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Asteroids and meteorites reveal family resemblance
September 11, 2006Asteroids and meteorites are supposed to be made of the same stuff - at least that's what earth science teachers have been telling their students for decades. But until re-cently, the data didn't quite fit the story. When researchers compared the near-infrared reflec-tance of asteroids (as measured from Earth) and meteorites (collected on Earth) they found enough differences to raise doubts about whether the asteroids really could be the source of Earth's meteorites.
A detailed new comparison of the near-Earth asteroid Itokawa with existing meteorite samples confirms that the process of space-weathering can explain the difference in reflectance pattern (spectrum) between asteroids and ordinary chondrites, the most common class of meteorites.
"They [chondritic meteorites] are so abundant, there have to be many, many asteroid sources, said Takahiro Hiroi, the paper's lead author, but we couldn't find any that matched so clearly, until now. These observations really let us see space weathering at work."
Over millions of years, the flow of high-energy ions and microscopic particles vaporizes the sur-face of asteroids, depositing a thin film that changes the asteroid's optical properties. Highly-weathered areas tend to appear dark and red. (The near infrared spectrum of such areas is shifted toward the red end of the spectrum.)
Takhiro Hiroi, a senior research associate at Brown University, visited several museums and col-lected dozens of samples of fresh, or newly fallen, meteorites. He rejected many samples because the oxidation caused by rain and air on the Earth's surface changes the rock's composition and interferes with the asteroid comparison. Together with other researchers from the Hayabusa mis-sion, Hiroi compared the near-infrared reflectance spectra of meteorite samples with spectra ob-served at specific locations on the asteroid.
One sample (from a meteorite dubbed Alta'ameem, for the area in Iraq where it fell) resulted in a near-identical match after correction for the changes that result from space weathering. Those changes include a reduction in mean optical path length - usually a sign of smaller grain size - and an increase in tiny iron particles known as nano-phase metallic iron or npFe0.
Hiroi was able to see the effects of space weathering by taking spectra from one light and one dark area on the asteroid's surface. Matching the observed spectra to that of the Alta'ameem me-teorite, he estimated that the highly-weathered site contained about 0.069 percent nanophase me-tallic iron and the less-weathered site contained about 0.031 percent. Because Alta'ameem is an LL chondrite, a class that represents only 10% of ordinary chondrite meteorites, Hiroi suggests that there must be many asteroids in near Earth orbit with compositions similar to the more-common L- and H-type meteorites.
Evidence of space weathering has been seen before on moons and larger asteroids, but such clear evidence is new for smaller asteroids, such as the 550-meter Itokawa. It had been thought that such bodies, with their smaller gravitational fields, would quickly be stripped of the weathered material. This new evidence shows that space weathered material does accumulate on small as-teroids, which probably are the source of most meteorites.
Brown University
Over millions of years, the flow of high-energy ions and microscopic particles vaporizes the sur-face of asteroids, depositing a thin film that changes the asteroid's optical properties. Highly-weathered areas tend to appear dark and red. (The near infrared spectrum of such areas is shifted toward the red end of the spectrum.)
Takhiro Hiroi, a senior research associate at Brown University, visited several museums and col-lected dozens of samples of fresh, or newly fallen, meteorites. He rejected many samples because the oxidation caused by rain and air on the Earth's surface changes the rock's composition and interferes with the asteroid comparison. Together with other researchers from the Hayabusa mis-sion, Hiroi compared the near-infrared reflectance spectra of meteorite samples with spectra ob-served at specific locations on the asteroid.
One sample (from a meteorite dubbed Alta'ameem, for the area in Iraq where it fell) resulted in a near-identical match after correction for the changes that result from space weathering. Those changes include a reduction in mean optical path length - usually a sign of smaller grain size - and an increase in tiny iron particles known as nano-phase metallic iron or npFe0.
Hiroi was able to see the effects of space weathering by taking spectra from one light and one dark area on the asteroid's surface. Matching the observed spectra to that of the Alta'ameem me-teorite, he estimated that the highly-weathered site contained about 0.069 percent nanophase me-tallic iron and the less-weathered site contained about 0.031 percent. Because Alta'ameem is an LL chondrite, a class that represents only 10% of ordinary chondrite meteorites, Hiroi suggests that there must be many asteroids in near Earth orbit with compositions similar to the more-common L- and H-type meteorites.
Evidence of space weathering has been seen before on moons and larger asteroids, but such clear evidence is new for smaller asteroids, such as the 550-meter Itokawa. It had been thought that such bodies, with their smaller gravitational fields, would quickly be stripped of the weathered material. This new evidence shows that space weathered material does accumulate on small as-teroids, which probably are the source of most meteorites.
Brown University
Meteorites Current Events and Meteorites News RSS'Ultra-primitive' particles found in comet dust
Dust samples collected by high-flying aircraft in the upper atmosphere have yielded an unexpectedly rich trove of relicts from the ancient cosmos, report scientists from the Carnegie Institution.
Meteorite from September 25 fireball event recovered and presented
When Tony Garchinski heard a loud crash just after 9 p.m. on Friday, September 25 he didn't think much of it. That is, until he awoke the next morning to find the windshield of his mom's Nissan Pathfinder with a huge crack in it. Making note of the 'unusual' rocks he later found on the car's hood, Garchinski chalked the incident up to vandalism and filed a police report.
Geologists point to outer space as source of the Earth's mineral riches
According to a new study by geologists at the University of Toronto and the University of Maryland, the wealth of some minerals that lie in the rock beneath the Earth's surface may be extraterrestrial in origin.
Western astronomers capture spectacular meteor footage and images
Astronomers from The University of Western Ontario have released footage of a meteor that was approximately 100 times brighter than a full moon. The meteor lit up the skies of southern Ontario two weeks ago and Western astronomers are now hoping to enlist the help of local residents in recovering one or more possible meteorites that may have crashed in the area of Grimsby, Ontario.
Nullarbor fireball cameras find rare meteorite
Using cameras which capture fireballs streaking across the night sky and sophisticated mathematics, a world-wide team of scientists have managed to find not only a tiny meteorite on the vast Nullarbor Plain, but also its orbit and the asteroid it came from.
Extraterrestrial platinum was 'stirred' into the Earth
Report author CSIRO Minerals Down Under Flagship researcher Dr Stephen Barnes said the study group collected a large body of data on the platinum content of lava flows called komatiites, which host some of the world's major nickel deposits.
Tiny diamonds on Santa Rosa Island give evidence of cosmic impact
Nanosized diamonds found just a few meters below the surface of Santa Rosa Island off the coast of Santa Barbara provide strong evidence of a cosmic impact event in North America approximately 12,900 years ago.
Apollo 11 moon rocks still crucial 40 years later, say WUSTL researchers
A lunar geochemist at Washington University in St. Louis says that there are still many answers to be gleaned from the moon rocks collected by the Apollo 11 astronauts on their historic moonwalk 40 years ago July 20.
New findings on the birth of the solar system
A team of international astrophysicists, including Dr Maria Lugaro from Monash University, has discovered a new explanation for the early composition of our solar system.
Primitive asteroids in the main asteroid belt may have formed far from the sun
Many of the objects found today in the asteroid belt located between the orbits of Mars and Jupiter may have formed in the outermost reaches of the solar system.
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