"Ballistic" Sand May Explain Shifting Martian Dunes

Anne Minard
for National Geographic News
April 29, 2008
Martian sand grains can make giant leaps compared to those on Earth, even though the red planet rarely experiences wind powerful enough to move them.

That's the key finding in a controversial new study led by Murilo Almeida, a physicist at Federal University of Ceara in Brazil.

The researchers found that a Martian dust storm only needs a short gust of wind to create a dramatic chain reaction. (Related: "Mars Warming Due to Dust Storms, Study Finds" [April 4, 2007].)

Even a small amount of sand can trigger a process called saltation, in which windblown dust grains crash into the surface in low ballistic trajectories and kick up other grains.

Almeida and his colleagues used computer simulations to mimic environmental conditions on Earth and Mars over a range of wind speeds. They found that sand grains on Mars can travel a hundred times higher and longer, and reach speeds five to ten times faster than sand on Earth.

"Because of the giant grain trajectories, sand on Mars can be effectively transported at surprisingly low wind velocities," said study co-author Jose Andrade, of the Federal University of Ceara and the Swiss Federal Institute of Technology in Zurich.

The results may help explain how dust storms can arise on a planet with virtually no atmosphere—and why scientists have recently observed sand dunes migrating on Mars without the lasting wind speeds previously believed to be necessary. (Related photo: windblown Mars dunes.)

The study appears in this week's issue of the Proceedings of the National Academy of Sciences.

Winding Down

Ever since Mariner 9 sent back the first images of Mars dunes in the 1970s, scientists have wondered whether the features could form under present-day conditions.

Confirmation came only recently, with noticeable changes on the surface of a few Martian dunes being reported in the past several years. (Related: "New Hi-Res Color Images of Mars Released" [October 11, 2007].)

Almeida and colleagues set out to explain the shifting Martian dunes by conducting simulations in a virtual wind tunnel. Their experiments compared sand saltation on Earth and Mars under environmental conditions on both planets over a range of wind speeds.

The findings suggest that saltation can be a major factor for triggering dust storms.

Wind speed must reach a certain uncommon threshold to begin the process of aeolian activity—windblown transport of sand particles—the authors write.

"But no matter if, after such a gust, the wind strength decreases to typical Martian values, dust will not cease spreading into the air—as long as sand saltates on Mars," they add.

However, John Merrison, an unaffiliated researcher from the University of Aarhus, Denmark, disagrees.

"On the basis of observations from all of the NASA lander missions, dust transport—without the saltation of sand grains—is the most active aeolian process on Mars," he said. "So unlike on Earth, dust apparently blows around on Mars without sand saltating."

Continued observations of dunes and ripples, as well as wind tunnel measurements that include the lighter gravity of Mars, will be needed, he added.

"The unanswered questions regarding the action of wind on the Martian surface, I think, will only be addressed by combined studies in the lab, on Mars, and by theorists."

Fresh Ideas?

Lori Fenton, a NASA Ames Research Center astronomer who is not affiliated with the new study, credited its authors with showing that it's much easier to sustain sand saltation than scientists thought.

"It's unclear how much this will help explain dust storm initiation, because there is so much we don't understand about the atmospheric processes that lead to these wind storms," she said. "But at least now we know that it should be easier to maintain the storms once they start."

Bruno Andreotti of the University of Paris Diderot, who also did not participate in the study, argues that the paper doesn't present many new findings.

"This article is mostly what we call a copy-paste paper" from previous work by the authors and others, he said.

"So one interest of the paper is perhaps to allow planetary researchers to discover the aeolian literature published in physics," he said.

Study co-author Andrade countered that the effect of gravity and air viscosity different from Earth had never been accounted for in wind tunnel experiments.

The paper also provides a new set of equations that can be used in order to calculate sand flux and wind-transported grain trajectories on any planet, he said.

"The proposed expressions are, therefore, universal."

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