December 20, 2012 | 10
Meteor astronomer Peter Jenniskens must move quickly to trap evidence of a fresh meteorite fall. In 2008, a small asteroid roughly three meters across struck Earth’s atmosphere over northern Sudan, producing a brilliant fireball in the sky. The asteroid’s orbit had been tracked before striking Earth, upping the chances that searchers would be able to locate pieces of the meteorite on the ground. So Jenniskens traveled to the Nubian Desert to recover fragments, as did dozens of searchers from the University of Khartoum.
In April of this year, he did not have to travel nearly so far to gather fresh meteoritic material. A bright fireball lit up the daytime sky April 22 over northern California’s gold country, a few hours’ drive from Jenniskens’s bases of operations in the San Francisco Bay Area: the SETI Institute in Mountain View and the NASA Ames Research Center in Moffett Field. The California bolide, like its African predecessor, made a well-documented entry—three Doppler radar stations picked up the track of the fireball, pointing the way to meteorite fragments on the ground. (The asteroid itself had not been spotted in space—such small objects usually escape astronomers’ notice.) Given the convenient location, the searchers were even able to marshal a slow-moving zeppelin to scan the area from the air, to look for impact scars on the terrain below caused by large meteorite fragments, but none were found.
Jenniskens and other searchers did ultimately locate 77 smaller pieces of the meteorite on the ground, according to a study he and his colleagues published in Science on December 21. The fragments total nearly one kilogram. But that is just a tiny fraction of the original mass of the Sutter’s Mill meteorite—named for the site of one of the finds, in Coloma, Calif. (Sutter’s Mill also happens to be the place where the California Gold Rush began in the mid-1800s.)
In the new study Jenniskens and his colleagues report that the asteroid that hit the atmosphere probably had a mass of some 40,000 kilograms, corresponding to a diameter of 2.5 to four meters. It streaked in from the east before detonating at an altitude of about 48 kilometers, releasing the energy equivalent of four kilotons of TNT in the process, or about one-quarter the yield of the nuclear weapon detonated over Hiroshima. The impact was seen and heard by many witnesses and was even picked up by two infrasonic (low-frequency sound wave) detector stations designed to monitor compliance with the Comprehensive Nuclear Test-Ban Treaty.
Given the violence of the reconstructed atmospheric entry, it’s remarkable that any fragments were recovered at all. Drawing on witnesses’ photos and videos of the fireball, the researchers have calculated that the parent object of the Sutter’s Mill meteorite entered the atmosphere at 28.6 kilometers per second (64,000 mph)—the highest such entry velocity recorded for recovered meteorites.
The recovered chunks revealed the Sutter’s Mill meteorite to be a rare variety called a carbonaceous chondrite. And in this case, rapid recovery proved critical—the researchers’ analysis found notable differences between samples recovered just two days after atmospheric entry and those found a few days later, after heavy rainfall. The rainwater reacted with sulfurous compounds in the meteorite, partly overwriting its original chemical makeup. The rapid alteration of meteorites by terrestrial water, the researchers conclude, “probably erases many vestiges of the internal and external process on the asteroid” and may mean that carbonaceous asteroids are more complex in composition than had been thought.
Photos courtesy NASA/Eric James
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