Wind gusts might be rare in the thin Martian atmosphere, but that doesn't stop the Red Planet from kicking up a mean sandstorm every five years or so. Physicists at the Federal University of Ceará in Fortaleza, Brazil, say the planet's gravity may hold the answer. They simulated "saltation"the way a windblown sand grain sprays other grains into the air when it landsin both Earthly and Martian conditions. They found that because Mars has only 40 percent of Earth's gravity, its ruddy sand grains can leap to heights of nearly 20 feet (five meters) and feel the full force of the wind for up to 40 seconds, allowing individual grains to travel the length of an entire football field in a single bound, they report in Proceedings of the National Academy of Sciences USA. The group says the results may explain how Martian sand dunes, such as these ripples observed by NASA's Opportunity rover on Meridiani Planum, can undulate across the planet's surface.