[meteorite-list] Research Offers Explanation for Titan Dune Puzzle
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 8 Dec 2014 17:11:00 -0800 (PST)
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.
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
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.
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
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)
Received on Mon 08 Dec 2014 08:11:00 PM PST