Saturn's giant moon Titan seems like a version of early Earth that ended up in a deep freeze. Two new studies extend the resemblance, showing that Titan even seems to have summer rains—but temperatures are so low that the rain probably consists of liquid methane.
In December 2001, Henry Roe, then a graduate student at the University of California at Berkeley, trained Hawaii's Gemini-North telescope at Titan. Through the bland orange haze, Roe spotted something never seen before: patchy clouds near the south pole. Around the same time, Michael Brown at Caltech measured those first clouds using the Keck II telescope, also in Hawaii. Both men have just released their results. The white puffs cover about 50 square miles, Brown found, and float 10 to 12 miles above the surface. They grow alternately brighter and dimmer before dissipating within a few hours to a few days. "A dimming cloud could be getting smaller because it is raining out methane drops," says Roe, now also at Caltech. This is the first evidence of rainfall anywhere beyond Earth.
Roe and Brown are now finding more clouds all the time. Titan may be experiencing a change in the weather as its southern hemisphere enters the heart of summer. Brown proposes that pockets of relatively warm air rising off the surface spark the formation of clouds, as happens over deserts on Earth. "This may be the only time and place clouds occur on Titan," says Roe.
