[meteorite-list] Solar Wind Tans Young Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 29 Apr 2009 15:56:17 -0700 (PDT)
Message-ID: <200904292256.PAA06225_at_zagami.jpl.nasa.gov>

http://www.eso.org/public/outreach/press-rel/pr-2009/pr-16-09.html

22 April 2009
For Immediate Release
ESO

Solar wind tans young asteroids

A new study published in Nature this week reveals that asteroid surfaces
age and redden much faster than previously thought - in less than a
million years, the blink of an eye for an asteroid. This study has
finally confirmed that the solar wind is the most likely cause of very
rapid space weathering in asteroids. This fundamental result will help
astronomers relate the appearance of an asteroid to its actual history
and identify any after effects of a catastrophic impact with another
asteroid.

"Asteroids seem to get 'sun tan' very quickly," says lead author
Pierre Vernazza. "But not, as for people, from an overdose of the Sun's
ultraviolet radiation, but from the effects of its powerful wind."

It has long been known that asteroid surfaces alter in appearance with
time - the observed asteroids are much redder than the interior of
meteorites found on Earth [1] - but the actual processes of this
"space weathering" and the timescales involved were controversial.

Thanks to observations of different families of asteroids [2] using
ESO's New Technology Telescope at La Silla and the Very Large Telescope
at Paranal, as well as telescopes in Spain and Hawaii, Vernazza's team
have now solved the puzzle.

When two asteroids collide, they create a family of fragments with
"fresh" surfaces. The astronomers found that these newly exposed
surfaces are quickly altered and change colour in less than a million
years - a very short time compared to the age of the Solar System.

"The charged, fast moving particles in the solar wind damage the
asteroid's surface at an amazing rate [3]", says Vernazza. Unlike
human skin, which is damaged and aged by repeated overexposure to
sunlight, it is, perhaps rather surprisingly, the first moments of
exposure (on the timescale considered) - the first million years - that
causes most of the aging in asteroids.

By studying different families of asteroids, the team has also shown
that an asteroid's surface composition is an important factor in how red
its surface can become. After the first million years, the surface
"tans" much more slowly. At that stage, the colour depends more on
composition than on age. Moreover, the observations reveal that
collisions cannot be the main mechanism behind the high proportion of
"fresh" surfaces seen among near-Earth asteroids. Instead, these
"fresh-looking" surfaces may be the results of planetary encounters,
where the tug of a planet has "shaken" the asteroid, exposing unaltered
material.

Thanks to these results, astronomers will now be able to understand
better how the surface of an asteroid - which often is the only thing we
can observe - reflects its history.
      
More information

This result was presented in a paper published this week in the journal
Nature, "Solar wind as the origin of rapid reddening of asteroid
surfaces", by P. Vernazza et al. The team is composed of Pierre Vernazza
(ESA), Richard Binzel (MIT, Cambridge, USA), Alessandro Rossi (ISTI-CNR,
Pisa, Italy), Marcello Fulchignoni (Paris Observatory, France), and
Mirel Birlan (IMCCE, CNRS-8028, Paris Observatory, France). A PDF file
can be downloaded here <nature07956_proof1.pdf>.

Notes

[1] Meteorites are small fragments of asteroids that fall on Earth.
While a meteorite enters the Earth's atmosphere its surface can melt and
be partially charred by the intense heat. Nevertheless, the meteorite
interior remains unaffected, and can be studied in a laboratory,
providing a wealth of information on the nature and composition of
asteroids.

[2] An asteroid family is a group of asteroids that are on similar
orbits around the Sun. The members of a given family are believed to be
the fragments of a larger asteroid that was destroyed during a collision.

[3] The surface of an asteroid is affected by the highly energetic
particles forming the solar wind. These particles partially destroy the
molecules and crystals on the surface, re-arranging them in other
combinations. Over time, these changes give formation of a thin crust or
irradiated material with distinct colours and properties.

Contacts

Pierre Vernazza
European Space Agency, Noordwijk, Netherlands
Tel: +31 71 565 3154
E-mail: pierre.vernazza (at) esa.int
Received on Wed 29 Apr 2009 06:56:17 PM PDT