[meteorite-list] Astronomers Find Evidence That Asteroids Change Color as They Age

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 19 20:28:18 2004
Message-ID: <200405200028.RAA03942_at_zagami.jpl.nasa.gov>

http://www.ifa.hawaii.edu/info/press-releases/Jedicke_asteroids5-17-04.html

University of Hawaii Astronomer and Colleagues Find Evidence That
Asteroids Change Color as They Age

University of Hawaii
Press Release
May 19, 2004

In an article published today in the journal Nature, a team led by Robert
Jedicke of the University of Hawaii's Institute for Astronomy
provides convincing evidence that asteroids change color as they age.

David Nesvorny, a team member from the Southwest Research Institute
in Boulder, CO, used a variety of methods to estimate asteroid ages that
range from 6 million up to 3 billion years. Accurate color measurements for
over 100,000 asteroids were obtained by the Sloan Digital Sky Survey
(SDSS), and catalogued by team members Zeljko Ivezic from the
University of Washington and Mario Juric from Princeton University.

Robert Whiteley, a team member from the USAF Space and Missile
Systems Center in Los Angeles, points out that "the age-color
correlation we found explains a long-standing discrepancy between the
colors of the most numerous meteorites known as ordinary chondrites
(OC) and their presumed asteroid progenitors." Meteorites are
chips of asteroids and comets that have fallen to Earth's surface.

According to Jedicke, "If you were given a piece of rock from the
Grand Canyon, you might expect that it would be red, like the colorful
pictures in travel magazines. You'd be forgiven for questioning its
origin if the rock had a bluish color. But if you were then told that
the rocks turn from blue to Grand Canyon red because of the effects
of weather, then everything might make sense. Your gift is simply a
fresh piece of exposed rock, whereas the pictures you've seen show
weathered cliff faces millions of years old."

Nesvorny explains that this is similar to the situation experienced by
asteroid astronomers. "The meteorites are gifts of the solar system to
scientists on Earth - pieces of asteroids delivered to their own
backyard. The mystery is that the OC meteorites have a bluish color
relative to the reddish color of the asteroids from which they were
supposedly released." Jedicke asks, "How could they possibly be related?"

About thirty years ago, a "space weathering" effect was proposed to
explain the color change. Meteorites, whose surface is affected by
their fall through Earth's atmosphere, are usually studied in
laboratories by observing their freshly cut and exposed interiors.
Billions of years of exposure of the same material on the surface of
an asteroid to solar and cosmic radiation and the heating effect of
impacts of tiny asteroids might alter the surface color of asteroids
in exactly the manner required to match the color of asteroids.

Jedicke said that they found that "asteroids get more red with time
in exactly the right manner and at the right rate to explain the
mystery of the color difference between them and OC meteorites." He
added, "Even though we have found a link between the two types of
objects, we still don't know what causes space weathering."

Once these researchers refine their analysis by obtaining more colors
of the youngest-known asteroid surfaces, it will be possible to
determine the age of any asteroid from its surface color. They are
currently searching for a space weathering effect on other types of
asteroids in the solar system.

The Institute for Astronomy at the University of Hawaii conducts
research into galaxies, cosmology, stars, planets, and the sun. Its
faculty and staff are also involved in astronomy education, deep
space missions, and in the development and management of the
observatories on Haleakala and Mauna Kea. Refer to
http://www.ifa.hawaii.edu/ for more information about the Institute.

Funding for the creation and distribution of the SDSS Archive has
been provided by the Alfred P. Sloan Foundation, the Participating
Institutions, the National Aeronautics and Space Administration,
the National Science Foundation, the U.S. Department of Energy, the
Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web
site is

http://www.sdss.org/.

The SDSS is managed by the Astrophysical Research Consortium (ARC)
for the Participating Institutions. The Participating Institutions
are The University of Chicago, Fermilab, the Institute for Advanced
Study, the Japan Participation Group, The Johns Hopkins University,
Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy
(MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico
State University, University of Pittsburgh, Princeton University, the
United States Naval Observatory, and the University of Washington.

[Image]

This enhanced false color image of Ida with its small satellite Dactyl
to the right gives a vivid example of the effect of space weathering on
asteroids. Blue regions on the asteroid tend to be associated with
fresh young craters where subsurface material has been recently exposed
to space. There are also blue regions associated with ridges and
steep hills where surface material falls down during small
"asteroid-quakes" to expose fresh surfaces. Red regions on the surface
correspond to old craters and flat surfaces that have not been disturbed
in a very long time. By measuring the rate of space weathering on
asteroids Jedicke and his collaborators can now estimate the ages of
the various colored regions on the surface of asteroids like Ida.
Photo courtesy of NASA/JPL.

Note to Editors:

A high-resolution photograph of Dr. Jedicke is available at
http://www.ifa.hawaii.edu/info/press-releases/jedicke.cmyk.300dpi.tif

Nature's Web site is http://www.nature.com/.

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Received on Wed 19 May 2004 08:28:05 PM PDT