16 December 2008

AGU Day 1: Titan

Posted by Ryan Anderson

I spent most of the morning in sessions on Titan, hearing the latest on everyone’s favorite methane shrouded moon. There were several talks looking at the first preliminary topography results and trying to draw some conclusions from them. According to Howard Zebker, Titan is more oblate than you would expect if it was just in its equilibrium shape, meaning its poles are lower in elevation than its equator. Oddly, the Xanadu region, which looks like mountains in radar, seems to be in a relatively low area. He also suggested that if Titan’s gravity field is more spherical than its actual surface, it could explain why there are more lakes near the poles, all other things being equal. The same thing happened with the lava lakes that form the dark patches on Earth’s moon. The side of the moon facing earth is lower in topography, so most of the maria formed on the near side.

Randy Kirk gave an interesting presentation about Titan’s topography, with lots of very cool 3D flyovers of features. One of the big pieces of news was that Ganesa Macula, a feature that many people had speculated was a volcanic dome, is not a dome at all. Emily has a full post about this, so I’ll just direct you there.

Radar image of Ganesa Macula (annotated by Emily Lakdawalla)

Radar image of Ganesa Macula (annotated by Emily Lakdawalla)

Jeff Moore gave a deliberately controversial talk in which he called Titan “Callisto with Weather”, implying that Titan may be much more frozen than people thought. He pointed out to everyone that the only landforms that have been identified unambiguously are ones that are “exogenic” meaning that they occur when something external happens to the surface: impact craters, fluvial erosion channels, and sand dunes. All of the features due to supposed internal processes, such as volcanoes and faults, are iffy.


Jeff compared our state of knowledge of Titan right now to the state of knowledge about Mars after Mariner 6 and 7: a few snapshots of a small fraction of the planet which may or may not be capturing the full story. He pointed out that, based on the morphology of impacts and other features, Titan seems to have a very stiff lithosphere. He also took a look at lobate deposits that people have been calling potential cryovolcanic features and compared them to radar images of fluvial deposits on earth that looked almost identical. His point was that Titan may actually be a very cold and dead world in terms of volcanism and tectonics.

I think that what Moore was trying to do was the same as what was done to great effect in the early days of Mars exploration: take a look at what data you have, and come up with “strawman” models to explain it. The idea is that you may not fully believe the model, but in the process of disproving it, you learn some valuable things. So basically what we saw today was Moore standing up and saying “disprove this!”

Alex Hayes and Oded Aharonson gave a pair of interesting talks about the polar lakes on Titan. Alex presented his mapping of the lakes and showed that the large seas and nearby small lakes have the same “water level”, which may imply that they are connected through a “water table”. (I put these in quotes because it’s actually methane/ethane, not water) Oded presented a hypothesis for why the lakes are almost all in the northern hemisphere, suggesting that variations in Saturn’s orbit are to blame. Right now, southern summer is shorter and more intense, which dries methane and ethane away from the south to condense in the north. In 10,000 years, this pattern will reverse, and he predicts that the lakes will migrate to the south. He made the interesting point that all of the craters identified so far are near the equator, which would support resurfacing of the poles.

One of the large methane/ethane seas near Titan's north pole.

One of the large methane/ethane seas near Titan's north pole.

Speaking of the lakes, Mike Brown presented evidence that some of the transient clouds near the north pole may be due to a lake effect. As a Michigan native and current resident of New York, I am all too familiar with the lake effect: cold air blows over warmer bodies of liquid and picks up heat and humidity, which causes cloud formation and precipitation when the air hits land again.

Sushil Atreya gave a presentation on the origin of Titan’s atmosphere. He showed that the nitrogen which makes up most of the atmosphere is probably not primordial, but likely formed when ammonia was split by sunlight into nitrogen and hydrogen. The methane in Titan’s atmosphere is also not primordial, and Atreya thinks that it can be explained by a process called “serpentinization” where volcanic minerals alter to form the mineral serpentine and give off methane in the process. Apparently titan has plenty of methane to support this process for many billions of years.