Methane drizzles down every morning on Titan, according to a new study of Saturn's largest moon.
The rain is probably widespread across Titan and may even close the loop in a methane cycle that closely resembles the water cycle on Earth, the researchers suggest.
Scientists involved with the Huygens probe, which landed on Titan's surface in late 2004, have long suspected such an atmospheric cycle.
"The most important part of these results is that there is a way to monitor methane condensation from ground-based telescopes," said Mate Adamkovics of the University of California at Berkeley, who led the new research.
"Monitoring how often and to what extent the drizzle occurs might be an indication of seasonal changes on Titan that is more sensitive than watching other types of clouds come and go."
The results will be published in this week's issue of the journal Science.
Exotic Moon
Titan is a mysterious and alluring ball of rock and ice surrounded by a thick, orange, methane-heavy atmosphere.
Some scientists believe that the moon is a likely place to harbor alien life. One reason is that several studies have suggested that Titan has long-lasting lakes most likely made of methane. (Related: "Alien Life May Be 'Weirder' Than Scientists Think, Report Says" [July 6, 2007].)
On Earth, methane is produced in gas form through biological reactions such as digestion. It can exist as a liquid only under very high pressures.
But at about minus 290 degrees Fahrenheit (about minus 180 degrees Celsius), the surface of Titan is so frigid that methane doesn't need pressure to remain in liquid form.
Using ground-based telescopes and computer models, Adamkovics and his colleagues found a solid methane cloud 15 to 21 miles (25 to 35 kilometers) from Titan's surface.
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