4 August 2008

Guest Blog: Lakes on Titan

Posted by rebecca

So, Saturn is in the news again, and I’d be remiss in my bloggerly duties if I didn’t mention it.

Or, rather Titan is in the news again. See, it has liquid on its surface.

But wait, you say, didn’t we already know that?

Well, kind of. What had happened is that the radar aboard Cassini showed that there were these dark spots near the poles. Now, interpreting radar is funny, because it’s not like visible light, but, in general when something is smooth, it looks dark unless you look right where the radar is reflecting. Think about shining a light onto a piece of window glass at night — if you view it straight on, you can see the light reflecting. Otherwise, it is dark. Conversely, a rough surface looks somewhat bright at all angles. Smooth and rough here is about the same size as the radar wavelengths — centimeters or so.

Re-projected false-color radar image of Titan’s South Pole, with areas of low radar reflectivity marked in blue. Ontario Lacus is the large blue spot in lower left.
Image credit: NASA/JPL
Image hosting via Photobucket

So, we have large smooth patches near the poles. Furthermore, they occur in what look like low points. That’s a pretty good indication that they are lakes, but it’s not foolproof.

Now, it would be nice to know what the lakes are made of. Problem is, Titan is covered in this hazy atmosphere which is made of nitrogen and methane. The nitrogen isn’t that bad, but the methane absorbs a lot of light, especially in the near-infrared (the best place to identify molecules). There’s a couple of clear patches, one around 2 microns, and another right around the spectrograph’s long cutoff (5 microns). So, we can use Cassini’s spectrograph (VIMS) to look at the ‘lake’, and examine how the view looks different than the surrounding ‘land’.

What scientists were expecting would be more hydrocarbons, but ones that are liquid at Titan temperatures — things like methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). All of those are gases at Earth temperatures and pressures, but Titan is cold enough that ethane and propane are liquids and methane is like water is here on Earth (it can be a gas or liquid, depending on conditions). And all of them would look slightly different from methane.

So, what the scientists working on this did was pretty much take the spectrum of the lake and subtract off the spectrum of nearby ‘dry’ terrain, to find the differences, especially in the clear patches. And, to be doubly sure, they did this with different amounts of atmosphere (airmass) in the way — that way, if an effect was caused by the atmosphere, it would change with the amount of atmosphere. They could also look for patterns that would let them extrapolate out to what would they see without the atmosphere.

What did they see? Well, they spotted a couple of absorption lines at around 2 microns, and a sharp drop off at 5 microns — exactly what a liquid hydrocarbon should look like. Moreover, the strengths of these features were inversely proportional to the airmass — the less of Titan’s atmosphere confusing things, the clearer the signal got, implying they are surface features, not cloud features. So, there’s pretty good evidence that not only are there flat, smooth things in low-lying areas, but they are contain liquid hydrocarbons. Hence, lakes.

This makes Titan the first place (besides Earth) with amounts of permanent (or long-lasting) liquid on the surface, and giving it the possibility of something like a water cycle, where precipitation and evaporation act like Earth. It’s just it happens at -170° C with hydrocarbons, rather than at 10° C with water. Which makes it one of the few places were we can compare hydrological features — Mars has some, but none that are currently active. (Mars, on the other hand, has the benefit of having a nice, clear atmosphere (aside from dust storms), and using more familiar geology than Titan.)

Titan is an interesting place. Cassini should be in place to make measurements for at least another 2 year, and there’s talk about extending the mission as far into the Saturnian (and Titanian) spring as we can. We’re hitting equinox right around now (well, next year), but the seasons are 7.5 terrestrial years long. Plus, Titan (and Enceladus) seem to have marked themselves as Interesting to NASA. I might be a ring person myself (a Titan mission would have a poor view of the rings), but I like seeing the outer solar system in general getting attention.