The Magellan spacecraft has found evidence that ocean-size floods of molten rock once inundated more than half the surface of Venus.

NASA scientists said they don't yet know if there was one great outpouring of lava from cracks in Venus or a series of smaller flows, each hundreds of thousands of square miles, that engulfed parts of Venus at different times.The scientists at the space agency's Jet Propulsion Laboratory reported the findings Wednesday.

Magellan uses radar to make pictures of cloud-covered Venus. The craft was launched from space shuttle Atlantis in May 1989, went into orbit around Venus on Aug. 10 and formally began making maps and pictures of the planet on Sept. 15.

Geologist Steve Saunders, Magellan's chief scientist, said he favors the view that the lava gushed out of cracks called vents in a global set of eruptions that happened all at once, maybe 400 million years ago.

Brown University geologist and Magellan scientist Jim Head said, however, the evidence is incomplete.

"There's no question that the vast majority of the surface of Venus has been formed by lava flows and volcanic activity," he said. "But the question is, did it all happen catastrophically?"

The lava covers roughly 60 percent of the planet, Saunders said.

Head said he began to suspect Venus was subject to such catastrophic volcanic activity because Magellan's pictures show the planet has fewer large meteorite impact craters than Earth's moon, and because "the ones we find are all relatively fresh."

That suggests "there's a process of very widespread volcanism that occurs from time to time that destroys all the craters - and everything else," Saunders said.

Saunders said Venus' widespread lava flooding was similar to, but much larger than, vast lava deposits such as India's Deccan Traps.