Karly M. Pitman

Global circulation as the main source of cloud activity on Titan

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 (∼40°) 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 ∼40° 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.

First high solar phase angle observations of rhea using cassini vims: Upper limits on water vapor and geologic activity

Using radiances acquired with Cassinis Visual and Infrared Mapping Spectrometer (VIMS), we construct high solar phase angle curves for Saturns second largest moon, Rhea. Ground-based studies of Rhea and Saturnian icy satellites are focussing on low phase angles; to our knowledge, these are the first solar phase curve data for Rhea on phase angles >70°. We compare these data to similar phase curves for Enceladus at near-infrared wavelengths to estimate the amount of water vapor that could possibly be generated and thus set an upper limit on the amount of geologic activity that may be occurring on Rhea. We find that Enceladuss plume manifests itself in the VIMS solar phase curve at a phase angle near 160° and peaks most strongly for -->λ = 2.017810 μm. No such peak can be found in the Rhea VIMS phase curve. Absence of a forward scattering peak supports the recent determination that particles in Rheas surrounding ring are not small. We calculate that the maximum water vapor column density that could be supplied from Rhea ranges from -->1.52 × 1014 to -->1.91 × 1015 cm−2, 2 orders of magnitude less than what is calculated by Cassini UVIS for Enceladus. This implies that for Rhea, the level of active internal (endogenic) processes is exceedingly small, if any.