The Lakes of Titan

2.2-cm Synthetic Aperture Radar imagery acquired during Cassini's (T16) July 22nd, 2006 fly-by of Titan relealed multiple dark patches in Titan's north-polar regions interpreted to liquid hydrocarbons (Stofan et al., 2007). These are the first active non-magmatic lakes discovered on an extraterrestrial body. Since then, six further fly-bys have shown that much of Titan's artic (~15% north of 65 degrees, ~2.3% globally to date) is covered in hundreds of lakes as varying size and morphology, as well as some that are large enough to be considered seas. The observations are consistent with the model of Mitri et al. (2007) in which the high methane relative humidity of Titan's lower atmosphere could be sustained by evaporation of seasonally-variable lakes (at least 0.2 – 2%) of mixed methane-ethane composition.

We find that much of the landscape is consistent with a karst-like model, in which methane and/or ethane liquids dissolve surficial solid organics of >~10^-6 solubility, enhancing existing topographic lows and often producing steep-rimmed depressions. Variations in lake morphology across the polar region are consistent with different drainage characteristics and net subsurface transport. Lakes are sufficiently transparent to 2.2-cm radar that we can often see extensive subsurface lake and sea floor structure, including old channels, indicating variations in sea levels. However, no changes in lake levels or shorelines have been detected over the 10 months of north polar observations, which is equivalent to ~3% of a Titan year. Two recent fly-bys also reveal lakes in the antarctic, although these appear to be less extensive than in the north, possibly due to seasonal differences. Future observations will allow us to improve our coverage, obtain topographic data and look for signs of change over a longer time period.