Martian sand dunes: blowing in the wind.

Martian sand dunes: blowing in the wind

First observed by a spacecraft in 1972, sand dunes are common over most of Mars. What hasn't been clear, however, is whether today's extremely thin Martian atmosphere could sculpt these features, or whether dunes could be relics from a time when the atmosphere was denser and wind could blow sand more easily.

Viewing the north polar region of Mars, an orbiting craft has found the first evidence that the appearance of sand dunes has changed over a matter of months. The snapshots, taken by Mars Global Surveyor, suggest that gusts of wind have recently blown sand across the frost-covered polar dunes, leaving streaks of freshly deposited sand over the frost.

Surveyor took the images from July through September, when its elliptical orbit brought it within 500 kilometers of the surface of the north polar region. The first images showed that the dunes had a thin coating of bright frost, a holdover from the northern winter that had ended just weeks earlier.

Some of the dunes have dark spots, with dark streaks emanating from them. Members of the Surveyor camera team, which includes Kenneth S. Edgett and Michael Malin of Malin Space Science Systems in San Diego, propose that the dark spots represent sites where gusts of wind have removed the frost and exposed dark sand beneath it. The streaks, they argue, consist of sand blown from the exposed patches. The wind gusts could reach 40 meters per second, Malin estimates.

The movement of Martian sand intrigues planetary scientists because it could be a prelude to the dust storms that sometimes engulf the planet. According to Malin, dust particles tend to stick together and hug the planet's surface too closely to be picked up by Martian winds. However, sand that bounces along the surface could propel the dust into the atmosphere, allowing winds to carry it far and wide.

Edgett presented his team's findings on Oct. 29 in Toronto at the annual meeting of the Geological Society of America.