Experiments Indicate Strong Winds Needed To Shape Titan's Dunes

Chuck Bednar for redOrbit.com – Your Universe Online

The presence of massive dunes, some over a mile wide and hundreds of yards high, on Saturn’s moon Titan has long puzzled scientists, but the authors of a new study appearing in the current edition of the journal Nature believe they have solved the mystery.

Titan, which has a dense atmosphere as well as lakes and rivers made up of components of natural gas, is home to dunes that can stretch out for hundreds of miles – even though data suggests the moon only experiences light breezes, not the strong winds required to move such massive amounts of sand.

However, Devon Burr, an associate professor in the Earth and Planetary Sciences Department at the University of Tennessee, and colleagues from Arizona State University have demonstrated that Titan’s winds must blow faster than previously believed in order to move the sand, thus potentially explaining how those dunes were formed.

Burr, James K. Smith, engineer and manager of ASU’s Planetary Aeolian Laboratory, and their fellow researchers conducted experiments at the Arizona State facility’s high-pressure wind tunnel and found that previous estimates of the wind seep required to blow sand-sized particles around on the moon are approximately 40 percent too low.

“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,” Burr, lead investigator on the study, explained in a statement Monday. “Before seeing the images, we thought the winds were likely too light to accomplish this movement.”

Image Above: 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

The dunes, which were first discovered by the Cassini-Huygens orbiter and lander in 2004, begin to form when the wind collects loose particles from the ground and causes them to saltate – or hop – downwind. To understand the dunes, it is essential to identify the threshold wind speed that causes the particles that comprise them to start moving.

While geologists had already determined the threshold wind speeds required for dust and sand under various conditions on Earth, as well as on Venus and Mars, the bizarre conditions present on Titan made it more difficult. For one thing, given that the moon’s surface temperature is negative 290 degrees Fahrenheit, it is unlikely the particles found on its surface behave the same way as sand found on Earth, Mars or Venus.

Based on the Cassini observations and other data, scientists believe that it is composed of small particles of solid hydrocarbons (or ice wrapped in hydrocarbons), with a density about one-third that of terrestrial sand, the researchers said. Furthermore, Titan’s low gravity (approximately one-seventh that of Earth’s) and the low density of the particles gives them a weight of roughly four percent that of terrestrial sand, they added.

Burr, Smith and their colleagues used a high-pressure wind tunnel previously used to study conditions on Venus, and increased its air pressure to about 12 times the surface pressure of Earth to recreate the conditions of Titan. They also compensated for the low density of Titan “sand” and the moon’s reduced gravity through numerical modeling.

Ultimately, the simulation “reproduces the fundamental physics governing particle motion thresholds on Titan,” the researchers explained, according to an ASU statement, adding that previous studies extrapolated data from experiments designed to mimic conditions on Earth and Mars and produced results considered to be questionable under Titan’s conditions.

The new wind tunnel experiments indicate “that the previous calculations for wind speeds necessary to lift particles were about 40 to 50 percent too slow,” the university said. “The new experiments show that near the surface of Titan, the most easily moved sand-size particles need winds of at least 3.2 miles per hour (1.4 meters per second) to start moving,” demonstrating that gusts would be needed to “blow them around and reshape the dunes.”

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Source: http://www.redorbit.com/news/space/1113295239/strong-winds-needed-to-shape-titan-dunes-120914/