October 5, 2009
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, which on October 5 dedicated a special session to this asteroid, "From the Heavens to the Earth: The 2008 TC3 / Almahata Sitta Ureilite Fall." The small asteroid, called 2008 TC3, was the first to have been spotted in space before hitting Earth.Editor's note:
magazine Associate Editor Liz Kruesi's reports from DPS09
"The asteroid now has a face," said SETI Institute astronomer Peter Jenniskens, chair of the special session. Last December, Jenniskens and Sudan astronomer Muawia Shaddad went to the crash site and recovered 300 fragments in the Nubian Desert (called meteorites) by bringing students of the University of Khartoum to the area for a careful sweep of the gravely desert. They found many different looking meteorites close to, but a little south, of the calculated impact trajectory.
"We have a gigantic jigsaw puzzle on our hands, from which we try to create a picture of the asteroid and its origins," Jenniskens said. "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 the Clay Center Observatory in Brookline, Massachusetts, tracked the asteroid with a telescope and captured the flicker of light during a 2-hour period just before impact. Scheirich combined these observations with others to work out the shape and orientation of the asteroid.
Other forensic evidence presented at the meeting is based on analysis of the recovered meteorites. These are of an unusual "polymict ureilite" type. Jason S. Herrin of NASA's Johnson Space Center in Houston confirms that the meteorites still carry traces of being heated to between 2100° and 2400° Fahrenheit (1150° and 1300° Celsius), before rapidly cooling down at a rate of tens of degrees Celsius 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 that the pore linings appear to be vapor phase deposits. According to Zolensky, "Almahata Sitta 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 among the most cooked of all known meteorites. Scientists detected carbon crystals of graphite and nanodiamonds. 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's Goddard Space Flight Center in Greenbelt, Maryland, now report that even some amino acids have survived.
To find more puzzle pieces, Jenniskens and Shaddad plan to revisit the scene of the crash in the Nubian Desert, after sharing notes during a 2008 TC3 Workshop at the University of Khartoum December 6-7.