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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 atmosphere1,2,3,4. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole5,6,7,8,9,10,11,12 and at mid-latitudes7,8,13,14,15 from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole16,17, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models1,2,3 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 hemisphere2. 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.

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Figure 1: Method of spectral detection of Titan’s clouds illustrated on a representative VIMS data cube.
Figure 2: Maps of Titan’s clouds derived from VIMS observations from July 2004 to December 2007.
Figure 3: Titan’s latitudinal cloud coverage over time compared with global circulation model 3 predictions.
Figure 4: The southern temperate cloud distribution as a function of longitude.

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We thank M. E. Brown for discussions that allowed us greatly to improve the quality of this study. This work was performed partly at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration. K.M.P. and J.W.B. are supported by the NASA Postdoctoral Program, administered by Oak Ridge Associated Universities. Calibrated VIMS data appear courtesy of the VIMS team. We thank the CNRS, CEA and CNES French agencies, as well as the Universities of Paris Diderot and Nantes for their financial support.

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Correspondence to Sébastien Rodriguez.

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Rodriguez, S., Le Mouélic, S., Rannou, P. et al. Global circulation as the main source of cloud activity on Titan. Nature 459, 678–682 (2009).

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