Violent methane storms on Saturn's largest moon Titan can solve the sand dune direction mystery on one of the most Earth-like places in the solar system.
Climate simulations indicate that Titan's near-surface winds -- like Earth's trade winds -- blow toward the west. So why do the surface dunes, reaching a hundred yards high and many miles long, point to the east?
The direction of the dunes has at times been attributed to the effects of Saturn's gravitational tides or various land features or wind dynamics but none quite explained their eastward slant.
"Violent methane storms high in Titan's dense atmosphere where winds do blow toward the east might be the answer," said astronomer Benjamin Charnay from University of Washington.
Using computer models after analysing data from NASA's Cassini-Huygens spacecraft, Charnay and co-authors hypothesise that the attitude of Titan's sand dunes results from rare methane storms that produce eastward gusts much stronger than the usual westward surface winds.
"These fast eastward gusts dominate the sand transport and thus dunes propagate eastward," Charnay explained.
The storm winds reach up to 10 metres a second, about 10 times faster than Titan's gentler near-surface winds. Titan has thick, hazy atmosphere, surface rivers, mountains, lakes and dunes.
Titan's atmosphere is in "super-rotation" above about five miles, meaning that it rotates a lot faster than the surface itself.
The model, Charnay said, suggests that these methane storms "produce strong down drafts, flowing eastward when they reach the surface" thus rearranging the dunes.
The dunes in question, which are linear and run parallel to Titan's equator, are probably not composed of silicates like Earth sand but of hydrocarbon polymers -- a kind of soot resulting from the decomposition of methane in the atmosphere.
Titan, discovered in 1655 by Christiaan Huygens, has long intrigued astronomers. Its atmosphere is 98.4 percent nitrogen and most of the rest is methane, and a bit of hydrogen.
Its gravity is one-sixth that of Earth's and its air density is four- to five-times higher, meaning that flight will be relatively easy for visiting spacecraft.
The paper was published in the journal Nature Geoscience.
