A vast tropospheric cloud of ethane has been sighted near the north pole of Saturn's largest moon, Titan, and scientists think it may partially explain why they haven't seen the expected liquid ethane oceans on that frigid world (Science 2006, 313, 1620). Titan's atmosphere consists mostly of nitrogen and methane. Ultraviolet light from the sun irreversibly dissociates the methane, producing primarily ethane and a host of other organic molecules. Scientists had reasoned that a considerable fraction of the moon's surface would be covered with liquid ethane as a result of eons of methane photolysis. Thus far, condensed ethane hasn't been detected there, although scientists have observed dunes that likely contain solid organic material. Now, judging from spectral images taken by the Cassini orbiter, it appears that at least some of the missing ethane has condensed in a vast cloud 30-50 km above the moon's north pole, according to a team led by Caitlin Ann Griffith of the University of Arizona. The scientists speculate that, under certain conditions, the precipitating ethane could accumulate as ice at the poles.