A Restless Venus

By MICHAEL D. LEMONICK

Venus is the closest planet to Earth, and yet it retains an aura of mystery. The thick layer of clouds hiding its surface has long frustrated efforts to take a close look at the Venusian landscape.

Now the veil is coming off. Last week NASA's Magellan spacecraft transmitted the most detailed pictures ever made of Earth's next-door neighbor. The radar images revealed a tortured topography with fault-like cracks in surprisingly regular patterns, craters as big as greater Los Angeles and volcanic mountains flanked by congealed rivers of lava at least 320 km (200 miles) long. Says James Head III, a Brown University geologist and member of the Magellan imaging team: "It's a revolutionary new view of Venus."

The revolution almost didn't happen. When Magellan first took up its elliptical orbit around Venus on Aug. 10, its communications system inexplicably stopped working. Then the equipment started up, letting the space probe send back a few tantalizing images -- and stopped once more. Fearful that the spacecraft would lose contact with Earth permanently, engineers at NASA's Jet Propulsion Laboratory, which is responsible for Magellan, put the imaging on hold while they tracked down the problem. It was apparently a faulty computer chip. Electronic signals have been rerouted to bypass the flaw, and meanwhile Magellan's control software is being entirely rewritten to make doubly sure there are no more lapses.

If the probe keeps working, scientists will by next summer have mapped some % 90% of the planet. Magellan's radar detectors can pick up features as small as 120 meters (394 ft.) across, 10 times smaller than anything ever seen before. That should be enough to begin answering some important questions about the geology and atmosphere of the planet. Although nearly the same size as Earth, Venus has an atmosphere thick with carbon dioxide and sulfuric acid. These gases have created an atmospheric pressure at the surface 90 times that of Earth and led to a greenhouse effect that keeps the temperature at 470 degrees C (900 degrees F), hot enough to melt lead and ensure that liquid water cannot exist.

That is a plus for scientists. Without flowing liquids to cause erosion, the geological history of Venus has been preserved and can now be observed as if by time-lapse photography. For example, the outline of one mountain peak clearly shows how it was formed over time by three separate upward thrusts of the planet's surface. The atmosphere preserves the land in another way as well: the clouds are so dense that all but the largest incoming meteorites burn up before striking the surface. Earth has absorbed far more hits from meteorites, although much of the evidence has eroded away.

Many scientists believe the Venusian volcanoes are still capable of erupting, and there is even a chance they may be caught in the act. After finishing a 243-day mapping mission, Magellan will start all over and do it again. Any change in the landscape that shows up in the second version -- new lava or other debris, for example -- would be strong evidence of volcanism, making Venus only the fourth body in the solar system, after Earth, Jupiter's moon Io and Neptune's moon Triton, where eruptions have been spotted.

It is still debatable whether Venus shares one trait with Earth that is not shared by any other planet or moon: a surface divided into "tectonic plates" that slide around on a subsurface sea of semimolten rock, forming mountains where they collide and trenches where they pull apart. Because Venus is so similar to Earth in size and weight, the notion is plausible, and the initial images seem consistent with such a structure. But, cautions Caltech planetary scientist Bruce Murray, "the whole debate is premature. We must be very careful with this evidence." In fact, it is dangerous to generalize much about Venus just yet. Magellan has so far mapped only 1.5% of the planet's surface. It is possible that other areas will look vastly different, and that Venus may have more secrets to reveal.

With reporting by Edwin M. Reingold/Los Angeles