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The Honolulu Advertiser
Posted on: Sunday, March 24, 2002

Asteroid theory explores impact on Earth life

By Jan TenBruggencate
Advertiser Science Writer

Talk about a big bang —Êa UH researcher offers evidence indicating that a swarm of asteroids slammed into Earth, the moon and Mars some 3.9 billion years ago.

Such an event could have wiped out life on Earth — or helped generate it, according to University of Hawai'i researcher Barbara Cohen.

Cohen and co-researcher David Kring, of the University of Arizona, put together diverse bits of information to develop a compelling theory that could help to explain why the oldest rocks on Earth, the moon and Mars are all the same age — hundreds of years younger than the main bodies themselves.

Their concept is that something jerked a large number of rocks out of the asteroid belt between Mars and Jupiter, and sent them careening through the inner solar system. Perhaps it was a burp in the orbit of Jupiter, changing the gravitation field of the region; perhaps it was the formation of Uranus and Neptune out of gas clouds; or perhaps it was any of a suggested series of other events.

"We're looking for the initiating event, and whether it could happen again," said Cohen, a planetary scientist with the Hawai'i Institute of Geophysics and Planetology. That's next in the series of investigations that led to a paper on the asteroid impacts, "Cataclysmic bombardment throughout the inner solar system 3.9-4.0 Ga," in the Journal of Geophysical Research (Planets).

Their initial take on the likelihood of recurrence is that this was an exceedingly rare event.

"We want to know what happened to the Earth because we want to know what could happen in the future," Cohen said. The evidence suggests that the asteroid catastrophe 3.9 billion years ago was an exceedingly rare event that may have been associated with the formation of the solar system, and therefore highly unlikely to happen again.

The scientists were intrigued by the finding that meteorites known to have come from pieces of the moon had the same age as a meteorite known to be from Mars, and the same age as the oldest known rocks on Earth. Because all three bodies are believed to be older than that, something terrible must have happened 3.9 billion years ago to cause the effects on all three of them.

Cohen and Kring feel that a disruption in the asteroid belt dislodged a cluster of stony bodies that "came winging in" toward the inner planets. There were so many of them, they crashed into multiple planets and probably into the sun as well. Cohen said no one has been able to test the theory, but she believes that Venus and Mercury were probably also hit.

"Eighty percent of the moon was resurfaced by remelting or settled dust," Cohen said — an indication of the severity of the impact.

An intriguing question is what impact this might have had on life. There are some scientists who believe they see the chemical signature of single-celled life already in place when the impacts took place. But fossils don't appear for nearly another half-billion years—about 3.5 billion years ago.

"One possibility is that life already existed, and it survived.

Another is that (the asteroid impacts) provided the Earth with heat sources and organic elements, and helped start life," she said. A third is that life existed before but was wiped out by the impacts, so it had to start again.

"The impactors likely delivered biogenic materials, although it is not clear if these were essential for life's origins," the authors said in their paper.

The rogue asteroids slammed into a planet or moon perhaps every 10,000 years or so — a heartbeat in geologic time. Most were probably sucked into the sun by its immense gravity. Earth, because its gravity is stronger than the moon's, probably got more impacts than the moon.

The craters that are the signatures of those impacts are still visible on the moon, but are gone from Earth. That's because the moon has no weather to modify the landscape through erosion and lacks Earth's plate tectonics.

On our planet, vast plates cover the surface like the pattern on a soccer ball. The plates are in constant movement, some sliding under others, with the rock remelting as it is driven into the depths. The process destroys and re-forms the land. Only one part of the surface has remained exposed and represents the oldest rocks on Earth. It's in Canada. On the Canadian shield, Cohen said, is rock 3.9 billion years old. It shows evidence of having formed earlier, then being transformed by an event in the time frame of the presumed asteroid impacts.

Like the moon's, the surface of much of this planet would have been re-formed by new lava flows and by the settling of the vast clouds of debris that would have been tossed into the atmosphere with each new impact, she said.