The Magellan spacecraft, mapping the surface of cloud-shrouded Venus for the second time, is returning images of huge volcanoes, pancake-shaped lava domes and craters gouged by rocks that smacked the planet at a speed of 12 miles a second.

"They literally are like nuclear bombs when they go off," Magellan scientist Steve Saunders said of the high-speed impacts that scarred a surprisingly young Venusian surface.He spoke Wednesday as the National Aeronautics and Space Administration released a new set of radar images taken by Magellan, which began mapping Venus last year.

"One of the small craters we showed you has the energy equivalent of thousands of times of the whole global nuclear arsenal," Saunders said. "You couldn't make a hole that big if you piled up every atomic bomb in the world."

The spacecraft recorded a volcano 250 miles in diameter and lava flows from 300 kilometers (186 miles) to 5,000 kilometers (3,100 miles) long. But Saunders said it is not known whether the volcanoes were active. Photographs also showed pancakelike lava structures 30 kilometers (18.6 miles) in diameter.

He said scientists will be looking carefully at future pictures for any change that might indicate still-flowing lava.

Magellan mapped 600 craters made by large chunks of rock whizzing through space at great velocities. The impacts - at speeds as great as 43,000 mph - occurred on an average of one every million years, Saunders said.

The big surprise to date, he said, is that the Venusian surface does not have ancient terrain. "Not anything older than a billion years," he said. "It might be what Earth looked like a billion or 2 billion years ago."

The first mapping phase, which surveyed 84 percent of the planet, ended May 15. During two succeeding nine-month phases, the radar will look at the surface from different angles, achieving a stereo effect. By the end of the third phase, Magellan will have mapped the entire planet.

"When we are finished, we will have a better global photo information data base of Venus than is presently available of Earth," said Wesley Huntress Jr., director of the agency's Solar System Exploration Division.

"The results have been spectacular," Huntress said. "Magellan has removed the veil from the planet, and we are able to see entirely through the Venus cloud cover."

Venus, which is about the size of Earth and is brighter than any object in the sky except the sun and moon, has a 60-mile-thick cloud cover of carbon dioxide that obscures any observation of the surface from Earth.