[meteorite-list] 'Cosmic Botox' Bashes Asteroid Wrinkles Away (Eros)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jul 20 18:39:27 2005
Message-ID: <200507202238.j6KMcXL26110_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/article.ns?id=dn7699
    
'Cosmic Botox' bashes asteroid wrinkles away
Zeeya Merali
20 July 2005

Meteoroid impacts can give facelifts to asteroids, knocking hundreds of
millions of years off their apparent age. The finding casts doubt on the
reliability of current asteroid dating methods.

The only way to date most asteroids, like other bodies in the solar
system, is to infer their ages from the number and appearance of craters
that pockmark their surfaces. Heavily cratered objects suggest ancient
surfaces that have long been pummelled by space rocks. The only
exception to the rule is the asteroid Vesta, which has been dated
radiometrically using meteorites known to have originated there.

But a new Nature study by Peter Thomas of Cornell University in Ithaca,
New York, and Mark Robinson of Northwestern University in Evanston,
Illinois, both in the US, hints the crater-based technique may not
always be accurate.

They mapped the craters on the 33-kilometre-long asteroid Eros using
more than 100,000 images taken by NASA's NEAR Shoemaker spacecraft
between 2000 and 2001. They noticed that 40% of the asteroid's rocky
surface bore far fewer craters than the rest of it.
          
Smooth and youthful

At first Thomas thought that a meteoroid impact might have created a
massive crater. That would have thrown debris into the air, which then
could have blanketed small craters in the area. "But there clearly
wasn't enough material in the central crash site to fill so many holes
by that process," he explains.

Now he believes a collision with a meteoroid a few hundred metres across
caused an asteroid-quake on Eros, which appears to be coated in 100
metres of soil-like regolith. The seismic waves set Eros shaking so hard
that the smallest craters within a 9 km radius of the crash site
collapsed, he suggests, giving it a smoother, more youthful appearance.

"One impact can completely re-set a surface," says Thomas. "If you only
look at part of an asteroid's surface, instead of at the whole thing,
you could be fooled into thinking it's a factor of 10 younger than it is."

Erik Asphaug, a planetary scientist at the University of California in
Santa Cruz, US, agrees. "This asteroidal Botox calls into question the
habit of dating asteroid surfaces through their cratering record," he
writes in an accompanying article.

Journal reference: Nature (vol 436, p 335, 364)
 
Received on Wed 20 Jul 2005 06:38:32 PM PDT