Cassini, Huygens, Titan...

So the Huygens probe will detach, by firing explosive bolts, from the Cassini Spacecraft today. ("/.: Cassini Robot Lab Successfully Separates") A significant part of my past involved astrophysics, a Master's in such from Berkeley Astronomy and a ton of research involving comparative planetology... most notably, the weather of Saturn's moon Titan.

Here's why Titan is so cool... and why you should pay attention on January 14 when the Huygens probe, released from Cassini today, makes its descent into Titan's atmosphere (Feel free to ask questions in the comments):

Titan is the second-largest moon in our solar system after Ganymede of Jupiter. It has a gigantic atmosphere -- 1.5 times as massive as Earth's! -- and one of only 4 rocky bodies in our solar system with a significant atmosphere (the others are Venus, Earth and Mars). Titan's atmosphere is 90% Nitrogen gas and appears to be very similar to Earth's early atmosphere, with one large difference: instead of a hydrologic cycle with H20 clouds, rain and seas, Titan has methane and ethane clouds, rain and bodies of liquid hydrocarbons (propane, gasoline, acetylene, etc.) on its surface. My undergraduate adviser, Caitlin Griffith, was the leading theorist and observer (she's a bit of both) that argued for this methane cycle... we found evidence of clouds -- see our old Science paper ("Detection of Daily Clouds on Titan" (PDF)) -- and this small amount of cloud coverage has been confirmed by Cassini (see, "Probe sees Titan's Methane Clouds").

So, what's the most intriguing question we still have about Titan? Modulo the recent surface observations, which are weird, I would say it is: What has been able to replenish such a massive atmosphere over time, as in equilibrium it should float off into space because Titan is too small to have enough gravity to hold onto it?