Turtle, E. P., Perry, J. E., McEwen, A. S., West, R. A., DelGenio, A. D., Barbara, J., Dawson, D. D., Porco, C. C. (2008). "Cassini Imaging Observations of Titan's High-Latitude Lakes" American Astronomical Society, DPS meeting #40, #23.07.


Abstract
Titan's substantial atmosphere consists primarily of nitrogen, with a few percent methane and lesser amounts of other species (e.g. Broadfoot et al., 1981). Methane and ethane are stable in the liquid state in Titan's lower atmosphere and at the surface (e.g. Tyler et al., 1981); clouds inferred to be composed of methane and ethane have frequently been detected (e.g. Griffith et al., 1998; Schaller et al., 2006; Porco et al., 2005). Photochemical processes acting in the atmosphere convert methane into more complex hydrocarbons, creating Titan's haze and destroying methane over relatively short timescales (e.g. Strobel et al., 1982). Therefore, it has been hypothesized that Titan has surface or subsurface reservoirs of liquid methane, which resupply the atmosphere (e.g. Lunine et al., 1983).

Cassini ISS imaged Titan's south polar region in July 2004 and June 2005, revealing dark surface features that were interpreted to be lakes, e.g. 235-km-long Ontario Lacus. Recent evidence from VIMS strongly suggests that Ontario Lacus contains liquid ethane and methane (Brown et al., 2008). Differences between the two ISS observations may be due to changes on the surface as a result of precipitation from a large cloud system observed in Fall 2004 (Schaller et al., 2006), although diffuse clouds or atmospheric scattering could also play a role. Images of northern latitudes have revealed larger dark features, one more than 1100 km long, which coincide with liquid-filled regions identified by Cassini RADAR (e.g., Lopes et al., 2007). Combined these features cover well over 600,000 km2, 1% of Titan's surface area; however, as shown by Lorenz et al. (2008), even if all of these features are filled with liquid, they do not appear to provide enough methane to keep Titan's atmosphere resupplied for a substantial amount of time, unless the lakes are unexpectedly deep or other subsurface reservoirs exist.