All moons are special of course. That is, you can probably dredge something interesting up about most of the large ones. All that said, of all the moons in the system, all the planets in fact, Titan must be near the start of any list ranked by interest. Writing this post is in fact quite daunting because I want to do it justice, and having written a couple of thousand words even on somewhere like Rhea, which let’s face it doesn’t strike me as one of the more intriguing places, I now feel obliged to do this amazing world justice, and I can do that, but I may go on and on, which I do a lot.
I live in Loughborough, and consider it a very boring town. To be fair, even when I lived in Canterbury I considered it boring even though it had that big pointy building in the middle. A more positive approach would lead to one casting around for sources of pride regarding the place, and in the case of Loughborough there are a few things. There’s the Great Central Railway, which is Britain’s only main line double track heritage railway. There’s the Bell Foundry, which produces a large proportion of church bells in Britain. Ladybird Books were based here. There’s also a university, which actually I found quite unimpressive, and it’s next to the National Forest. The Carillon is also quite special. Some people also say Loughborough is where the North starts. As you become familiar with a place, you get to realise what makes it individual and special. Consequently I can imagine people living on Rhea, perhaps working for the Rhean Tourist Board, and coming up with bits and pieces which might attract sightseers such as the possible ring system, but there would probably be a lot of time and work spent on trying to promote the moon. Rhea is in a sense the Loughborough of Saturn’s system. Titan, by contrast, sells itself. It’s the London or NYC of the system. You don’t need to push it because it’s amazing.
The illustration at the top of this post, perhaps surprisingly, is in the public domain. It’s by Chesley Bonestell, whose matte painting for ‘2001 – A Space Odyssey’ I used on this blog yesterday. Bonestell was a prominent mid-century space artist who also worked on films. He also designed a number of prominent buildings and used his skill in cinematography to create masterful depictions of space-related scenes. He was influenced by the frontier style of art, where beautiful and almost deserted landscapes in North America would have small figures, horses and wagons depicting the European pioneers travelling across the continent to settle and raise food, so many of his pictures show astronauts, spacecraft and bases on the surface of various bodies throughout the Solar System and in space itself. They also serve as a record of the state of knowledge and expectations at the time. For instance, before the Apollo program it was expected that a non-staged rocket ship would land on the lunar surface and return in one piece. The staging concept is so familiar to us nowadays that we find it quaint to imagine anything else, but there was a time when a much more straightforward vision saw a finned and streamlined craft perhaps a hundred metres high setting out from Earth and coming to rest somewhere like the Sea of Tranquility. This is what Bonestell depicts.
In the case of views from the different moons of the Solar System, artists at the time had very little to go on. They had the angles of the orbits, the distances from the planets and a rough estimate of the sizes of the moons in question. There was a lot that could be concluded from the data available but on the whole this was quite tentative. Bonestell produced a series of paintings from the major moons of Saturn, which might be expected to be quite spectacular given the planet’s rings, but in fact like most satellite systems, most of the moons orbit close to the equatorial plane, particularly the closer ones, which has the unfortunate result that either Saturn is big in the sky but has hardly visible rings because they’re seen from edge-on or does show the rings from a suitable angle but is so far away that they’re not that impressive. Moreover, although he was, I’m sure, assiduous in collecting as much information as possible about all of his subjects, he didn’t have much to go on apart from those few facts. Therefore it’s not surprising that all these paintings focus on the appearance of Saturn in these moons’ skies.
As I say, I was very surprised that his view of Titan is in the public domain. It seems to me that this is one of his most iconic and famous paintings, and just being able to post it like that, though presumably in a lower resolution than is available online for a price, is quite amazing. Incidentally, this picture occupies a significant position on the wall of a NASA office in the film version of ‘The Martian’. I don’t know if it’s on the wall anywhere in the real offices but it is quite inspiring and classic, so I wouldn’t be surprised.
All that said, it is of course inaccurate. In particular, Titan is much cloudier than that and it’s unlikely that Saturn is ever visible from the surface. Moreover, this is a daytime picture and stars are visible in the sky. In reality they wouldn’t be because not only is there copious smog in the atmosphere, in a good way, but even if there wasn’t the atmosphere has several times the density of our own air at sea level, so there’s no chance, even above the cloud deck, that stars would be visible during the day. Moreover, Bonestell has a tendency to depict ice as it would appear in terrestrial conditions rather than how they are actually likely to be on the bodies concerned. By the time you get out to Titan, the temperatures involved are so far below freezing that water ice is basically just another rocky mineral. The average surface temperature on Titan is around -182°C. This is ninety-one degrees above absolute zero, and freezing point is three times that temperature. In proportion, Earth’s mean temperature is 22°C, and three times that is 612°C, and certain common terrestrial minerals have a melting point around that, such as quartz and mica. Water ice is not just frozen water, even on Titan’s surface. The shiny, snowy look is interesting but speculative, and turns out to be wrong. I feel bad criticising his art in this way, and I want to stress that I still think his paintings are amazing and wonderful.
The landscape is also craggy in a similar way to Bonestell’s representation of the lunar surface. This is also inaccurate. Not only did it turn out to be wrong in the case of Cynthia, substantially because of moondust and micrometeoroid impacts, but it’s even less accurate for Titan because in the latter world’s case there is liquid-based erosion there. Here’s the famous image from the Huygens lander:
These are basically pebbles, at least in the foreground, and this is because of the rôle of liquid in their erosion. This, of course, is part of what makes Titan so fascinating. It’s in some ways the most Earth-like world in the whole Solar System.
This statement, though it has a lot of truth to it, can also be quite misleading. Yes, Titan is quite Earth-like but also has important differences. In the novel ‘Imperial Earth’, Arthur C Clarke illustrated the difference between the two with a burning plume of flame. On Titan, it was an oxygen spout burning in a methane atmosphere but on Earth it could be a methane spout burning in oxygen. Nowadays we realise that the rôle of methane in the Titanean atmosphere is not as a main gaseous constituent, but it still works as a good metaphor. The same kinds of phenomena often exist – rivers, lakes, seas, rain – but not in the same way. These pebbles are eroded into rounded shapes just as they would be in a stream or on a beach on Earth, but they’re likely not made enitirely of silicates but ice and the liquid eroding them is methane, gaseous on our home world. It is possible that they’re mixtures of ice and stone, so we might think of them as lumps of frozen mud or clay, but that’s considering them in terrestrial terms. In Titanean terms this planet is a furnace covered in oceans of molten rock with clouds of the same in the sky raining liquid as hot as fire, at least as far as the surface is concerned.
Titan is the second largest moon in the Solar System after Ganymede. Unlike Ganymede, and uniquely among moons, not only does it have an atmosphere, but said atmosphere is somewhat denser than Earth’s and the surface pressure is almost twice as high as ours. It is in fact the only moon in the system with a proper, collisional atmosphere like our own. This raises the question of how come Ganymede has no real atmosphere and yet the slightly smaller Titan has such a thick one. I imagine the answer is twofold. Firstly, Titan’s a lot colder than Ganymede, and secondly it’s less exposed to the solar wind because it’s twice as far from the Sun, making it only a quarter of the strength. The molecules would be moving much more slowly in the vicinity of Titan than Ganymede’s, and consequently don’t escape its gravitational pull.
Although it used to be thought to have a methane atmosphere, and a considerably more tenuous one to boot, it turned out that the main constituent of the atmosphere is the same as ours: nitrogen. This presumably means there are plenty of worlds in the Universe with a mainly nitrogen atmosphere like our own. Methane, being liquid, performs the same kind of antics as water does on Earth, making Titan the only other world in the system with liquid bodies of water and also land on its surface. There are several planets with liquid on their surfaces, but none with both liquid and solid. The fact that there is liquid flowing over a solid surface presumably means the latter is shaped, as Earth’s is, into river valleys, oxbow lakes, potholes, caverns, perhaps fjords and so forth. However, there are other factors which make it quite different.
Titan’s surface gravity is about the same as Cynthia’s, though somewhat lower at a little under a seventh of Earth’s to Cynthia’s sixth. Although it’s larger than Mercury, that planet is joint densest with Earth so Titan, with a density less than twice water’s at 1.88, has considerably less pulling power. This is due to its higher volatile content, such as water and ammonia. This also means that if Cynthia were a moon of Saturn, it too would have an atmosphere, actually a denser one even than Titan’s, and like Titan, liquids on its surface. Due to the lower gravity, the appearance of Titan’s lakes and rivers is somewhat different to Earth’s. For instance, the lakes seem to be more “spidery” in appearance, as if they have fjords. Liquid methane also appears to be more viscous than water, which combined with the much lower gravity would lead to more slowly moving rivers and less response to winds. The waves would also be different. The most important difference between methane and water, and in fact between most other liquids and water, is that the latter expands and therefore floats when it freezes whereas the former doesn’t. This means that freezing lakes on Titan would solidify from the bottom upward, making them less liable to melting or insulation from ice. Water is also slightly blue, but methane is almost perfectly colourless, so even without the distinctly orange lighting of the surface there would not be the usual bluish vista of the sea on this moon, but it is very slightly green. It’s also got a slightly lower refractive index than water, which would have some influence on the apparent distance to the horizon in humid air. However, that’s pure methane and the seas of Titan are not pure.
On Earth, we have two kinds of water. Most of our water is salty because it’s dissolved minerals from the sea bed and elsewhere, but when it first lands on the surface as snow, rain, hail, dew or frost it’s fresh. A similar division exists on Titan. The large standing bodies of water have had time to dissolve ethane and are in fact solutions of ethane in methane. They are also blackened by other hydrocarbn impurities dissolved from the crust into them. I’m guessing that this means there are “tar flats” there like Earth’s salt flats, and also the equivalent of hypersaline lakes but with ethane instead of salt.
Titan’s appearance from space is vivid orange because of the photochemical smog, similar to the reddish tholins found on many small objects far from the Sun, and they are in fact tholins themselves. In the case of the moon, it’s actually possible to image the horizon and the changing colours of the atmosphere from orbit like it is with Earth:
This is actually an ultraviolet image but has been colourised to resemble what would be seen by the human eye. Leaving its air’s composition and density aside for a bit, Titan is an important model for how an atmosphere behaves on a cold, fairly uniformly heated and slowly rotating spheroidal body. This came up recently in discussions I had with flat Earthers, because they attempt to explain the movement of Earth’s atmosphere based on the assumption that it doesn’t rotate and try to find another model which doesn’t use the Coriolis Effect. Titan and Venus provide such a model, and theoretical simulation of this moon’s atmosphere doesn’t rely on its actual existence. Like many moons, Titan has captured rotation and always shows the same face to Saturn during its sixteen day orbit, giving it a sixteen-day rotation. On a world much closer to the Sun, such a slow day would lead to winds in the atmosphere being dominated by the temperature differential between the night side and the subsolar point, leading to an “eyeball planet” to some extent, although unlike a genuine such planet it would still be rotating a little. There would be winds blowing from the tropics on the day side towards all parts of the night side, radially arranged. Above Titan, the atmosphere develops similar bands to what’s found on Jupiter, although they’re not visually apparent due to the relative homogeneity of the atmosphere. There are basically longitudinally-oriented rings around the planet with convection currents circulating between higher and lower altitudes and preventing mixing between latitudes. This is very indirect evidence that Earth is round, because if our planet wasn’t spinning this is how our atmosphere would behave, ignoring heat sources, and it doesn’t. In fact I wonder if that also causes the distinctive layers in this image. Perhaps there are multiple rotating “tubes” of air which don’t interact with each other.
The atmosphere is not horizontally homogenous. There is a “polar hood”. Titan’s orbit adds about twenty minutes to Saturn’s axial tilt of 27°, meaning that both have seasons, but in Titan’s case there is little or no significant internal heat influencing the weather, so Titan would exhibit seasons around seven years long each. The polar hood is a dark zone around the pole extending quite some way towards the equator, 70°, which appears in the local winter. It appears over both poles at different times of the “year”, i.e. the thirty-year period of Saturn’s and therefore Titan’s trip around the Sun. It seems to be caused by down-welling, which is the tendency for haze to build up in the winter at high altitudes which is then transported to the other hemisphere during spring.
Due to the lower gravity, the atmosphere is much deeper (or higher) than ours. Our “scale height”, the altitude over which density decreases by a factor of ε, or roughly 2.718. . . , is around eight and a half kilometres. The Titanean scale height is from fifteen to fifty kilometres. Now might be a good time to talk about scale height in more detail. It’s common knowledge that the further up you go on Earth, the thinner the air is. Most people cannot breathe at the top of Mount Everest without help although one can acclimatise oneself, and the air pressure inside an airliner is noticeably lower than at sea level, although it is also somewhat pressurised. The Kármán Line is the official boundary between Earth’s atmosphere and space, but is no more “real” than the borders between countries. It’s a hundred kilometres above sea level. However, the atmosphere doesn’t just suddenly cut off at that height, but gradually fades out. However, it doesn’t do that in a linear fashion. The air pressure 8.5 kilometres up is around 370 millibars, and at seventeen kilometres it’s 135 millibars, i.e. 2.718 times lower. At the Kármán line it’s about eight microbars. This actually means that were it not for the low temperature and lack of oxygen, it would be possible to survive at a much greater altitude above Titan than above Earth. The Armstrong Limit is the height at which the boiling point of water is equivalent to human body temperature, and the pressure is 62 millibars. This is about eighteen or nineteen kilometres above sea level. On Titan, taking the higher sea level (!) pressure of the atmosphere into consideration, this occurs at a minimum altitude of almost fifty kilometres up, which on Earth is the maximum height a balloon can rise to before pressure within it is equivalent to pressure around it, giving it neutral buoyancy. This also means that said balloons, airships etc, could operate at a much greater height above Titan than on Earth, at about a hundred and thirty kilometres, which on Earth would be well into space.
Methane rising into Titan’s upper atmosphere is broken down by radiation into hydrogen and ethane, which is effectively a dimer of methane with a hydrogen atom missing (in other words two methyl groups). Although it might be expected that this hydrogen would leave the atmosphere entirely, and I’m sure a lot does, what mainly happens is that the hydrogen expands and occupies a greater range of heights than it starts off at, and this leads to it moving down into the lower atmosphere. It would usually then be expected to rise back up again and leave, or perhaps react with something else, but in fact it seems to disappear. It’s been suggested that this hydrogen is being used by living organisms lower in the atmosphere. Once again, this series of posts is not supposed to be about life, but it would be weird to ignore it at this point so I think I have to say something about hydrogenosomes.
Cells with nuclei usually contain a number of bodies referred to as plastids. These include chloroplasts and mitochondria. Both of these evolved from independent microörganisms and provide their host cells with functions they would otherwise have to evolve or do themselves. Chloroplasts are of course former blue-green algæ and responsible for the kind of photosynthesis which produces oxygen as a waste product. Mitochondria use this oxygen to release energy from glucose in a controlled manner known as the Krebs Cycle. Hydrogenosomes are similar to mitochondria, are thought to have evolved from them, and do a similar job, but are found in anærobic environments, which is of course what Titan and almost everywhere else in the Universe is. They release energy by converting protons to molecular hydrogen. This is the opposite of what organisms on Titan would be doing with it, but it suggests that there is a potential source of energy there and it would explain why the hydrogen seems to vanish. Chloroplasts and mitochondria effectively have opposite functions, so maybe these are the opposite to hydrogenosomes.
Titan’s surface has now been completely mapped:
Perhaps surprisingly, in spite of the dense atmosphere and liquid and gaseous erosion, there are a number of craters on the surface, although they’re very sparse. These are the red patches on the map, all in the same hemidemisphere. The blue patches are lakes, and it’s notable that they’re within the polar circle, mainly the “Arctic”. Near the equator are dune fields, the purple bits. The green areas, plainly the largest, are in fact plains. Finally, the orange bits are described as “hummocky”. This is a cylindrical projection albedo map:
The impression one gets when looking at Titan is of a planet rather than a mere moon. It doesn’t feel like a mere adjunct to Saturn. This is clearly partly due to its size and mass, but it’s also the presence of a proper atmosphere. With the other moons, some of which technically have atmospheres which consist of sparse atoms and molecules bouncing around and perhaps orbiting, the surfaces are open to space and there’s less sense of “special space” with them. Titan’s not like that, and nor is Earth. Earth’s surface, ocean and atmosphere count to some extent as a “special space”. I will probably explain that in more depth at some point, but the gist is that there are some regions which count as special spaces for us, such as the Holy of Holies, an operating theatre, backstage or the parts of shops customers have no access to. Although they’re continuous with the rest of the Universe, there’s also a sense in which they’re kind of “roped off”, and I get that impression from Titan, but not any other moon. Conceptually it may be linked to liminal spaces and in a contemporary sense the “backrooms”. In a way, the whole of Titan’s surface is a huge “backroom”, since we’re trans its atmosphere and Titan is cis to it. It’s an arduous endeavour to reach sea level here, and it’s also kind of doing its own thing. For instance, it actually does have a sea level, or perhaps a mean sea level, since there seem to be at least two separate systems of liquid bodies. Tides will inevitably occur in these lakes, raised by Saturn and the other moons to some extent, and will be higher than is obvious due to the lower gravity. In a way, Titan is also a “desert world”, since although it does have bodies of methane on its surface they don’t form an extensive ocean. Perhaps somewhere out there are moons or planets with proper continents and oceans.
The presence of nitrogen in both Titan’s and Earth’s atmospheres suggests something further. Maybe there are planets and moons out there with oceans of liquid nitrogen.
Titan’s surface area is over eighty-three million square kilometres. This is far larger than any country or continent and getting on for the total land surface area of Earth. Next to it, even Rhea is small. It’s larger than Mercury and about the same size as Ganymede. Due to the lakes, its own land surface area is a little under that, and the greatest distance between two points on its surface is just over eight thousand kilometres, which is about the same as London to Los Angeles. This is not just some trivial moon you can give the brush-off to. It’s a massive great hulking world in space, getting on for the size of Mars, but far more distant. Similar colours too. Unlike Mars, however, Titan is constantly active and busy, with probable volcanic eruptions, though not to the extent on Io, but with water instead of lava, mixed with ammonia. It has gullies, branching streams and rivers with tributaries and evidence of tectonic activity. Basically the same stuff happens on Titan as on Earth, geologically, but with different materials involved. That said, although the surface is constantly being remodelled, it does seem that the occasional impact crater can persist. I have to say I don’t understand how.
There is more organic material and more complex organic chemistry going on there than on any other body apart from Earth. I’ve said before that tholins are like organic life’s cousin. It’s like the original complex mess of organic compounds which exist on or in a solid body have two alternatives as to how to develop, one being life and the other tholins. In Titan’s case, tholins have gone further than in any other known situation. the atmosphere is a case in point. On Earth, most of the complex chemistry going on in our atmosphere is in some way linked to life. Apart from that, there’s oxidation, almost completely inert nitrogen and completely inert argon. Lightning can cause nitrogenous compounds to form and ozone forms in the upper atmosphere, but most of what goes on here is physical. The organic chemistry is highly complex but mainly goes on inside organisms. This is not so on Titan, and may well not have been so when Earth was young and less organic material was locked up inside the biosphere, so although it’s much colder and therefore less reactive, Titan may be a passable model for what used to happen here before life evolved.
Broadly, what’s going on in the Titanean atmosphere, which remember is very deep compared to ours and therefore has a lot of stuff in it to react with each other in any case, is similar to what happens over a major polluted city in a hollow on a warm sunny day, one difference being that there’s no industry to inject the stuff into the air. Æons ago, all of the sludge we’ve dredged up with oil rigs and put into the atmosphere and water cycle wasn’t yet incorporated into the bodies of organisms, and may have been in a similar form, so we’re kind of returning our planet to the state it used to be in before life appeared on it, hence the resemblance to Titan. On Earth, vehicle exhausts form nitric oxide, which combines with organic compounds from the likes of paint, glue, weedkiller and other industrial and domestic chemicals along with the secondary pollutant peroxyacetyl nitrate formed from vehicle exhaust and fossil fuel power stations to form nitrogen oxides and ozone at a low level due to the action of sunlight on the chemicals. This turns out to be harmful to air-breathing organisms living in that environment.
The big difference with Titan is that there’s no free oxygen at all, although there is some locked up in compounds, so the process is rather different. It’s said to be possible to explain every detected compound in the atmosphere from the action of sunlight on a mixture of nitrogen and methane, although I don’t understand how because some compounds contain oxygen. Titan’s atmosphere is 94% nitrogen, six percent helium (which does nothing and therefore makes no contribution to the chemistry), 0.01% methane, and also acetylene, ethane, propane, diacetylene, methylacetylene, hydrogen cyanide, cyanoacetylene, cyanogen, carbon dioxide and carbon monoxide. In particular, there are several cyanide-based gases and the similar carbon monoxide, though in small amounts. Cyanogen is quite an interesting gas because it can behave as if it’s a halogen like chlorine or bromine. Several constituents also have nitrile groups, which also exist in superglue and an artificial rubber – I have a box full of nitrile gloves upstairs for the purpose of dealing with certain other organic materials. Although nitriles basically are cyanides, but properly organic as opposed to happening to include a couple of carbon atoms which might as well be any other lightish element, they tend to be a lot less toxic, possibly because the molecules are larger. Hydrogen cyanide in particular is a key intermediate in the synthesis of amino acids. As the chemical reactions proceed, I imagine the compounds get heavier and precipitate out of the sky onto the surface, so there will be substances vaguely resembling synthetic rubbers and glues, among other chemicals, on the ground and in the lakes and rivers, not at pollutants but as part of the uninterfered-with environment. All of this stuff will be in an unholy mess, all being mixed together, and it’s also hard to work out how it will behave at such a low temperature, but once again this is how Titan is the reverse of Earth. On Earth, all the plastic and other stuff is pollution. On Titan it’s a pristine part of the cycle: “natural”, to use that useless word. Deconstructing that word, though, maybe our seas being full of plastic and our air full of extra greenhouse gases is just as natural and it just took a convoluted path between a Titan-like original situation, a few thousand million years of evolution, the emergence of a technological species and a rapid return to Titaneanism.
Life, therefore, rears its head at this juncture. Titan has not one but two chances of being a life-bearing world because of its interior and its surface. There’s a whole load of stuff going on in its atmosphere and seas of course. Complex organic chemistry is a fact of (non-)life on Titan, but there is a problem: there is only rarely liquid water on the surface. It probably does happen, during volcanic eruptions, but the water emerging from these will freeze quickly. I suppose it’s possible that there would be microbes flitting around from site to site in these situations, waiting to take advantage of the brief periods that tiny area of the moon is above freezing, and in a way the combination of salty water and complex organic molecules almost seems to guarantee that life will find a way, but at this point we don’t know if life always happens when it can or if it’s a quadrillion-to-one chance that we exist on this planet, lost in the depths of a lifeless cosmos. But maybe water isn’t necessary to life anyway. Isaac Asimov, who was officially a biochemist, suggested that methane could replace water if instead of protein biochemistry used lipids. The crucial thing about water is its polarity. Water molecules are negatively charged on one side and positively charged on the other, which enables water to be a good solvent and to form cages around enzymes and extend their actions, among other things. This kind of life on Titan would use up molecular hydrogen by combining it with hydrocarbons, which would explain why there’s less hydrogen than expected in the lower atmosphere. And life gets a second bite of the cherry in Titan’s case, because as well as having an active and chemically complex surface, Titan is like many other outer moons in apparently having a hypersaline ocean underneath its icy crust, meaning that organisms could exist there too, with more familiar biochemistry. The mantle is a eutectic mix of water and ammonia, with some carbon dioxide, and is liquid. Immediately above it is a soup or sticky blend of complex organic molecules and the surface is tectonically active, meaning that these chemicals could be pushed into that ocean by movements of the crust and possible plates, if it goes that far. In the meantime, Titan appears to have many partially-assembled substances industries and chemists on Earth have expended considerable efforts in synthesising, such as the aforementioned artificial rubber monomers and components of superglue, as well as immense amounts of the same kind of hydrocarbons we use to power our entire civilisation, and I wonder whether it would be economically viable to fetch them from the moon and bring them back. It wouldn’t be a good thing though, due to the need for a low-carbon economy, but the presence of such compounds and their accessibility could ultimately lead to cheaper “fossil” fuels. Just as an example, the atmosphere contains twenty parts per million of propane. That’s more than seventeen millard tonnes. It’s notable that Russia is this planet’s largest supplier of natural gas. Even so, Titan is a long way away at one and a half light hours on average.
About an eighth of the surface is covered in dunes, which is about the size of the Sahara. This, again, is only possible on a world with a substantial atmosphere and some solid surface because they’re formed by winds. Mars has dunes but I’m not sure about Venus. They’re most similar to those in Namibia, which is where Earth’s highest dunes are, average a hundred metres high and can be hundreds of kilometres long. They give a good indication of the wind direction and are probably large in scale due to the low gravity and it also suggests that there are effectively desert conditions in those regions, emphasising the confusing fact that although Titan has seas, it’s actually a desert world. The dunes are around the tropics and cross the equator, although there are some other patches such as near the northern seas. It was initially speculated that these dark regions, which have a kind of fluid outline, were actually a surface ocean of methane and ethane, and they do flow around higher ground, but it’s actually some kind of organic “sand” being pushed around by the wind. The actual dues themselves are fairly widely separated and also quite steep and narrow themselves, like the dunes in the Namib Desert. It could even be that these grains are effectively plastic granules like those hoisted into hoppers and extruded, and personally I think this would make them suitable building materials.
Also mainly in the tropics is the “hummocky” terrain. Hummocks are small knolls or mounds which on Earth are formed by landslides or in permafrost-rich areas. These cover a further seventh of the world and are made of ice, which is like bedrock on Titan. They’re likely to have formed soon after the body itself and represent wrinkles in a solidifying surface due to contraction through cooling. Again, the hummocks turn up away from the tropics as well and are found in particular in the southern hemisphere.
There are also small regions of “labyrinth terrain”. These are maze-like structures (back to the backrooms?) cut by methane rivers, either through dissolving the surface or physically eroding it, and occur in areas of greater rainfall, often near high ground. On Earth, the Indonesian region of Gunungkidul is similar, consisting of limestone hills riddled with horizontal and vertical caves. The fact that this region on Earth is limestone suggests to me that methane rain may be dissolving the solid surface rather than just eroding it, but I’m no geologist.
The majority of the surface is covered by plains.
The illumination of Titan’s surface during the day is only 1% of Earth’s. This sounds very dim, but in fact it isn’t. Being around ten times Earth’s distance from the Sun, Titan already receives only a hundredth of the sunlight we get per unit area. Nine-tenths even of this is filtered out by the smog. The photo from ground level taken by the Huygens lander gives a fair impression of the murkiness as it would be seen by someone coming out of the kind of sunlight we experience on Earth, but it should also be remembered that the Sun is around sixty thousand times brighter than Cynthia at maximum brightness, so this is like a world with sixty “full moons” in its sky, and nobody could call that dim. The chances are you wouldn’t even notice after a while, although it would be overcast.
There may be clathrate hydrates in the makeup of the crust. These are also present at the bottom of the sea on Earth, and consist of ice which has “imprisoned” methane molecules in its own molecular cages. On Earth, these present a potential major risk of climate change because methane is such a powerful greenhouse gas that it could raise global temperatures catastrophically. On Titan, this is not an issue due to the low temperature.
The crust is around 150 kilometres thick, which makes the kind of missions suggested to Europa’s or Enceladus’s internal oceans less feasible in Titan’s case. Beneath the ocean, the same kind of process may be occurring as is apparent in the depths of Ganymede, with unusual (for us) allotropes of ice such as the cubic form. On the ocean bed there is probably hydroxide “mud” on top of a large rocky globe.
I feel this is such a huge and involved subject that although there’s still a lot I haven’t covered, some of which is very important, I’m going to stop here. Just be aware that Titan is in some ways as sophisticated and complex as Earth and is far more than just another moon.
Next time, the very different and much smaller Hyperion.
A Box Of Chocolates 