Hop, skip and a jump

The Martian atmosphere is thin, but it can whip up a mean sandstorm: On the Red Planet, wind-driven grains travel up to 10 times faster than those blowing along Earth’s surface, new analyses suggest.

As on Earth, most windblown sand grains on Mars get where they’re going by saltating, or repeatedly bouncing along the surface. On Mars, however, sand grains can hop higher than they do on Earth because gravity at the planet’s surface is much weaker than it is here on Earth, says Eric J.R. Parteli, a physicist at the Federal University of Ceará in Fortaleza, Brazil. That, in turn, stretches the length of each hop, a parameter that influences the spacing of ripples and other features on dunes, which are massive accumulations of tiny grains (SN: 10/19/02, p. 248).

Using a detailed model of turbulent winds, as well as data gathered by Mars rovers and satellites, Parteli and his colleagues analyzed how saltating sand grains behave on the Red Planet. Grains of Martian sand are made of basalt, which is denser than the quartz that makes up much of our planet’s sand, suggesting they wouldn’t bounce as high. However, the gravity at Mars’ surface is less than 40 percent that experienced at ground level on Earth.

That decreased downward pull is why Martian sand grains bounce so high and blow so fast, says Parteli. On Earth, saltating sand grains typically bounce no more than 15 centimeters high, a diminutive hop that doesn’t carry the particle out of the ground-hugging layer of air where winds are significantly slower than those at higher altitudes. On Mars, however, sand grains can reach heights of 5 meters, which exposes them to the full force of the wind.

As a result, says Parteli, a wind-driven grain of Martian sand can fly the length of a football field in a single bounce. Each time it strikes the ground, it does so at speeds five to 10 times faster than a saltating sand grain on Earth. Each high-energy impact blasts more grains into the air, until the atmosphere hugging the ground is saturated with sand. Parteli and his colleagues report their findings in the Apr. 29 Proceedings of the National Academy of Sciences.

Although sand grains on Mars travel fast once they’re airborne, in the thin atmosphere there, they need a big push to get started: Data gathered by Mars rovers indicate that the minimum wind speed needed to kick up sand grains — around 220 kilometers per hour at a height 1.5 meters above the ground — only occur, on average, once every five years and last no more than 40 seconds. At that rate, a 100-meter-by-100-meter dune would take 7,000 years to migrate one meter.

On the other hand, the new analyses suggest that a sandstorm doesn’t abate until winds drop to about 90 kilometers per hour — a much lower speed than expected, says Lori K. Fenton, a planetary scientist at NASA Ames Research Center at Moffett Field, Calif. “This is a major finding” that’s relevant to predicting the scope and extent of Martian dust storms, she notes.

The team’s detailed model is “a nice advance … that provides an interesting look into the dynamics of another planet,” says H. Eugene Stanley, a physicist at BostonUniversity.