Seasonal Changes in Titan's Meteorology Documented by Cassini's Imaging Science Subsystem (ISS)

Changes in cloud distributions and morphologies have accompanied the change in Titan’s season from southern summer at the time of Cassini’s arrival in July 2004 to early southern fall. Early on, large convective cloud systems were common over Titan’s South Pole [e.g., 1-2]. However, observations by Cassini’s Imaging Science Subsystem (ISS) and Earth-based observers have shown that such storms have been less common since 2005 [e.g., 3]. Elongated streaks of clouds, several hundred km long and also exhibiting convective behavior at small scales [4], have been observed consistently at mid-southern latitudes (~40° S). Such clouds became common at high northern latitudes (>55° N) starting in 2007. Only recently, and very infrequently, have clouds been detected at mid-northern latitudes (~40° N). The variations in the distribution of clouds over time suggest that Titan's clouds, and thus its general circulation, are influenced by both sub-seasonal temperature variations of a low thermal inertia surface and the much longer radiative time scale of Titan’s cold, thick troposphere. North polar cloud outbreaks are often associated with locations of lakes and seas, suggesting the possibility that local variations in methane concentration and/or lifting generated by surface roughness gradients may promote cloud formation. We will present observations of Titan’s atmospheric behavior, documenting changes observed over the past six years and their implications for Titan’s atmospheric circulation. References: [1] Schaller et al. (2006) Icarus 182, 224-229. [2] Porco et al. (2005) Nature 434, 159-168. [3] Schaller et al. (2006) Icarus 184, 517-523. [4] Griffith et al. (2007) Science 310, 474-477.