Titan’s organic cloud

An infrared detector on the Cassini spacecraft has imaged a huge cloud that engulfs most of the north pole of Saturn’s icy moon Titan. Particles of ethane, methane, and other organic compounds in the cloud could be a source of the liquid hydrocarbon—most likely methane—that makes up the lakes that Cassini’s radar detector recently found near the moon’s north pole (SN: 8/5/06, p. 83: Titan’s Lakes: Evidence of liquid on Saturn’s largest moon).

Methane may cycle back and forth between the lakes and the cloud, just as water recycles between the atmosphere and surface of Earth. Ellen Stofan of the University College London and Proxemy Research in Rectortown, Va., and her colleagues describe details of the lakes in the Jan. 4 Nature.

The cloud is roughly 2,400 kilometers in diameter and extends from the pole to latitude 62° north. The craft’s visual and infrared mapping spectrometer imaged the cloud during a Titan flyby on Dec. 29, 2006, and saw it again on Jan. 13. NASA released an image from the earlier set of observations on Feb. 1.

Observations from Earth show that Titan’s clouds wax and wane with the moon’s seasons, each of which lasts about 7 Earth years. As winter comes to the southern reaches of Titan, the clouds and lakes will probably migrate from the north pole to the south, says Cassini researcher Christophe Sotin of the University of Nantes in France. The craft will fly past Titan 16 more times this year, enabling scientists to track the cloud’s evolution.