Wednesday, January 27, 2010

Odd connection: Hawaiian suntans and near-Earth asteroids

Like a Hawaii visitor to Alaska, near earth asteroids approach our planet with a tan, and depart looking pale.

(Image: The asteroid Itokawa, viewed from the Japan spacecraft Hayabusa. Credit & Copyright: Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency.)

For the human tourist, it's about the sun, or the lack thereof.

For the asteroid, it appears to be the Earth's gravity, according to a team of astronomers that inclkudes two from the University of Hawai'i at Manoa.

Schelte Bus and Alan Tokunaga, of the university's Institute for Astronomy, are part of an international group looking into the change in the shade of asteroids.

Their assessment is that the darker color is the color of ancient, weathered rocks that have been passing through space for a very long time. The paler color appears to be that of unweathered stone. And they found that paler asteroids appeared to be those that had survived a near-miss with the Earth.

Their conclusion is that the Earth's gravity causes landslides on the asteroids, burying the dark rock and exposing new stone surfaces.

"We now suspect that most asteroids are loose conglomerations of rocks and boulders, rather than strong, monolithic objects," Bus said.

“Landslides on the asteroid cause the dark weathered areas to be covered by fresh, lighter colored rocks. Hence the asteroid's color, after the encounter, will appear paler than before."

Their paper, in the Jan. 21 issue of the journal Nature, is entitled, Earth encounters as the origin of fresh surfaces on near-Earth asteroids.

The researchers studied the color of the asteroids. Rocks that have been rolling around in space for as little as a million years “weather” to a redder color than normal. But some rocks that have passed near our planet don't have that same reddish tint.

“Tidal stress, strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process,” the team wrote in Nature.

“Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long puzzle of the difference in colour of asteroids and meteorites.”

How is this information useful?

For most of the public it may be the stuff of science fiction movies, but it could play a role in some future threatened impact of the Earth by an asteroid.

"The more we can learn about what holds an asteroid together, the better chance we have to reduce or eliminate damage to Earth," Tokunaga said.

© Jan TenBruggencate 2010

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