2017-01-23 09:57:10
Trilobites: Ancient Bits of Rock Help Solve an Asteroid Mystery

09:57, January 23 234 0

An asteroid is a smaller-than-a-planet rock orbiting in the inner solar system. A meteor is the streak of light of a space rock plunging into the atmosphere, and a meteorite is the remnant of space rock that survives the fiery descent and comes to rest on the ground.

Thus, one might think that meteorites that fall on Earth ought to be just like the asteroids that pass through Earth’s neighborhood.

“That’s what everybody would have expected,” said Philipp R. Heck, the curator in charge for the meteorite and physical geology collections at the Field Museum of Natural History in Chicago.

Planetary scientists were surprised almost a decade ago when they discovered that the most plentiful types of meteorites they had collected and studied on Earth were actually not common in space.

In a paper published Monday by the journal Nature Astronomy, an international team of researchers led by Dr. Heck says it has uncovered part of the explanation. Mineralogical evidence in some meteorites had already pointed to a cataclysmic collision in the asteroid belt about 466 million years ago — long before dinosaurs, when multicellular animals were still fairly new. (Dr. Heck estimated that any skywatchers back then would have seen about 100 times as many shooting stars as crisscross the night sky today.)

Dr. Heck and his colleagues wanted to examine meteorites that landed on Earth before that, and for that they looked at ancient limestone from Russia — one-time sediment of a seafloor transformed to rock — that was about a million years older.

They dissolved almost 600 pounds of rock to extract 46 minuscule crystals containing the mineral chromite. The crystals persist unchanged for hundreds of millions of years, and from the mix of elements in the crystals, the scientists could tell that 41 of them came from space and what types of asteroids they were once part of.

The mix of meteorites was very different. Back then, more than a third of them belonged to a type known as primitive achondrites; today, less than half a percent of them are.

“It’s really inventive, their approach,” said Tasha L. Dunn, a geology professor at Colby College in Maine who studies meteorites and who was not involved in the research.

The findings fits in with the new understanding that a mix of meteorites is determined more by the history of collisions in the asteroid belt rather than by the mix of asteroids whose orbits around the sun are close to Earth’s.

“It opens new roads of research,” Pierre Vernazza, a researcher at the Laboratory of Astrophysics of Marseille, France, who first noted the discrepancy between asteroids and meteorites in 2008, said of the new paper.

For one, many of the achondrites then came from Vesta, a large asteroid that suffered a powerful impact about a billion years ago. But then there are other achondrites that did not. “In the case of the other achondrites, we don’t have any well-known source so far,” Dr. Vernazza said.