Asteroid Impact Helps Trace Meteorite Origins (Forwarded)
Andrew Yee
Washington, D.C.
Contact:
Douglas Rumble, 202-478-8990
March 25, 2009
Asteroid Impact Helps Trace Meteorite Origins
The car-sized asteroid that exploded above the Nubian Desert last October
was small compared to the dinosaur-killing, civilization-ending objects that
still orbit the sun. But that didn't stop it from having a huge impact among
scientists. This was the first instance of an asteroid spotted in space
before falling to Earth. Researchers rushed to collect the resulting
meteorite debris, and a new paper in Nature reports on this first-ever
opportunity to calibrate telescopic observations of a known asteroid with
laboratory analyses of its fragments.
"Any number of meteorites have been observed as fireballs and smoking meteor
trails as they come through the atmosphere," says Douglas Rumble of the
Carnegie Institution's Geophysical Laboratory, a co-author of the paper.
"It's been happening for years. But to actually see this object before it
gets to the Earth's atmosphere and then to follow it in -- that's the unique
thing."
The chemical compositions of asteroids can be studied from Earth by
analyzing the spectra of sunlight reflected from their surfaces. This
provides enough information to divide asteroids into broad categories, but
does not yield detailed information on their compositions. On the other
hand, meteorites recovered on Earth can be analyzed directly for chemical
composition, but researchers generally have no direct information on what
type of asteroid they came from.
The asteroid, known as 2008 TC3, was first sighted October 6, 2008, by
telescopes of the automated Catalina Sky Survey near Tucson Arizona.
Numerous observatories followed its trajectory and took spectrographic
measurements before it disappeared into the Earth's shadow the following
day. A recovery team led by Peter Jenniskens of the SETI Institute in
California and Muawia Shaddad of the University of Khartoum then searched
for meteorites along the projected approach path in northern Sudan. They
recovered 47 fragments, one of which was selected for preliminary analysis
by laboratories, including the Carnegie Institution's Geophysical
Laboratory.
"This asteroid was made of a particularly fragile material that caused it to
explode at a high 37 kilometer altitude, before it was significantly slowed
down, so that the few surviving fragments scattered over a large area,"
explains Jenniskens, the lead author of the Nature paper. "The recovered
meteorites were unlike anything in our meteorite collections up to that
point."
Carnegie's Andrew Steele studied the meteorite's carbon content, which
showed signs that at some point in its past the meteorite had been subjected
to very high temperatures. "Without a doubt, of all the meteorites that
we've ever studied, the carbon in this one has been cooked to the greatest
extent," says Steele. "Very cooked, graphite-like carbon is the main
constituent of the carbon in this meteorite." Another form of carbon Steele
found in the meteorite, nanodiamonds, may give clues as to whether the
heating was caused by impacts on the parent asteroid, or by some other
process.
Oxygen isotopes in the meteorite give other information about its parent
body. Each source of meteorites in the solar system, including planets such
as Mars, has a distinctive signature of the three isotopes 16O, 17O, and
18O. This signature can be recognized even when other variables, such as
chemical composition or rock type, differ. "Oxygen isotopes represent the
single most decisive measurement in determining the parental or family
groupings of meteorites," says Rumble who performed the analysis.
According to Rumble's analysis, 2008 TC3 falls into a category of very rare
meteorites called ureilites, all of which may have originally come from the
same parent body. "Where that is, we don't know," says Rumble. But because
astronomers took spectral measurements of 2008 TC3 before it hit the Earth,
and can compare those measurements with the laboratory analyses, scientists
will be better able to recognize ureilite asteroids in space. One known
asteroid with a similar spectrum, the 2.6 kilometer-sized asteroid 1998 KU2,
has already been identified by researchers as a possible source for 2008
TC3.
Rumble's work was funded by NASA Cosmochemistry grant NNX07AI48G. Steele was
supported by NASA's Sample Return Laboratory Instruments and Data Analysis
Program (SRLIDAP) , NASA's Astrobiology Science and Technology for Exploring
Planets (ASTEP) program , and the NASA Astrobiology Institute (NAI).
IMAGE CAPTION:
[http://www.ciw.edu/sites/www.ciw.edu/files/news/PRRumbleSteeleAsteroid-ImageAMedforWeb.jpg
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This fragment of Asteroid 2008 TC3 provided scientists with the first-ever
opportunity to calibrate telescopic observations of a known asteroid with
laboratory analyses.