"Ballistic" Sand May Explain Shifting Martian Dunes

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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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