. . . 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.
I was going to blog about the larger asteroids at this point, but in recent days it’s been borne in upon me that there’s a current issue in astronomy, perhaps over-emphasised but definitely there, over whether Pluto was unfairly demoted. The reason I mention this now is Steve’s comment about what the difference between Phobos and Deimos and asteroids might be. It’s a very good question and I’ll address this first.
Phobos and Deimos, the moons of Mars, are a little puzzling. There are two hypotheses about where they come from. One is that they’re main-belt asteroids which were captured by Mars. At first glance this sounds very sensible and logical. After all, Mars is next to the asteroid belt, it could be expected to gather up a few stones from it from time to time and the pair seem to be only the latest representatives of a whole series which have scarred Mars with chains of craters as they broke up and impacted. However, there are problems with it. Firstly, the common type of asteroid found near the edge of the belt closest to Mars is different from the type of asteroid Phobos and Deimos would be if they are asteroids. That type is found near Jupiter. This is due to the inner belt being warmer than the outer belt, so the composition differs because temperature makes a difference to them. Secondly, both moons have almost perfectly circular orbits over the Martian equator, and if they were captured, they would usually have come in at a high angle to the equator and have markèdly elliptical orbits. This can be seen with Nereid, Neptune’s third largest moon, and Saturn’s moon Phoebe orbits backwards compared to most other bodies in the system. Therefore, if Mars’s moons are asteroidal in origin, something needs to be evoked to explain that. A simpler explanation would be that they emerged from the cloud which was forming Mars. This would be spinning in the same plane as any moons which formed from it, and if they were formed in situ they would be more likely to have almost circular orbits. However, as Steve astutely pointed out, the actual nature of the bodies themselves is very close to being asteroidal, and in fact is asteroidal, so maybe it doesn’t matter in most ways. In the sense of the physical nature of the two moons, they basically are asteroids. The way in which they aren’t is to do with their history and orbits, which may not be a sensible thing to focus on. The only thing which goes against this is that both are directly affected by orbiting Mars. Phobos has streaks because of the tidal forces of its planet, and Deimos accumulates fragments and dust from itself as it moves through its rather short orbit. If they were orbiting in the asteroid belt itself, neither of these things would be happening. All that said, I can totally see the argument that they are in fact just asteroids in an unusual place which are also moons rather than minor planets. So I agree with you Steve.
This connects to a wider issue which affects Pluto, and it also affects a number of other worlds in the system which if addressed could solve the problem of knowing what to call the big round things in our Solar System. It could also address the peculiarity of our own “moon”. The 2006 CE definition of a planet by the International Astronomical Union is:
The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
This definition was motivated by the discovery of a number of relatively large trans-Neptunian objects. Eris, discovered at the start of the previous year, has now been established to have a diameter of 2326 kilometres and a mass of 1.6466 x 1022 kilogrammes. Sedna, discovered in 2003, has a diameter of around a thousand kilometres and an unknown mass because unlike Eris it seems to have no moons. Sedna is less of a threat to the status quo but Eris was initially thought to be larger than it has now turned out to be. For comparison, Pluto is 2376.6 kilometres and it has a mass of 1.303 x 1022 kilogrammes, so it’s actually slightly larger than Eris but also less massive, so the question arose of whether it would be acceptable to admit a potential host of newly discovered planets, thereby reducing the “specialness” of planets, or to invent a new category. This last idea, of “dwarf planets”, seems very odd to me because the category of “minor planet” had existed for a very long time up until that point and instead of inventing an entirely new class of object, it would’ve made more sense, if they were going to do this. Whether or not I agree with the decision, there seems to be no merit in creating a whole new category of “planet” when “minor planet” already existed. I honestly don’t know why they did this.
Many people have disagreed with the decision to demote Pluto. It did elevate Ceres, previously considered a mere asteroid, at the same time. Before that point, for most of its history since discovery Ceres was considered an asteroid, but it’s the only body in the asteroid belt which has managed to make itself round due to its own gravity (there might be other bodies which just happen to be round-ish through chance because asteroids are irregular and could hypothetically be many shapes, including spheroidal), so it probably does deserve special recognition.
In spite of this definition, which is quite unpopular, a paper has recently been published on the subject arguing that Pluto, among other worlds, does in fact merit planethood. The paper can be found here. It’s sixty-eight pages long and I haven’t read the whole thing but the general gist of it seems to be that there used to be a scientifically arrived-at understanding of what a planet was, but over a period in the early twentieth century when astronomers focussed more on what was happening outside the Solar System, the popular uneducated public understanding of what a planet was took over. I have to say this doesn’t reflect my perception of what happened based on my knowledge of astronomy. I’m aware of the controversy about the canals, the discovery of Pluto, the idea that Mercury always faced the Sun and so on, all ideas which resulted from astronomical research at around that time. I’m aware of the research that was being done at the time about stellar evolution and the realisation that there were other galaxies, but it really doesn’t seem like they were concentrating that much on that more than this Solar System, but anyway, that’s what this paper claims.
Further, it claims that because they adopted a kind of folk understanding of what a planet was, it had led to them adopting earlier, non-scientific ideas about it. So for example, the public was really into astrology and had only recently got used to the idea that the Sun was at the centre of the Solar System rather than Earth. The authors of the paper give examples of how scientific classifications differ from public ones. For instance, most people think of fruit and vegetables as two different things but when it comes to botany, vegetables include fruits, which are the reproductive organs of plants, so from a culinary viewpoint fruit and veg are separate but scientifically they aren’t. To this I would add a couple of things which are I hope relevant to astronomy. One is that I think of a lot of things as fruit, such as tomatoes, aubergines, courgettes, peppers and tomatoes, which other people seem to think of as vegetables because it makes sense to me to think of them nutritionally and in terms of flavour in that way. The other is that the culinary arts are also sciences, and it seems a bit hierarchical to see them as inferior to botany for some reason. After all, we all need to eat. Applying that to astronomy and planets, that would mean that although some things are planets and some things aren’t according to astronomers of a particular vintage, that doesn’t mean there isn’t another branch of science which would view them differently. For instance, everything is subject to the laws of physics, and geology would seem to apply pretty much equally to planets, moons and asteroids in their own way. They’re just bodies in space like everything else. Therefore, I’m not convinced about this. Also, the general public were specifically irritated at the idea of Pluto not being a planet any more, so I don’t see how exactly they were using the public view of what planets were if they managed to annoy so many non-astronomers with their assertion that Pluto wasn’t one.
What seems to have happened is that the problem crept up on astronomers and they kind of panicked and made a fairly slapdash and hasty decision. As various large bodies were discovered on the edge of the Solar System, they became uncomfortable with the idea that they were probably going to end up with a very long list of planets, which seemed unwieldy and not very “neat”, and they also perceived it as an imposition on education that people were going to have to learn about so many worlds. They seemed to feel like this would be regarded as off-putting. The paper compares the situation with how mammals are defined. The official definition of a mammal is now rather abstruse, because it actually hinges on how many bones are in the jaws and the ears, but this is partly because of the need to identify fossil mammals. The widely-used definition is “animals who suckle from their mothers as infants, maintain a different body temperature from their environment, are often covered in fur or hair and mostly give birth to live young”, and the first criterion is the most important. There are exceptions to most of these. For instance, some hibernating mammals don’t keep their body temperatures above their surroundings and humans, whales and elephants are largely hairless, but this is a fairly good definition. However, claim the authors, astronomers have taken a weird approach to planets, having concentrated on whether they dominate their local region, which is in any case vague because what’s local? They’ve also looked at how they move. If mammals had been defined in this way, as warm-blooded vertebrates who walk in herds on land for example, a lot of mammals would’ve been excluded. Bats and whales would then not be mammals and any mammal who has a largely solitary life, such as leopards or sloths, would not then count as mammals either.
Looking at the history of the idea of planets, for a long time any round object in the sky which didn’t appear to stay in the same place was a planet. This used to include Cynthia and the Sun, when people thought Earth was at the centre of the Universe, and it didn’t include Earth. Later on, the four largest moons of Jupiter were discovered and also referred to as planets, and even the thick parts of the rings on either side of Saturn due to the poor quality of telescopes at the time. Later still, Ceres was called a planet because it seemed to fit into Bode’s Law, and turned up where it was expected. By that time, however, the known satellites had been relegated to moons, and soon after Ceres was also demoted because it was realised that there were thousands of other bodies between Mars and Jupiter, some even quite large.
The 2006 definition also has a rather silly consequence which a few people have noticed: it means Earth isn’t a planet! As I’ve mentioned before, from the Sun’s perspective Cynthia doesn’t orbit Earth, but the two weave in and out of each other’s orbits. I’m not completely clear what the astrological influence is supposed to be, but I think it’s the emphasis on orbits, i.e. the kind of definition which would’ve excluded bats, whales and leopards from being mammals. Whatever the definition of a mammal is, it seems to make more sense to use their anatomy and physiology than other more dubious criteria. Both of the definitions I mentioned above do this. The first is rather abstract and strange to most people, although there are good reasons for it – mammal jaws and teeth survive better than the rest of their bodies so it’s like identifying a body by dental records – but both of them focus on what their bodies are like, which seems entirely sensible compared to that fictional other definition.
What, then, is proposed as a more sensible definition of a planet? Well, it’s closer in spirit to that way of defining a mammal. A planet is a geologically active body. I have to admit I’m not sure about this because of various things, such as “eggshell planets”, and I’d also want planets to be round and I can’t tell if they also stipulated that. What it means (I’ll get back to eggshell planets in a moment) is that Pluto’s Sputnik Planitia which is created by frozen nitrogen and is active even though the Sun isn’t strong enough at that distance to have that effect. In talking about asteroids, I’ve mentioned the fact that the larger ones tend to be layered like Earth is, but the smaller ones are either rubble piles or mixtures of different minerals and other substances which aren’t separated out in the same way. A geological process has done this sorting in the larger ones, and consequently Ceres, for example, could count as a planet: it has been geologically active.
This applies also to some moons. Io, the innermost large moon of Jupiter, is intensely active with continual volcanic eruptions, to the extent that it’s thought to “turn itself inside out” every few years – some much of its interior is spewed onto the surface that the former surface becomes the interior and proceeds to get thrown out itself a few years later. This is because of the tidal forces effectively “wringing” the moon all the time, with the other large moons in the Jovian system along with Jupiter itself wreaking havoc on the place. By this standard, Io is definitely a planet, albeit a planet which is also a moon.
I’ll now permit myself a digression into eggshell planets. An eggshell planet is a surprising kind of “planet” which kind of “does nothing”. It isn’t necessarily possible to tell from a distance which planets are like this. Earth’s crust is divided into plates, and other planets have a thick, solid layer all the way round, but there is another possibility or which at least three examples may have been found already. This is where the crust is thin and fragile, and so cannot have plates or thick layers, and also can’t even support mountains or hills, so the surface is solid and also smooth, and nothing happens there – no volcanic eruptions, continental drift or erosion, because there’s nothing to erode. The question arises of whether this even counts as a planet under this new definition, since it isn’t geologically active. However, there are no such planets in our Solar System as far as anyone knows, and they’re probably quite rare.
There are three categories of planets suggested in this new definition: terrestrial planets; giant planets; satellite and dwarf planets. The last category is the largest. It includes the large moons of Jupiter, Ceres, Titan, Pluto, Charon, Eris and Sedna, and in fact there are more than a gross of these. Far from the expected response, apparently people tend to be quite excited at the idea that there are so many planets around the Sun. The giant planets include Jupiter, Saturn, Uranus and Neptune, so no surprises there, although this clear-cut division may be an artifact of how our own Solar System is, with its complete absence of the very commonest type of planet, the mini-Neptune, intermediate between Earth and Neptune in size.
There are five planets in the terrestrial category rather than four, because once the criterion for dominating its orbit has been removed, Cynthia becomes eligible, which makes me very happy! Cynthia is not even in the same group as the satellite and dwarf planets, but a planet just like Mars and Mercury. This also means that the Apollo astronauts landed on another planet, not just our moon. As well as that, Earth now has no moon!
It seems that the process leading to the decision to redefine planets was not very scientifically grounded and was in fact rather acrimonious. The orbital dynamics people took umbrage at the geophysical definition and there were only a few days available for debate, forcing people to take sides quickly without due consideration. Planetary scientists were underrepresented because they’re apparently not officially astronomers, which is a bit astonishing. Another motivation was to keep the number of official planets low because the IAU didn’t expect the alternative to go down well with the public because previously, i.e. in Victorian times, they’d felt more comfortable with a small number of planets. They were used to seven at that point, including the Sun and Cynthia. This is probably no longer the case, so in 2006 they made a decision based on misjudging the mood of the general public.
To finish, I’m going to make a commitment. Henceforth I will be referring to every spheroidal body in the Solar System as a planet, although I will also acknowledge what kind of planet it is, such as a moon or dwarf planet. And Pluto is a planet!
If you say “Vulcan” to most people nowadays in an Outer Space context, the chances are they’ll think of Spock, and that’s an entirely valid thing to do. However, if you were to say it to anyone with much knowledge of astronomy in the nineteenth century, it would’ve called something completely different to mind: a planet which orbits the Sun even more closely than Mercury. I’m going to cover both in this post.
Firstly, the ‘Star Trek’ Vulcan, whose Vulcan name is Ni’Var. This is reputed to orbit the star 40 Eridani A, a member of a trinary star system also known as ο2 Eridani (Omicron-2 Eridani – that isn’t an “O”) sixteen and one quarter light years from here, and therefore also quite close to 82 Eridani, which is said to be one of the most suitable nearby stars for life, around which a possibly habitable planet orbits in real life. Of the stars, A is an orange dwarf, B a white dwarf and C a red dwarf which is also a flare star. Because B would previously have been a red giant and exploded, the chances are that any habitable planets orbiting A would have been sterilised by B’s outburst, and since C is a flare star, this is also unsuitable, although there would be nothing to stop an interstellar civilisation settling a planet in A’s habitable zone, which would of course be Vulcan.
As I’ve mentioned, I don’t pay much attention to either ‘Discovery’ or the new ‘Star Trek’ films, but I’m aware that Vulcan has been destroyed in revenge for the destruction of Romulus. I find this a bit annoying and I’m not sure what the point of it was plot-wise, but it doesn’t alter the in-universe fact that Vulcan was the homeworld of the first species to make open contact with humans when Zefram Cochrane first activated the warp drive. I’m also aware that that is inconsistent with the depiction of Cochrane in TOS. It is interesting, though, that any real planet in the habitable zone of 40 Eridani A would have been severely damaged by the 40 Eridani B supernova.
I understand Vulcan to have no moons, higher gravity than Earth and no surface oceans. I’m also aware that Romulans and Vulcans are the same species. It irritates me that they’re humanoid but also interests me that some of their anatomy and physiology is known, such as their copper-based respiratory pigment. Then again, although the in-universe explanation of widespread humanoid aliens is that we are all descended from humanoid ancestors who existed around the time our own Solar System formed, it’s also conceivable that convergent evolution would lead to similar body forms among sentient tool-using species. Back to Vulcan itself though. It has a thinner atmosphere than Earth’s, which I think justifies the copper-based blood pigment, and the sky and much of the surface is red. There are seas, i.e. large landlocked lakes, rather than oceans continuous with each other. Depending on the total surface coverage of bodies of water, I think this would probably make the planet uninhabitable for humans although clearly not for native life. 40 Eridani A is a K-type star, with a longer lifetime than the Sun’s in terms of being able to support a habitable planet, which, if orbiting at the distance necessary to receive the same quantity of light and head from its primary as we do from our Sun as a planet, would have a mean orbital radius of about 0.68 AU, i.e. sixty-eight percent of Earth’s distance from the Sun, and 223-day year. However, Vulcan is supposed to be hotter than Earth and might therefore be closer to its sun or have more greenhouse gases in its atmosphere, or it could just reflect less heat back into space, and in fact it probably would due to less ice on its surface. The difficult thing to account for with Vulcan is the combined higher gravity and thinner atmosphere, but there is another reason than gravity why a body might lose some of the gas surrounding it, which is consistent with what we “know” about Vulcan. Earth’s strong magnetic field is generated by our own large moon, Cynthia, which raises tides in our iron-nickel core and magnetises it like stroking a bar of iron with a magnet does, and that generates our magnetosphere, which traps ionising radiation from the solar wind which might otherwise reach Earth’s surface and strip away our atmosphere. Hence Vulcan, with no pre-existing satellites, would not have this benefit but would on the other hand still be able to hold on to some atmosphere because of its higher gravity, so maybe that is in fact realistic. Venus has no magnetic field but an extremely dense atmosphere, although not one hospitable to life at the solid surface, due to photolysis – the action of light on rocks releasing carbon dioxide gas. However, we’re basically aware that Vulcan’s atmosphere has enough oxygen to support human life without their own oxygen supply, and not enough carbon dioxide to poison us, which is 0.5% at our own atmospheric pressure. 170 millibars partial pressure of oxygen is required for this and CO2 cannot be making a significant contribution to the pressure, so we can surmise that the rest of Vulcan’s atmosphere substantially consists of other gases. It isn’t pure oxygen. In fact, it’s quite likely to be nitrogen if Vulcan physiology is anything like ours and their bodies consist partly of protein, as the nitrogen has to come from somewhere, so I’m going to say the mean surface air pressure is about 0.25 bars. I’ve plucked this figure out of the air, so to speak. There probably is no such thing as sea level there because of the various lakes with different presumed depths and heights, so this would be defined as some kind of mean distance from the centre of the planet or a level at which gravitational pull is close to a particular standard. The boiling point of water on Vulcan is therefore about 60°C, but we know from McCoy’s mouth that Vulcan is very hot compared to Earth, so this puts an upper limit on its surface temperature unless it’s so hot at the equator that it causes water to evaporate.
40 Eridani A is orange. The sky is likely to be close to a complementary colour, such as teal, given that, but because of the dusty surface it’s entirely feasible that it would in fact be pinkish due to small particles high in the atmosphere. Also, the general ruddiness of the planet as shown on screen gives the impression of heat and dryness, so artistically that does seem to be a good decision. The same features make some people think of Mars as a hot planet when in fact it’s often colder than Antarctica. Regarding sparse water cover, a thin atmosphere might make sense here too, particularly if water is regularly evaporating from the surface at the equator, since some might then be lost into space.
Vulcan would also lack plate tectonics if it’s like this, since that’s fuelled by water. The planet has no continents as such, but it does have active volcanoes and lava fields, which is to some extent to be expected as it corresponds to the “hot spot” situation in the centre of the Pacific plate on Earth, where magma seems to need to vent. Here, this results in Hawaiʻi, but on Vulcan a mountain range could be expected because there are no oceans. There would be nothing like the Pacific Ring Of Fire, and also no fold mountains because those are caused by the collision of continental plates.
Vulcan’s colour is depicted differently in different manifestations of the series. In TOS and Enterprise, it’s red. In TAS it’s yellower, and in TNG brownish. However, on Mars there is variation in colour from space due to a dust storm season, and this can be imagined on Vulcan too. Maybe one way to think of Vulcan is as a larger, hotter version of Mars.
The real 40 Eridani A does have a planet. This is, as usual, called “b”, and orbits much closer to the star than the inner edge of the habitable zone. It has a roughly circular orbit 0.22 AU from the star and a mass estimated at 8.5 times Earth’s (both those figures are rounded off). At Earth’s density, this would give it a diameter of around 25 000 kilometres, which is a type of planet unknown in our own solar system at any distance from us, and it’s classed as a “Super-Earth”, but it has a period of 43 days and would be like Mercury on its surface during the day, if it rotates at all. It’s also the closest known Super-Earth. Its orbit differs considerably from Mercury’s, which will become relevant later in this post, in being much less elliptical, which to me, in my probable naïveté, suggests there are no planets larger than it in at least the inner solar system.
This brings me to the other Vulcan. In the nineteenth Christian century, the French astronomer Urbain Le Verrier came up with a particularly accurate model of planetary motion within the Solar System. It had been noted that the most recently discovered planet, Uranus, tended to drift slightly behind and ahead of its predicted position given its distance from the Sun and shape of its orbit. From this, Le Verrier calculated mathematically that there was likely to be another planet further out pulling at it, and predicted its position, which turned out to be correct. In fact he almost had it named after him, but they eventually decided to call it Neptune. This established his reputation and consequently, when he turned his attention to the orbit of Mercury, people paid attention and took his views seriously.
Mercury’s orbit is quite unusual compared to the other planets, particularly if you ignore the period of time when Pluto was regarded as one. It’s the most eccentric orbit by a long way compared to the others, with a variation in distance from the Sun of around twenty percent. Le Verrier also noted that the movement of the “points” of the orbit precessed around the Sun much faster even when compared to its year of eighty-eight days than those of other planets. Just as he had with Neptune, Le Verrier proposed that there was either an as-yet undiscovered planet even closer to the Sun or a number of smaller bodies like asteroids within the orbit of Mercury, and since it would’ve been so close and so hot, he called it Vulcan after the Roman god of fire, Vulcanus. The planet’s existence could be confirmed in two ways. Either it could be detected in transit, as most planets are detected at the moment, or it could possibly be glimpsed during a total solar eclipse. A number of astronomers then reported that they had indeed seen this planet transiting the Sun. For instance, Edmond Lescarbault, a doctor, described a tiny black spot moving across the Sun faster than a sunspot, moving with the rotation of the Sun, would, and also lacking a sunspot’s penumbra. The observations even seemed to confirm Le Verrier’s prediction of Vulcan’s size and orbit. However, it was difficult to predict when these transits would occur because that depended on the tilt of Vulcan’s orbit compared to ours. Mercury, for example, can only be seen to transit the Sun in May or November because only then is the tilt of both its and our orbits aligned such that it can get between us and the Sun. The observations did seem to occur fairly randomly, but at first glance Mercury’s do as well, if you didn’t know anything about its movements already.
There was a total eclipse of the Sun in 1883, shortly after Le Verrier’s death in 1877, during which Vulcan was not observed. It was still possible that the planet was either behind or transiting the Sun at the time, but six further such observations, the last in 1908, also failed to turn it up, making it increasingly improbable that the planet existed. However since that time astronomers have claimed that close ups of the Sun’s surface do sometimes show small black dots which are not sunspots, although these may be imperfections of photographic plates, and there are asteroids which approach the Sun more closely than Mercury does, such as Icarus. It strikes me that it’s not only possible but probable that there are asteroids which orbit entirely within the orbit of Mercury, although they would have to be very small and would be difficult to observe or confirm. These are known as Vulcanoids, and would have to be between six kilometres and a couple of hundred metres in diameter. Every region of the Solar System which is not severely perturbed by the gravity of known objects has been found to contain objects like asteroids or comets, so if the innermost region of the system doesn’t have any this must be due to a non-gravitational effect. It is in fact possible that the light from the Sun is so strong at that distance that it would push smaller bodies away from it over a long period of time, so this may be the explanation. This might sound far-fetched, but it’s been proposed that this effect could be used to divert asteroids which would otherwise crash into Earth by painting them white in order that the pressure of light from the Sun would change their orbits, and this is also the principle used in a solar sail. The MESSENGER probe took photographs of the region but this was limited because damage from sunlight needed to be avoided. Much closer in than Mercury, asteroids are likely to vaporise of course.
Vulcan was considered to orbit 26 million kilometres from the Sun, giving it a sidereal period (“year”) of twenty-six days. At another point, observations appeared to show it had a year of 38.5 days. I think it was also supposed to be very small but I can’t track this down: possibly about a thirtieth the mass of Mercury, which with the same density would’ve given it a diameter of around 1 600 kilometres, probably meaning that if it had been found to exist it would’ve been demoted from planethood by now in the same way as Pluto was. In fact, if it did exist, it would indeed have perturbed the orbit of Mercury but the other factors which turned out to be the explanation for this phenomenon would still be in play, meaning that there would’ve been an even greater anomaly unless the planet happened to be exactly the right mass and in exactly the right place, and possibly retrograde. Some kind of pointless immense astroengineering project could probably achieve that to some extent, but why? Possibly to prevent us from being aware of relativity?
The fact is that the planets don’t simply orbit the Sun alone without influencing one another and the Sun. This is the famous three-body problem, that it’s impossible to work out in almost all cases how three bodies would orbit each other, and even more so the much larger number of massive bodies in the Solar System. It’s possible to work out how much gravitational influence the planets would have on each other if they were the only two bodies in the Universe though, and if initial conditions are known. For instance, Venus and Earth approach each other to within fifty million kilometres and have roughly the same mass, so left to themselves they would orbit each other at roughly twenty-five million kilometres from their centre of gravity once in forty-five millennia if I’ve calculated that correctly, and at the closest approach, which would be during a transit of Venus, that’s the gravitational pull we’re exerting on each other – about forty-five thousand times less than the Sun’s. Mercury is the least massive planet, being just over half the mass of Mars, the next smallest. Pluto is of course far lower in mass, and if Cynthia is considered a planet in its own right, that would be considerably less massive. Anyway, this means that Mercury is pulled around a lot by the other planets. Venus approaches it to within about 38 million kilometres but without doing the maths it isn’t clear if that’s the biggest gravitational influence because of Jupiter being so much more massive than the other planets, even though it’s far further away. Jupiter is over three hundred times the mass of Earth but would get within 4.8 AU of Mercury, which actually gives it roughly the same influence as Venus. But this is not the only reason Mercury’s orbit precesses as much as it does.
Albert Einstein listed a number of ways to test his theory of general relativity, one of which was the orbit of Mercury. The pull of the other planets is insufficient to explain precession in Newtonian terms. There’s still a bit left over if you try to do this. It’s at least seven percent larger than it “should” be. The explanation for this was instrumental in getting general relativity accepted. Einstein made three suggestions about how general relativity could be corroborated. One was that light would be red shifted if it passed through a gravity well. Remarkably, although it took something like four decades, the observation of 40 Eridani B eventually showed that this was so, I’m guessing because of the other stars in its system. Gravity stretches light because it distorts space. The second proposition was that stars observed near the Sun during a total solar eclipse (Again! They’re useful things) would appear to be in a different position because their light would be bent by the solar gravity, and this was indeed found to be so a few years later. However, the world had to wait for these two findings. The other one was that Mercury’s orbit would precess at the rate it did having taken into account the perturbations of all the other planets, and again this was found to be so, but in this case it was already known that this happened because Le Verrier had observed it in the previous century and the existence of Vulcan had been refuted. The reason this happens, I have to admit, I don’t really understand, but I can provide a kind of visual model of it which could show this.
The Rubber Sheet Theory is a model of space as if it’s two dimensional left to itself with weights representing stars and planets which, if placed on such a sheet would create dents in it. Obviously this is not an adequate explanation as such of general relativity for several reasons, one of which is that it uses gravity to explain gravity – that’s what’s pulling down the weights. It also makes space appear to be a substance, something which physicists had worked heavily against when they disproved the existence of the luminiferous æther, which since it was supposed to be extremely rigid wouldn’t work in this situation anyway. It shouldn’t be mistaken for Einstein’s theory itself, but it is a useful way of looking at it. In any case, if you imagine the kind of dent which shows up in the title sequence of Disney’s ‘The Black Hole’:
. . . which is like one of those charity coin collection things, space around the Sun is distorted to a limited extent like that, and attempting to do a “wall of death”-style orbit around it, which would in any case be elliptical rather than perfectly circular because the Universe is imperfect like that, would lead to your bike describing a series of ellipses which were not perfectly congruent with each other but were more like a spirograph pattern. Having written that paragraph with its references to a number of very ’70s things makes me wonder if it’s going to make any sense to someone born after Generation X.
Now I can see that this does happen, but I am also puzzled by it. Whereas I’m sure that I couldn’t aim a coin at one of those charity collection things in such a way that it would just circle around at the same level until friction interfered, and that at best if I could make it describe an elliptical path for a few revolutions, the bits of the ellipse furthest from and closest to the hole would precess, I would put that down to the fact that I, and anyone else to a lesser extent, can’t aim perfectly rather than simply due to the geometry of the hole. Nevertheless, this appears to be what I’m being asked to believe with this: that it isn’t only one’s inability to aim perfectly, or for that matter the friction the coin (or ball bearing – let’s take the instability of the coin out of the picture), that leads to this precession. But apparently not. Apparently, if you were to have too much time on your hands and designed some kind of precision ball bearing throwing machine for charity coin collectors, and it wouldn’t be popular because they want coins, not ball bearings, it would do the wobble thing even if it stayed circular enough not to fall down the hole immediately, and it would wobble more the closer it was to the whole. So they say, and this is what got general relativity accepted.
There have been other Vulcans. For instance, one of the many hypothetical planets in Western astrology is the intramercurial Vulcan, seen as the soul ruler of Taurus and orbiting once every twenty days. This Vulcan would go retrograde more often than Mercury. It’s fiery and urges the individual to look for non-physical knowledge, which makes sense given its history in astronomy. It was also suggested in a poll as the name of one of the moons of Pluto, and actually won the most votes but that was then named Kerberos after the Hadean dog, which was the runner up. Vulcan actually doesn’t seem like a very good name for a moon of an icy planet way out in the outer reaches of the Solar System, but I don’t know the reasons it wasn’t used. Maybe the IAU just didn’t want to be reminded of what they might regard as an embarrassing phase in the recent history of their science. In the Second Doctor story ‘The Power Of The Daleks’, there’s a planet called Vulcan which is settled by humans and highly volcanic with pools of fuming mercury on its surface. Doesn’t sound very nice at all really. There does not, however, seem to be an asteroid named Vulcan, which is quite surprising.
I’ve sometimes wondered if there’s a story behind the naming of the ‘Star Trek’ Vulcan and if it’s in any way connected to the hypothetical planet, but I don’t know. How about you?
Before I get going on this, I want to explain the noun game with this title. Humpty-Dumpty’s dictum, “When I use a word, it means just what I choose it to mean—neither more nor less”, is clearly absurd, which is one reason why non-standard personal pronouns are such a struggle. I try to avoid using the term “the Moon”, because I think it encourages small-mindedness. It isn’t “the moon”. There are hundreds of moons in this Solar System, many of which have proper names. Ours also has proper names, in the form of what it (or she?) has been called in various different cultures, not in the sense of “that big hunk of rock up there” but in terms of deities associated with her. In a sense, we don’t appreciate her enough because there are two contrary forces involved in not giving her a name. On the one hand, it gives a sense of her being special in a way which excludes other special moons, such as those of Jupiter and Saturn. On the other, it paradoxically makes her generic: she’s just “a moon”, the nearest one. When we look at that luminary in the sky, we fail to appreciate that she is both special and not special, and whatever else we do we fail to appreciate her individuality (I’ll come to gender in a minute).
Although there are many deities associated with our sleeping satellite and there’s a good case to be made for calling celestial bodies by non-Western names such as Sedna, the fact is that she’s been associated with several Greco-Roman divinities, including Diana, Selene, Artemis and of course the name I chose for her, Cynthia. Using a name which is in common use as a human given name may seem odd at first but it does happen the other way round with other names for celestial bodies in English-speaking cultures, as with Venus Williams, so it isn’t unknown. Of the names listed, Diana and Cynthia are the most prosaic, although I’m sure there are people out there called Artemis and Selene. The big issue, of course, is that every time I mention Cynthia to people who might not have heard me do so before, I have to explain by doing something like putting “(‘The Moon’)” immediately thereafter, which is quite inelegant.
The gender issue is a side thing. Six of the seven known planets besides Earth in the Solar System have masculine names, as did Pluto when he was a planet. There’s a risk of it sounding like I’m objectifying women by using “she” for Cynthia the satellite, but I do this as part of my aim to reduce the association with sexes and would therefore equally refer to the likes of Mercury and Mars as “he”. Also, in a sense we are all “it” because underlying the interpersonal and emotional elements of our relationships, we are also conscious objects. It is, however, annoying that there are so many masculine celestial bodies.
And I’ve used that word again. “Celestial” bothers me too. Earth is in space. All there is is sky. Space is as much below us as above us. Everyone knows this of course, unless they’re Flat Earthers, but it’s easily forgotten and we have a tendency to revert to the sandwich model of the Universe we probably grew up with as a species. Earth is a celestial body – a heavenly body if you like.
The main theme of this post, though, is to consider Cynthia as a heavenly body among others that we know, that is, other planets and moons in our particular star system, and decide whether she’s boring. That is, if we were observing her as a moon of Venus, say, and sending space probes there and the like, what would we think? Or as a planet in her own solar orbit (which she nearly is)? It definitely seems that some worlds are more interesting than others, and it is quite diverting to have a large world orbiting us at close quarters, whatever it might be. Sarada is very captivated by seeing her sometimes, and I wish I could see past what I think might be disappointment that people haven’t got further into space, or rather further from Earth, than they currently have, because for me that taints it. But imagining Cynthia replaced with Io for instance, with that moon staying in her current state, which arises from the tidal forces acting upon her in the Jovian system due to the proximity of other satellites and the fact that she orbits within the planet’s magnetosphere, it would seem much more interesting stuff would be going on there, such as the volcanic eruptions and the multicoloured surface. Compared to that, our own moon just seems to sit there not doing very much. One of the people who went there said that if he wanted to see something which was exactly the same colour as the surface, he’d go out and look at his concrete driveway. It’s mainly various shades of grey, which is not exciting. Colour isn’t everything of course, and Apollo XVII is known for having discovered orange soil at Taurus-Littrow. Also, one of the most remarkable things about Cynthia is her maria and their distribution – the smooth lava plains which were deposited after the Late Heavy Bombardment which scarred the whole globe with craters, as it did elsewhere in the Solar System, such as on Callisto. The oddest thing about the maria is that until the 1960s CE, nobody realised there were practically none on the far side which can’t be seen from Earth. There are small smooth areas at the bottoms of craters, but no extensive plains. Nobody knows why.
Another mysterious feature is Transient Lunar Phenomena, which I know I’ve mentioned before on here, but anyway. These are temporary changes in light, colour or other appearance, and have the distinction of being something Patrick Moore was the world expert on. Explanations include outgassing, meteorite impacts and statically charged dust being repelled from the surface. Most TLPs have been observed in craters with cracked floors or around the edges of maria. A whole third of them are observed around the crater Aristarchus. They’re difficult to confirm because the same area would have to be observed at the same time by different people, which doesn’t often happen. NASA monitors meteor impacts, which are sporadic but also occur more often around the times of the famous regular meteor showers such as the Leonids or Quadrantids, because the two worlds are in practically the same place.
There’s also an “atmosphere” of dust. Sunlight ionises particles on the surface and they become statically charged, as mentioned above, then fall down to the surface and may bounce. This is happening all the time, with the result that there’s a constant fine mist of dust constisting of transient specks of dust. Additionally there’s a real atmosphere, though very, very thin. It’s estimated that the Apollo engines temporarily added more to the local atmosphere than was there before the landings. There are about a thousand million atoms and molecules per cubic decimetre just above the lunar surface, which is so sparse as to constitute a high vacuum in terrestrial terms. It’s also a ballistic gas: the particles are so far apart that they hardly ever encounter each other and undergo the same kind of bouncing trajectories as dust does there. It consists of helium, which is I imagine the result of alpha particles getting ionised, argon, which is also common in our atmosphere, potassium and sodium, which are relatively high in our upper atmosphere, ammonia and carbon dioxide. In fact, the atmosphere is similar in many characteristics to our own as it blends into cis lunar space, i.e. it’s an exosphere, the difference being that it’s at ground level.
The distance and size of Cynthia are also quite remarkable. She’s proportionately the largest natural satellite by far of an actual planet in the Solar System, as opposed to Pluto whose satellite is considerably larger but is not officially a planet. The closest rival is Neptune and Triton, with a mass ratio around 750:1 compared to Cynthia’s 81:1 ratio compared to Earth’s. In the inner system, only Mars has moons and they’re captured asteroids and very much smaller. The other thing about Cynthia’s size and distance is that she happens to be exactly the same size as the Sun in the sky, which is unknown for any other moon seen from the surface of their planet, although I understand Callisto comes fairly close on Jupiter. This makes solar eclipses possible in the sense that the Sun’s visible surface can be perfectly hidden while still allowing the corona, the solar atmosphere, to be visible. This doesn’t always happen because sometimes Cynthia is too far away, in which case the result is an annular eclipse with the Sun’s surface visible as a thin ring. Eclipses do occur elsewhere but not with such a perfect match, and this is very improbable. Nor does it seem to be directly connected to the necessity for life as we know it to exist on this planet. Although we may well need a large moon, it needn’t as far as anyone can tell be exactly the right size for that kind of eclipse, and this fact has been cited as an example of a possible Easter Egg if it turns out we’re living in a simulation. It could also be thought of as a sign of divine favour. The other thing about this is that because we’re moving apart at about a centimetre a year, it’s a temporary situation which will end in several hundred million years and didn’t happen until a few hundred million years ago, although that was before complex multicellular life existed.
Although Cynthia is very likely to be important for the existence of complex land life on Earth, I don’t want to consider this in this post as this is about her, not life. Just briefly though, tides within the planet’s iron core act as a dynamo and generate the magnetic field which keeps ionising radiation away from our surface and she also has a rôle in stabilising the axis of rotation. However, as with the above considerations, these are things to do with the relationship between the two rather than Cynthia herself. Before I leave thisses orbit entirely though, it’s worth pointing out that she’s the result of the outer layers of the planet which became Earth getting “chipped off” through a collision, and as a result she’s less dense than Earth by a considerable margin. Interestingly, and that’s what we’re looking for, the large bodies of the inner solar system fall into two neat categories regarding density. Earth, Venus and Mercury are all something over five times the density of water, and Cynthia and Mars around three. I don’t know why Mars is less dense, although it may be to do with the increasing temperature as one approaches the Sun causing lighter materials to evaporate. However, there was once an alternate explanation of the formation of Cynthia, where she was formed along with Mars from an initial body whose remnants are found as Earth. This kind of means Mars isn’t so much a smaller Earth as a larger Cynthia with a proper atmosphere, which is a little depressing.
There is magnetism, although Cynthia as a whole has no significant magnetic field, individual parts of the surface do have various magnetic fields, which are based in the crust rather than the core. The field tends to be weakest under the maria, particularly Oceanus Procellarum, and strongest at their antipodes, so the far side is more strongly magnetic than the side we can see. There may also be temporarily magnetised regions when meteorites hit the surface and cause melting. This lack of a global field also means that helium-3 is likely to have managed to accumulate, which is important if anyone can ever get nuclear fusion power to work.
When I look up, I never see a face in Cynthia. In fact, I’d consider it to be somewhat disturbing to see what would amount to a giant skull orbiting our planet, so I’d say that was a blessing. What I do see, independently of the other cultures which claim to see something like this, is a rabbit. This is of course the Far Eastern intepretation, along with a similar native American view that it’s a horned toad (not the lizard but an actual toad with horns). However, I don’t think I see the same bits as corresponding to these for the far eastern and western (who are of course linked) cultures. I see Oceanus Procellarum as the body, Mare Imbrium as the thigh, Maria Nubium & Humorum as the feet, Mare Tranquilitatis as the head and Maria Crisium & Fecunditatis as the ears. As I understand it, the Far Eastern interpretation is more upright and makes Maria Nubium & Humorum a mortar, which may be because they tend to view it from a different angle. I don’t know how the Native American projection works, or indeed if there’s more than one version although I suspect there would be. Cynthia holds the distinction of being the first body to be recognised as celestial to have her features named, and of course the selenography, as it’s known, is known in much greater detail than the “geographies” of anything in trans lunar space, although that gap narrowed somewhat from the 1960s onward.
It’s been claimed that even experts can’t tell the difference between closeup images of Mercury and Cynthia in some areas, and the two bodies bear comparison. Were it not for the maria, a casual observer wouldn’t be able to tell the difference between images of the two, particularly if Mercury was compared to our satellite’s far side. I’m not sure this is so because up until fairly recently only one quality source existed for images of Mercury, the Mariner 10 probe which flew by in September 1974 (and incidentally, briefly appeared to show that Mercury had a moon, which is not so). Compare and contrast these two pictures:
Ignoring external clues, are these interchangeable? Much of the appearance of the two bodies suggests that there is a kind of “standard” small rocky planet, possibly found throughout the Universe, which looks like Mercury, and Cynthia is one of these too. Callisto is a fairly good example:
Although Mercury does have smooth plains, they’re the same shade as the rest of the surface and don’t stand out. Mercury’s surface gravity is close to that of Mars but there’s no substantial atmosphere, partly because the planet is smaller and this makes it easier for molecules to escape the gravitational pull. This higher gravity makes a difference to the appearance of the craters, because when meteorites hit Mercury’s surface, the ejecta and hummocky rings are closer to the centre than they are on Cynthia, and are also more crowded because the material kicked up doesn’t rise as high or go as far. Mercury’s surface is also a lot more varied than Cynthia’s, but I don’t want to get too diverted onto the issue of the planet as opposed to the satellite.
There are quakes, which have several causes. One is the impact of meteorites, and it’s becoming clear that this is a very significant aspect of the place. The other can be divided into several more detailed causes, including tides, which start deep under the surface, and changes in temperature causing rocks to expand and contract. All of them are very mild compared to earthquakes, but they are associated with TLPs. The total energy involved is much less than a thousand millionth of those here, because we have tectonic plates and continental drift and Cynthia hasn’t.
There are “mascons” in the centres of the maria. These are regions of higher density detected when spacecraft orbited during the ’60s. In fact, these are unusually pronounced on Cynthia compared to other bodies in the solar system and amount to variations in gravity of over one percent. This is actually a distinctive feature not directly related to her position or relationship with us. They’re thought to be buried asteroids which have been there since soon after formation.
The dust is definitely worth mentioning. It used to be thought that it might be so deep that there was a risk of astronauts sinking into it like quicksand, but it turned out to be quite shallow, and this has been used as evidence by young Earth creationists. It also gets everywhere and is an inhalation risk like asbestos. It’s never been exposed to oxygen or moisture, so it has different characteristics than the kind of dust found on Earth. It’s technically just very fine particles of lunar soil or regolith, so there’s no definite cutoff between dust and soil. Due to the lack of exposure to water or air, it’s more reactive when it actually does come into contact with living tissue or just a wide variety of substances with which it comes into contact, which makes it a health risk in ways which silicate dust here wouldn’t be, and it hasn’t been smoothed by erosion and is therefore more jagged and has a larger relative surface area over which such chemical reactions can take place. It can jam mechanical equipment and interfere with wiring, and it also becomes statically charged quite easily. It’s nasty stuff, but probably not unusual because the same processes which generate it, meteorite impacts (again!) and radiation gradually breaking up the rocks, and I presume variations in temperature which are much more extreme than they are here due to the almost non-existent atmosphere, will also be operating on Mercury and inner system asteroids, and that implies that there will be similar processes going on once again all over the Universe.
The rock itself resembles certain rocks found on Earth but here it tends to be much rarer because we have weathering and erosion along with continental drift. Here, it tends to be of Precambrian origin and is therefore most common in places like Canada. Unsurprisingly, the maria and highlands are of different composition. In the former, pyroxene is the most common mineral, at about fifty percent of the surface there, and is made up of calcium, magnesium, iron and silicate. It forms yellow-brown crystals. The other common minerals are olivine, gabbro, breccias and anorthosites such as plagioclase. Olivine has pale green crystals and is a mixture of silicates of magnesium and iron which doesn’t survive long on Earth’s surface but is actually the most common mineral here. It releases heat as it combines with carbon dioxide or water and is therefore a potential fuel for heating which is actually carbon-negative, and is found copiously on the surface – a good reason to go back there I’d say. Plagioclase is a feldspar found in the highlands and is also the most common Martian mineral. It’s light grey or blue, so I presume that when we look at Cynthia, that’s what we see away from the maria. It consists of a framework of silicate groups in which are embedded aluminium, sodium and calcium atoms.
Much of the rock and dust is composed of glass, which also acts as an adhesive binding together fragments of other minerals. This is again because of meteorite impacts heating the surface which then cools rapidly, and this makes me wonder whether the same is true on Mercury because it’s hotter during the day but just as cold at night. When I say glass, I don’t mean the sodium silicate used to make windows and bottles on this planet, although lunar glasses do sometimes contain sodium and usually, possibly even always, silicates, but they would be less pure than what we use as glass.
I don’t honestly know if this is interesting or not. It seems plausible that there would be semiprecious stones and crystals in some places, which is quite appealing. Olivine looks quite nice:
This is actually a gemstone, when it’s known as peridot, and can be cut as such:
However, these kinds of olivine are close to being pure magnesium silicate and I don’t know if that applies to lunar olivine.
The existence of crystals gives me to wonder if there are any objects whose formation resembles living things, such as dendritic patterns in rocks or something like desert rose gypsum. It seems unlikely though, since there’s no water to act as a solvent and the place is very uneventful. It’s also likely that if olivine became easily available, since it’s so common on the surface, that it would cease to be a precious stone and if it can be used as fuel it would more or less have to drop in price.
There are no carbonates or sulphates on Cynthia, and also no hydrated minerals. However, there is a little ice in polar craters, which counts as a mineral there more than it does here in a way. These are craters in permanent shadow, and the same is true on Mercury. There is a general problem with raw materials due to the importance of water in concentrating useful mineral deposits here on Earth. Although the metals are often there in some shape or form, they don’t seem to be in the usual readily-extractable ores found here, and this of course reduces the incentive for going back.
Thus far I’ve mentioned the maria, highlands and craters, but there are other selenographical features. For example, although the maria are not real seas, they are smoother than the highlands, darker and have “shores” like the real oceans and seas here have. This means in particular that they have bays and tend to flood craters, the former being craters on the border of maria. One famous example of this is the Sinus Iridum – the “Bay Of Rainbows” – at the northwest of Mare Imbrium. The maria themselves also have features, including craters which are more recent than their formation and other craters which have become submerged under lava when the maria formed, sometimes called “ghost craters”. Lunar domes are present, previously thought to be “blisters” which had not ruptured to form craters in the days before their formation was attributed to meteorite impact, but they are in fact shield volcanoes with central non-meteoritic craters. The most concentrated collection of these is the Marius Hills, which range between two and five hectometres above the surface of the mare in Oceanus Procellarum. Of course, just saying they’re in Oceanus Procellarum is a bit like saying a geographical feature on this planet is “in the Pacific Ocean”, but unsurprisingly they’re near a crater named after the astronomer Marius, who may have discovered the Galilean satellites of Jupiter although their name suggests he didn’t. Among the hills is a forty metre wide pit apparently opening into a lava tube or rille. Lava tubes are basically long, sinuous lunar caves which form when lava solidifies on the outside but continues to flow out of the hollow tube thus formed. Their roofs can later fall in, forming a channel referred to as a valley or rille. Rilles can also form when parallel faults allow the ground between them to fall, in which case they will be roughly straight.
There are also ridges on maria, formed from the contraction of cooling magma, and these are also found on Mars and other moons and planets. They’re officially known as dorsa. Catenæ are also common elsewhere and consist of chains (hence the name) of craters, formed when tidal forces cause meteors to break up before impact. My impression is that catenæ are not as common on Cynthia as on some other bodies in this solar system, but that may be my imagination.
And there are mountains of course, although here a problem arises. On Earth, the height of a mountain is easily expressed as above sea level, though this can be misleading as it makes Everest seem to be the highest, which it may not be because Earth is not perfectly spherical. On Cynthia, a fairly arbitrary decision has to be made which has been given different values at different times, involving deviation from a presumed diameter of a sphere. Height of the peaks above the surrounding surface is easy to measure because they cast shadows and since the angle of the Sun and the distance are both known, it can be straightforwardly estimated. Mountains can be isolated or parts of ranges. The tallest mountain is Huygens, at 5.5 kilometres. This is a little surprising, as one might expect a body with lower gravity to be able to form higher mountains, which would then be fairly immune to erosion due to the practically absent atmosphere. The highest possible elevation of granite on the surface would be something like eighty kilometres, so this is very much in need of an explanation in my mind, and I would guess it has something to do with there not being the same kind of mountain-forming processes on Cynthia as there are here. That said, Mars lacks them too and yet has a mountain over twenty kilometres high. Isolated peaks unlike anything found on Earth occur in the centre of craters.
So to conclude, is this interesting or not? Mere proximity enables us to observe features likely to be found everywhere, even on planets and moons gigaparsecs from here, but as a body Cynthia does have distinctive features too. The maria being confined to the visible side, the presence of mascons to a greater extent than in other known worlds and the presence of transient lunar phenomena are all interesting. The greyness and familiarity make her seem dull, but there’s more to her than might at first be supposed. If she were a continent, her surface area would be second largest, somewhat larger than Afrika, and maybe in a way that’s a profitable way to think of her. She’s like a seventh continent which happens to be in orbit around the rest, more drastically different from them than Antarctica is to the other five, or perhaps a feature of Earth like the bottom of the oceans, and she is interesting. If you could drive there, she’d only orbit the rest of us nine times, and Concorde could travel that distance in eight days, so she really isn’t that far away, and definitely worth going back to. But I can relate to the dullness.
A Box Of Chocolates 