Titan's violent methane storms may solve dune direction mystery

Eric Hopton for redOrbit.com – Your Universe Online

The Cassini-Huygens spacecraft has studied Titan for many years. But its tantalising discoveries about Saturn’s largest moon have led to new mysteries. One of those involves what seem to be wind-created sand dunes spotted by Cassini near the moon’s equator. These dunes are up to one hundred yards high, many miles in length, and point to the east. Climate simulations indicate that Titan’s near-surface winds, similar to Earth’s trade winds, blow toward the west. What’s going on?

Titan has a dense, hazy atmosphere, surface rivers, mountains, and methane seas and lakes. It’s one of the most Earth-like places in the solar system though its atmosphere, 98.4 percent nitrogen with the rest mainly methane and hydrogen, would be hostile to terrestrial life. Gravity is one-sixth that of Earth’s but its atmospheric density is four or five times higher. So modelling Titan’s climate and surface systems is challenging, making previous attempts at explaining the direction of the dunes harder.

Saturn’s gravitational tides, and various land features or wind dynamics on Titan have been suggested as causes but none were convincing.

Benjamin Charnay, a University of Washington post-doctoral researcher, and his co-authors, have a new theory, published in a paper in the journal Nature Geoscience.  Charnay and his team believe that violent, eastward blowing methane storms high in Titan’s dense atmosphere might be the answer. These methane storm gusts are much stronger than the usual westward surface winds.

“These fast eastward gusts dominate the sand transport, and thus dunes propagate eastward,” said Charnay.

The storm winds reach up to 10 meters a second (22 mph), about 10 times faster than Titan’s gentler near-surface winds. Although the storms happen only when Titan is in equinox and its days and nights are of equal length, about every 14.75 years, they strong enough to realign Titan’s dunes. Titan was last in equinox in August 2009.

Super-rotation station

Cassini’s observations also show that Titan’s atmosphere is in “super-rotation” above about 5 miles. In other words, the atmosphere there rotates a lot faster than the surface itself. The UW model suggests that these methane storms “produce strong downdrafts, flowing eastward when they reach the surface.” These downdrafts could be rearranging the dunes.

The linear dunes run parallel to Titan’s equator. Unlike Earth’s dunes made of sand, Charnay believes Titan’s dunes are made of hydrocarbon polymers formed from the decomposition of methane in the atmosphere.

Only direct observation by Cassini would confirm this hypothesis. Unfortunately, the Cassini mission will end in 2017 and Titan’s next equinox is not until 2023.

“But there will be other missions,” said Charnay. “There are still a lot of mysteries about Titan. We still don’t know how a thick nitrogen atmosphere formed, where the methane comes from nor how Titan’s sand forms. And it is not completely excluded that life can be there, perhaps in its methane seas or lakes. So Titan really is a fascinating and evolving world, which has to be understood as a whole.”

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