Magellan captures landslides on Venus.

Scientists didn't catch these rolling stones in action, but they do have pictures of the show after it went down.

This week, NASA released the first radar images clearly showing evidence of landslides on Venus. The Magellan spacecraft snapped the pictures while orbiting the planet. Although researchers don't know exactly when the landslides occurred, they believe the activity took place thousands or even millions of years ago. "Clearly it was sometime in the past -- probably even deep in the geologic past. We didn't catch a landslide in the act," says Jeffrey Plaut, a member of the Magellan team at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Two Magellan images released last fall showed what scientists initially thought were before-and-after shots of a plateau on Venus undergoing a landslide (SN: 9/7/91, p.149). These pictures appeared to provide the first visible evidence of current geologic activity on a planet other than Earth. After further analysis, however, the researchers discovered they had mistaken image distortion for geologic destruction and withdrew the claim (SN: 10/26/91. p.269).

The landslides in the new images formed on volcanoes. JPL scientists propose three basic theories to explain what caused the avalanches. It's possible that a buildup of cooled lava on the steep slopes succumbed to gravity and broke loose. Or, as with Mount St. Helens in 1980, an eruption could have sheared off the side of a mountain. Finally, a "Venusquake" may have shaken loose tons of unstable debris.

Spotting landslides by satellite, says Plaut, is an exercise in "photogeology." Viewed from overhead, most landslides have a distinct, recognizable geologic footprint. In the Venus images, "you see a gouged-out area on the hillside, and downslope from there you see a big pile of rubble," says Plaut. "It's very similar to what you'd see if you flew Magellan over some of the large landslides on Earth."

Magellan coordinator R. Stephen Saunders suggests that since Venus has no rainfall to cause erosion, landslides probably helped sculpt much of the mountainous terrain on the planet.

Studying ancient landslides on planets such as Mars and Venus helps piece together "the big geological picture" of their evolution, says Plaut. "We are trying to understand the history of other worlds, and what it comes down to is geology."