The
asteroid that crashed in northern Sudan last year was shaped like a loaf of
walnut-raisin bread, according to astronomer Peter Scheirich and colleagues at
Ondrejov Observatory and Charles University in the Czech Republic.
Scheirich
reported his findings at the Division for
Planetary Sciences of the American Astronomical Society meeting in Puerto Rico on Oct. 5, 2009 in a special
session dedicated to this asteroid one year after the fall. The small
asteroid, designated "2008
TC3", was the first to have been spotted in space before
hitting Earth.
Last
December I traveled to Sudan. With the help of Sudan astronomer Muawia Shaddad,
and 45 students of the University of
Khartoum, we went to the crash site in the Nubian Desert and recovered 300
fragments (called meteorites) by carefully sweeping the gravely desert. We
found many different looking meteorites, slightly south of the calculated
impact trajectory.
We now have
a gigantic jigsaw puzzle on our hands, from which we try to create a picture of
the asteroid and its origins. Now, Scheirich and colleagues have provided us
with a composite sketch of the culprit, cleverly using the eyewitness accounts
of astronomers that saw the asteroid sneak up
on us.
An
irregular shape and rapid tumbling caused Asteroid 2008 TC3 to flicker when it
reflected sunlight on approach to Earth. Astronomers Marek Kozubal and Ron
Dantowitz of Clay Center Observatory in Brookline, Massachusetts, tracked the
asteroid with a telescope and captured the flicker of light
during a two-hour period just before impact. Scheirich combined these
observations with others to work out the shape and orientation of the asteroid.
Other
forensic evidence, which was presented during the special session at the
AAS/DPS meeting, chaired by Jason S. Herrin of NASA Johnson Space Center and me, is based on analysis of the recovered meteorites. These are of an unusual
"polymict ureilite" type. Herrin confirms that the meteorites still
carry traces of being heated to 1150-1300 ºC, before rapidly cooling down at a
rate of tens of °C per hour, during which carbon in the asteroid turned part of
the olivine mineral iron into metallic iron. Hence, Asteroid 2008 TC3 is the
remains of a minor planet that endured massive collisions billions of years
ago, melting some of the minerals, but not all, before a final collision
shattered the planet into
asteroids.
Mike
Zolensky of NASA's Johnson Space Center first pointed out that, as far as
ureilites are concerned, this meteorite is unusually rich in pores, with pore
walls coated by crystals of the mineral olivine. He now reports from X-ray
tomography work with Jon Friedrich of Fordham University in New York, that
those pores appear to outline grains that have been incompletely welded
together and the pore linings appear to be vapor phase deposits. According to
Zolensky, "2008 TC3 may represent an agglomeration of coarse- to
fine-grained incompletely reduced pellets formed during impact, and
subsequently welded together at high temperature."
The carbon
in the recovered meteorites is one of the most cooked of all known meteorites.
Carbon crystals of graphite and nano-diamonds have been detected. Still, it
turns out that some of the organic matter in the original material survived the
heating. Amy Morrow, Hassan Sabbah, and Richard Zare of Stanford University have found polycyclic aromatic hydrocarbons in high abundances. Amazingly, Michael
Callahan and colleagues of NASA Goddard Space Flight Center now report that
even some amino acids have survived.
To find
more puzzle pieces, Muawia Shaddad and I plan to re-visit the scene of the
crash in the Nubian Desert on the one-year anniversary of the find this
December 6-7, after sharing notes during a 2008 TC3 Workshop at the University of Khartoum.
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