Triton

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:

Earth:Cynthia – 81
Jupiter:Ganymede – 12 808
Saturn:Titan – 4 222
Hamlet:Titania – 25 294
Neptune:Triton – 4 768

In terms of equatorial diameter, the ratios work out thus:

Earth:Cynthia – 3.67
Jupiter:Ganymede – 26.54
Saturn:Titan – 22.62
Hamlet:Titania – 32.15
Neptune:Triton – 18.19

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.

The Honeymoon

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.

Planet Cueball

The first time I saw images of Jupiter’s moon Europa, it reminded me, for some reason, of a softball. I realise it looks a lot more like a cue ball than that, and I can’t explain why I got that association rather than the other. Because I was thinking of a relatively pristine object, it always makes me feel that it’s a bit worn out, scuffed, dirty and in particular scratched, and it makes me feel like I’ve got dusty hands like I’ve just picked up a mucky ball in dry but dirty conditions, as prevailed in our sports hall at school. I may be wrong about this, but my impression of Kent generally is that it’s rather dustier and sandier than the English Midlands, and that does make sense given its slightly warmer, drier climate. Over the channel it seems to become slightly more so, but I don’t know because it doesn’t seem like the difference is that big. The average annual temperature in Canterbury is 11°C and precipitation is 728 mm. Compare this to a place I don’t live (because I don’t want to doxx myself) but do live fairly near, Oakham is slightly drier at 716 mm precipitation annually and slightly cooler at 9.8°C, so in fact it seems not to be true.

But this post is not about the climate of East Kent but if anything, the climate of Jupiter’s moon Europa. Europa is in some ways very Earth-like in a way no other planet (see here for why I’m calling it that) is. It’s the smallest Galilean at 3 126 kilometres in diameter, which makes it slightly smaller than Cynthia. There are of course more than six dozen still smaller Jovian moons and if we could see Europa from the distance we see the lunar surface from, it would look about the same size, but would be four and a half times brighter and lacks the shadows our satellite has due to its flatter relief.

The “accident” of its naming opens it up to comparisons to the pretend continent with a similar name, and it’s also worth explaining why it has the same name, so let’s start with that. Europa the mythical, or possibly historical, figure was King Minos of Crete’s wife. There have been attempts to connect the name to the Akkadian word for “west”, ‘ereb, and that’s quite neat because it then allows Asia to be connected to a word for “east” and Afrika to a word for “south” (I think), but it may not work. It might also mean “wide face”, which is how it sounds in Greek. As usual for these stories, Zeus abducted or raped Europa, and this time he was in the form of a bull hiding in her father’s herds. This was commemorated as the constellation Taurus. The association with Europe is therefore somewhat surprising, but the way it worked was that it was initially applied to cis Balkan Thrace by the Greeks, then became the name of a Roman province including that area, which was then used to supplant the division which had emerged between the eastern and western Roman Empire. I have to say this explanation really feels like it has a lot missing from it. The element Europium is named after it, and just in passing I want to say that Europe is a fake continent. It’s actually just Eurasia’s biggest peninsula, and from that rejection, Asia is also a misleading name. There’s just Eurasia. That said, I regard myself as Northwestern European, while recognising that this doesn’t refer to my origins in a part of a continent but just as from that part of that peninsula. (This may be enlightening). This is the convoluted route whereby Europa came to refer to two such different things.

The surface of the roughly Cynthia-sized Europa is three times the size of the terrestrial region at thirty million square kilometres. This makes the planet’s surface twice the size of Antarctica. Another way of thinking of this is that Europa’s surface is equal in area to the combined area of Antarctica and the Arctic Ocean. We kind of have our own Europa right here, as well as our own Europe, but the Europa orbiting Jupiter is colder even than the South Pole in midwinter, at least on the solid surface, at a temperature of -160°C. The temperature at the equator varies daily between -141 and -187°C. The poles are actually warmer than the equator at night, and the north pole is warmer than the south at those times. This range of temperature happens to be the one (below freezing) where the properties of water ice change most.

Europa is very bright, having a surface of water ice, although it doesn’t reflect as much light as Enceladus as its surface is “dirtier”. Compared to the other Galileans, it’s composed much more like the inner planets, being mainly silicate rock with an iron core. The chief difference is that its surface is solid water ice with an ocean of salt water underneath. Back in a period referred to as the Cryogenian, Earth was in a somewhat similar state with a crust of ice covering a salty ocean over silicate rock and an iron core of course, although Earth is much larger than Europa and it had continents and oceans underneath the ice, unlike the moon, which is probably more homogenous. This was 700 million years ago, and is sometimes thought to have stimulated evolution enough to trigger the Cambrian Explosion.

It’s difficult to talk about Europa without talking about the possibility of life, so I’m going to break my self-imposed rule here and do that. It wasn’t initially clear whether the ice was simply frozen solid or covered a water ocean, but the latter appears to be so. Salt water can be detected by space probes because of its ions, which being charged behaves differently in terms of magnetism than fresh water. The surface, though mainly water ice, is also covered in sulphates and there is some sulphuric acid, but these may well be from Io’s volcanism. Like most moons, Europa faces the planet it orbits at all times, giving it a leading and a trailing hemisphere, and the sulphates, which include Epsom salts, and sulphuric acid are mainly deposited on the latter, indicating that it doesn’t come from the ocean but from Io, or it would be evenly distributed. The leading hemisphere, by contrast, has sodium chloride on its surface. This would lower the freezing point of the water, making it more likely that “life as we know it” could exist there. There is a “found footage” film, ‘Europa Report’, which takes pains with accuracy and depicts complex multicellular life in the ocean, and ‘2010’ also shows complex life there. The main difficulty as I see it is that although the situation isn’t as bad as on Io, the radiation belts are still significant, but I presume the ice provides shielding. As well as the other constituents, there’s dry ice and frozen hydrogen peroxide, the latter of which is thought to be formed by the radiation.

If there is life, it’s likely to derive its energy from deep-sea vents, as also happens on Earth, and like Io, the energy for this volcanism comes from the flexing of the crust and planet from tidal forces of Jupiter and the other Galileans. This is thought to be responsible for the cracks on the surface. Also like Io, Europa’s surface is almost devoid of craters, strongly suggesting that it was liquid more recently than Ganymede and particularly Callisto, the two outer Galileans. When the Voyagers visited, the encounter was relatively distant and the moon wasn’t mapped in as much detail as the others, so the knowledge and research done into the moon lagged behind that on the others. Three types of feature were identified: lineæ, which are the “cracks”, flexūs and maculæ. It was from “macula” used in this naming that I first learnt the Latin word for spot, as in “immaculate”. None of the features are very high or low and the surface is unusually smooth. There are currently forty-five named lineæ, formed when cracks appear in the surface and material seeps up from the interior to fill them, which then freezes. Salt is highest in the lineæ.

Europa takes three days and thirteen hours (plus a bit) to orbit Jupiter. Like most other moons its day lasts as long as its orbit. This period is significant because it’s almost exactly twice Io’s. Roughly every three and a half days, Io and Europa are within a quarter of a million kilometres of each other, making them larger than Cynthia in each other’s skies and this causes them to pull on each other, raising tides in their surfaces and elsewhere and heating each other independently of solar radiation. Perhaps surprisingly, although Europa is the least massive moon of the four Galileans, it has the second highest gravity at 0.134 g, somewhat lower than Cynthia’s. The next moon out, Ganymede, also the largest moon in the Solar System but I’ll come to that later, again has almost exactly double Europa’s period. The Darian calendar, originally designed for Mars, has been adapted for use with the Galileans.

The surface is covered in icy regolith, substantially broken down by the radiation, with grains about the same size as snowflakes, though presumably not so regularly formed. This means it would be possible to ski on Europa, although there are no real slopes. Also the radiation would quickly kill you unless you had really good shielding on your ski suit. Maybe one day. Incidentally, radiation shielding doesn’t have to consist of lead or some other heavy metal, and synthetics work quite well. That said, I don’t know how powerful the radiation is there. It’s weaker than on Io though, and unlike Io, Europa doesn’t have the flux tube. However, although it was long considered quiescent, it does have cryovolcanism. There are domes on its surface which may have volcanic origins and of course it seems to have actual volcanism, or rather volcanism like Earth’s, in the form of deep sea vents. The cracks in the surface, which rapidly freeze over, expose water which evaporates into the atmosphere like steam. And yes, it has an atmosphere, though even thinner than Io’s, but unlike Io’s the main constituent is oxygen. This is generated by the radiation splitting the steam and Europa’s gravity being insufficient to hang onto the hydrogen.

Finally, the Galileo probe was deliberately pushed into Jupiter’s atmosphere to destroy it because of its own discovery of a salt ocean on Europa, to protect any potential life which might exist there.

That’s Europa then. Next: Ganymede.