[meteorite-list] A Nutty Effect on Eros

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:39 2004
Message-ID: <200103151731.JAA27663_at_zagami.jpl.nasa.gov>


A Nutty Effect on Eros
March 15, 2001

A report from this year's Lunar and Planetary Science Conference in Houston
on the mysterious surface of the now-famous asteroid.

by Vanessa Thomas

High-resolution images of asteroid 433 Eros taken by the Near Earth Asteroid
Rendezvous (NEAR) Shoemaker spacecraft reveal a surface strewn with large
boulders. Where these boulders came from is an interesting topic for debate.
At the Lunar and Planetary Science Conference in Houston on Tuesday, a group
of scientists suggested that these boulders might have been shaken to the
surface from inside Eros.

A logical explanation for the presence of these boulders is that they are
blocks of material ejected by impacts. If this is the case, planetary
scientists expect to see craters near boulder groupings, and do see this
association on another large asteroid, 243 Ida. On Eros, however, no such
correlation exists.

"When you look at the surface, you see all these blocks littering the
surface and there are very few craters," University of California (Santa
Cruz) planetary scientist Eric Asphaug pointed out on Tuesday. Either
something wiped out the craters, or some other mechanism transported these
boulders to their present positions on the surface.

At the conference, Asphaug described a problem common in the pharmaceutical
sciences. "Whenever you try to mix powders, they don't mix," he said. "They
segregate according to grain properties." Asphaug and his team think a
similar situation might be taking effect on Eros, "sifting" the largest
rocks to the surface.

Asphaug's group likens the process to shaking a can of mixed nuts. In a
typical can of mixed nuts, Brazil nuts are the largest. When the can is
shaken, the Brazil nuts eventually rise to the surface and the small peanuts
end up at the bottom. Repeated shaking of Eros caused by impacts could cause
the grains and rocks of the asteroid's regolith to sort themselves according
to size.

Like the Brazil nuts, Eros's large boulders rise to the top when the
asteroid is shaken by impacts, and like the peanuts, the fine grains of
Eros's regolith falls through the fractures, grooves, and other crevasses on
the asteroid's surface.

"Everything's undergoing random motions during shaking, and a small grain
can find its way underneath a big grain," Asphaug explained. But, he said,
the opposite will never happen. "The big grain's never going to find a big
enough hole to fall back into." Several small grains would have to all move
out of the way at the same time to allow room for a large grain to move
down. "So statistically, the big grain works its way to the top," Asphaug

But how often would Eros need to be shaken to raise all of the boulders seen
on the surface? According to Asphaug's team, that statistic is hard to pin
down. They do know, however, that it is easier for material to move around
in a low-gravity environment. "Asteroids with very low gravity are more
prone to particle size segregation," Asphaug said. The "Brazil Nut Effect"
would have a more difficult time on a larger body such as Earth or the moon.

If Eros's boulders actually have been shaken to the surface like Brazil nuts
in a can, Asphaug's team suggests that scientists could predict the original
depth of the boulders. "You could make a prediction, if you were bold
enough, to say how deep the asteroid's regolith is by looking at the size
distribution of the crowded surface," Asphaug said. If this is the case, the
boulders sitting on Eros could provide a window to the secrets of the
asteroid's interior.
Received on Thu 15 Mar 2001 12:31:27 PM PST

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