title: 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.
authors.affiliation: Laboratoire de Planétologie et Géodynamique, CNRS, UMR 6112, Université de Nantes, 2 rue de la Houssinière, 44000 Nantes, France. sebastien.rodriguez@cea.fr
authors.forename: Sébastien
authors.initials: S
authors.lastname: Rodriguez
authors.forename: Stéphane
authors.initials: S
authors.lastname: Le Mouélic
authors.forename: Pascal
authors.initials: P
authors.lastname: Rannou
authors.forename: Gabriel
authors.initials: G
authors.lastname: Tobie
authors.forename: Kevin H
authors.initials: KH
authors.lastname: Baines
authors.forename: Jason W
authors.initials: JW
authors.lastname: Barnes
authors.forename: Caitlin A
authors.initials: CA
authors.lastname: Griffith
authors.forename: Mathieu
authors.initials: M
authors.lastname: Hirtzig
authors.forename: Karly M
authors.initials: KM
authors.lastname: Pitman
authors.forename: Christophe
authors.initials: C
authors.lastname: Sotin
authors.forename: Robert H
authors.initials: RH
authors.lastname: Brown
authors.forename: Bonnie J
authors.initials: BJ
authors.lastname: Buratti
authors.forename: Roger N
authors.initials: RN
authors.lastname: Clark
authors.forename: Phil D
authors.initials: PD
authors.lastname: Nicholson