Near-surface winds on Saturn’s largest moon, Titan, blow toward the west. So why do its dunes, that can be a hundred yards high and many miles long, point east? Researchers think they may finally have an answer.
With its thick, hazy atmosphere and surface rivers, mountains, lakes, and dunes, Titan is one of the most Earthlike places in the solar system.
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.
Atmosphere in ‘super-rotation’
Violent methane storms high in Titan’s dense atmosphere, where winds blow toward the east and are much stronger than westward surface winds might be the answer, researchers say.
“These fast eastward gusts dominate the sand transport, and thus dunes propagate eastward,” says Benjamin Charnay, a postdoctoral researcher at University of Washington and coauthor of the study that is published in Nature Geoscience.
The storm winds reach up to 10 meters a second (22 mph), about 10 times faster than Titan’s gentler near-surface winds. And though the storms happen only when Titan is in equinox and its days and nights are of equal length—about every 14.75 years—they are of sufficient power to realign Titan’s dunes. Titan was last in equinox in August 2009.
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.
A mystery with few clues
A new model suggests that these methane storms “produce strong downdrafts, flowing eastward when they reach the surface,” thus rearranging the dunes.
Charnay says he first tried to solve the problem with a global climate model that didn’t factor in methane clouds, then realized that it was impossible, hinting that methane could be part of the solution.
“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.”
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.
An earlier study published in Nature showed that it would take winds of at least 3.2 mph to lift and transport sand across Titan’s surface—40 to 50 percent stronger winds than previous estimates.
The measurement of such a high wind speed threshold was a pleasant surprise, Charnay says: “That means that only fast winds transport Titan’s sand, compatible with our hypothesis of strong storm gusts controlling the orientation and propagation of dunes.”
Is there life on Titan?
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 European Space Agency’s Huygens probe, which rode along on Cassini, successfully landed on Titan in 2005 and sent back the first photo of the moon’s stone-strewn surface.
Direct observations by Cassini would be the way to confirm his hypothesis, Charnay says. Unfortunately, the Cassini mission will end in 2017 and Titan’s next equinox is not until 2023.
“But there will be other missions,” he says. “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.”
Researchers from the Southwest Research Institute in Boulder, Colorado, the Laboratory of Dynamic Meteorology, and Paris Diderot University are coauthors of the study.
The NASA Postdoctoral Program and the French National Research Agency funded the work.
Source: University of Washington