Global circulation as the main source of cloud activity on Titan

Abstract

Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes ( approximately 40 degrees ) in the winter hemisphere. The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified. Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude approximately 40 degrees N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions. This suggests that Titan's equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected.

Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT

“…However, the polar clustering might also be related to higher precipitation at the poles (Rodriguez et al, 2009;Brown et al, 2010;Rodriguez et al, 2011) relative to the dunefilled equatorial desert (Lorenz et al, 2006;Radebaugh et al, 2008;Le Gall et al, 2011;Rodriguez et al, 2014), which may be caused by circulation (Rannou et al, 2006;Friedson et al, 2009). The lower elevations of the poles relative to equatorial regions may also play a role (Iess et al, 2010;Lorenz et al, 2013).…”

AVIATR—Aerial Vehicle for In-situ and Airborne Titan Reconnaissance

“…Titan's cloud menagerie is most diverse over the poles. During southern summer frequent, large conglomerations of convective storms were present [20,113,122,123]. With the onset of southern autumn these have petered out.…”

“…There is a persistent cloud belt located at almost all longitudes on Titan at 40 • S since 2004 [122][123][124]. The question is still fully open as to whether feedbacks exist between surface features and cloud occurrence in these mid-latitudes.…”

“…Heavy rainfall may ultimately cause the midlatitudes' bland contrasts. Convective storms [45] have been seen regularly at 40 • south latitude throughout the Cassini mission [25,122,123] and are predicted to move to 40 • north in the coming season [103]. While this storm belt and the blandlands spatially correlate, whether they are causally related is unclear.…”

“…Cassini has gauged the global distribution of Titan's clouds [25,122,123,158,160]. AVIATR would offer a fresh perspective, sampling particles from within clouds, watching clouds evolve vertically, and measuring raindrops.…”