Titan

Sometimes I think I should just go through all the major bodies in the Solar System on this blog because so many of them suggest themselves. This post in particular has an interesting origin. I sometimes try to generate blog ideas using an AI technique called a generative adversarial network. When I did this yesterday, I put a number of post titles in and was rewarded not with a list of titles but some body text which looked like it came from a post about Titan, Saturn’s largest moon, and it actually made quite a bit of sense. I’ve since deleted it, but it was the inspiration for this.

When the Voyager probes reached Saturn in the early ’80s CE, I was really surprised to find that Titan had a mainly nitrogen atmosphere like Earth’s which was actually denser than ours. I don’t know if it was news to astronomers or not. Titan had been thought to be potentially the largest moon in the Solar System, but I think it was the Voyagers that established that it wasn’t. Unlike the Jovian satellite system, which has four planet-sized moons and a large number of assorted objects orbiting it at various distances, Saturn has one very large one, several medium-sized moons and an even larger number than Jupiter of much smaller ones. At first glance, Titan seems to be a bit anomalous, and it is in fact quite unique in the Solar System in being the only moon with a substantial atmosphere. Other moons do have tenuous gaseous envelopes, but Titan’s is thousands or millions of times thicker than any of the others, and even thicker than ours. Among the terrestrial planets, only Venus has a more substantial atmosphere.

In fact there are ways in which Titan is the most similar world to Earth in the entire system. Not only is its atmosphere of similar composition, being mainly nitrogen like ours, but it’s also the only place other than here where bodies of liquid, including lakes and rivers, can be found on the solid surface along with islands. There’s even a similar division as we have here between fresh and salty water in the form of methane containing ethane and purer methane, and it also rains there and there are forms, including pebbles, which are shaped by liquid erosion. The last is also true of Mars but there it’s more historical. At the same time, the surface gravity there is about the same as on Cynthia (“the Moon”). This would seem to make Titan this weird floaty place where not only is the gravity weak, but also the atmosphere is thick enough to slow the fall of really quite massive objects. Dust particles float in our air. On Titan, they could be at least the size of grains of sand and do the same. The surface level pressure of Titan’s atmosphere is over 1½ times ours at sea level, but is not merely fifty percent denser than ours because the gravity is also lower. It’s actually something like nine times the density, with correspondingly higher buoyancy, further enhanced by the lower pull downwards. This would make any winds much stronger at the same velocity.

The surface temperature averages out at around -182°C, although it also varies a fair bit. Because Titan is orbiting near Saturn’s equatorial plane and Saturn is itself tilted, its seasons are determined by its planet’s orbit more than its own, as it usually the case for moons. This gives it seasons around 7½ years long. A spacecraft would need to stay in the vicinity for almost three decades to observe all the seasonal changes. There is also photochemical smog formed in a similar process to that in urban areas, and this reflects some of the heat from the Sun back into space, making the moon colder than it would be if its atmosphere were clearer, but methane is also a greenhouse gas so the overall effect is that it’s warmer than a bare moon such as Dione would be at that distance from the Sun. Like Earth, Titan has a layer which absorbs ultraviolet light high in its atmosphere. Like many other moons Titan’s spin is locked to its planet, giving it a day lasting just over a fortnight. Saturn would appear ten times the diameter of the Sun from Earth in Titan’s sky, except that this would only be visible above the haze. However, eclipses would only occur rarely because of the angle at which Titan and Saturn orbit the Sun. The rings would also be practically invisible because it’s so close to their plane. This is true of most of the moons of Saturn, sadly, and they only become visible from the outer satellites, by which time the planet looks quite small. Eclipses would occur at the same time of day and only at the equinoctes, their maximum duration being about five hours.

Although there is no direct evidence for life on the surface of Titan, it resembles the larger moons of Jupiter in having a watery mantle like the rocky mantle of Earth, which could constitute a habitat for life as we know it. However, thicker layers of water may be less suitable for life because they’re further from energy sources and may not have very concentrated salts. The fact that this is a possibility, however, raises the question of whether inorganic life forms might exist in the magma mantles of rocky worlds, which I’ve mentioned elsewhere. The surface is less promising, at least if one expects a polar solvent and organic life, because there simply are no liquid polar solvents on it. To explain, some liquids consist of molecules which are more positively charged on one side and more negatively charged on the other. A very good example is water:

This difference in electrical charge allows water to dissolve a very wide variety of substances, which are then able to react with each other. There are plenty of other polar solvents in existence, including ammonia:

and formaldehyde:

All of these molecules are quite common in the Universe, although water is the most. All of them have a certain asymmetry to them which renders them polar. However, the melting point of ammonia is -77°C and that of formaldehyde is -19°C, so neither would be liquid on Titan. The liquids which are plentiful there are ethane and methane, both of which are non-polar and not good solvents. Hence they can cycle around like water, evaporating from the lakes into the clouds which then become saturated and fall as rain, and possibly snow, forming streams and rivers, and generally behave rather like water, but that’s never going to contribute to biochemistry like ours, which is all we know.

One unusual chemical which is found on Titan is cyclopropenylidene. This consists of molecules made up of a triangle of carbon atoms, two with double bonds and linked to hydrogen atoms: C₃H₂. It has two free electrons and is therefore highly reactive. It’s unique to Titan in this solar system although it is also found in interstellar space. It’s of the kind of molecule which might form purines and pyrimidines in combination with nitrogen, which are the information-carrying parts of DNA and RNA – genes in other words. However, although such substances might form on Titan, they’re unlikely to get anywhere due to the absence of polar solvents, so the likelihood is that the chemistry of Titan is going to be constantly on the brink of producing life but never quite getting there, until that is the Sun heats up in thousands of millions of years time and melts the water ice. At that point, the whole moon will become a very deep ocean which will slowly boil into space, so life might just start briefly and then get cooked to death, but it’s not going to happen for something like fifteen hundred million years if it does. This could be taken as a salutary demonstration of the fragility of life in the Cosmos but it probably won’t be.

Other gases or aerosols in the atmosphere include propane, hydrogen cyanide, cyanogen, diacetylene, methylacetylene, carbon dioxide and carbon monoxide. All of these are at less than one part per million. Much higher is the six percent of helium present. All of the hydrocarbons are more concentrated than they are on Jupiter and Saturn. The moon orbits within a torus of rarefied hydrogen which has escaped from its gravitational pull but not from Saturn’s, extending in to the orbit of the next large moon in, Rhea. The list of gases includes a couple which contain nitrogen and are, I’m guessing, the result of reactions with cyclopropenylidine. Hydrogen cyanide in particular is a vital step on the way to amino acids and DNA. The hemispheres differ in colour, the northern hemisphere being darker and redder at the time of the Voyagers’ encounters.

NASA considered Titan so important that of the two Voyager missions, Voyager 1 was sent over Saturn’s south pole to bring it as close to Titan as possible, which sent it out of the ecliptic (the plane of the Solar System) and ended its mission to the planets. Had that failed, Voyager 2 would’ve been sent on a similar trajectory and Uranus and Neptune would still never have been visited. I’m personally glad that this didn’t happen although there’s not a lot going on with those two compared to the two larger gas giants. This means that the two spacecraft are heading in very different directions and are already very far apart. Voyager 1 is heading towards the red dwarf star Gliese 445 in Camelopardalis, which is towards the next arm in of the Milky Way, although it isn’t currently in the position where it will be encountered and it will pass 1.6 light years from it in four hundred centuries’ time. Voyager 2 will pass near Ross 248, another red dwarf in Andromeda, and that name of course indicates that it’s heading out of the Galaxy in the approximate direction of the Andromeda Galaxy. Voyager 2 is further out than Voyager 1 right now by about nine times the distance of Earth from the Sun.

The Cassini-Huygens mission made Titan the first moon to be landed on by a terrestrial space probe. Huygens sent back this image of the surface:

The geography of Titan is now quite well-known thanks to both the Cassini-Huygens mission and the Hubble space telescope. One bright area, about the size of Australia, that springs to mind is Xanadu, and one of the seas is called Ligeia Mare. Unlike the lunar “seas” or maria, these are real bodies of liquid. Another sea is called Kraken. The seas can be seen to glint in infrared sunlight from orbiting spacecraft, or rather the one orbiting spacecraft which has been there to detect that. There are wind-blown streaks of particles hundreds of kilometres in length. The rivers have carved out canyons. There are flash floods and torrential downpours interspersed with long periods of drought. Craters are practically absent due to liquid and wind erosion.

Economically speaking, Titan is a potentially useful place. It has easily available hydrocarbons on the surface in vast quantities for use in petrochemical industries and as fuel, whether or not that would be environmentally desirable. These resources, unlike the vastly greater ones on and in the gas giants, are also only weakly anchored to the moon gravitationally, although there is a thick atmosphere above them. However, the atmosphere itself is a potential resource of the same kind. Whether that’s desirable is another question entirely, and of course I believe in orbital solar power, which would be much easier to achieve than acquiring resources from this moon. Nonetheless this formed the basis of Arthur C. Clarke’s classic 1976 novel ‘Imperial Earth’, where a third-generation clone of the original governor of Titan travels to Earth to have himself cloned again and celebrate the US quincentennial. This is kind of Clarke’s equivalent of Asimov’s ‘Bicentennial Man’, written to celebrate that event. Another prominent science fictional reference to the place is found in Kurt Vonnegut’s ‘The SIrens Of Titan’, which inspired the Al Stewart track of the same name. This novel was pieced together at a party in a few hours off the top of Vonnegut’s head. It’s the first work of his, as far as I know, to feature the Tralfamadoreans who appear much more prominently in ‘Slaughterhouse Five’. Although it is a good novel, the moon is not very prominent in the story and its characteristics don’t drive the plot. As far as I can recall, the world’s richest man Niles Mumfoord has become trapped in a ‘Chrono-Synclastic Infundibulum’ on the way to Mars and is now distributed in a spiral stretching from the Sun to Betelgeuse, becoming manifest on bodies as they cross its path. On Titan, however, the spiral is in perfect sync with its orbit and therefore Mumfoord is permanently manifested there, meaning that he effectively lives on the moon full-time. He is able to see the past and the future since he’s no longer localised in space-time. However, this is not the main plot of the novel, although Mumfoord does influence events. A major theme in the novel is that of free will and determinism, which kind of links back to yesterday’s post. Here’s Al Stewart’s take on it:

For some reason I’ve never been able to understand, the novel is used on risk management courses. The book also has the catchphrase “I was the victim of a series of accidents, as are we all.”

There are two successful SF novels simply entitled ‘Titan’, one by Stephen Baxter, the other John Varley. Both are too optimistic in their timeline. I haven’t read either, but the Baxter novel forms the second volume of his NASA Trilogy, whereof ‘Voyage’ is absolutely awesome. That I have read. I seem to remember the film ‘GATTACA’ ends with a mission to Titan but on the whole I regard cinematic SF as a minor and inconsequential cul de sac compared to the main written tradition. I wish it wasn’t though.

Leaving fiction behind, Titan’s magnitude from here maxes out at 8.2 which is something like eight times too faint to be seen by the naked eye (and maybe a hundred or so for me because of my poor eyesight), meaning that alone among the Saturnian satellites it can be seen through binoculars with at least 60 mm aperture. Right now it’s in Capricorn and very close visually to Saturn itself, meaning that it might be easy to identify but on the other hand might be obscured by the planet’s glare. That’s the real Titan, discovered in 1655 by Christiaan Huygens, hence the name of the lander. I always think it’s important to anchor astronomy in what you can actually experience personally, but since I live in England and have terrible eyesight this is unlikely in this case. Nobody will ever go there of course, but they could and it wouldn’t be that difficult, and there’s even a crass commercial reason for doing so. Long after we’ve become the victim of a series of accidents ourselves in some easily avoidable but still inevitable way which nobody could be bothered to do anything about, life might just flicker into being briefly in the last days of the Solar System, just before being boiled to death. So that’s something at least. And after all, not taking any risks is the greatest risk of all.