Turtle, E.P., Perry, J.E., McEwen, A.S., DelGenio, A.D., Barbara, J., Dawson, D.D., Porco, C.C. (2008). "Cassini Imaging Observations of Titan's High-Latitude Lakes." Asia Oceania Geosciences Society Meeting PS05-A022.


Abstract
Titan substantial atmosphere consists primarily of nitrogen, with a few percent methane and lesser amounts of other species [e.g. 1]. Methane and ethane are stable in the liquid state in Titan's lower atmosphere and at the surface [e.g. 2]; clouds inferred to be composed of methane and ethane have frequently been detected [e.g. 3-5]. 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. 6]. Therefore, it has been hypothesized that Titan has surface or subsurface reservoirs of liquid methane, which resupply the atmosphere [e.g. 7].

Cassini ISS imaged Titan's south polar region in July 2004 and June 2005, revealing >50 dark surface features tens to hundreds of km long. Differences between these two observations may be due to changes on the surface as a result of precipitation from a large cloud system observed in Fall 2004 [4], 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 [8]. Combined these features cover well over 600,000 km^2, ~1% of Titan's surface area; however, as shown by [9], 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.

References
[1] Broadfoot et al., Science 212, 206-211 (1981).
[2] Tyler et al., Science 212, 201-206 (1981).
[3] Griffith et al., Nature 395, 575-578 (1998).
[4] Schaller et al., Icarus, 182 224-229 (2006).
[5] Porco et al., Nature 434, 159-168 (2005).
[6] Strobel, Planet. Space Sci. 30, 839-848 (1982).
[7] Lunine et al., Science 222, 1229-1230 (1983).
[8] Lopes et al., EOS 88, 569-570 (2007).
[9] Lorenz et al., GRL 35, DOI 10.1029/GL032118 (2008).