[meteorite-list] NEAR Mission Images Give Clues to Composition of Asteroid Eros

[Ron Baalke]

http://www.news.cornell.edu/stories/July05/Thomas.Eros.lxg.html

NEAR mission images give clues to composition of asteroid Eros
Cornell University News Service
July 22, 2005

Writer: Lauren Gold
Phone: (607) 255-9376
E-mail: lg34 at cornell.edu

Media Contact: Press Relations Office
Phone: (607) 255-6074
E-mail: pressoffice at cornell.edu


ITHACA, N.Y. -- An asteroid's external features, when analyzed 
carefully, can say a lot about its interior. So it was while he was 
mapping the surface of the asteroid 433 Eros that Peter Thomas, a 
senior research associate in astronomy at Cornell University, found a 
simple solution to an earlier puzzle about the asteroid's composition.

Thomas was using images collected by the Near Earth Asteroid 
Rendezvous mission in 2001 to create a digital map of Eros. On the 
asteroid's surface, predictably pock-marked with thousands of craters 
accumulated from impacts over its lifetime, he saw a feature first 
noticed by Cornell graduate student Marc Berthoud: that a few 
particular patches were inexplicably smooth. That observation had led 
to various theories -- but none that seemed completely satisfying.

In a letter appearing in the current issue of the journal Nature 
(Vol. 436, No. 7049, p. 366), Thomas and Northwestern University 
geologist Mark Robinson show that the asteroid's smooth patches can 
be explained by a seismic disturbance that occurred when the crater, 
known as the Shoemaker crater, was formed.

The fact that seismic waves were carried through the center of the 
asteroid shows that the asteroid's core is cohesive enough to 
transmit such waves, Thomas says. And the smoothing-out effect within 
a radius of up to 9 kilometers from the 7.6-kilometer Shoemaker 
crater -- even on the opposite side of the asteroid -- indicates that 
Eros' surface is loose enough to get shaken down by the impact.

Asteroids are small, planetlike bodies that date back to the 
beginning of the solar system, so studying them can give astronomers 
insight into the solar system's formation. And while no asteroids 
currently threaten Earth, knowing more about their composition could 
help prepare for a possible future encounter.

Eros, whose surface is a jumble of house-sized boulders and small 
stones ("what geologists call 'poorly sorted,'" says Thomas), is the 
most carefully studied asteroid, in part because its orbit brings it 
close to earth.

Thomas and Robinson considered various theories for the regions of 
smoothness, including the idea that ejecta from another impact had 
blanketed the areas. But they rejected the ejecta hypothesis when 
calculations showed an impact Shoemaker's size wouldn't create enough 
material to cover the surface indicated. And even if it did, they 
add, the asteroid's irregular shape and motion would cause the ejecta 
to be distributed differently.

In contrast, says Thomas, the shaking-down hypothesis fits the 
evidence neatly. "The classic light bulb goes on in your head," he 
says; the crater density of small craters increases with the distance 
from the Shoemaker crater. "Simple geometry says something like a 
simple seismic wave."

The NEAR mission, in which a NASA spacecraft landed on the asteroid's 
surface in 2001 after orbiting it for a year, yielded more than 
100,000 images of the small asteroid. (Eros is about 33 kilometers 
long, 13 kilometers wide and 8 kilometers thick). Since the mission's 
conclusion 16 days after the landing, scientists from institutions 
around the world have been sorting through the data.

That process is expected to continue for years. "Careful mapping of 
things on the surface can give you a good clue as to what's inside," 
says Thomas. "And in one sense, we've barely begun."


Related World Wide Web sites:

Cornell University Department of Astronomy: <http://www.astro.cornell.edu/>
Seismic resurfacing by a single impact on the asteroid 433 Eros 
(Nature) 
<http://www.nature.com/nature/journal/v436/n7049/full/nature03855.html> 


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