Shaking Asteroid Sorts, Instead of Sheds, Its Rubble

Brian Handwerk
for National Geographic News
April 24, 2007
Images of the near-Earth asteroid Itokawa may shake up conventional ideas about what the space rocks are made of and how they "weather" over time.

High-resolution snapshots show that the asteroid's surface materials—a mix of gravel and boulders called regolith—are sorted by shaking and vibrations that occasionally rock the entire object.

Fine particles gather in the areas of lowest gravity, creating patches that look smooth from afar. Larger boulders are grouped together and stranded on the surface in clumps.

The findings are the first to reveal clear evidence that global-scale resurfacing processes occur on relatively small asteroids.

"Asteroids smaller than a kilometer [0.6 mile] are considered not [able] to hold granules on their surfaces due to low gravity," said study leader Hideaki Miyamoto of the University of Tokyo.

Surface gravity on the 1,640-foot-long (500-meter-long) Itokawa is about a hundred thousand times less than it is on Earth (related: explore a virtual model of the solar system).

"[It was believed that] granules would have easily escaped from the surface, so the piles we found on Itokawa were very surprising."

All Shook Up

Miyamoto and colleagues used images from the Japanese spacecraft Hayabusa, which landed on Itokawa in 2005.

The shots were detailed enough to reveal individual pieces of gravel just centimeters in size.

The team's analysis appears in the current issue of the journal Science.

"I think this paper on Itokawa is an epoch-making, classic work, which reveals for the first time how regolith on a low-gravity body develops," said Takahiro Hiroi, a Brown University researcher unaffiliated with the study.

Scientists can't be sure what forces shake up Itokawa. One theory holds that impacts from other objects play a role.

"We show that meteoroids as small as centimeter-scale can shake the entire asteroid sufficiently to create landslides," study leader Miyamoto said.

(Related news: "Asteroids Spin Faster Due to Solar Power, Studies Show" [March 7, 2007].)

And if, as some experts think, Itokawa was once a binary asteroid, shaking could also have been induced when the two objects merged.

Erik Asphaug is a planetary scientist at the University of California, Santa Cruz, who was unaffiliated with the research.

He said the new ideas are convincing but sure to be controversial.

"This is just one more example of an asteroid laughing in our faces," he said.

"We keep thinking we understand asteroids, and then we take a closer look and find them to be stranger than before."

Near-Earth asteroids like Itokawa are of special interest to scientists.

They may contain unchanged remnants from the ancient clusters of cosmic rock that formed Earth and other solid planets some 4.5 billion years ago.

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