Asteroid Crash Sent "Meteorite Express" to Earth, Study Says

Stefan Lovgren
for National Geographic News
July 14, 2004
Small meteorites impact our planet several times a year, and—despite Hollywood's doomsday scenarios—life on Earth survives. Almost all of these meteorites are from asteroid collisions that occurred 3 to 60 million years ago.

A new study suggests, however, that the violent destruction of an asteroid can create a long-lasting rain of meteorites. The space rocks would be delivered to Earth by a "meteorite express," the study says.

Total travel time to our planet: a mere hundred thousand years.

The research, which is described in tomorrow's edition of the science journal Nature, was conducted by scientists who studied meteorites preserved in 480-million-year-old sediments in southern Sweden.

The scientists found that the meteorites contained gases produced by cosmic rays that suggest the meteorites' transfer to Earth occurred much faster than for more recent ones.

The data led researchers to conclude that a violent collision in our solar system's distant asteroid belt 500 million years ago produced meteorites that traveled to Earth in a relatively scant hundred thousand years.

"It's the first physical confirmation of unusually short transfer times [of meteorites] after a major asteroid collision," said Philipp Reza Heck, the report's lead author. Heck studies cosmochemistry at the Swiss Federal Institute of Technology in Zurich.

His research was partially funded by a grant from the National Geographic Society's Committee for Research and Exploration.

Diverted by Jupiter

The general term "meteor" comes from the Greek meteoron, meaning "phenomenon in the sky." Meteoroids, in particular, are fragments of asteroids produced by collisions.

Meteoroids in our solar system spend most of their time in the asteroid belt between Mars and Jupiter, unless they encounter a phenomenon known as orbital resonance and change course.

A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized by the Earth's atmosphere.

While most meteoroids are rocky in composition, some are almost pure metal.

Intense meteorite showers can only occur after a very large collision between asteroids. Such events have been rare in the last billion years of the solar system's history.

Scientists say that meteorite showers are also rare because Jupiter's titanic gravitational pull diverts much of the space debris before it has a chance to venture toward Earth.

The phenomenon known as orbital resonance plays a key role in the "meteorite express" described in the new study. Resonance occurs when two orbiting bodies exert a gravitational influence on each other.

As Heck explained: "If a [meteoroid] goes into an important orbital resonance with Jupiter, for example the 3:1 resonance, then while Jupiter goes once around the sun, the [meteoroid] circles the sun three times."

"So the small object experiences periodically a gravitational tug from Jupiter, and its orbit gets changed quite fast," he said.

The efficient transfer of asteroid material into the inner solar system—the basis of the so-called meteorite express—only happens when an asteroid collision occurs close to a resonance, however.

"Once the fragments are injected into the resonance, their orbital shape is changed rather quickly until they become Earth-crossing," Heck said. This is what has happened in the distant past, scientists believe.

L Chondrites

Heck and his colleagues studied meteorites that measured several centimeters in diameter. They were found in Thorsberg, a rock quarry in southern Sweden.

Around 500 million years ago the meteorites, of a type known as chondrites, fell into a shallow sea. There, they were covered with fine-grained marine sediments and well preserved.

A chemical analysis showed the meteorites were all of the same class, the so-called L chondrites class. Scientists believe that L chondrites come from a precursor to the Flora family of asteroids, a prominent group of asteroids circling the sun near the inner edge of the asteroid belt.

Most L chondrites experienced a major gas loss 500 million years ago, which is best explained by a violent collision in space.

"The transfer time [of these meteorites] to Earth is around a hundred thousand years, consistent with a long-lasting rain of meteorites after the destruction of an asteroid and the existence of at least one trajectory [in which] material was flung towards the inner solar system," Heck said.

The findings do not contradict present scientific thinking about the rate at which meteoroids can travel. But the short transfer times suggested by the new study are at the very low end of theoretical predictions.

"This analysis shows again what happened hundreds of millions of kilometers away can eventually make for a real bad day on Earth," said Bruce Betts, director of projects at the Planetary Society in Pasadena, California.

For more asteroid news, scroll down.

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