If astronauts ever visit the asteroid Lutetia, they may have to strap on snowshoes to avoid sinking into its nearly half-mile-thick layer of dust.

Dusty debris shrouds the huge asteroid to a depth of at least 2,000 feet (600 meters), scientists have calculated. The dust probably resembles the regolith found on the moon, and it's a result of the intense cosmic pummeling Lutetia has endured from other space rocks since the birth of the solar system. [Photo of Lutetia craters.]

"It must have been produced by impacts," said Rita Schulz of the European Space Agency in a media briefing yesterday (Oct. 4) in Pasadena, Calif. The announcement came at a conference organized by the American Astronomical Society's Division of Planetary Sciences.

A visit to Lutetia

The new look at Lutetia, which is located in the asteroid belt between Mars and Jupiter, is the result of a close fly-by made by ESA's Rosetta spacecraft in July. Rosetta, whose chief task is chasing down and studying a comet, zoomed to within 1,900 miles (3,162 km) of Lutetia, making it the largest asteroid ever to be visited by a space probe.

Rosetta snapped some detailed photos and probed the space rock from afar with a suite of instruments. The data confirmed that Lutetia is an elongated body with its longest side spanning approximately 81 miles (130 km), ESA officials have said.

But it took some time for scientists to process and analyze much of Rosetta's data. Over the last few months, they scrutinized some of Lutetia's many craters, measuring how deep they are. Researchers then compared that information with predictions of how deep such craters are expected to be based on theoretical models.

The craters were much shallower than researchers had anticipated, Schulz said, because of the thick layer of dusty debris. Indeed, one of Rosetta's most dramatic photos shows a landslide of dust at the bottom of an enormous crater.

Based on Lutetia's size, Schulz said, the asteroid probably has a relatively high escape velocity — the speed needed to overcome the rock's gravitational pull. So about 90 percent of the debris ejected by Lutetia's many collisions with other cosmic bodies probably falls back onto the space rock's surface. Over the course of 4.6 billion years or so, this would add up to a pretty hefty dust coating.

Scientists think the regolith covering the moon was produced the same way.

A solid piece of rock

Rosetta has revealed other intriguing details about Lutetia. Using data gathered by the probe's instruments, scientists were able to estimate the asteroid's density. And the results suggest Lutetia is a solid piece of rock, not a loosely agglomerated "rubble pile" that many other asteroids are suspected to be.

Rosetta also studied Lutetia's color.

"It is boringly gray," Schulz said. But that doesn't mean the asteroid is any less interesting, she added. "That means it's like the moon, which isn't boring at all."

Schulz said more Lutetia revelations will be forthcoming after the research team spends more time with Rosetta's data.