The surface of Venus has never seemed very hospitable. Temperatures hover around 470 degrees C (900 degrees F), the result of a runaway greenhouse effect, and the pressure of its atmosphere, thick with carbon dioxide and sulfuric acid, is some 90 times that of Earth's. Lead would flow like water on Venus, and water cannot have existed in liquid form for perhaps a billion years.
Now NASA's Magellan spacecraft seems to have found one more horror in the nasty landscape: active volcanoes. Last week the space agency released the first detailed map of Venus and the most spectacular images ever made of its surface. The pictures offer the best evidence to date that a planet once presumed dead is actually a lively cauldron of geological change.
The most stunning image is of Venus' second tallest mountain, Maat Mons, which rises 8 km (5 miles). Most of the planet's many peaks, including 9.5-km- (6-mile-) high Maxwell Montes, look bright in the radar pictures Magellan takes from its orbit above the perpetual cloud cover. That means they are strong reflectors of radar waves. But Maat Mons is dark; like the Stealth bomber, it absorbs much of the radar falling on it.
This intriguing fact, say project scientists, is a strong hint that the mountain has recently been covered with lava. Rock that sits on the surface of mountaintops appears to weather quickly in the hot, chemically reactive atmosphere, creating a soil that is rich in iron sulfide. It is this mineral, the scientists believe, that shows up easily on radar. If Maat Mons doesn't have any, it has presumably been resurfaced, perhaps within the past few years.
Such resurfacing has undoubtedly taken place in Venus' lowlands: earlier images of the planet showed vast areas that are remarkably free of craters. That would be easy to explain on a planet like Earth, where cratering from meteor strikes is erased by steady erosion. But while there is some evidence of wind erosion on Venus, the best explanation for the lack of cratering is periodic lava flows. Magellan has found direct evidence of such flows, including domelike upwellings and hardened streams of rock trailing down the sides of Venusian peaks. There are also signs of other geologic activity, including dramatic faulting and several distinct episodes of mountain building. But until last week the evidence didn't indicate whether the activity was still going on or had ceased millions of years ago. The case for active Venusian volcanoes is not yet proved, but Magellan, which is now well into its second complete survey of the planet's surface, may eventually settle the issue.
While NASA studied Magellan's images, another space explorer made history last week. Moving out beyond Mars, Galileo became the first spacecraft to have a close encounter with an asteroid. But pictures of the mysterious planetary fragment, called Gaspra, are unavailable because Galileo's main antenna for sending out images is frozen in the wrong position. Not until 1992, when Galileo swings back by Earth, can smaller antennas on the craft successfully transmit the missing pictures. The frustrating delay makes scientists all the more grateful for Magellan's reliable -- and revealing -- signals from Venus.