"...in this cool and slothful atmosphere..."

Clouds, Rain May Make Saturn Moon Homey But Methane, Not Water, Probably Falls From Sky

By Maggie Fox Reuters

WASHINGTON (Oct. 20) - Clouds and rain may make Saturn's moon Titan look a little bit like home, but its spring showers would produce drops of methane, not water, scientists say.

They said images taken from the Earth strongly suggest that Titan, the moon considered to be most like our planet, has clouds made of methane.

The clouds move about and disappear, which suggests that they produce rain, Caitlin Griffith of Northern Arizona University in Flagstaff and colleagues report in Friday's issue of the journal Science.

"Their short lives point to the presence of rain," the astronomers wrote. "We propose that Titan's atmosphere resembles Earth's, with clouds, rain and an active weather cycle."

Except, instead of water, this weather is produced by the stuff of cow burps and swamp gas -- methane.

For their study, Griffith and colleagues analyzed near-infrared light emitted by Titan. Such spectral analyses can show what elements and compounds are present, and in what amounts.

They found a lot of methane in the atmosphere, and found its presence waxed and waned. One big fluctuation looked like nothing so much as a hurricane-sized cloud system.

One mystery is how the clouds and weather systems form. Earth gets a lot of energy from the sun and spins quickly, both of which cause weather systems to move across its surface.

"Compared to Earth, Titan's atmosphere is cooler, more massive and thus essentially more sluggish," the researchers wrote. "Titan spins slowly, providing little vorticity to cloud systems. In addition, Titan receives about 100 times less power from the sun."

And no one knows yet just what goes on at the surface of the moon.

"The local terrain, topography, winds and humidity on Titan are unknown," they wrote. "The mysterious instigator of activity in this cool and slothful atmosphere will be examined with future ground-based observations and by the Cassini spacecraft encounter with Titan in 2004."

Ralph Lorenz of the University of Arizona in Tucson said ultraviolet light from the Sun should be burning off the methane in Titan's atmosphere.

"For the methane we see today not to be a bizarre fluke, it must be continuously resupplied from a surface reservoir or by cryovolcanism (that is, volcanism where the molten 'rock' is just water ice)," he wrote in a commentary on the findings.

But he said the report paints an interesting picture of what Titan's drizzle might look like.

"In Titan's thick atmosphere and moon-like gravity, methane raindrops could be over 9 mm wide (.36 inches) -- rather larger than terrestrial raindrops -- yet would fall at a languid 1.6 meters (5.2 feet) per second, not unlike snowflakes on Earth (a picture already embraced by science fiction)."