Titan: A Moon with Atmosphere
Talk by Chris McKay, Part I
Titan is the only moon of Saturn that you can see clearly through a small telescope. Jupiter has four big moons, the Galilean moons. Saturn only has one, Titan, and it's much larger than all the others. Saturn has a lot of moons, but from my point of view there's really only one: Titan. It's the one with the atmosphere.
We don't really know why. That is one of the main things that is going to come from the probe. A clue to where Titan's atmosphere came from will be in the noble gases. If you look at our atmosphere, the presence of trace amounts of argon, krypton, xenon and neon tell us how the volatiles that came to Earth got here. They came in comets, where these gases, as well as nitrogen and oxygen, were trapped. Our thought was that if Titan had these noble gases, too, that would say that its atmosphere was primordial and it came in with comets. And then there'd be a question: Why didn't Ganymede and Callisto also form atmospheres like that?
But the probe results so far seem to indicate that there are none of these trace gases there. The only noble gas we've seen on Titan is argon-40, which is the radioactive decay product of krypton-40. It's not a gas that comes in with comets, it's a gas that's produced by the decay of potassium inside the interior of the planet and then it outgases. The result seems to be that Titan's atmosphere is not primordial, the way Earth, Mars and Venus's are, but it's a secondary atmosphere that's come from outgassing from the interior.
So Titan seems to have condensed with an interior which had nitrogen and methane in it, and then it's outgassed those. The nitrogen and methane probably have come from the decomposition products of something in Titan's interior. That's as far as we've gotten in terms of understanding. The further analysis of the probe results, in particular things like nitrogen isotopes, and the carbon isotopes in the methane, might help us unravel that. But that's going to take a couple more years of work.
We also think that our moon also formed without an atmosphere, but in a very different way. The Earth formed by the accretion of a variety of different rocks, along with asteroids and comets. Some of those asteroids and comets had water and air and other things in them. So Earth formed with a complement of volatiles, light molecules. We think that the moon formed by the impact of a Mars-sized object smashing into the Earth and literally knocking a big chunk of the Earth's mantle into space, as a liquid slab. That liquid slab of Earth's crustal rocks then condensed to form the moon. So the material from which the moon formed did not have any volatiles - ever. It was formed out of molten rock, condensing in Earth orbit. It never had the same endowment of material that Mars, Venus and Earth had when they formed. It didn't form the way those planets did.
Now, even if the moon had volatiles, it wouldn't hold them as well as Titan does. It's a lot warmer, because it's 10 times closer to the sun. So any atmosphere it had, it would lose it faster.
Titan is losing its atmosphere, too. We think that there must be mechanisms to recharge it. Cryovolcanism may be one such mechanism. So Titan's atmosphere may be a bit misleading. People think it's been there forever; it's a stable, thick atmosphere. But maybe not. It may be, in fact, losing it every 10 million years - it's just replenishing it. That's a mystery that we really don't know the answer to yet. It's one of the things that we're trying to work on from the probe results.