Clouds in the Tropics

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Science  04 Sep 2009:
Vol. 325, Issue 5945, pp. 1182
DOI: 10.1126/science.325_1182b

Previous observations of Saturn's largest moon showed methane clouds at high southern latitudes, where summer prevailed, and ethane clouds in the winter north polar region. The tropics, by contrast, tended to appear virtually cloudless. Observations from Cassini and ground-based telescopes have now revealed a greater recent incidence of clouds in Titan's tropical atmosphere. Using Cassini observations taken between 3 July 2004 and 28 May 2008, Griffith et al. detected the presence of five tropical clouds with characteristics similar to those of the summer polar methane clouds. This increasing cloud presence provides evidence for the beginning of an overturn in Titan's atmospheric circulation; Titan's pole-to-pole circulation is expected to reverse near the equinox, which occurred this August, inducing the polar clouds to swap hemispheres.

High-latitude clouds can reach altitudes of 45 km, but the Cassini observations show that tropical clouds are confined to altitudes below 26 km. Although this finding points to a dry and stable tropical climate, another recent study implicates rather different weather patterns. Using images from two ground-based telescopes, Schaller et al. report large cloud outbursts starting at southern mid-latitudes in April 2008, triggering cloud activity near the south pole and the equator of Titan over the next 3 weeks. The cause of this outburst is unknown, though it is thought to be associated with stormy weather conditions and substantial amounts of methane rain.

Astrophys. J. 702, L105 (2009); Nature 460, 873 (2009).

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