Seasonal changes in Titan's meteorology

Abstract

The Cassini Imaging Science Subsystem has observed Titan for ∼1/4 Titan year, and we report here the first evidence of seasonal shifts in preferred locations of tropospheric methane clouds. South‐polar convective cloud activity, common in late southern summer, has become rare. North‐polar and northern mid‐latitude clouds appeared during the approach to the northern spring equinox in August 2009. Recent observations have shown extensive cloud systems at low latitudes. In contrast, southern mid‐latitude and subtropical clouds have appeared sporadically throughout the mission, exhibiting little seasonality to date. These differences in behavior suggest that Titan's clouds, and thus its general circulation, are influenced by both the rapid temperature response of a low‐thermal‐inertia surface and the much longer radiative timescale of Titan's cold thick troposphere. North‐polar clouds are often seen near lakes and seas, suggesting that local increases in methane concentration and/or lifting generated by surface roughness gradients may promote cloud formation.

Possible temperate lakes on Titan

“…Throughout Titan's cloud history, clouds are both predicted and observed in southern mid-latitudes during summer, some even specifically around 40 S (Roe et al, 2005;Rodriguez et al, 2009Rodriguez et al, , 2011Brown et al, 2010). Some clouds have recently been observed around 40 N as well after equinox (Rodriguez et al, 2011;Turtle et al, 2011a). The presence of clouds around Polaznik Macula (41.1°S) suggests at least the possibility that the area could be either a source or a sink for the moisture in the 40 S clouds.…”

“…However, Stofan et al (2007) and Tan et al (2013) state that liquid methane is thermodynamically stable anywhere on the surface of Titan. Clouds have been seen to concentrate around the 40 S latitude mark (Griffith et al, 2005;Roe et al, 2005;Brown et al, 2010;Rodriguez et al, 2009Rodriguez et al, , 2011Turtle et al, 2011a) whereas no persistent surface liquids had been previously identified equatorward of 70 S or 53 N latitude. Rainfall has been shown to modify Titan's surface reflectance (Turtle et al, 2011b;Barnes et al, 2013), and recent observations suggest that equatorial lakes may also exist (Griffith et al, 2012).…”

Precipitation-induced surface brightenings seen on Titan by Cassini VIMS and ISS

“…Heavier cloud cover, and presumably higher rainfall, occurs at the poles and at 40°latitude in the summer hemisphere [21][22][23][24]. However the correlation between clouds and surface rainfall is not obvious.…”

Review of Exchange Processes on Ganymede in View of Its Planetary Protection Categorization

“…Modeling of convection within the icy shell permits a prediction of the velocities within the convective zone. By using the simplest context of a pure ice mantle with constant viscosity, velocities are directly related to the vigor of convection when using the global Rayleigh number (Turcotte and Schubert, 2002). This assumption of constant viscosity is in fact close to reality, because the fluid in the convective sublayer is almost isothermal [see Fig.…”