Martian rocks offer a windy tale.

It's not the rosy sunrises or pinkish sunsets that thrill geologists Michael Malin and Ronald Greeley when they view the latest images of Ares Vallis, the Martian valley where Pathfinder landed on July 4. It's close-up pictures of rocks suggesting that huge gusts of wind once screamed across the undulating terrain. Such winds, more than 10 times the speed of the gentle, 20-kilometer-per-hour late summer breeze that now blows through the valley, would have driven sand into rocks with enough force to carve new shapes.

Pictures taken by Sojourner, Pathfinder's tiny rover, provide the first clear evidence that Martian rocks have been chipped away by windblown sand. While researchers had already seen rocks that appear sandblasted, stripped of a veneer of dust and debris, they had never seen material gouged from rock, says Malin, a Pathfinder investigator at Malin Space Science Systems in San Diego.

If the rocks were eaten away by storms similar to those Malin has studied in Antarctica and Iceland, the erosion could have occurred in just tens of seconds. More generally, the finding may force scientists to rethink how the Martian surface has been altered over time, adds Greeley, a mission scientist at Arizona State University in Tempe.

Examining images taken by Sojourner 5 meters from the landing site, as it faced a sand dune dubbed Mermaid, Malin spotted two centimeter-size rocks marked with grooves. These streamlined depressions are aligned with the direction in which prevailing winds elsewhere on Mars have blown drifts of soil and scrubbed the faces of boulders, he told Science News.

Before 1976, when the Viking landers and orbiters arrived at Mars, researchers assumed that fierce winds had scalloped the Martian surface, Greeley notes. The Viking craft turned that idea around when it revealed only heavily cratered terrain that seemed untouched by wind erosion. Now, with Sojourner taking a closer look at rocks, wind-sculpted features are evident, reversing once again the thinking about the importance of wind and sand in shaping the planet.

Wind erosion requires a copious supply of sand. The source of Martian sand is unknown, although on Earth sand typically comes from granite, Greeley notes. No one has found granite, a rock that forms as a result of repeated cycles of heating and cooling, on Mars. Indeed, before the current mission, scientists felt certain that the geological processes within the Red Planet did not produce granite.

However, the rover's study of a Martian rock called Barnacle Bill indicates that it resembles andesite, a volcanic rock often formed by several cycles of heating and cooling, although not as many as granite. Although the data are extremely preliminary, researchers are now speculating that Mars "may have a much more complicated generation of magma and [surface] evolution than we suspected," says Greeley.

Tying in with that suggestion, the latest analysis of another rock, Yogi, indicates that it, too, may contain andesite, says Harry Y. McSween Jr. of the University of Tennessee in Knoxville. Scientists had initially classified Yogi as basalt (SN: 7/19/97, p. 39), but they hadn't accounted for the dust that coats the rock, he says.