Image courtesy NASA/SSI

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Clouds build up near the equator of Saturn's moon Titan in an October 2010 Cassini picture.

Image courtesy NASA/SSI

Spring Rains Darken Saturn's Moon Titan

Clouds sprinkled methane on desert dunes, study says.

They probably won't bring May flowers, but April showers do fall on Saturn's largest moon Titan, according to a new study.

On frigid Titan, these rains aren't balmy showers—they're made of liquid methane, a substance found on Earth as the principal ingredient in natural gas.

The finding is just the latest evidence that Titan has a "methane cycle" similar to Earth's water cycle.

Previously scientists had found methane lakes near Titan's poles, and later discoveries of fog over the lakes and lake effect clouds suggested that, as water does on Earth, the lakes' liquid methane may evaporate and enter the atmosphere.

What's more, the appearance of new lakes at the south pole showed that Titan's clouds likely produce cold methane rain over the poles.

But so far the equatorial regions on Titan have been dry, featuring not much more than vast expanses of rippling dunes.

Then, last September, scientists using NASA's Cassini spacecraft watched a massive cloud form near Titan's equator.

"We'd not seen anything like this before," said study leader Elizabeth Turtle, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.

"That really got our attention."

Titan Rain a Torrent or a Drizzle?

Subsequent images revealed that a huge stretch of dunes—1,200 miles (2,000 kilometers) long and at least 80 miles (130 kilometers) wide—had darkened since before the cloud appeared.

Two weeks after that, the dunes were beginning to lighten. The most likely conclusion, Turtle said, is that the clouds dumped methane rain that is now slowly evaporating.

Other possibilities, such as a big windstorm changing the surface of the dunes, are unlikely, Turtle added. The slow return of the dunes to their normal brightness strongly suggests evaporation of some liquid, presumably methane.

In addition, pictures of the dunes had previously shown river-like channels that are tantalizingly similar to dry streambeds in Earth's deserts. But until now there were no clues whether these channels were relics of an ancient, wetter time or modern channels carved by seasonal storms.

Equatorial storms hadn't been observed on Titan before because Cassini has seen only a fraction of the moon's year, which—due to its distance from the sun—is 29 times longer than Earth's.

"We got there [in 2004] in the equivalent of mid-January," Turtle said. "It's now the equivalent of early April."

And just as seasonal changes bring unsettled weather on Earth, they appear to be doing the same on Titan, she said.

The scientists don't know how much rain fell. "In some places it may have flooded," Turtle said. But the rains also could have been nothing more than a widespread drizzle.

On Earth, she noted, "if it starts raining, it doesn’t take much for concrete to get darker."

The Titan rain study appears in this week's issue of the journal Science.