It moves. Dark streaks of sand on the frosted slope of a martian dune (left center) mean that the sand slid off the dune crest in recent months.

Geologists Find Action in Martian Still Life

WASHINGTON, D.C.--Planetary geologists examining some of the sharpest pictures ever returned from Mars are seeing the first ongoing geologic activity detected on the red planet. At a NASA press conference here today, researchers showed streaks of dark sand splayed across frosted dunes. "This is proof positive the sand is moving," said planetary scientist Peter Thomas of Cornell University, a member of the camera team for the Mars Global Surveyor spacecraft. "We see real action on geologic features."

Mars Global Surveyor could nearly catch moving sand in the act because its orbiting camera, developed by Michael Malin of Malin Space Science Systems in San Diego, can make out features only a few meters across. Mariner 9 imaged the dark dunes in Proctor Crater in 1972, but the resolution of Mariner's camera was just 62 meters. Nothing in Proctor Crater has moved that far during the 14 martian years since then, but Surveyor's camera can make out narrow dark streaks running from the crests of dunes across downwind slopes still covered by frost. Because that frost comes and goes each year, that bit of dune sand must have moved downwind in recent months.

"To see changes in sand dunes suggests Mars is a dynamic environment," says Malin. "We're quantifying it for the first time." That can only help geologists trying to understand martian geology. "The middle history of Mars seems to be dominated by [windblown] features," he says. A billion years or more ago, Malin says, a wind seemingly stronger than the one blowing today appears to have moved giant dunes and gouged out ridges and grooves.