Titan Rocks

Contributed by
Sep 21, 2005

On Monday of this week, I had the pleasure of attending a talk by NASA scientist Chris McKay. Back in January 2005, a lander called Huygens (pronounced "HOY-gens") touched down on the surface of Titan, Saturn's biggest moon. It was an incredible achievement! McKay is a co-investigator on an instrument onboard Huygens. He came to my university to talk about that weird moon. Whenever I hear a good talk, I try to walk away with one cool thing I learned. This time I really did, and now I'm gonna share it with you.

Titan is big, as moons go. It's 50% wider than our Moon, and 80% more massive. That means it's less dense than our Moon. The Moon is mostly rock, and so Titan must be mostly material less dense than that. We know there's a lot of water ice out there in that part of the solar system, so it's a good bet that Titan has a lot of it.

But Titan is cold. Really cold: -180 C! At that temperature, ice gets very hard, like rock on Earth.

Now let me digress for a second. Titan has an atmosphere. It's mostly nitrogen (like Earth's!) with about 10% methane. At that temperature and pressure, it's possible for methane to exist as a liquid, too. So methane can rain down from the air as a liquid, collect in depressions, and flow like water does on Earth.

Still with me? Great! Now look at this awesome picture:

That's the surface of Titan, as seen by a camera on Huygens. The flat rock to the left of center is about 15 cm (6 inches) across, and the rounder one next to it is roughly 4 cm (1.5 inches) across. Now, those aren't rocks: they're ice! Like I said, on Titan, ice is like rock. To be honest, it's not certain those are ice; some instruments on Huygens indicated they are, others said they aren't. I suspect they are covered with some sort of material on their outside which is throwing off the instruments, but I'm no expert. Still, it seems likely they are mostly ice.

But look how round they are! Almost as if they're eroded. It's known also that there are riverbed-like features not far from the rocks, and the plain they sit in looks a lot like a dry stream bed. Could this have been the site of running liquid methane?

There's more, and this is totally cool: there is an accelerometer on board Huygens, an instrument which measures how hard the probe was decelerated as it fell to the surface. When it hit, it measured that as well. If the surface had been solid rock, then the accelerometer would get a big spike as the probe smacked into the surface, and that's it. If it hit a liquid, then you'd get up-and-down wiggles as it bobbed in the liquid.

But what the accelerometer showed was that the impact had a big spike, and then a single rebound. This is not what you'd expect from a solid or liquid surface, but just what would happen if it hit something gloppy like mud.

Mud! It seems as if it recently rained methane where Huygens hit. The methane would flow down, like a river, and drain into a basin someplace. As this happens many times over millions of years, the "rocks" of water ice erode and get round. But every time, just after it rains, the ground is soggy.

Did Huygens land right after a rain, maybe only a few months after a wet season? No one knows, and McKay only speculated. But what an amazing thought! We have a rainy season here in northern California, and if an alien probe landed here in, say, April, and plopped into the mud that causes Mrs. Bad Astronomer and me such grief in the garden every spring, they might get a reading much like we got from Huygens. But while ours has silicon, oxygen, dirt, and biological organic compounds in it, Titan's mud has methane, ethane, and simple hydrocarbons... all at a seasonal 94 Kelvin.

So that's my one thought from the talk: a billion miles away, but maybe Titan is not so alien after all.

I've said it before, and I'm sure I'll say it many times hence: I love this stuff!

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