be seen as a mixture of the wrinkly deflating balloon and the cracks of an expanding soufflé (but without the bubbles). The plains are probably similar to lunar maria, in this case involving the eruption of a thick liquid, possibly a mixture of ammonia and water. There are no large craters, suggesting that the surface is younger than the Late Heavy Bombardment period early in the system’s history. The largest crater is the 78 kilometre-wide Yangoor. Ariel has similarities with Saturn’s Dione.
Those, then, are all the large moons. To summarise that bit of the system, they are in order Miranda, Ariel, Umbriel, Titania and Oberon. Their spacing corresponds to a law similar to the Titius-Bode Series relating to the spacing of the planets, if that is indeed valid. Mary Blagg’s 1913 generalisation of Bode’s Law yielded the formula A(1.7275)n(B+f(α+nβ)), where A for this system was 2.98 and B 0.0805. Hence there seems to be something orbital resonance-related going on here. Some of them were probably warmer in the past due to having less circular orbits and so more vigorous tides.
I want to mention a slight personal peculiarity at this point. As a small child I used to delight in memorising the names of the moons of the outer planets. This led to the oddness of Jupiter’s moons having their names changed to my considerable confusion in the late ’70s. In the case of “Hamlet”, the seventh planet, the planet whose name one dare not speak, the list was rather short and didn’t really stick in my memory, but oddly it had an extra member according to my unreliable recollection: Belinda. I didn’t think much of this because the subject of those moons rarely or never arose until 1986, and even then it wasn’t all that, partly due to the Challenger disaster. Belinda is a small moon orbiting below Miranda which wasn’t discovered until 1986. I had no knowledge of ‘The Rape Of The Lock’ at this time, so I can’t account for the fact that for well over a decade I thought there was a moon called Belinda when it didn’t even get named until after the Voyager 2 mission. This seems to be rather akin to a Mandela Effect, such as the placement of single releases in my memory being several years different than in reality. For what it’s worth, Belinda is an elongated moon 128 kilometres long by sixty-four kilometres wide and extremely dark, and it may collide with other moons in a hundred million years or so, so it could be a future ring. There are thirteen known moons within Miranda’s orbit and many of them are elongated, although I personally wonder if that’s the reality or whether it’s motion blur. Presumably that’s been taken into account though. These cis Mirandan moons are known as the “Portia Group” and are named Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, (the second largest, at 156 kilometres maximum diameter), Rosalind, Cupid, Belinda, Perdita, Puck and Mab. Puck is the largest, with a diameter of 162 kilometres and was the first discovery after the larger moons, in 1985, by Voyager 2 shortly before it began the main part of its mission. It’s heavily cratered, dark and has water ice on its surface. Because it was the first moon to be discovered, there was time to program the probe to get more information on it than the other small moons. Three of its craters are named: Butz, Lob and Bogle, named after impish spirits in European mythologies.
Then there are the nine known outer moons, which are trans Oberonian: Francisco, Caliban, Stephano, Trinculo, Sycorax, Margaret, Prospero, Setebos and Ferdinand. Sycorax is the largest of these at 157 kilometres diameter. It’s more than twenty times further out than Oberon and is light red in colour. It has its own rotation period of seven hours, not locked to the planet and takes three and a half years to orbit. It averages twelve million kilometres from Hamlet. All of the outer moons orbit backwards with respect to the planet, which itself technically rotates in the opposite direction to all other official planets except Venus. The orbits are not in the equatorial plane. The outermost moon is Ferdinand, orbiting on average twenty million kilometres from the planet and taking almost eight years to do so. Margaret is unique among this group in orbiting in the same direction as the large moons.
When the large moons were first discovered they were numbered in order of their discovery. This was then changed to the order of their distance from the primary because of course they’d change the system because it’s “Hamlet” isn’t it? Hence there are two different numbering systems.
It isn’t that the moons are less distinctive or interesting than those of Jupiter and Saturn, although they may in fact be, so much that little is known about them. The larger ones certainly seem to be more similar to each other than those of the two largest gas giants and there isn’t as much interaction between them. They are also rather unlike the moons of Neptune, which include a major anomalous member. The general impression they give is of a system of remarkably unremarkable moons of average dimensions, although in a way this is surprising considering that they all effectively have days lasting seven dozen years.
I’m not sure what to do next. I will probably more on to the rather similar Neptune, but there might be something interesting going on between the orbits of the seventh and eighth planets so I might also consider that.
Look here for an explanation of the post title. At least for this post I shall be calling this planet Hamlet rather than the silly name. So far as I know, nobody has ever called it that before and it may not function well as a viable official name, although I think it would. Although there may be issues of cultural imperialism, the character as portrayed in the play in question is in a sense global property. On a different note, it has an even lower population than a hamlet.
Hamlet used to fascinate me inordinately as a child, probably for two reasons. One is that it’s blue. In fact, Neptune is if anything bluer, the image above being false colour, but James Muirden the astronomer commented in his book that he definitely saw it as having a blue tinge even though everyone else seemed to see it as green. The border between green and blue seems to be more disputed than most colour differences, and it’s worth remembering that colour terms in other languages often vary, and also tend to occur in a particular order. I presume that Japanese calls the colour in question “青”, as does Mandarin (kind of). The other reason is that for whatever reason, Hamlet is the most obscure planet, being mainly used as the butt of jokes because of its name, which makes it intriguing and a target for the imagination. Hamlet is also only a little denser than water, and at the time of the 1930s (CE) encyclopædia I was getting my info from, its density seems to have been estimated as the same as water, suggesting to astronomers at the time that the planet was a globe of liquid. In 1977, I wrote a story called ‘A Holiday On Uranus’ about exactly that, set in 2177. I remember it fairly vaguely, but in it Hamlet was inhabited by intelligent fish-like beings living in its vast ocean and there was a security scanner used at the spaceport which used terahertz radiation to reveal the surface of the body in clothed people, which was eventually invented for real. Travel to the planet was at near the speed of light. I also imagined slavery in the Saturnian system and cruel and oppressive measures being taken to modify the bodies of Saturnians to make it impossible for them to rebel in an analogy to the Atlantic slave trade. I still have it somewhere I think.
At that time it was still possible to project one’s imagination onto the outer Solar System in such a way, although my view was clearly influenced by the fact that most of what I’d read about Hamlet had been written in the ’30s. Also, in one of those odd random associations one gets as a child, Bing Crosby’s ‘Little Sir Echo’, about a personified echo who was “ever so far away”, always used to make me think of someone living there, and I even went so far as to calculate how long it would take sound to travel the distance from Earth to the planet and back, which is around five and a half centuries. I also imagined a steam locomotive travelling there, which would probably take about a millennium, though that’s a guess. It strikes me that all my imaginings about Hamlet were extremely outdated even for the time I was making them.
Back in Stapledon’s day, and he was chiefly active in the 1930s as far as popular fiction was concerned, the giant planets weren’t considered to be gas giants, but extremely large rocky planets with thick and deep atmospheres. Consequently he was able to imagine Neptune in particular, and also to a limited degree Hamlet, as planets inhabited both by native life and the descendants of life from Earth, and given the increased radiation from the Sun æons in our future, Hamlet has agriculture at its poles, the equator being too hot, suggesting that at that point its peculiar rotation had yet to be discovered.
This brings me to the first real point about the planet: it “rolls around” on its side. Hamlet does not rotate “upright” like most other planets. It doesn’t even rotate at a somewhat tilted angle. Instead, each pole spends a season of the seven dozen-year long orbit pointing towards and at another time away from the Sun, as its axial tilt is 98°. This means that for most of the surface, with the exception of the equatorial region, there are forty-two years of daylight followed by another forty-two years of night. Hamlet does, however, rotate properly every seventeen hours, so at the equator it would have a normalish day with sunrise and sunset. This zone is about fourteen thousand kilometres wide. If it was much closer to the Sun, this peculiar arrangement would lead to very extreme seasons, but Hamlet is actually colder than the next planet out, Neptune, at -224°C. It has the coldest average temperature of any of the planets in the system. This anomalous situation is thought to be caused by the same incident which tilted it so extremely. It’s believed that a major impact or close encounter between a massive object and Hamlet knocked it onto its side and stirred up its atmosphere to the extent that the warmer layers nearer the centre of the planet, where the temperature is about 5000°C, ended up circulating towards the cloud tops and radiating the heat which in other gas giants is insulated from space by thousands of kilometres of not very conductive fluid. It might be thought that the reason is that half the planet is in darkness for forty-two years at a time, but this is not in fact the reason. Hamlet is so far out that it doesn’t really make as much difference to the temperature, and like many outer worlds the internal heat is a major contributor to the climate and weather. However, Hamlet is smaller than the two inner gas giants and has no significant tidal forces to generate heat, so it would in any case have a much cooler interior even without the incident which stirred it up.
When he discovered the planet, William Herschel thought it was probably a comet. It’s remarkable in being the first planet to be consciously discovered in historical times. There is a sense in which Venus was discovered when it was realised that the Morning and Evening Star were identical in the thirteenth century, which also led to it being given that name because the Morning Star was dedicated to the goddess, but an entirely new planet had never been discovered before. Remarkably, Herschel lived to the age of eighty-four, which is the same length as Hamlet’s year. Asteroids began to be discovered about twenty years later. The planet often seems to be passed over. For instance, there are relatively few works of SF which feature it. One exception is Fritz Leiber’s ‘Snowbank Orbit’, a 1962 short story in which the spaceship Prospero ejected from the inner system by an explosion in a battle attempts a slingshot orbit around Hamlet to bring it back inward. This was before such a manœuvre had been attempted for real as far as I know, but is now common, though not round the planet in question. Leiber tends to focus on Shakespeare, so his inclusion of Hamlet in that tale is probably due to its own naming theme. I haven’t read it all, but suspect that the planet only really participates in the plot as a distant “roundabout” rather than a planet in its own right. To be fair, so little was known about the place back then that it might not have had much opportunity to be anything else, although it’s all about imagination and Leiber was substantially a sword and sorcery author as much as an SF one. Cecelia Holland’s ‘Floating Worlds’ novel does have it as a proper location though. I actually owned that book for decades but never got around to reading it before I ended up giving it away, so I can’t enlighten you on its content.
The key concept here, then, seems to be that Hamlet tends to be ignored to a much greater extent than other planets, except for the obvious occasional puerile comment. Is this fair? Is it just that the silly name puts people off taking it seriously, or is there something about it, or perhaps all the other planets, which lends itself to being ignored? Is it the Basingstoke of the Solar System? Come to think of it, is Basingstoke really that boring? Am I being unfair? All that said, Hamlet as a planet, as opposed to our relationship with it, is indeed unusual because of the fact that it orbits on its side, if for no other reason. It’s also the first planet to be found with rings after Saturn, within my lifetime in fact, and its rings are notably different to Saturn’s, being darker, thinner and more widely spaced. Its moons are, uniquely in the Solar System, not marked by any outstanding features. Neptune has the kudos of being the outermost planet if Pluto isn’t counted as one, and for twenty years at a time Neptune really is the outermost due to Pluto’s peculiar orbit. Neptune also has unusual moons and the fastest winds in the system, but I’ll deal with all that when I come to it.
It is, however, worth comparing the two worlds, as they’re probably the two most similar planets in the Solar System. I’ve kind of been here before. Both are roughly the same size, very cold, the same density and have similar day lengths. They also have similar colours and compositions, and their size and density dictate that their cloud top gravity is similar. Although Hamlet is the colder, the difference is only about ten degrees, bearing in mind, however, that ten degrees is a bigger difference at such a low temperature than it is at room temperature and more like a difference of thirty degrees for us.
Here’s the picture I posted last time:
This is Hamlet as it looked to Voyager when it got there in ’86. The equinox occurred in 2007 so this is something like twenty years off from that, a quarter of a “year” or so away from that point. It’s exceedingly featureless and fuzzy looking, unlike the much clearer and more vivid Neptune:
It’s possible that the haze in the atmosphere of the closer planet is seasonal, but this rather uninspiring view is enough to make one understand why it tends to be ignored. After all, just imagine if a space probe costing millions had been dispatched all the way to the place and it had come up with nothing but for the greenish cueball image shown above. Fortunately, Voyager visited all four gas giants and is to date the only spacecraft ever to have visited either Hamlet or Neptune. It took four and a half years to travel the distance from Saturn to Hamlet and at the time it got there, January 1986, the planet was invisible to the naked eye. Hamlet dips in and out of visibility because of its distance and orientation, but is bright enough to be visible as a faint “star” some of the time to people with good eyesight who know where to look. In order to get a good look at Titan, Voyager 1 had manœuvred itself out of the plane of the Solar System and visited no planets after Saturn in late 1980, but Voyager 2 went on to cover Hamlet and Neptune. This means, of course, that the planet didn’t get as much attention as the previous two in any case. There were also imaging challenges. The rings are as dark as coal and the moons are not only dark but also dimly-lit compared to Jupiter’s and Saturn’s. Moreover, the velocity with which Voyager 2 moved through the system marred many of the images with motion blur. This brings up an important issue often raised by conspiracy theorists about NASA. Images taken by space probes are, as far as I know, always processed from the raw form in which they’re received, for this kind of reason. There may be too much or too little contrast, and in this case the problem was that the blur had to be filtered out. I have little idea regarding how this was done, as I would’ve thought that blurring would mean that some features would have obliterated others completely owing to brightness, but maybe not. I do know it seems impossible to get rid of a different kind of blur with processing in that way, namely when things are out of focus, because otherwise an out of focus image could be drawn which would appear to be in focus to someone with myopia, and that doesn’t happen, I’m guessing because of entropy. However, motion blur is not the same thing. Techniques of processing the blur have also improved since 1986, so it’s been possible to extract new information from the data received at the time. In the case of Hamlet I’m tempted to say that it hardly matters because so little detail is apparent, due not to motion blur but the basic appearance of the planet itself.
Another aspect of Hamlet’s appearance is that for human eyes the green-blue colour tends to dominate and make details hard to see. This is similar to the way a clear daytime sky on Earth, so to speak, looks bluer than it really is to many people. This sounds like nonsense, but I have to interject a personal note here that I don’t actually see the sky just as blue, and this is an issue which has come up repeatedly and I haven’t been able to resolve satisfactorily. When I look at a cloudless blue sky in broad daylight, I see large purple “splotches” all over it. These are not directly linked to my vision because they stay in the same parts of the sky when I look around, so it isn’t a question of glare creating an optical illusion due to the blood in my retinæ. It may be connected that in fact the Rayleigh scattering responsible for the bluish colour of the sky isn’t confined to blue wavelengths but actually affects indigo and violet light even more, and I suspect that what I’m seeing is uneven scattering of these higher frequencies. I don’t know why I would notice this more than other people. I wouldn’t go so far as to say that I see the sky as purple or indigo, but it definitely doesn’t look merely blue to me, and for some reason nobody else has ever mentioned this, so I presume they don’t or can’t see it. Nonetheless, if the human eye were equally sensitive to all wavelengths of visible light, the sunlit sky wouldn’t look blue to anyone but more indigo.
I’ve never seen Hamlet with a telescope or anything else, but only via images processed imperfectly for human colour vision. Through violet, orange and red filters, the globe is banded in the same way as Jupiter and Saturn are, though more subtly. The green and blue colour of the atmosphere, however, drowns this out to the unaided human eye. I’ve previously mentioned conspiracy theorists in connection with the question of NASA image processing. Flat Earthers would have the same problem explaining models of Hamlet’s atmosphere as Titan’s, because of the dominance of the Coriolis Effect. Hamlet is very cold indeed, unlike Jupiter and Saturn has only a weak internal heat source, and unlike all other planets in this system orbits on its side. This means that models of its atmosphere correctly show the movements of clouds in a counterclockwise direction dominated by the Coriolis Effect. Note also that these models do not depend on the actual existence of the planet itself, since they’re merely an extrapolation of what happens in a fluid body of Hamlet’s character. The movements are dominated by the movements of the planet itself and not by heat from inside or outside, in spite of the fact that entire hemispheres are daylit for forty-two years at once while their antipodes are nocturnal for the same period, and it might be thought there would be a big temperature difference driving the winds, but there isn’t. This is difficult for flat Earthers to explain because of the rotation of weather systems in our own atmosphere being clockwise on one side of the Equator and counterclockwise on the other.
Hamlet has a number of unusual features which are difficult to explain simply. It rotates on its side, the magnetic field is neither oriented towards the poles or particularly away from them and originates from a location about a third out from the planet’s centre. It’s also colder than expected, and the moons are unusual as well. The most popular explanation is that a roughly Earth-sized body collided with the planet and still has much of its material within it, knocking Hamlet off its axis, changing its composition and causing the formation of carbon monoxide from some of the methane, in other words burning the atmosphere via incomplete combustion due to low oxygen level. Although this is also used to explain the strange magnetic field, I don’t know the connection. Maybe no-one does. This peculiarity also means that unlike any other known planet, Hamlet’s auroræ are equatorial rather than polar, although they do occur around two localised areas on opposite sides of the equator.
One thing I seem to have been right about is that Hamlet contains a vast water ocean, although it is mixed with ammonia, altering its freezing point. Of Neptune, a rather similar planet in many ways, Olaf Stapledon once said, “. . . the great planet bore a gaseous envelope thousands of miles deep. The solid globe was scarcely more than the yolk of a huge egg”. Hamlet and Neptune are by far the two most similar planets in the System, and this is equally true of both. A major fact about both which is almost completely ignored is that it rains diamonds. What happens is that methane is compressed, squeezing out the hydrogen and causing the carbon left behind to form into diamonds under the extreme pressures. These then fall through ever-hotter layers towards the core, where they vapourise, bubble up through the ocean and recrystallise at the top. This also means there may be “diamond-bergs” floating on the ocean. I used the tendency for gas giants to form diamonds in this way in my novel ‘Replicas’, where diamonds have become a monetarily worthless byproduct of the deuterium and helium-3 mining industry on those planets. ROT13’d text spoiler: Zryvffn raqf hc bjavat n qvnzbaq znqr sebz ure cneragf’ erznvaf, fuvccrq onpx ng terng rkcrafr sebz Nycun Pragnhev gb Rnegu, juvpu vf cevpryrff gb ure ohg nf n cenpgvpny bowrpg vf cenpgvpnyyl jbeguyrff. https://rot13.com/. The diamonds may also be floating in a sea of liquid carbon. If this is so, or if there’s a whole geological layer of diamond, it could explain why the magnetic field is so different.
It takes over two and a half hours for a radio signal to pass between Hamlet and Earth, and the round trip is of course twice as long. Voyager 2’s transmitter is about as powerful as the light bulb in a fridge at 23 watts. This is stronger than a mobile ‘phone signal but way weaker than most radio stations. It works over such a long distance because the dishes used are aimed directly at each other, the frequency is free of interference by other human-made signals and the antennæ are very large. This could’ve been mentioned at any point in a number of my recent posts, but it may as well be here. In the case of Hamlet, this single spacecraft is responsible for practically everything the human race knows about the planet, and it relies on that tiny gossamer thread of a radio signal sent in the mid-’80s from two light hours away by a transmitter as weak as a dim filament light bulb. The initial baud rate was about 21 kilobaud, reduced in the end to a mere one hundred and sixty bits per second. They’re pretty amazing ships.
The Voyager mission to Hamlet was overshadowed by tragedy. Its closest approach took place on 24th January 1986, when I was at the height of my arguments with the fundamentalist Christians I met at university (that is relevant, as you’ll see). The Challenger disaster occurred on 28th, and was reported some time in the afternoon. I first heard of it as I was queuing for dinner at my hall of residence, and the kind of “head honcho” Christian student responded that it was “good” because it would persuade people to focus on and spend money on more pressing things. Whereas that’s a common and valid opinion I happen not to share, there’s a time and a place, and I get the impression he was saying that for shock value, which doesn’t seem very Christian by any internal standard. That, then, is my abiding memory of the Challenger disaster, and regardless of the value or priorities of NASA’s Space Shuttle program, the fact remains that seven people lost their lives that day, and of course anyone’s death diminishes us all.
A tangential result of Challenger was that it eclipsed the news from Voyager 2. It was also intimately connected with it in that NASA was inundated with letters requesting that the newly discovered moons be named in memory of the Challenger astronauts. This didn’t happen, even through coincidental Shakespearian characters having the same names. It was a factor in this naming proposal that there was a teacher on board, as many people who were children at the time were watching the launch live on TV due to this connection. It’s also a little-known fact that NASA almost sent Big Bird of Sesame Street, in character, on this flight. In 1988, the IAU, an organisation I currently like less and less the more I hear about it but maybe I’m being unfair, and it is after all an organisation and those are usually bad in some way, voted not to adopt the names of the astronauts for moons because they weren’t international enough. This might seem to make some sense until you consider that they’re actually named after Shakespearean (sp?) characters, which are of course associated with England, so their decision didn’t actually make much sense. However, at least some craters on the far side of Cynthia got named after them.
Hamlet has rings. Although they seem quite different to Saturn’s from a distance, close up pictures are hard even for experts to distinguish between at first glance once the image’s dynamic range has been boosted, because they show the same ringlet structure and there are also at least two shepherd moons, Ophelia and Cordelia. The rings are labelled using Greek letters and numbers, apparently without particular regard to their order. From inner to outer they’re referred to as ζ, 6, 5, 4, α, β, γ, δ, λ, ε, μ and ν. I presume this anomalous order is connected to their order of discovery because the way I remember them from the early ’80s they were named from α to ε. This also seems to continue the tendency to call things to do with the planet odd names, as it seems more logical either to number them or give them letters but not mix the two. The outermost two are red and blue respectively and the rest are dark. The first five, α to ε, were discovered on 10th March 1977 when the planet crossed in front of the star SAO 158687 and it blinked on and off regularly on either side of the planetary disc. However, a ring had been reported much earlier, by William Herschel, although this may have been imaginary because they’re very dark. The ν (Nu, not “Vee”) ring is between the moons Rosalind and Portia, so they also count as shepherds. The fact that most of the rings remain very narrow but don’t have shepherds is unexplained. Before their discovery, only Saturn was thought to have rings. After Jupiter was also discovered to have a ring in 1979, the question was whether Neptune would be the odd one out in lacking them. From that point onward, I assumed Neptune had them. Nobody knows what they’re made of, except that they can’t be ice, because their colours are unusual and don’t yield definite spectra to go on. Their darkness suggests they’re carbon-rich, and in conjunction with the probable diamond-bergs and liquid carbon ocean show that Hamlet is well on its way to being a carbon planet.
Most of the light is reflected by the ε Ring, which is also the most elliptical and the one closest to the equatorial plane. It’s brighter in some areas than others due to that eccentricity and varies in width. It’s possible that this variation translates into arcs – curves – rather than rings for other planets, perhaps orbiting other stars, or maybe Neptune. I can assure you that by the time I come to Neptune I will know if this is so. This is the ring with the first discovered pair of shepherds. The next brightest rings are α and β, which also vary in width, being widest 30° from their furthest points from Hamlet and narrowest 30° from their nearest. It’s probably coincidence that these angles correspond to those of the planetary magnetic field, or if not, something to do with a similar but separate dynamic process. Both these rings are somewhat tilted and are ten kilometres wide in some places, which raises the issue that they were detectable from three milliard kilometres away even though they were smaller than the Isle of Wight. The γ Ring (I’m just going to deal with these in alphabetical order, which means mentioning the 1977 ones first) is narrow, almost opaque and thin enough to make no difference to stars crossing when it’s edge on. This also means it isn’t dusty. The inner edge particles orbit six times for five of Ophelia’s orbits, so there seems to be a relationship there. As for δ, it’s circular, slightly tilted and may contain a moonlet because it seems to have waves in it. It has a more opaque and narrower outer part and a wider and more transparent inner side, which seems to be dustier.
Before Voyager 2 got there, the team who discovered these first five rings found a further three rings by the same method. For some reason these are known as 4, 5 and 6 even though five were already known by that point and there was a Greek letter naming scheme going on from the same team. I don’t understand this, but there it is. Voyager 2 found another two, fainter, rings, the naming scheme going back to Greek letters, and in this century the Hubble Space Telescope found two more. Rings 4, 5 and 6 are up to dozens of kilometres away from the equatorial plane and are inner and fainter to the ones discovered in ’77. They’re also narrower and don’t occult starlight edge-on. The μ Ring is blue and contains the moon Mab, around which it’s also brightest so the chances are it’s made of bits of that moon. These rings are dusty. Finally there’s 1986U2R, because of course it would be called that wouldn’t it?
The rings don’t form a stable system and given what’s known about them should disperse within a million years. However the fact that all the other gas giants have rings suggests either that having rings is normal for such planets or that they’re temporary but very common. Hamlet’s system generally, including the moons, is not so dominated by ring-related factors as Saturn’s although there are several harmonies, operating between small inner moons and the rings rather than the larger classic moons observable from Earth. A moon the size of Puck would be enough to provide the material for the rings, and Mab is actually currently breaking up and forming another ring, so it isn’t that peculiar. There are probably moonlets up to ten kilometres across within each of the rings. I presume the dimness of the sunlight out there combined with the darkness of the satellites and other material makes them harder to detect optically than small moons of Jupiter and Saturn.
Getting back to Hamlet itself, it’s methane which gives it that colour, but the atmosphere is in fact mainly hydrogen and helium like the other gas giants. It’s the second least dense planet and has a cloud top gravitational pull of only 89% of our sea level gravity. There are four layers of cloud corresponding to increasing temperature and atmospheric pressure. At slightly above sea level pressure, there are methane clouds. Considerably further down are the deepest clouds which have been actually observed, where the pressure is equivalent to the Earth’s ocean’s sunlit layers’, and are made of hydrogen sulphide. Appropriately for the planet’s official name, these would stink of rotten eggs. These share the layer with clouds of ammonia, which has an acrid, stinging odour. Below that is ammonium hydrosulphide, and finally, at a level where the pressure is equivalent to about four dozen times our sea level pressure, there are clouds of water vapour. The atmosphere is probably the most featureless of any solar planet’s, but does show the occasional white cloud, as can be seen in the photo at the top of this post. It’s also quite clear compared to all the other gas giants’, Titan’s and Venus’s, though not ours or the Martian one. I would expect there to be a level where one would find oneself completely surrounded by blue-green with various species of cloud. There are also traces of complex hydrocarbons as would be found in mineral oil and natural gas on Earth. Unlike other collisional atmospheres, Hamlet lacks a mesosphere, which is normally found between the stratosphere and thermosphere. There is a hydrocarbon haze in the stratosphere.
The chief distinguishing feature of Hamlet’s atmosphere is its featurelessness. Voyager 2 only detected ten clouds over the entire planet as it flew past. All the other gas giants have more stuff going on in theirs, and this is probably why it took so long to work out its rotational period of seventeen hours. There is a whiter polar cap from around half way between the equator and the poles, which swaps over between north and south as the orbit wears on. Voyager 2 was unable to observe the northern hemisphere because it was night there when it passed, so not only has Hamlet only been visited once but also half of it hasn’t been observed close up at all. In the decade or so after Voyager left, things started happening in its atmosphere but of course they couldn’t be seen as well as they would’ve if they’d taken place when it was there. I feel like there’s a kind of theme emerging here. Also, astronomy has only been advanced enough to make much meaningful sense of what’s going on since the 1950s, which is less than an entire orbit ago, so a whole cycle of seasons has yet to be observed. There has been a dark spot like the one on Neptune, and there are thunderstorms. It’s also possible that there’s a convection layer blocking the internal heat from the outer reaches of the planet.
So that’s Hamlet, such as it is. Next time I’ll be talking about its moons. I have two questions for you though. Did you feel that avoiding the name “Uranus” made you feel differently about this planet? I’m not sure about calling it “Hamlet” either, but that does at least circumvent the issue. Could you think of a better name or is it a bad idea to fixate on it so much?
Ever since I first saw ‘Friends‘ back in the mid-1990s CE, I’ve wondered about the choice the writers made with Phœbe’s name. Is it connected with the moon’s name or not? If it is, it must be an extremely obscure in-joke because I imagine that most people had no idea what Phœbe was at the time, and of course the name is originally from Greek mythology, which raises the question of whether there was something about the titan herself which brought eccentricity or oddness to mind. Because Phœbe the moon is odd. It orbits the opposite way round to the majority of other moons in the Solar System, which is expressed in the stats as having a high orbital tilt, and Phœbe the ‘Friends’ character kind of does the same thing. She’s the odd one out and in the model of ‘Friends’ characters which approximates each to a personality disorder, she’s the schizotypal one. Not that I agree with that particular approach to personality disorders because they may be better characterised as combinations of unusually pronounced traits (which means that on the OCEAN model there could be thirty-two of them), but it’s been said a lot recently that there are various ways in which the sitcom has not aged well.
As is often so, Greek myths include several figures named Phœbe, but the moon is unequivocally named after the titan because it’s a satellite of Saturn and that’s how the naming scheme there went. That Phœbe is the grandmother of Artemis and Apollo, this last also known as “Phœbus” in Latinised form, who are respectively deities associated respectively with the lesser and greater luminaries. Hence it’s possible that naming a child Phœbe associates her with shining beauty, perhaps even a woman “with hair brighter than the Sun”. Phœbe’s daughter is Leto, alias Latona, goddess of night, chiefly known for being in labour for nine days owing to Hera keeping the midwife goddess Ilythia away from her when she birthed Apollo.
The question therefore arises as to why Phœbe the moon’s name was arbitrary beyond the order of its discovery leading to the need to seek decreasingly significant titans. This in itself raises an interesting question: does this mean that smaller Saturnian moons are more likely to have feminine names? If so, does that reflect a bias in classical times or more recently? This moon is the first to be discovered through photography alone, the second-largest retrograde satellite and as such is bumped down the scale of discovery, being likelier to be found later. It was first confirmed on photographic plates on 18th March 1899, the plates having been taken on the sixteenth of August the previous year. I find it a little surprising that it only took one night of plates to detect the moon’s movement and presume that it must’ve been quite far from maximum elongation at the time.
Just to return once more to the cultural aspects of this body, Phœbe is difficult to type on a computer. Although English, French and Latin all use the “œ” digraph, the letter isn’t present on the French AZERTY layout so far as I can tell, so on a typewriter it would have to be double-struck, and English lacks it entirely. I think of it as a letter in the French version of the Latin alphabet and it’s also used in the International Phonetic Alphabet with its French value. In Latin, the diphthong it represented shifted before the classical period into the sound /e/, and combined with our own vowel shift we now pronounce the moon’s name as “FEE-bee”, which incidentally also means we’ve conceded the shift from /ph/ to /f/, even though in classical Latin it would’ve been pronounced in the former way, at least for a while. In fact the only sound which has stayed the same in the name is /b/. I am, in any case, acutely aware of the fiddliness of typing the name as I’m writing this post
When I first heard about Phœbe in I think 1973, more than two decades before ‘Friends’ but millennia after the end of Dodekatheism, all that I and presumably most other people knew about it was that it was a small irregular moon, the outermost of Saturn’s, orbiting backwards compared to the other known moons, and was considered to be a captured asteroid. This last bit puzzled me because the asteroid belt is something like 750 million kilometres from Saturn. In the next few years, Chiron was discovered, and for a while this puzzled astronomers because it appeared to be an out-of-place asteroid. I will be talking about Chiron in a future post. Chiron, being named after a centaur, was just the first discovered minor planet of the “centaur” class, which I will eventually mention. There’s also Hidalgo, which is odd in that its aphelion is almost as far out as Saturn and its perihelion not so far from Mars’s, so it’s almost as if it belongs to both asteroid belts, as it were. But I’m getting ahead of myself.
Although Chiron was the first centaur to be discovered, in about 1977 I think, Phœbe seems to be a former centaur. This wasn’t picked up for getting on for a century since its discovery because they were otherwise unknown, but some of the characteristics of its orbit are highly compatible with this designation. Before I go any further, just as a centaur is a half-equine, half-human creature, astronomically centaurs are intermediate between comets and asteroids, which is also what Phœbe seems to be. The moon has an eccentricity of almost sixteen percent and averages almost thirteen million kilometres from Saturn. Since Saturn itself has an aphelion of 10.1238 AU, this means Phœbe reaches out to 10.2238 AU from the Sun. Chiron’s perihelion is actually inside Saturn’s orbit, so it’s entirely feasible to imagine Phœbe as a centaur.
While I’m at it, I may as well mention the other features of its orbit. It’s inclined to Saturn’s equatorial plane by 151° 47′, which actually just means it orbits backwards at a tilt of around thirty degrees, taking a year and a half to go all the way round. This distance also means it approaches Sinope, Jupiter’s outermost moon, to within about three and a half AU, which sounds like a lot, being slightly greater than the diameter of the orbit of Mars, but this is the outer Solar System where distances are increasingly larger than the inner. It’s about half a light hour. This is not a hugely consequential fact unless there were perhaps some kind of “moon-hopping” means of transport for getting between systems. There is of course The Solar Mass Transit System I mentioned a while back, but the gravity involved is insignificant. Nonetheless, out there somewhere is a neutral gravity point between the two, much closer to Sinope than Phœbe. That moon would also feel Jupiter’s magnetosphere most strongly out of any of the moon
I ask myself, is Phœbe genuinely the most distant of Saturn’s moons? Are there any bits and pieces in beyond it which still orbit it? Saturn’s Hill Sphere is bigger than Jupiter’s because it’s further from the Sun even though it’s also less massive, at sixty-one million kilometres in radius, which is almost five times the radius of Phœbe’s orbit. Nevertheless, matter is sparser out there than further in. And in fact there are fifty-five further moons, though some are extremely small. Some are only fourteen metres across, and it seems both hardly fair to include them as moons and also quite amazing that they’ve been detected at all. However, even the outermost moon is only half way to the surface of the Hill sphere, so it seems possible there will be even more. It’s thirty-four metres across and has no official name.
The moon is the largest of the so-called “Norse Group” of irregular satellites with retrograde motion. It’s over a thousand times the volume, ten times the diameter, of the next largest such moon, Ymir. Since it was discovered before the invention of this grouping, Phœbe has a Greco-Latin rather than a Norse name, Ymir being the frost giant nourished by the milk of the primordial cow and from whom the world was made in Norse mythology, thereby providing a possible link with Hinduism. There are probably a number of subgroups among the Norse moons. Among all of them, however, Phœbe is in a league of its own in terms of size, averaging about two hundred kilometres across, and as can be seen from the image at the top of this post, it somewhat approximates sphericality, more so in fact than the rather larger Hyperion. Other comparisons with Hyperion are worthwhile too. For instance, Phœbe lacks Hyperion’s spongy appearance and looks to me more like Deimos or a small asteroid. It’s also more massive than Hyperion, which is in fact connected to the appearance as it’s less porous too, and therefore denser. Phœbe is also as black as soot, reflecting only six percent of the light falling on it, which is darker than any other of Saturn’s largish or large moons. This would make it warmer than most of the other small moons which don’t experience substantial tidal forces, and certainly warmer than Hyperion, which is quite a bit paler and reflects a lot of light and therefore heat, being five times brighter than this moon.
Although there are maps of the place, it kind of makes more sense to label the “globe” because it’s too irregular to map without considerable distortion compared to a spheroidal object:
The craters are named after the story of ‘Jason And The Argonauts’, hence the very large crater called Jason at top left of this panel. This is more than eighty kilometres across and has walls sixteen kilometres high.
This moon is an exception to the exploration of the satellites undertaken by the Voyager probes. This is the best image taken at the time:
Hence research on the moon is rather behind that on the others. One thing which is noticeable about it is that it’s higher in dry ice than the others, which is one reason why it’s thought to be a centaur. It’s also the only such object which has been imaged as more than a dot, even by the Hubble Space Telescope. No space probe has been anywhere near any of the others, which basically means Chiron. It’s difficult, really, to talk about it without talking about the other, proper centaurs, which I want to leave until I get to Chiron.
Phœbe is unusual in having its own rotation period. Unlike Hyperion, whose rotation is chaotic, it does have a proper axis and takes nine and a quarter hours to rotate on it. This makes it the only sizeable moon of Saturn which has a proper day of its own, and Saturn will rise and set in its sky. Saturn is also usually in a position where its rings are fully visible, but unfortunately the planet is also very small and far-away.
Phœbe also has a ring, although unlike Rhea’s possible rings and the remnants of the one around Iapetus, if that’s what that is, it doesn’t encircle the moon but its orbit, through which the moon travels. It’s one of those irritating technical truths, like the fact that Alaska is the easternmost state in the US because of the Aleutian Islands crossing into the Eastern Hemisphere, that Saturn’s biggest ring is actually this one, which is so sparse as to be practically non-existent. It’s technically 23 million kilometres across, and may be the cause of the dark hemisphere on Iapetus, due to dark material leaving Phœbe’s surface and spreading inward as far as the two-faced moon. It’s probably caused by meteorites hitting Phœbe’s surface and the moon’s gravity not being strong enough to pull them back. It is, however, entirely within the moon’s orbit, suggesting that like the inner moonlets near Saturn’s more substantial and visible rings most of the way in, it also acts as a ring shepherd, although a particularly large one with a particularly diaphanous though large ring. Some of the larger impacts may also have caused bigger fragments to escape the moon’s pull and become other Norse moons in their own right, some of which have similar orbital characteristics.
That, then, is not only it for Phœbe but for the entire Saturnian system. Although most of the moons haven’t even been mentioned, these are all the moons discovered before the twentieth century. My impression of Saturn’s system is that it’s characterised by clutter. It has the rings, numerous small moons orbiting in unexpected places and a fair bit of matter exchange. It’s also quite light and of low density, with the exception of Titan. Due to being both quite massive, even given its low density, and far out in the system, it has a large sphere of influence and has managed to retain quite a lot of matter.
The next post on the Solar System will be about the initially mysterious and surprising object Chiron, discovered in 1977, and its “relatives”, the centaurs. These form a kind of second, outer asteroid belt. More on them in a couple of days.
. . . but not all! Hyperion’s no planet of course, but its situation could apply as much to a planet as it does to this moon.
Hyperion is the next largish moon out from Saturn after the big one, and is in a way a pair with Mimas. Mimas is the smallest world in the system which is roughly spherical. By contrast, Hyperion is the largest object in the system which isn’t. It has quite a distinctive appearance besides this, in that its craters are oddly deep for their diameter, giving the impression of being like coral or pumice, or maybe chimneys or organ pipes, and in fact it is like pumice in that it’s unusually porous, so this may be more than coincidence. If it were a small object, say a decimetre or so across at its largest width, I can imagine holding it in my hand and finding it to be very light for its size. If I licked said object, I would expect it to try to suck my tongue in with capillary action. It just looks very odd. Kind of delicate and easily crushed.
In fact Hyperion is bloody huge! Not perhaps by the standards of spheroidal bodies elsewhere in the Solar System, but considered as an object in its own right. It’s 360 x 205 x 266 kilometres in size, and was the first known decidedly non-round moon, discovered in 1848 CE. Hence a box containing Hyperion would have a volume of nineteen and two-thirds million cubic kilometres. Its extreme ends are as far apart as Glasgow and Nottingham or Cork and Donegal. Not huge on a global scale by any means, but still massive. Enough to cast a shadow over most of Ireland. It’s the kind of size which would constitute a reasonable and fairly arduous road trip which you’d need a toilet break from. Also, that largest axis is actually quite close to the diameter of Mimas, which like any moon or planet is not perfectly round. The least diameter of that moon is only twenty kilometres greater than thisses greatest. Hyperion is so nearly round. It’s a runner-up in the sphericality stakes, and you can see that from its rather ovoid shape. Its gravity has proven to be enough to smooth it out but not quite enough to finish the job and make it round.
When I was a teenager I used to think of Hyperion as the largest possible size for a cylindrical space station. It’s special in that way because once an artificial object exceeds its dimensions, its design becomes at least somewhat constrained by the force of gravity to being made approximately spherical. A cylindrical space habitat could exist which was 360 kilometres in length and 205 kilometres in diameter, giving it a habitable internal surface the size of Laos, about which I thought I’d blogged at some point but apparently didn’t. Maybe I should. The surface area of Hyperion itself is rather imponderable because not only is it irregular but it also has many craters and is very porous. Nearly half of it is empty space, more or less, meaning that its real volume is quite a bit smaller than it seems. It doesn’t just look like a sponge. However, this is a common or even universal characteristic for small irregular bodies in the system and is also found with, for instance, Phobos and Deimos. Its shape also means that it has three times the gravity at its narrowest diameter than at its most elongated locations, although that gravity is still quite low regardless of whereabouts on the surface you are. It’s only 54% as dense as water, sharing that low density with Saturn itself and a number of other local moons. Like Saturn, it would float on water but unlike Saturn it would actually stand a chance of finding a body of water large enough to float on.
Getting back to the title, “lots of planets have a north”. That is, on the whole planets and moons in the Solar System, and presumably beyond, rotate around a single axis, wobbling only slightly over a long period of time compared to the length of their day. Most or all of the moons I’ve been into in any depth on here have captured rotation, where they always present one face to their planet but still have day and night because they orbit that planet without facing the Sun at all times. Titan, for example, has a day about two weeks long, but above its haze Saturn hangs in the same place in its sky at all times, or is invisible due to being below the horizon. The Sun, though, rises in the east and sets in the west like on most other planets, meaning that as you stand on the surface at the equinox with the setting Sun to your left, you are facing north. Titan, like many other places, has a north. However, the next “large” moon out from Saturn hasn’t. Every time the Sun rises and sets on Hyperion, it does so in a different place from the previous day, chaotically. Therefore, Hyperion has no north or south. There is no way, based on either magnetic polarity or rotation, that a map of this moon could be oriented, and it tumbles through its orbit with no simple pattern.
Hyperion occupies an intermediate position in moons’ relationships with their planets. Moons closer to Saturn, including Titan but all the others, have captured rotation. Of moons further away, Iapetus at least also has captured rotation. However, Phœbe, which is still further out, has its own rotation period. There seems to be a set of circumstances which leads certain bodies not to have compass directions. It isn’t clear what they are because Iapetus once again shows the same face to Saturn at all times. What, then, is going on with Hyperion’s rotation, and can these circumstances happen to planets? Are there planets without a “north” too?
One possibility for Hyperion’s peculiar shape is that it’s a chip off the old block, that is, a remnant of a much larger but shattered moon. It’s another of those bodies, like Vesta, with a large feature which almost makes it a vignette for it. In this case it’s a crater-like ellipse occupying one entire side of the moon, although it seems to have no name. It has a rim and a central peak like a conventional crater but is itself so heavily cratered it no longer really counts as one itself. Personally, I wonder if this impact was in some way connected to its formation, and that there was some kind of “proto-Hyperion” which was destroyed by that very impact, but I can’t work out the dynamics of such an event so maybe not. The moon does have a latitude and longitude system though, which is hard to understand because it doesn’t have an axis of rotation or a magnetic field. I’m guessing that an arbitrary feature was chosen, possibly the central peak of the area surrounded by Bond-Lassell Dorsum, which is the rim of the apparent large crater. The other features are labelled with latitude and longitude even though this has little meaning, so basically the compass directions have been chosen at random for the sake of convenience so far as I can tell.
The moon’s orbit has a fairly high eccentricity for a fairly large moon at 0.1, i.e. its distance from Saturn varies by about ten percent. It also orbits once every three weeks compared to Titan’s fortnight, meaning that Titan is likely to have a gravitational influence on it, and keeps its orbit from becoming more circular. Just as the probability that Enceladus would solidify and become a quiet moon is low, so is the probability that Hyperion would rotate conventionally. Even very slight influences on its movement push it into states where it won’t spin on an axis. I would expect this to be partly linked to its shape. The real oddity is not so much that it’s in this intermediate state as that the next large moon out, Iapetus, does still have captured rotation despite the increased distance from Saturn. Hyperion takes thirteen days to return approximately to its previous orientation, which is close to Titan’s period, but this may not be simply related.
As well as consisting mainly of water ice and empty space, the moon probably also contains frozen methane and dry ice. Being covered in a dark substance, it’s possible that heat from sunlight has caused some of this to evaporate and contribute to the porosity. Impacts on its surface probably crunch through to a considerable depth and throw débris free of the moon, hence the single central peak and dorsum, which suggests to me that they were formed when Hyperion was part of a larger moon. The reddish colour of the dark material possibly responsible for this heating is similar to that on Iapetus, which I will shortly cover. It’s also concentrated in the bottoms of the craters, so it isn’t immediately apparent that the moon averages as dark as it does.
The composition of the moon is likely to be the same all the way through due to its low gravity. If it formed part of a larger body in the past, it might be expected to show traces of stratification, but it’s also a rubble pile and very porous, so the chances are it would be jumbled up by that calamity in the same way as Cynthia probably formed from Earth’s disrupted outer layers, although it that case the stronger gravity would have sorted the fragments.
The name “Hyperion” is very popular and applied to many different things in the wider world. It’s the name of a series of SF novels by Dan Simmons, a classical record label and an investment company. The original Hyperion is, unsurprisingly, a titan in Greek mythology and the name literally means “the one that goes on high”, and is therefore associated with the Sun. One of the craters on the moon is named Helios. Keats abandoned a poem on the titan. There is a possibly projected tale that Hyperion was the first person to understand the movements of the Sun and Cynthia and their effects on the seasons. If there was such a person at any point, Hyperion would be an appropriate name for them.
That, then, is Hyperion. The next moon is one whose reputation precedes it and was noticed as having a very distinctive appearance long before any spacecraft visited it: Iapetus.
All moons are special of course. That is, you can probably dredge something interesting up about most of the large ones. All that said, of all the moons in the system, all the planets in fact, Titan must be near the start of any list ranked by interest. Writing this post is in fact quite daunting because I want to do it justice, and having written a couple of thousand words even on somewhere like Rhea, which let’s face it doesn’t strike me as one of the more intriguing places, I now feel obliged to do this amazing world justice, and I can do that, but I may go on and on, which I do a lot.
I live in Loughborough, and consider it a very boring town. To be fair, even when I lived in Canterbury I considered it boring even though it had that big pointy building in the middle. A more positive approach would lead to one casting around for sources of pride regarding the place, and in the case of Loughborough there are a few things. There’s the Great Central Railway, which is Britain’s only main line double track heritage railway. There’s the Bell Foundry, which produces a large proportion of church bells in Britain. Ladybird Books were based here. There’s also a university, which actually I found quite unimpressive, and it’s next to the National Forest. The Carillon is also quite special. Some people also say Loughborough is where the North starts. As you become familiar with a place, you get to realise what makes it individual and special. Consequently I can imagine people living on Rhea, perhaps working for the Rhean Tourist Board, and coming up with bits and pieces which might attract sightseers such as the possible ring system, but there would probably be a lot of time and work spent on trying to promote the moon. Rhea is in a sense the Loughborough of Saturn’s system. Titan, by contrast, sells itself. It’s the London or NYC of the system. You don’t need to push it because it’s amazing.
The illustration at the top of this post, perhaps surprisingly, is in the public domain. It’s by Chesley Bonestell, whose matte painting for ‘2001 – A Space Odyssey’ I used on this blog yesterday. Bonestell was a prominent mid-century space artist who also worked on films. He also designed a number of prominent buildings and used his skill in cinematography to create masterful depictions of space-related scenes. He was influenced by the frontier style of art, where beautiful and almost deserted landscapes in North America would have small figures, horses and wagons depicting the European pioneers travelling across the continent to settle and raise food, so many of his pictures show astronauts, spacecraft and bases on the surface of various bodies throughout the Solar System and in space itself. They also serve as a record of the state of knowledge and expectations at the time. For instance, before the Apollo program it was expected that a non-staged rocket ship would land on the lunar surface and return in one piece. The staging concept is so familiar to us nowadays that we find it quaint to imagine anything else, but there was a time when a much more straightforward vision saw a finned and streamlined craft perhaps a hundred metres high setting out from Earth and coming to rest somewhere like the Sea of Tranquility. This is what Bonestell depicts.
In the case of views from the different moons of the Solar System, artists at the time had very little to go on. They had the angles of the orbits, the distances from the planets and a rough estimate of the sizes of the moons in question. There was a lot that could be concluded from the data available but on the whole this was quite tentative. Bonestell produced a series of paintings from the major moons of Saturn, which might be expected to be quite spectacular given the planet’s rings, but in fact like most satellite systems, most of the moons orbit close to the equatorial plane, particularly the closer ones, which has the unfortunate result that either Saturn is big in the sky but has hardly visible rings because they’re seen from edge-on or does show the rings from a suitable angle but is so far away that they’re not that impressive. Moreover, although he was, I’m sure, assiduous in collecting as much information as possible about all of his subjects, he didn’t have much to go on apart from those few facts. Therefore it’s not surprising that all these paintings focus on the appearance of Saturn in these moons’ skies.
As I say, I was very surprised that his view of Titan is in the public domain. It seems to me that this is one of his most iconic and famous paintings, and just being able to post it like that, though presumably in a lower resolution than is available online for a price, is quite amazing. Incidentally, this picture occupies a significant position on the wall of a NASA office in the film version of ‘The Martian’. I don’t know if it’s on the wall anywhere in the real offices but it is quite inspiring and classic, so I wouldn’t be surprised.
All that said, it is of course inaccurate. In particular, Titan is much cloudier than that and it’s unlikely that Saturn is ever visible from the surface. Moreover, this is a daytime picture and stars are visible in the sky. In reality they wouldn’t be because not only is there copious smog in the atmosphere, in a good way, but even if there wasn’t the atmosphere has several times the density of our own air at sea level, so there’s no chance, even above the cloud deck, that stars would be visible during the day. Moreover, Bonestell has a tendency to depict ice as it would appear in terrestrial conditions rather than how they are actually likely to be on the bodies concerned. By the time you get out to Titan, the temperatures involved are so far below freezing that water ice is basically just another rocky mineral. The average surface temperature on Titan is around -182°C. This is ninety-one degrees above absolute zero, and freezing point is three times that temperature. In proportion, Earth’s mean temperature is 22°C, and three times that is 612°C, and certain common terrestrial minerals have a melting point around that, such as quartz and mica. Water ice is not just frozen water, even on Titan’s surface. The shiny, snowy look is interesting but speculative, and turns out to be wrong. I feel bad criticising his art in this way, and I want to stress that I still think his paintings are amazing and wonderful.
The landscape is also craggy in a similar way to Bonestell’s representation of the lunar surface. This is also inaccurate. Not only did it turn out to be wrong in the case of Cynthia, substantially because of moondust and micrometeoroid impacts, but it’s even less accurate for Titan because in the latter world’s case there is liquid-based erosion there. Here’s the famous image from the Huygens lander:
These are basically pebbles, at least in the foreground, and this is because of the rôle of liquid in their erosion. This, of course, is part of what makes Titan so fascinating. It’s in some ways the most Earth-like world in the whole Solar System.
This statement, though it has a lot of truth to it, can also be quite misleading. Yes, Titan is quite Earth-like but also has important differences. In the novel ‘Imperial Earth’, Arthur C Clarke illustrated the difference between the two with a burning plume of flame. On Titan, it was an oxygen spout burning in a methane atmosphere but on Earth it could be a methane spout burning in oxygen. Nowadays we realise that the rôle of methane in the Titanean atmosphere is not as a main gaseous constituent, but it still works as a good metaphor. The same kinds of phenomena often exist – rivers, lakes, seas, rain – but not in the same way. These pebbles are eroded into rounded shapes just as they would be in a stream or on a beach on Earth, but they’re likely not made enitirely of silicates but ice and the liquid eroding them is methane, gaseous on our home world. It is possible that they’re mixtures of ice and stone, so we might think of them as lumps of frozen mud or clay, but that’s considering them in terrestrial terms. In Titanean terms this planet is a furnace covered in oceans of molten rock with clouds of the same in the sky raining liquid as hot as fire, at least as far as the surface is concerned.
Titan is the second largest moon in the Solar System after Ganymede. Unlike Ganymede, and uniquely among moons, not only does it have an atmosphere, but said atmosphere is somewhat denser than Earth’s and the surface pressure is almost twice as high as ours. It is in fact the only moon in the system with a proper, collisional atmosphere like our own. This raises the question of how come Ganymede has no real atmosphere and yet the slightly smaller Titan has such a thick one. I imagine the answer is twofold. Firstly, Titan’s a lot colder than Ganymede, and secondly it’s less exposed to the solar wind because it’s twice as far from the Sun, making it only a quarter of the strength. The molecules would be moving much more slowly in the vicinity of Titan than Ganymede’s, and consequently don’t escape its gravitational pull.
Although it used to be thought to have a methane atmosphere, and a considerably more tenuous one to boot, it turned out that the main constituent of the atmosphere is the same as ours: nitrogen. This presumably means there are plenty of worlds in the Universe with a mainly nitrogen atmosphere like our own. Methane, being liquid, performs the same kind of antics as water does on Earth, making Titan the only other world in the system with liquid bodies of water and also land on its surface. There are several planets with liquid on their surfaces, but none with both liquid and solid. The fact that there is liquid flowing over a solid surface presumably means the latter is shaped, as Earth’s is, into river valleys, oxbow lakes, potholes, caverns, perhaps fjords and so forth. However, there are other factors which make it quite different.
Titan’s surface gravity is about the same as Cynthia’s, though somewhat lower at a little under a seventh of Earth’s to Cynthia’s sixth. Although it’s larger than Mercury, that planet is joint densest with Earth so Titan, with a density less than twice water’s at 1.88, has considerably less pulling power. This is due to its higher volatile content, such as water and ammonia. This also means that if Cynthia were a moon of Saturn, it too would have an atmosphere, actually a denser one even than Titan’s, and like Titan, liquids on its surface. Due to the lower gravity, the appearance of Titan’s lakes and rivers is somewhat different to Earth’s. For instance, the lakes seem to be more “spidery” in appearance, as if they have fjords. Liquid methane also appears to be more viscous than water, which combined with the much lower gravity would lead to more slowly moving rivers and less response to winds. The waves would also be different. The most important difference between methane and water, and in fact between most other liquids and water, is that the latter expands and therefore floats when it freezes whereas the former doesn’t. This means that freezing lakes on Titan would solidify from the bottom upward, making them less liable to melting or insulation from ice. Water is also slightly blue, but methane is almost perfectly colourless, so even without the distinctly orange lighting of the surface there would not be the usual bluish vista of the sea on this moon, but it is very slightly green. It’s also got a slightly lower refractive index than water, which would have some influence on the apparent distance to the horizon in humid air. However, that’s pure methane and the seas of Titan are not pure.
On Earth, we have two kinds of water. Most of our water is salty because it’s dissolved minerals from the sea bed and elsewhere, but when it first lands on the surface as snow, rain, hail, dew or frost it’s fresh. A similar division exists on Titan. The large standing bodies of water have had time to dissolve ethane and are in fact solutions of ethane in methane. They are also blackened by other hydrocarbn impurities dissolved from the crust into them. I’m guessing that this means there are “tar flats” there like Earth’s salt flats, and also the equivalent of hypersaline lakes but with ethane instead of salt.
Titan’s appearance from space is vivid orange because of the photochemical smog, similar to the reddish tholins found on many small objects far from the Sun, and they are in fact tholins themselves. In the case of the moon, it’s actually possible to image the horizon and the changing colours of the atmosphere from orbit like it is with Earth:
This is actually an ultraviolet image but has been colourised to resemble what would be seen by the human eye. Leaving its air’s composition and density aside for a bit, Titan is an important model for how an atmosphere behaves on a cold, fairly uniformly heated and slowly rotating spheroidal body. This came up recently in discussions I had with flat Earthers, because they attempt to explain the movement of Earth’s atmosphere based on the assumption that it doesn’t rotate and try to find another model which doesn’t use the Coriolis Effect. Titan and Venus provide such a model, and theoretical simulation of this moon’s atmosphere doesn’t rely on its actual existence. Like many moons, Titan has captured rotation and always shows the same face to Saturn during its sixteen day orbit, giving it a sixteen-day rotation. On a world much closer to the Sun, such a slow day would lead to winds in the atmosphere being dominated by the temperature differential between the night side and the subsolar point, leading to an “eyeball planet” to some extent, although unlike a genuine such planet it would still be rotating a little. There would be winds blowing from the tropics on the day side towards all parts of the night side, radially arranged. Above Titan, the atmosphere develops similar bands to what’s found on Jupiter, although they’re not visually apparent due to the relative homogeneity of the atmosphere. There are basically longitudinally-oriented rings around the planet with convection currents circulating between higher and lower altitudes and preventing mixing between latitudes. This is very indirect evidence that Earth is round, because if our planet wasn’t spinning this is how our atmosphere would behave, ignoring heat sources, and it doesn’t. In fact I wonder if that also causes the distinctive layers in this image. Perhaps there are multiple rotating “tubes” of air which don’t interact with each other.
The atmosphere is not horizontally homogenous. There is a “polar hood”. Titan’s orbit adds about twenty minutes to Saturn’s axial tilt of 27°, meaning that both have seasons, but in Titan’s case there is little or no significant internal heat influencing the weather, so Titan would exhibit seasons around seven years long each. The polar hood is a dark zone around the pole extending quite some way towards the equator, 70°, which appears in the local winter. It appears over both poles at different times of the “year”, i.e. the thirty-year period of Saturn’s and therefore Titan’s trip around the Sun. It seems to be caused by down-welling, which is the tendency for haze to build up in the winter at high altitudes which is then transported to the other hemisphere during spring.
Due to the lower gravity, the atmosphere is much deeper (or higher) than ours. Our “scale height”, the altitude over which density decreases by a factor of ε, or roughly 2.718. . . , is around eight and a half kilometres. The Titanean scale height is from fifteen to fifty kilometres. Now might be a good time to talk about scale height in more detail. It’s common knowledge that the further up you go on Earth, the thinner the air is. Most people cannot breathe at the top of Mount Everest without help although one can acclimatise oneself, and the air pressure inside an airliner is noticeably lower than at sea level, although it is also somewhat pressurised. The Kármán Line is the official boundary between Earth’s atmosphere and space, but is no more “real” than the borders between countries. It’s a hundred kilometres above sea level. However, the atmosphere doesn’t just suddenly cut off at that height, but gradually fades out. However, it doesn’t do that in a linear fashion. The air pressure 8.5 kilometres up is around 370 millibars, and at seventeen kilometres it’s 135 millibars, i.e. 2.718 times lower. At the Kármán line it’s about eight microbars. This actually means that were it not for the low temperature and lack of oxygen, it would be possible to survive at a much greater altitude above Titan than above Earth. The Armstrong Limit is the height at which the boiling point of water is equivalent to human body temperature, and the pressure is 62 millibars. This is about eighteen or nineteen kilometres above sea level. On Titan, taking the higher sea level (!) pressure of the atmosphere into consideration, this occurs at a minimum altitude of almost fifty kilometres up, which on Earth is the maximum height a balloon can rise to before pressure within it is equivalent to pressure around it, giving it neutral buoyancy. This also means that said balloons, airships etc, could operate at a much greater height above Titan than on Earth, at about a hundred and thirty kilometres, which on Earth would be well into space.
Methane rising into Titan’s upper atmosphere is broken down by radiation into hydrogen and ethane, which is effectively a dimer of methane with a hydrogen atom missing (in other words two methyl groups). Although it might be expected that this hydrogen would leave the atmosphere entirely, and I’m sure a lot does, what mainly happens is that the hydrogen expands and occupies a greater range of heights than it starts off at, and this leads to it moving down into the lower atmosphere. It would usually then be expected to rise back up again and leave, or perhaps react with something else, but in fact it seems to disappear. It’s been suggested that this hydrogen is being used by living organisms lower in the atmosphere. Once again, this series of posts is not supposed to be about life, but it would be weird to ignore it at this point so I think I have to say something about hydrogenosomes.
Cells with nuclei usually contain a number of bodies referred to as plastids. These include chloroplasts and mitochondria. Both of these evolved from independent microörganisms and provide their host cells with functions they would otherwise have to evolve or do themselves. Chloroplasts are of course former blue-green algæ and responsible for the kind of photosynthesis which produces oxygen as a waste product. Mitochondria use this oxygen to release energy from glucose in a controlled manner known as the Krebs Cycle. Hydrogenosomes are similar to mitochondria, are thought to have evolved from them, and do a similar job, but are found in anærobic environments, which is of course what Titan and almost everywhere else in the Universe is. They release energy by converting protons to molecular hydrogen. This is the opposite of what organisms on Titan would be doing with it, but it suggests that there is a potential source of energy there and it would explain why the hydrogen seems to vanish. Chloroplasts and mitochondria effectively have opposite functions, so maybe these are the opposite to hydrogenosomes.
Titan’s surface has now been completely mapped:
Perhaps surprisingly, in spite of the dense atmosphere and liquid and gaseous erosion, there are a number of craters on the surface, although they’re very sparse. These are the red patches on the map, all in the same hemidemisphere. The blue patches are lakes, and it’s notable that they’re within the polar circle, mainly the “Arctic”. Near the equator are dune fields, the purple bits. The green areas, plainly the largest, are in fact plains. Finally, the orange bits are described as “hummocky”. This is a cylindrical projection albedo map:
The impression one gets when looking at Titan is of a planet rather than a mere moon. It doesn’t feel like a mere adjunct to Saturn. This is clearly partly due to its size and mass, but it’s also the presence of a proper atmosphere. With the other moons, some of which technically have atmospheres which consist of sparse atoms and molecules bouncing around and perhaps orbiting, the surfaces are open to space and there’s less sense of “special space” with them. Titan’s not like that, and nor is Earth. Earth’s surface, ocean and atmosphere count to some extent as a “special space”. I will probably explain that in more depth at some point, but the gist is that there are some regions which count as special spaces for us, such as the Holy of Holies, an operating theatre, backstage or the parts of shops customers have no access to. Although they’re continuous with the rest of the Universe, there’s also a sense in which they’re kind of “roped off”, and I get that impression from Titan, but not any other moon. Conceptually it may be linked to liminal spaces and in a contemporary sense the “backrooms”. In a way, the whole of Titan’s surface is a huge “backroom”, since we’re trans its atmosphere and Titan is cis to it. It’s an arduous endeavour to reach sea level here, and it’s also kind of doing its own thing. For instance, it actually does have a sea level, or perhaps a mean sea level, since there seem to be at least two separate systems of liquid bodies. Tides will inevitably occur in these lakes, raised by Saturn and the other moons to some extent, and will be higher than is obvious due to the lower gravity. In a way, Titan is also a “desert world”, since although it does have bodies of methane on its surface they don’t form an extensive ocean. Perhaps somewhere out there are moons or planets with proper continents and oceans.
The presence of nitrogen in both Titan’s and Earth’s atmospheres suggests something further. Maybe there are planets and moons out there with oceans of liquid nitrogen.
Titan’s surface area is over eighty-three million square kilometres. This is far larger than any country or continent and getting on for the total land surface area of Earth. Next to it, even Rhea is small. It’s larger than Mercury and about the same size as Ganymede. Due to the lakes, its own land surface area is a little under that, and the greatest distance between two points on its surface is just over eight thousand kilometres, which is about the same as London to Los Angeles. This is not just some trivial moon you can give the brush-off to. It’s a massive great hulking world in space, getting on for the size of Mars, but far more distant. Similar colours too. Unlike Mars, however, Titan is constantly active and busy, with probable volcanic eruptions, though not to the extent on Io, but with water instead of lava, mixed with ammonia. It has gullies, branching streams and rivers with tributaries and evidence of tectonic activity. Basically the same stuff happens on Titan as on Earth, geologically, but with different materials involved. That said, although the surface is constantly being remodelled, it does seem that the occasional impact crater can persist. I have to say I don’t understand how.
There is more organic material and more complex organic chemistry going on there than on any other body apart from Earth. I’ve said before that tholins are like organic life’s cousin. It’s like the original complex mess of organic compounds which exist on or in a solid body have two alternatives as to how to develop, one being life and the other tholins. In Titan’s case, tholins have gone further than in any other known situation. the atmosphere is a case in point. On Earth, most of the complex chemistry going on in our atmosphere is in some way linked to life. Apart from that, there’s oxidation, almost completely inert nitrogen and completely inert argon. Lightning can cause nitrogenous compounds to form and ozone forms in the upper atmosphere, but most of what goes on here is physical. The organic chemistry is highly complex but mainly goes on inside organisms. This is not so on Titan, and may well not have been so when Earth was young and less organic material was locked up inside the biosphere, so although it’s much colder and therefore less reactive, Titan may be a passable model for what used to happen here before life evolved.
Broadly, what’s going on in the Titanean atmosphere, which remember is very deep compared to ours and therefore has a lot of stuff in it to react with each other in any case, is similar to what happens over a major polluted city in a hollow on a warm sunny day, one difference being that there’s no industry to inject the stuff into the air. Æons ago, all of the sludge we’ve dredged up with oil rigs and put into the atmosphere and water cycle wasn’t yet incorporated into the bodies of organisms, and may have been in a similar form, so we’re kind of returning our planet to the state it used to be in before life appeared on it, hence the resemblance to Titan. On Earth, vehicle exhausts form nitric oxide, which combines with organic compounds from the likes of paint, glue, weedkiller and other industrial and domestic chemicals along with the secondary pollutant peroxyacetyl nitrate formed from vehicle exhaust and fossil fuel power stations to form nitrogen oxides and ozone at a low level due to the action of sunlight on the chemicals. This turns out to be harmful to air-breathing organisms living in that environment.
The big difference with Titan is that there’s no free oxygen at all, although there is some locked up in compounds, so the process is rather different. It’s said to be possible to explain every detected compound in the atmosphere from the action of sunlight on a mixture of nitrogen and methane, although I don’t understand how because some compounds contain oxygen. Titan’s atmosphere is 94% nitrogen, six percent helium (which does nothing and therefore makes no contribution to the chemistry), 0.01% methane, and also acetylene, ethane, propane, diacetylene, methylacetylene, hydrogen cyanide, cyanoacetylene, cyanogen, carbon dioxide and carbon monoxide. In particular, there are several cyanide-based gases and the similar carbon monoxide, though in small amounts. Cyanogen is quite an interesting gas because it can behave as if it’s a halogen like chlorine or bromine. Several constituents also have nitrile groups, which also exist in superglue and an artificial rubber – I have a box full of nitrile gloves upstairs for the purpose of dealing with certain other organic materials. Although nitriles basically are cyanides, but properly organic as opposed to happening to include a couple of carbon atoms which might as well be any other lightish element, they tend to be a lot less toxic, possibly because the molecules are larger. Hydrogen cyanide in particular is a key intermediate in the synthesis of amino acids. As the chemical reactions proceed, I imagine the compounds get heavier and precipitate out of the sky onto the surface, so there will be substances vaguely resembling synthetic rubbers and glues, among other chemicals, on the ground and in the lakes and rivers, not at pollutants but as part of the uninterfered-with environment. All of this stuff will be in an unholy mess, all being mixed together, and it’s also hard to work out how it will behave at such a low temperature, but once again this is how Titan is the reverse of Earth. On Earth, all the plastic and other stuff is pollution. On Titan it’s a pristine part of the cycle: “natural”, to use that useless word. Deconstructing that word, though, maybe our seas being full of plastic and our air full of extra greenhouse gases is just as natural and it just took a convoluted path between a Titan-like original situation, a few thousand million years of evolution, the emergence of a technological species and a rapid return to Titaneanism.
Life, therefore, rears its head at this juncture. Titan has not one but two chances of being a life-bearing world because of its interior and its surface. There’s a whole load of stuff going on in its atmosphere and seas of course. Complex organic chemistry is a fact of (non-)life on Titan, but there is a problem: there is only rarely liquid water on the surface. It probably does happen, during volcanic eruptions, but the water emerging from these will freeze quickly. I suppose it’s possible that there would be microbes flitting around from site to site in these situations, waiting to take advantage of the brief periods that tiny area of the moon is above freezing, and in a way the combination of salty water and complex organic molecules almost seems to guarantee that life will find a way, but at this point we don’t know if life always happens when it can or if it’s a quadrillion-to-one chance that we exist on this planet, lost in the depths of a lifeless cosmos. But maybe water isn’t necessary to life anyway. Isaac Asimov, who was officially a biochemist, suggested that methane could replace water if instead of protein biochemistry used lipids. The crucial thing about water is its polarity. Water molecules are negatively charged on one side and positively charged on the other, which enables water to be a good solvent and to form cages around enzymes and extend their actions, among other things. This kind of life on Titan would use up molecular hydrogen by combining it with hydrocarbons, which would explain why there’s less hydrogen than expected in the lower atmosphere. And life gets a second bite of the cherry in Titan’s case, because as well as having an active and chemically complex surface, Titan is like many other outer moons in apparently having a hypersaline ocean underneath its icy crust, meaning that organisms could exist there too, with more familiar biochemistry. The mantle is a eutectic mix of water and ammonia, with some carbon dioxide, and is liquid. Immediately above it is a soup or sticky blend of complex organic molecules and the surface is tectonically active, meaning that these chemicals could be pushed into that ocean by movements of the crust and possible plates, if it goes that far. In the meantime, Titan appears to have many partially-assembled substances industries and chemists on Earth have expended considerable efforts in synthesising, such as the aforementioned artificial rubber monomers and components of superglue, as well as immense amounts of the same kind of hydrocarbons we use to power our entire civilisation, and I wonder whether it would be economically viable to fetch them from the moon and bring them back. It wouldn’t be a good thing though, due to the need for a low-carbon economy, but the presence of such compounds and their accessibility could ultimately lead to cheaper “fossil” fuels. Just as an example, the atmosphere contains twenty parts per million of propane. That’s more than seventeen millard tonnes. It’s notable that Russia is this planet’s largest supplier of natural gas. Even so, Titan is a long way away at one and a half light hours on average.
About an eighth of the surface is covered in dunes, which is about the size of the Sahara. This, again, is only possible on a world with a substantial atmosphere and some solid surface because they’re formed by winds. Mars has dunes but I’m not sure about Venus. They’re most similar to those in Namibia, which is where Earth’s highest dunes are, average a hundred metres high and can be hundreds of kilometres long. They give a good indication of the wind direction and are probably large in scale due to the low gravity and it also suggests that there are effectively desert conditions in those regions, emphasising the confusing fact that although Titan has seas, it’s actually a desert world. The dunes are around the tropics and cross the equator, although there are some other patches such as near the northern seas. It was initially speculated that these dark regions, which have a kind of fluid outline, were actually a surface ocean of methane and ethane, and they do flow around higher ground, but it’s actually some kind of organic “sand” being pushed around by the wind. The actual dues themselves are fairly widely separated and also quite steep and narrow themselves, like the dunes in the Namib Desert. It could even be that these grains are effectively plastic granules like those hoisted into hoppers and extruded, and personally I think this would make them suitable building materials.
Also mainly in the tropics is the “hummocky” terrain. Hummocks are small knolls or mounds which on Earth are formed by landslides or in permafrost-rich areas. These cover a further seventh of the world and are made of ice, which is like bedrock on Titan. They’re likely to have formed soon after the body itself and represent wrinkles in a solidifying surface due to contraction through cooling. Again, the hummocks turn up away from the tropics as well and are found in particular in the southern hemisphere.
There are also small regions of “labyrinth terrain”. These are maze-like structures (back to the backrooms?) cut by methane rivers, either through dissolving the surface or physically eroding it, and occur in areas of greater rainfall, often near high ground. On Earth, the Indonesian region of Gunungkidul is similar, consisting of limestone hills riddled with horizontal and vertical caves. The fact that this region on Earth is limestone suggests to me that methane rain may be dissolving the solid surface rather than just eroding it, but I’m no geologist.
The majority of the surface is covered by plains.
The illumination of Titan’s surface during the day is only 1% of Earth’s. This sounds very dim, but in fact it isn’t. Being around ten times Earth’s distance from the Sun, Titan already receives only a hundredth of the sunlight we get per unit area. Nine-tenths even of this is filtered out by the smog. The photo from ground level taken by the Huygens lander gives a fair impression of the murkiness as it would be seen by someone coming out of the kind of sunlight we experience on Earth, but it should also be remembered that the Sun is around sixty thousand times brighter than Cynthia at maximum brightness, so this is like a world with sixty “full moons” in its sky, and nobody could call that dim. The chances are you wouldn’t even notice after a while, although it would be overcast.
There may be clathrate hydrates in the makeup of the crust. These are also present at the bottom of the sea on Earth, and consist of ice which has “imprisoned” methane molecules in its own molecular cages. On Earth, these present a potential major risk of climate change because methane is such a powerful greenhouse gas that it could raise global temperatures catastrophically. On Titan, this is not an issue due to the low temperature.
The crust is around 150 kilometres thick, which makes the kind of missions suggested to Europa’s or Enceladus’s internal oceans less feasible in Titan’s case. Beneath the ocean, the same kind of process may be occurring as is apparent in the depths of Ganymede, with unusual (for us) allotropes of ice such as the cubic form. On the ocean bed there is probably hydroxide “mud” on top of a large rocky globe.
I feel this is such a huge and involved subject that although there’s still a lot I haven’t covered, some of which is very important, I’m going to stop here. Just be aware that Titan is in some ways as sophisticated and complex as Earth and is far more than just another moon.
Next time, the very different and much smaller Hyperion.
As soon as I saw the first pictures of Enceladus from the Voyagers, I realised it was special. For almost a decade, Enceladus had just been another name on the list of Saturn’s moons to me. The most interesting moons in the system up until then had probably been Titan, Iapetus and Phoebe, for reasons which will emerge when I get to them. One thing which was known far in advance of any spacecraft approaching it was that Enceladus is the brightest world in the Solar System. In order to explain how bright it is, the distinction between bolometric and geometric albedo needs to be made clear.
Albedo is the proportion of light reflected back from a surface, and is measured in two different ways, giving two different figures. One is bolometric albedo, also known as Bond albedo. This is the fraction of power in the total light, visible or otherwise, reflected back into space. In the case of Enceladus this is 81%. Geometric albedo is something else, by which I mean it’s a little odd. It’s the ratio of the brightness of a surface seen from the light source illuminating it to an idealised flat surface which reflects back with uniform brightness all over its surface. For some reason I don’t understand, Enceladus is so bright by this measure that its geometric albedo is actually greater than one! It’s 1.38. This is the visual geometric albedo, which only takes the ratio of visible light reflected back into consideration, so there are various ways in which it could be brighter. For instance, a fluorescent surface could have a figure greater than one. Enceladus is of course not fluorescent or luminous, but it does actually have such a brightness. This is because the surface of the moon reflects light back directly to its source without scattering, so whereas a piece of white paper might bounce light off to the sides, Enceladus doesn’t. It’s as if you’re standing on the night side shining a torch straight down at the surface.
One result of this extreme reflection of light is that Enceladus is unusually cold for a Saturnian moon. It hardly turns any of the light reaching its surface into heat and also reflects heat, so its surface temperature is only -189°C. Even so, it has a liquid water interior, but a 60:40 water ice:rock ratio like several other of the moons. This is what makes it so interesting. I first realised this was so during the Voyager mission, and thought to myself that this would make it suitable for life. However, this would require an energy source and more complex chemicals than just water. The place reminds me a little of Europa, but it’s a lot cleaner. The surface is mainly white, unsurprisingly, with pale aqua streaks on it. It’s a lot smaller than Europa though, having a diameter of only 504 kilometres. This gives it a surface gravity only one percent of Earth’s because of its much lower density. This is what led me to write a story about a couple who honeymooned on Enceladus, having got married on Titan, but I didn’t finish it and it didn’t come together. There’s something very “honeymoony” about the place to me, being so white and pure like a wedding dress, and also very floaty, floatier by far than Saturn. In my story there was a hotel with a glass wall stretching all the way down, populated by life from the moon’s ocean, with which it was continuous. I don’t really feel I can discuss the place without mentioning the possibility of life. Of all the possible habitats for “life as we know it”, Enceladus seems to be the most neglected.
As I’ve said, there needs to be an energy source for life to exist. This is less an assumption than a law of physics. In this moon’s case, the highly reflective surface rules out sunlight, but the interior is nonetheless liquid so energy must be coming from somewhere. The interiors of planets and moons are often heated either because they haven’t cooled down yet or because of radioactivity. Enceladus is both too small and too “watery” for this to be so here, but what does seem to be happening is similar to the Galileans in the Jovian system, notably Io and Europa, and again, more like the latter than the former. However, it isn’t clear where this is coming from. It’s undoubtedly there because of the geysers, or volcanoes depending on how you think of eruptions of water from the surface, in the southern “tiger stripes” region. This can’t be happening without a heat source. One possible explanation is the largish moon two orbits out from it, Dione, although it’s also been suggested that it’s due to Janus. Resonances between various moons in Saturn’s system need to take into account the fact that Enceladus is heated but Mimas apparently isn’t.
Whatever the cause, the churning interior of the moon has a major effect on its surface. The terrain has been divided into six different types. The cratered areas are the oldest and are of two types. They differ from Mimas and Tethys in not having any relatively large impact basins but there are lots of smaller craters between ten and thirty kilometres in diameter. The difference between the two areas is that one has well-preserved craters and the other shows signs of collapse, with lower rims and smoother central peaks, which suggests that only the latter has undergone heating since they formed. Although both areas are cratered, it’s only on a par with the least-cratered parts of the other Saturnian moons such as the smooth plains of Dione’s leading hemisphere. Three other types of landscape are intermediate between the cratered and craterless kinds, with both grooves and craters, and the final type only has grooves.
These grooves can also be thought of as valleys and ridges, and they indicate that the crust moves and are possibly formed by water seeping out from inside. However, unlike Europa, which has what look like (but presumably aren’t) scratches on its surface, Enceladus looks clean except for a bluish tinge to some of them. It’s the smallest active moon known.
The moon has a plume, as seen above. This was only discovered by Cassini, since the Voyager spacecraft only approached to about ninety thousand kilometres whereas Cassini got within 175 kilometres, mainly because even the first approach of 1 200 showed something weird going on. It was discovered that the moon deflects Saturn’s magnetic field but only at the south pole, where it had been night time when the Voyagers took a look. It turned out to be the newest terrain on the moon and to be strewn with house-sized blocks of ice. There was a relatively dense cloud of water vapour over it too. Moreover, it was found that the surface temperature at the south pole was around -163°C. Like Saturn itself, Enceladus is warmer at the south pole than at the equator. All the jets of ice are from the tiger stripes. They also contribute to a very tenuous ring around the orbit of the moon referred to as the E Ring, and I’m not sure if it’s called that because of Enceladus beginning with an E or it was just allocated that letter because it was the fifth ring to be discovered. It couldn’t be seen easily from here if at all.
Cassini was flown through the plumes and found them to be mainly ice with some ammonia, methane and carbon dioxide and monoxide, all under one percent. Later on, amines were discovered. These are organic compounds which have an NH2 group at one end, which includes the amino acids from which protein is made, and includes some other important biochemical compounds such as neurotransmitters, hormones and some alkaloids. It’s always “some alkaloids” incidentally, as it’s a family resemblance definition. Hence there are geysers on Enceladus which spew out chemicals associated with life as found on this planet, which could be evidence of life there. Then again, maybe not.
The tiger stripes themselves are four sulci which bend at the side facing away from Saturn and branch on the side facing it. They’re five hundred metres deep and about 130 kilometres long, and are called Cairo, Baghdad, Alexandria and Damascus. Along with the other sulci, they’re named after cities mentioned in ‘The Arabian Nights’. Almost the whole surface of the moon is covered in a substance resembling snow, although it isn’t clear that it’s made of snowflakes and it probably isn’t. The ice around the tiger stripes is different, being larger crystals and absorbing red light, which is what gives them their faint turquoise appearance. They contain dry ice and organic material, which sounds to me like it stains them that colour, and it occurs to me that blue-green algæ are also that colour. The fact that the crystals are larger and unlike the surface ice grains elsewhere also means they can only be a maximum of a few thousand years old, as otherwise the magnetic field of Saturn would’ve converted them into the other form. Presumably they’re being constantly replenished by the geysers.
Whether or not there is life in or under the geysers, it’s probable that the extremophile organisms living in some unusual environments on Earth, such as geothermal vents, and these organisms do produce methane, which is found in the emissions from these geysers on Enceladus. They also contain sodium, chlorine and carbonate ions, indicating that the water is salty and contains washing soda. The presence of ammonia within the plumes means that the mixture of compounds could be liquid at as low as -103°C, but even without it the internal heating is sufficient to cause them. The presence of the ocean can only be guaranteed in the south pole region, and it may be around thirty-five kilometres deep, which is favourable to life as it means geothermal vents would be relatively close to the surface and not covered in compressed ice.
The situation where a mixture of ammonia and water freezes at a lower temperature than either, which is one possibility here, is known as a eutectic mixture. This also occurs with salt water, which freezes at -21.3°C and is used to thaw ice on roads. In the case of the liquid here, not only might it be a mixture of the two, but also salt is involved. Ignoring the salt, a 36% concentration of ammonia would be sufficient for being liquid at this temperature. However, if this is true, I’d expect there to be more ammonia in the E Ring, which there isn’t, and the water ice content of the geysers is something like 99%, so I don’t really get why they think this would be a solution. Maybe the salt makes a big difference.
The total volume of Enceladus is about a twentieth that of the World Ocean, and it isn’t entirely water, so it isn’t one of those moons with more water than Earth for once. Its diameter is such that it would almost cover the North Sea, and its surface area is slightly smaller than Mozambique and somewhat larger than Turkey.
Compared to Mimas, Enceladus is very active, but being closer to Jupiter the “death star moon” should experience more tidal forces and therefore activity than the “honeymoon” (yes, I know it doesn’t really work. Just play along), but in fact it’s very much the other way round. This is all the more mysterious because Enceladus has a more circular orbit than Mimas, which ought to render it less active due to less variation in tides. It’s also odd that the south pole is hotter than the rest of the moon. Although it makes sense that Enceladus is partly heated due to its orbital residence with Dione, Mimas has the same relationship with Tethys. It looks like it must have started off very liquid at an early stage and stayed that way. Theoretically, Enceladus could be a quiet moon like Mimas, but it has two stable states whereas Mimas only has one, so even if Mimas started off active, it would’ve frozen through, whereas Enceladus would not. Unlike Mimas, Enceladus is constantly losing mass and this could have led to subsidence under the south pole. If Enceladus is the same age as most of the objects in the Solar System, it would’ve started off 30% more massive. However, it’s also been suggested that the entire Saturnian satellite system only formed during the Cretaceous because the orbital dynamics of the moons is not stable enough to have lasted since the formation of Saturn. Against this is the modelling done of the system which seems to show that the ocean has been there since Precambrian times, although only back into the Cryogenian, when Earth itself seems to have been frozen over, although that’s long enough for life to appear there.
One final issue: Because Enceladus is active and internally heated, it’s possible that it would be suitable for a human settlement, even maybe a honeymoon hotel.
That’s it for Enceladus then. I’m not sure what I’ll post next, but after that I’ll be talking about Tethys. I’m conscious of ignoring the global situation rather heavily and can’t decide whether I should persist in doing this or not.
A Box Of Chocolates 