A huge storm has been spotted raging over the biggest moon in the solar system, Titan. It is thought to be producing heavy rain - but of liquid methane, not water.
It comes from clouds that may be formed by sudden belches of methane gas from the moon's rocky surface.
Astronomers are excited because such downpours explain why Saturn's main satellite has features similar to lakes, rivers and coastlines on Earth.
Titan, which is half as big again as our own Moon, is the only planetary satellite known to have a dense atmosphere which usually appears as a bland orange smog.
A European spaceprobe called Huygens landed on its surface in 2005 and photographed channels and deltas as well as Titan's own "lake district". But what there did not appear to be was clouds.
But a giant telescope on Hawaii has spotted a cloud system explode in the atmosphere causing ripples of other clouds to form around the moon, scientists report in the journal Nature this week.
Professor Mike Brown, of the California Institute of Technology, said Titan was usually a "very bland place, weatherwise." He added: "We can watch for years and see almost nothing happen."
Time on giant telescopes is too valuable to be taken up watching nothing happen. But Brown's team began to monitor Titan with a smaller NASA infrared telescope for a hint of activity. When they found it, they were able to book time for the giant Gemini North telescope on Hawaii to take a closer look.
Emily Schaller, who was a graduate student of Brown's at the time, observed a large system of storm clouds appear in what were thought to be dry mid-latitudes of Titan in April last year.
They spread in a south-eastward direction, eventually generating a number of bright but fleeting clouds over Titan's tropical latitudes, a region where clouds had never been seen and where they were thought very unlikely to occur.
Brown said: "The first cloud was seen near the tropics and was caused by a still-mysterious process, but it behaved almost like an explosion in the atmosphere, setting off waves that travelled around the planet, triggering their own clouds.
"Within days a huge cloud system had covered the south pole, and sporadic clouds were seen all the way up to the equator."
He added: "The month-long event has many important implications for understanding the hydrological cycle on Titan. But one of the reasons I am most excited about it is that it shows clouds near the equator--where the Huygens probe landed--for the first time. For a while now, people have speculated that the equatorial regions are simply too dry to ever have significant clouds."
Experts had long speculated how there could be streams and channels in a region with no rain. The new discovery reveals that it probably rains in bucketloads.
Brown adds some "rampant speculation" in his blog: "What I think is going on is that Titan occasionally burps methane, and I think Emily found one of the burps. For many years scientists have wondered where all of the methane in Titan's atmosphere comes from, and, I think, here is the answer. The surface occasionally releases methane.
"Call it what you want. Methane geysers? Cryovolcanoe? Titanian cows? Whatever happens, the methane gets injected into the atmosphere and, at that location, instantly forms a huge methane cloud. Massive rainout ensues downwind. The stream channels, the shorelines, and everything else in the otherwise desert-seeming regions are carved in massive storms."
Titan may have a vast underground ocean containing the basic ingredients for life to form, scientists believe. That is if it is not teeming with life already. New findings from Huygens' companion probe Cassini, which is orbiting Saturn, suggest that the moon's environment resembles Earth's at the time that life first got a foothold.
Two close flybys gathered fresh evidence that ammonia, probably mixed with water ice, has recently erupted onto the surface of Titan. The likely presence of ammonia, combined with plentiful methane and nitrogen in the moon's thick atmosphere, suggest that Titan may host a prebiotic brew, says Cassini scientist Robert Nelson of NASA's Jet Propulsion Laboratory in California.
Photo: Top: The huge storm erupts in the desert tropics of Titan. (Credit: Emily Schaller et al./Gemini Observatory). Bottom: How Titan and Saturn appear with the Gemini North telescope. (Credit: Gemini Observatory/AURA/Henry Roe, Lowell Observatory).
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