SAN FRANCISCO, DEC. 3 -- The Magellan robot explorer has sent back data showing that the face of Venus is not an ancient death mask, like those of Mars and Mercury, but has been remodeled piecemeal by volcanic flows, scientists reported today.

"Venus is not dead," Steve Saunders of the Jet Propulsion Laboratory in Pasadena, the project's chief scientist, told a gathering here. "It did not shut down 400 million years ago" as scientists had suspected, but shows evidence of continuing geologic change.

"The planet still appears to be active" volcanically, said Roger Phillips of Southern Methodist University. Scientists hope to catch an eruption in progress during Magellan's mission, he said. Only Earth and two planetary satellites, Jupiter's moon Io and Neptune's moon Triton, are known to have any form of active volcanism.

The first major crop of papers based on the data pouring in from the Magellan spacecraft as it circles Earth's hot and toxic sister planet was delivered here at the fall meeting of the American Geophysical Union.

Magellan's radar eyes peered through the planet's veil of clouds to show 10 times more detail than previous investigations. It has completed about 90 days of its $500 million, 240-day mapping mission, but because of an expected interruption when the sun blocked it from Earth and a series of technical problems, it has mapped only about 15 percent of the planet so far, scientists said.

Part of the evidence for volcanism on Venus is that the planet's surface in some areas is unexpectedly smooth. Magellan's data show significant gaps in the distribution of craters caused by falling meteorites, with vast expanses where there are no craters. Phillips said this indicates that the pockmarks were obliterated by more recent lava flows -- upwellings of molten rock from the planet's core -- some of which extend over an area the size of Australia.

However, there is an "apparent contradiction," he added. The gaps in the peppering of craters suggest "the planet is being resurfaced locally," but "we can't find craters in the process of being resurfaced."

One possible answer to the riddle is that "resurfacing is taking place very fast" at a given point.

Globally, the resurfacing process is very slow compared to that on Earth. On average, he said, Venus produces about one-tenth of a cubic mile of lava each year, while Earth's output is about 7.7 cubic miles a year. "Maybe only 10 percent of the planet is active at any one time," he said.

Some large volcanoes have several impact craters nearby, while others have virtually none, and these clues could point scientists to the most likely areas of current volcanic activity, said James Head of Brown University. "This is very exciting. We can pick our targets," he said.

There appears to be a wide variation in the thickness of the molten rock flowing on Venus, he added. It ranges from the consistency of toothpaste, as exhibited in features that resemble pancakes, to waterlike liquid in extensive river channels hundreds of miles long, indicating differences in the chemical composition of the hot rock.

Earth and Venus were blasted out of the same material nearly 5 billion years ago, and are about the same size and distance from the sun, but they have evolved quite differently. Venus's surface temperature is almost 900 degrees Fahrenheit, hot enough to melt lead; its thick, lethal atmosphere has a pressure at the surface 90 times that on Earth.

In order to explode volcanically, Head said, molten lava on Venus must have a concentration of volatile gases high enough to overcome the high atmospheric pressure.

"Venus quakes" also appear likely, said Sean Solomon of the Massachusetts Institute of Technology. "We see faults" on Venus, like those that cause earthquakes, he said. "I expect they slip episodically and generate seismic waves."

Scientists have debated whether the crater known as Cleopatra, on the slope of the highest mountain on Venus, was caused by a meteor impact or volcanic activity. Magellan has resolved that question, showing Cleopatra is an impact crater. But it is "pristine," implying that it is either very young or that there has been no activity to alter its form in millions of years, said Phillips.

After years of waiting for planetary scientists, "it's Christmas every day," said Head.