Cloudy evidence for Martian landslide.

Examining two images of Mars taken less than three minutes apart by the Viking I orbiter in 1978, researchers say they have found evidence that a landslide rolled down a cliff in the interval between snapshots. If their interpretation proves correct, it may provide one of the few clues that Mars is geologically active, reports Baerbel K. Lucchitta of the U.S. Geological Survey (USGS) in Flagstaff, Ariz.

She and USGS colleague Holly M. Ferguson studied Viking photos of a cliff in Valles Marineris, a 2,400-kilometer-long complex of troughs bounded by crustal faults south of the equator. The first image, taken around 1:30 p.m. Mars time on Sept. 10, 1978, shows an uneventful view of the edge of the plateau Baetis Mensa and a bright slope beneath. But a second image, taken 2 minutes, 23 seconds later, reveals the sudden appearance of a bright cloud above the plateau's edge, as well as a dark blob -- possibly a shadow cast by the cloud -- on the plateau. The team suggests that the cloud, about 1,100 meters long and 500 meters wide, represents dust kicked up by a landslide.

Several features make other explanations for the cloud unlikely, Lucchitta says. For example, if it were composed of vapor rather than dust, the cloud could not have formed as quickly, she observes. And if it represented a chance updraft of material from the slope, several clouds would most likely have appeared instead of only one, she adds.

Lucchitta conjectures that loosening of soil by the afternoon sun could have triggered a landslide. Alternatively, a Mars-quake might have cause the event. "It would be unlikely that Viking could have captured the sliding in the act unless landslides are common in the Valles Marineris," she concludes.