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.

When Voyager flew by Titan in 1980, it was a little bit of a disappointment to the geologists. They’re used to flying by worlds and looking down and looking at surface features like craters and channels and volcanoes. But when they flew by Titan, it looked like the L.A. basin: just smog covering it, orange haze. They couldn’t see a thing. But from an atmospheric science point of view, that’s great. It’s the only moon with an atmosphere, and the atmosphere has organic smog in it. So for understanding planets and life and organic chemistry, Titan is a big hit.

In many ways Titan is similar to the Earth. It’s about the size of our moon, so its gravity is less than Earth’s, one-seventh. But it’s the only world in the solar system that’s got an atmosphere that has anywhere near the atmospheric pressure on Earth. Mars has 100 times less than Earth’s pressure, and Venus has 100 times more. Those are our so-called “sister” planets – and they’re 100 times different in either direction. Titan’s atmosphere is only one and a half times as thick as Earth’s.

It’s also the only atmosphere in the solar system that, like Earth’s, is dominated by nitrogen. And that’s actually a big mystery – both on Earth and on Titan. Why is there so much nitrogen? If you look at Venus and Mars, the amount of nitrogen is very small, only a few percent. On Earth it’s 80 percent. On Titan it’s 95 percent.

The interesting thing about Titan’s atmosphere is not just that it’s made out of nitrogen, but it has all these other compounds in it, all produced from methane and nitrogen photochemistry. Sunlight is hitting the methane molecules, breaking it up , and then they’re reacting to form all of these compounds: benzene, acetylene, propane and so on. In fact, I was telling somebody the other day, there ought to be a sign there that says, “This world is known to the State of California to contain chemicals that cause cancer.” And these are just the ones that we can detect. We know there are a lot more organic compounds there.

Titan’s dark region that appears lake-like. Click image for larger view.
Image Credit: NASA/JPL

The question is, Why does Titan have an atmosphere and why is it so unusual? The natural comparison is between Titan and the Galilean moons of Jupiter – Ganymede, Callisto and Europa, in particular. Why does Titan haven an atmosphere and these moons that are roughly the same size and the same composition have no atmosphere at all? Not just a thin atmosphere, they have none; zero; 10-15 atmospheric pressure. Titan has 1.5 times Earth’s atmospheric pressure.

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.

Scientists puzzle over images that resemble complex landforms.
Image Credit: NASA/JPL

That was one of the major scientific results of Apollo, this understanding of the relationship between the Earth and the moon. Before Apollo, one of the theories was that the moon formed as a separate planet, like Mars or Venus or Earth. But then the moon should have formed with water and things like Earth has, although less of them. But Apollo, particularly the oxygen-isotope data from Apollo, showed that the moon was formed basically out of crustal rocks from the Earth. It took people a while to figure out how that happened. The consensus theory now is the moon formed by an impact that knocked off crustal rock that condensed to form the moon. So it was born without any volatiles. It is dry, dry, dry.

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.

See: Part II Part III Part IV