Titan Forecast: Cold, Drizzly Mornings

Anne Minard
for National Geographic News
October 11, 2007
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.

A constant methane rain pelts Xanadu, the moon's brightest continent, each morning, the researchers discovered.

It's the first time researchers have seen differences between day and nighttime weather on Titan, Adamkovics pointed out.

"They were unexpected, because day-to-night temperature changes are expected to be small," he said.

Still Mysterious

Martin Tomasko, an astronomer at the University of Arizona in Tucson who designed Huygens' camera system, has long suspected methane smog and condensation occurred on Titan.

He has proposed that methane rains from Titan's sky as a thick, tarry gunk. The chemical then travels through channels until it reaches reservoirs visible as dark patches on the moon's surface.

The exact nature of the dark patches remains in dispute, however.

If methane rain is indeed widespread, it could be the main method that atmospheric methane returns to the surface, the study authors write.

But Adamkovics said there's still a long way to go to understand the process.

"Many predictions are being made, but we are far from conclusions," he said, "and I would wager that more surprises are going to be discovered before we have a textbook description of Titan's atmospheric cycling."

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