Asteroid photos show complex surface with ‘ponds’ of thick dust

Photos taken by the first spacecraft to land on an asteroid show a landscape littered with boulders, small rocks and other debris that appear to have partly eroded and settled into mysterious "ponds" of thick dust.

The photos taken by the Near Earth Asteroid Rendezvous spacecraft, called NEAR, add evidence to the theory that even the weak gravity of an asteroid can hold on to much of the flying debris created when struck by another object such as a meteorite or comet.

But the photos of the asteroid Eros also created a new puzzle for astronomers who wonder why much of that debris appears to have eroded – without any wind or water in the airless void of space.

"It continues to surprise us, and to add to our amazement about how diverse the surface of an asteroid can be," said Joseph Veverka, a Cornell University astronomer and lead author in a series of three articles on Eros in the journal Nature.

NEAR was never designed to land on an asteroid. But after it orbited Eros for a year, taking thousands of high-resolution images of the 34-kilometer-long asteroid, scientists decided to gently crash the spacecraft into the surface last February.

Transition

The spacecraft sent back 70 photos of Eros during its descent, including a final sequence that shows a transition from a rough surface to a smooth pond of fine, loose material.

Veverka said photos of the asteroids Gaspra and Ida, and of Phobos, a satellite of Mars, show areas comparable to Eros but none of the pond-like deposits at the bottom of craters.

The fine material in the deposits appears to have been sorted from the upper portion of the potato-shaped asteroid’s regolith, or loose soil. The close-up photos of Eros also show that some blocks of rocky debris appear to have weathered and collapsed.

"Clearly, there is something else going on that is moving things downhill," Veverka said.

The astronomers believe the erosion may partly be due to an electrostatic effect observed on the moon’s surface. Tiny particles can build up photoelectric charges with long exposure to the sun, causing dust to rise and settle. But the effect would be unlikely to produce deposits the size of those found on Eros, researchers say.

Large crater

The main source of debris and rocks ejected on the surface of Eros came from a large crater that was created perhaps a billion years ago, well after asteroids settled into a belt of broken rock orbiting between Mars and Jupiter.

Scientists say the findings suggest the composition of asteroids is more complex than previously believed, making it more difficult to decide how to handle any threat if one ends up on a collision path with Earth.

"Things are certainly evolving and the science and evaluation of the potential threat are changing," said Eleanor Helin, one of the pioneers in asteroid tracking at the Jet Propulsion Laboratory in Pasadena, California.

In an accompanying commentary, Erik Asphaug of the University of California at Santa Cruz, said the heavy layer of dust could pose a problem for robot or even human explorers in the future. But it also could represent a relatively easy way to mine asteroids for their abundant metals, including precious metals such as gold and platinum.

"They’ll be making soda cans out of platinum if they’re successful," Asphaug said.

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On the Net:

Near Earth Asteroid Rendezvous project site: http://near.jhuapl.edu