The shattered remains of a high-profile space rock are oddly low in organic materials, the raw ingredients for life. The discovery adds a slight wrinkle to the theory that early Earth was seeded with organics by meteorite impacts.
In April a van-sized meteor was seen streaking over northern California and Nevada in broad daylight. The fireball exploded with a sonic boom and sprayed the region with fragments. Videos, photographs and weather radar data allowed the meteor's trajectory to be reconstructed, and teams quickly mobilised to search for pieces in the foothills of the Sierra Nevada in northern California.
Researchers readily identified the meteorites as rare CM chondrites, thought to be one of the oldest types of rock in the universe. "Because the meteorites were discovered so freshly, for the first time we had a chance to study this type of meteorite in a pristine form," says Peter Jenniskens of the SETI Institute in Mountain View, California, who led the search effort and the subsequent study of the space rocks.
Jenniskens personally found a fragment in a parking lot, where it remained relatively free of soil contaminants. "That's the best you could hope for, other than landing in a freezer," says Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Battered past
CM chondrites make up only about 1 per cent of known meteorites. Most of them contain plenty of organic materials, including amino acids, the building blocks of life on Earth.
Jenniskens and colleagues found that the California fragments also have amino acids, including some not found naturally on Earth. But in three rocks collected before a heavy rainstorm, which bathed the other pieces in earthly contaminants, organics are less abundant by a factor of 1000 than in previously studied CM chondrites.
These three rocks could not have lost organics due to space "weathering": analysis of the meteorites' exposure to cosmic rays suggests the original meteor was flying through space for only about 50,000 years before hitting Earth.
Based on its trajectory and its relatively short flight time, Jenniskens thinks the meteor can be traced back to a family of asteroids dominated by 495 Eulalia, a group known as a possible source of CM chondrites. It is probably a piece that broke off during an impact, revealing the relatively pristine material inside.
So what happened to its organics? Jenniskens' team found that the meteorites are breccia – smaller rocks cemented together – which suggests that the asteroid from which they came took a series of beatings. Those impacts, or possibly other processes inside the asteroid, could have heated it enough to destroy most organic material.
Limited delivery
The result might have implications for the organics delivery theory, says Bill Bottke of the Southwest Research Institute in Boulder, Colorado.
"It shows that not all asteroids can deliver sufficient quantities. One of the disappointments is that, from a prebiotic organic chemistry perspective, it was very limited," says Bottke. "But this is an unusual case. Most [CM chondrites] are loaded with organic compounds."
Still, studying the space rocks will help us prepare future missions to asteroids such as OSIRIS-Rex, scheduled to take off for asteroid 1999 RQ36 in 2016 and bring a sample back in 2023.
"In some ways, we've had a sample, a very fresh one, come to us," says Bottke. "This is a test bed for the techniques we'll use in that mission."
Journal reference: Science, DOI: 10.1126/science.1227163
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