Meteorite
fragments of the first asteroid ever spotted in space before it slammed into
Earth's atmosphere last year were recovered by scientists from the deserts of Sudan.
These precious
pieces of space rock, described in a study detailed in the March 26 issue of
the journal Nature, could be an important key to classifying meteorites
and asteroids
and determining exactly how they formed.
The
asteroid was detected by the automated Catalina Sky Survey telescope at Mount Lemmon , Ariz., on Oct. 6, 2008. Just 19 hours after it was spotted, it collided
with Earth's atmosphere and exploded 23 miles (37 kilometers) above the
Nubian Desert of northern Sudan.
Because it
exploded so high over Earth's surface, no chunks of it were expected to have
made it to the ground. Witnesses in Sudan described seeing a fireball, which
ended abruptly.
But Peter
Jenniskens, a meteor astronomer with the SETI Institute's Carl Sagan Center, thought it would be possible to find some fragments of the bolide. Along with Muawia
Shaddad of the University of Khartoum and students and staff, Jenniskens
followed the asteroid's approach trajectory and found 47 meteorites strewn
across an 18-mile (29-km) stretch of the Nubian Desert.
"This
was an extraordinary opportunity, for the first time, to bring into the lab
actual pieces of an asteroid we had seen in space," Jenniskens said.
Classification
Astronomers
were able to detect the sunlight reflected off the car-sized asteroid (much
smaller than the one thought to have wiped
out the dinosaurs) while it was still hurtling through space. Looking at
the signature of light, or spectra of space rocks is the only way scientists
have had of dividing asteroids into broad categories based on the limited
information the technique gives on composition.
However,
layers of dust stuck to the surfaces of the asteroids can scatter light in
unpredictable ways and may not show what type of rock lies underneath. This can
also make it difficult to match up asteroids with meteorites found on Earth —
that's why this new discovery comes in so handy.
Both the
asteroid, dubbed 2008 TC3, and its meteoric fragments indicate that it could
belong to the so-called F-class asteroids.
"F-class
asteroids were long a mystery," said SETI planetary spectroscopist Janice
Bishop. "Astronomers have measured their unique spectral properties with
telescopes, but prior to 2008 TC3 there was no corresponding meteorite class,
no rocks we could look at in the lab."
Cooked
carbon
The
chemical makeup of the meteorite fragments, collectively known as
"Almahata Sitta," shows that they belong to a rare class of
meteorites called ureilites, which may all have come from the same original
parent body. Though what that parent body was, scientists do not know.
"The
recovered meteorites were unlike anything in our meteorite collections up to
that point," Jenniskens said.
The
meteorites are made of very dark, porous material that is highly fragile (which
explains why the bolide
exploded so high up in the atmosphere).
The carbon
content of the meteorites shows that at some point in the past, they were
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," said study team member Andrew
Steele of the Carnegie Institution in Washington, D.C. "Very cooked,
graphite-like carbon is the main constituent of the carbon in this
meteorite."
Steele also
found nanodiamonds in the meteorite, which could provide clues as to whether
heating was caused by impacts to the parent asteroid or by some other process.
Rosetta
Stone
Having
spectral and laboratory information on the meteorites and their parent asteroid
will help scientists better identify ureilite asteroids still circling in
space.
"2008
TC3 could serve as a Rosetta Stone, providing us with essential clues to the
processes that built Earth and its planetary siblings," said study team
member Rocco Mancinelli, also of SETI.
One known
asteroid with a similar spectrum, the 2.6-km wide 1998 KU2, has already been
identified as a possible source for the smaller asteroid 2008 TC3 that impacted
Earth.
With
efforts such as the Pan-STARRS
project sweeping the skies in search of other near-Earth asteroids,
Jenniskens expects that more events like 2008 TC3 will happen.
"I
look forward to getting the next call from the next person to spot one of these,"
he said. "I would love to travel to the impact area in time to see the
fireball in the sky, study its breakup and recover the pieces. If it's big
enough, we may well find other fragile materials not yet in our meteorite
collections."
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