Study offers explanation for Titan dune puzzle

December 8, 2014
Cassini radar sees sand dunes on Saturn's giant moon Titan (upper photo) that are sculpted like Namibian sand dunes on Earth (lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes. Credit: NASA

Titan, Saturn's largest moon, is a peculiar place. Unlike any other moon, it has a dense atmosphere. It has rivers and lakes made up of components of natural gas, such as ethane and methane. It also has windswept dunes that are hundreds of yards high, more than a mile wide and hundreds of miles long—despite data suggesting the body to have only light breezes.

Research led by Devon Burr, an associate professor in the Earth and Planetary Sciences Department at the University of Tennessee, Knoxville, shows that winds on Titan must blow faster than previously thought to move sand. The discovery may explain how the dunes were formed.

The findings are published in the current edition of the academic journal Nature.

A decade ago, Burr and other scientists were amazed by the Cassini spacecraft's pictures of Titan that showed never-before-seen dunes created by particles not previously known to have existed.

"It was surprising that Titan had particles the size of grains of sand—we still don't understand their source—and that it had winds strong enough to move them," said Burr. "Before seeing the images, we thought the winds were likely too light to accomplish this movement."

The biggest mystery, however, was the shape of the dunes. The Cassini data showed that the predominant winds that shaped the dunes blew from east to west. However, the streamlined appearance of the dunes around obstacles like mountains and craters indicated they were created by winds moving in exactly the opposite direction.

To get to the bottom of this conundrum, Burr dedicated six years to refurbishing a defunct NASA high-pressure to recreate Titan's surface conditions. She and her team then turned up the tunnel's pressure to simulate Titan's dense atmosphere, turned on the wind tunnel fan, and studied how the experimental sand behaved. Because of uncertainties in the properties of sand on Titan, they used 23 different varieties of sand in the wind tunnel to capture the possible sand behavior on Titan.

Sediment inside the Titan wind tunnel for testing. Credit: University of Tennessee

After two years of many models and recalibrations, the team discovered that the minimum wind on Titan has to be about 50 percent faster than previously thought to move the sand.

"Our models started with previous wind speed models but we had to keep tweaking them to match the wind tunnel data," said Burr. "We discovered that movement of sand on Titan's surface needed a wind speed that was higher than what previous models suggested."

The reason for the needed tweaking was the dense atmosphere. So this finding also validates the use of the older models for bodies with thin atmospheres, like comets and asteroids.

Degraded sand dune patterns seen in this radar image taken by the Cassini spacecraft show evidence of shifting winds on Titan. Credit: Cornell/Laboratoire AIM Paris-Diderot

The discovery of the higher threshold wind offers an explanation for the shape of the dunes, too.

"If the predominant winds are light and blow east to west, then they are not strong enough to move sand," said Burr. "But a rare event may cause the winds to reverse momentarily and strengthen."

The high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory was originally built to simulate winds at the surface of Venus, where atmospheric pressures are about 90 times greater than on Earth. After modifications, it became the Titan Wind Tunnel, with researchers using it to determine how fast winds on Titan need to blow to start dune particles moving. Other wind tunnels and chambers at the laboratory let scientists simulate conditions on Earth and Mars. Credit: Devon Burr

According to atmospheric models, the wind reverses twice during a Saturn year which is equal to about 30 Earth years. This reversal happens when the sun crosses over the equator, causing the atmosphere—and subsequently the winds—to shift. Burr theorizes that it is only during this brief time of fast winds blowing from the west that the are shaped.

"The high might have gone undetected by Cassini because it happens so infrequently."

Explore further: Cassini Maps Global Pattern of Titan's Dunes

More information:Nature paper: dx.doi.org/10.1038/nature14088

— Related Nature Geoscience paper: dx.doi.org/10.1038/ngeo2323

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5 comments

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interceptor
1 / 5 (2) Dec 08, 2014
No mention of the vastly different (less) gravity?
mbee1
1 / 5 (2) Dec 08, 2014
As pointed out no compensation for the less gravity makes the whole thing pointless. If they reduced the particle size and upped the wind to compensate than the whole study is at best a guess not an answer to anything.
Urgelt
1 / 5 (3) Dec 08, 2014
The know-nothing fringe trolls have spoken...

Wait, I didn't speak yet! Don't I get a say, too? I'm a know-nothing fringe troll, pick me, pick me!

Interceptor and mbee1 are *exactly* right. Hah. Take that, NASA!

What NASA *should* have done is mount that wind tunnel contraption on a plane (get rid of the snakes first) and take it on a low-gravity-emulating flight profile, *then* blow fake Titan atmosphere over fake Titan sand particles, *then* firmly concluded that... that...

That we have no way to confidently emulate on Earth what's really going on at the surface of Titan. Too many unknowns! We don't know what the sand is, we don't know how a hydrocarbon atmosphere affects little things like adhesion between sand particles, we don't have much weather data... not a single rain gauge or anemometer or barometer anywhere on the moon.

A study like this one isn't interesting for its answers, which are probably wrong. Only the questions matter.
richard_f_cronin
1 / 5 (2) Dec 08, 2014
Higher winds than anticipated ?? Shocked ! Shocked, I say ! Perhaps, perhaps, there is higher heat driving the climate of Titan, coming from within Titan. Say, something like Dr. Herndon's GeoReactor -- the naturally occurring fission breeder reactor within the Earth and within all major bodies in our Solar System. Yeah, I know, Dr. Herndon has been completely dismissed since he first published in 1992. Competing scientists refuse to peer review him. He is too far "out of the mainstream." www.nuclearplanet.com
Urgelt
5 / 5 (2) Dec 08, 2014
Richard wrote, "...the naturally occurring fission breeder reactor within the Earth and within all major bodies in our Solar System."

I have no problem with people spinning hypotheses without evidence. It's fun!

I have a problem with people *believing* hypotheses without evidence. That's just dumb.

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