Dunes on Titan need firm winds to move, experiments at ASU show

December 8, 2014

Saturn's largest moon, Titan, is one of the few solar system bodies – and the only planetary moon – known to have fields of wind-blown dunes on its surface. (The others are Venus, Earth and Mars.)

New research, using experimental results from the high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory, has found that previous estimates of how fast winds need to blow to move sand-size particles around on Titan are about 40 percent too low. radar image showing lines of dunes on Titan Download Full Image

A team of scientists led by Devon Burr of the University of Tennessee, Knoxville reported the findings Dec. 8 in a paper published in the journal Nature. James K. Smith, engineer and manager of ASU's Planetary Aeolian Laboratory, is one of the paper's co-authors.

Saturn and Titan orbit about ten times farther from the sun than Earth. Scientists got their first detailed information about Titan when the Cassini/Huygens orbiter and lander arrived in 2004. The short-lived Huygens lander took photos when it reached the surface and as it was descending through Titan's dense, smoggy atmosphere, which has 1.4 times greater pressure than Earth's. These images, plus studies using instruments on the Cassini orbiter, revealed that Titan's geological features include mountains, craters, river channels, lakes of ethane, methane and propane – and dunes.

Dunes begin to form when the wind picks up loose particles from the ground and drives them to hop, or saltate, downwind. A key part of understanding dunes is to identify the threshold wind speed that causes dune particles to start to move. Geologists have found threshold speeds for sand and dust under various conditions on Earth, Mars and Venus. But for Titan, with its bizarre conditions, this remained unknown.

Particles of 'sand' as light as freeze-dried coffee

On Titan, where the surface temperature is negative 290 degrees Fahrenheit, even "sand" is probably unlike sand on Earth, Mars or Venus. From the Cassini observations and other data, scientists think it is composed of small particles of solid hydrocarbons (or ice wrapped in hydrocarbons), with a density about one-third that of terrestrial sand. In addition, Titan's gravity is low, roughly one-seventh that on Earth. Combined with the particles' low density, this gives them a weight of only about four percent that of terrestrial sand, or roughly as light as freeze-dried coffee grains.

The scientists led by Burr began their study with carefully designed wind tunnel experiments. "We refurbished the high-pressure wind tunnel previously used to study conditions on Venus," Smith explains. To recreate in the tunnel on Earth the wind conditions on Titan, the scientists had to increase the air pressure in the wind tunnel to about 12 times the surface pressure of Earth. And they compensated for the low density of Titan "sand" and the moon's reduced gravity through numerical modeling.

In the end, the Burr team explains, "This simulation reproduces the fundamental physics governing particle motion thresholds on Titan." They add that previous studies, which had extrapolated from wind tunnel experiments designed to mimic conditions on Earth and Mars, produced results that were questionable under Titan's conditions.

The outcome of the wind tunnel experiments show that the previous calculations for wind speeds necessary to lift particles were about 40 to 50 percent too slow. 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.

That doesn't sound like much, says Nathan Bridges of the Johns Hopkins University Applied Physics Laboratory, one of the co-authors, "but it makes more sense when you realize this is a dense atmosphere blowing against particles that are very light."

A higher threshold wind speed for making particles move creates an either/or situation in which weak, everyday winds do little or nothing to surface particles, but occasional strong ones readily blow them around and reshape the dunes. The pattern of dunes on Titan shows that despite prevailing winds blowing from the east, the dunes appear shaped by winds from the west, which occur more rarely. Thus, the new work indicates that Titan's dunes are seldom stirred into motion – only whenever conditions produce strong westerly winds.

For simplicity, the wind-tunnel modeling ignored some factors, among them whether Titan dune particles are sticky. If they are, the paper's scientists note, then it will take yet-stronger winds to get the particles moving, and the contrasts will be even greater between the normal east wind pattern and the stronger west winds that shape the dunes.

Bridges says, "Titan is a strange place indeed."

The facility that has grown to become ASU's Planetary Aeolian Laboratory was founded in the mid-1970s by the late Ronald Greeley of ASU. The laboratory, located at NASA's Ames Research Center in Mountain View, California, has been used for many studies of how wind interacts with particles of sand, dust and rock. Scientists have also used it to investigate what blowing sand and dust do to Mars spacecraft, such as NASA's Opportunity and Curiosity rovers. ASU operates the laboratory through an agreement with NASA.

The School of Earth and Space Exploration is an academic unit of ASU's College of Liberal Arts and Sciences.

Robert Burnham

Science writer, School of Earth and Space Exploration


Applied math is undergrad's ticket to law enforcement

December 8, 2014

Arizona State University’s applied mathematics for the life and social sciences program is designed to equip students to use math modeling and computation in the resolution of real-world problems. Many of its students focus on areas like sustainability and epidemiology.

Undergraduate Nathan Brockman is using it to build a solid foundation for a career in law enforcement, a field where problem-solving and critical thinking skills may mean the difference between life and death. Arizona State University undergrad Nathan Brockman Download Full Image

As a child, Brockman admired police officers and firefighters, and hoped to grow up to be one. “I wanted to be just like them, going around every day protecting and helping out others who need it,” he says.

As he grew older, he leaned more toward the police side because of his interest in justice and his athleticism – he was MVP of his high school team and received the First Team All-Section MLB.

Brockman is aiming for a career with the force and eventual placement as a homicide detective or SWAT team member.

The Scottsdale, Arizona, native is already working with his hometown police department. As a high school sophomore, he entered Scottsdale’s Police and Fire Public Safety Teen Academy, where he got a taste of the training required of police officers and firefighters, as well as an idea about these professionals’ everyday lives. He took to the material immediately and was presented with the Overall Recruit award at the end of the program.

The next year, he was invited back to participate in the Advanced Teen Academy as one of a small and select group of high-performing youth who are interested in careers in law enforcement.

Last year, Brockman was asked to step into a different role in the academy by serving as a junior training officer.

Moving from high school to college was challenging for Brockman, who viewed some of the changes as good and some as bad but most as simply different. Now, as he nears the end of his first semester at ASU, he has adjusted to his new life.

“A big help has been all of the tutoring and resources that ASU presents its students with in order to succeed,” he notes.

Though he almost pursued a criminal justice major, math-lover Brockman turned to the applied mathematics for the life and social sciences program because of its innovation and location.

Offered by the School of Human Evolution and Social Change in the College of Liberal Arts and Sciences, the major offers a different approach to – and way of applying – mathematics.

“It's like taking math and putting it into real-life situations and doing a lot of statistics work,” says Brockman, who was intrigued by the concept and saw the value of that knowledge base in everyday life. He feels confident about the program's ability to provide him with a secure launching pad for his career.

He is also glad to be at the Tempe campus because of its convenience for ASU football tryouts. Brockman is working out to increase speed and strength for a 2015 go.

Rebecca Howe

Communications Specialist, School of Human Evolution and Social Change