Here's the problem: Climate simulations indicate that Titan's near-surface winds - like Earth's trade winds - blow toward the west, according to a press release. 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 nothing 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, according to new research by University of Washington astronomer Benjamin Charnay and co-authors in a paper published in the journal Nature Geoscience.
Using computer models, the authors hypothesized that the tilt 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 said, according to the press release. It probably helps that, according to Cassini's observations, Titan's atmosphere is in "super-rotation" above about 5 miles, meaning that it rotates a lot faster than the surface itself. Their model, Charnay said, suggests that these methane storms "produce strong downdrafts, flowing eastward when they reach the surface," thus rearranging the dunes.
"It was a kind of detective game, as often is the case in planetary sciences, where we have many mysteries and a few clues to solve them," Charnay said, according to the press release. The dunes in question, which are linear and run parallel to Titan's equator, are probably not composed of silicates like Earth sand, Charnay said, but of hydrocarbon polymers - a kind of soot resulting from the decomposition of methane in the atmosphere.
Charnay said direct observation by Cassini would be the way to confirm his hypothesis, according to the press release. Unfortunately, the Cassini mission will end in 2017 and Titan's next equinox is not until 2023.
"But there will be other missions," he said. "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."
Reference:
"Methane Storms as a Driver of Titan's Dune Orientation," Benjamin Charnay et al., Nature Geoscience, 2015 April 13 [https://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2406.html].
Charnay's co-authors are Erika Barth and Scot Rafkin of the Southwest Research Institute in Boulder, Colo.; Sébastien Lebonnois of the Laboratory of Dynamic Meteorology; and Sylvain Courrech du Pont, Clément Narteau, Sebastian Rodriguez and Antoine Lucas of Paris Diderot University.
The research was done in part through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group, and funded by the NASA Postdoctoral Program and the French National Research Agency; grant #ANR-12-BS05-001-03/EXO-DUNES.
