[meteorite-list] Research Offers Explanation for Titan Dune Puzzle

[Ron Baalke]

http://tntoday.utk.edu/2014/12/08/ut-research-offers-explanation-titan-dune-puzzle/

UT Research Offers Explanation for Titan Dune Puzzle
University of Tennessee
December 8, 2014
 
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.

[Photo]
Sediment inside the Titan wind tunnel for testing.

Research led by Devon Burr, an associate professor in UT's Earth and Planetary 
Sciences Department, 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 
previously not 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 
that 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.

[Photo]
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

To get to the bottom of this conundrum, Burr dedicated six years to refurbishing 
a defunct NASA high-pressure wind tunnel 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.

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.

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."

According to atmospheric models, the wind reverses twice during a Saturn 
year which is equal to about thirty 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 dunes are shaped.

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

This research was supported by grants from NASA's Planetary Geology and 
Geophysics Program and the Outer Planets Research Program. A new grant 
will allow Burr and her colleagues to examine Titan's winds during different 
climates on Titan as well as the effect of electrostatic forces on the 
sand movement.

Burr's team included UT Earth and Planetary Sciences Assistant Professor 
Josh Emery as well as colleagues from the Johns Hopkins University Applied 
Physics Laboratory, SETI Institute, Arizona State University, and the 
University of California, Davis.


Photo Caption 1: Sediment inside the Titan wind tunnel for testing.

Photo Caption 2: 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


C O N T A C T:

Whitney Heins (865-974-5460, wheins at utk.edu)