Study: Asteroids missed most microbes

In a study accepted by the Journal of Geophysical Research, Arial Anbar, assistant professor of earth and environmental sciences and of chemistry, argues that during the extraordinarily violent era of 4 billion years ago--when Earth was being pummeled so often by interstellar objects that scientists refer to the period as "Late Heavy Bombardment"--microbes found living conditions quite hospitable most of the time.

The work makes it feasible that life on Earth's surface existed earlier than scientists have considered.

"It's been the conventional wisdom that with all this bombardment, life should be very hard to maintain, and some scientists have argued that Earth's surface wasn't habitable," says Anbar. "But if microbes could find places to ride out the big impacts, there is no reason that they wouldn't be able to repopulate the surface and flourish."

Analyzing levels of the spaceborne element iridium in sediments around the world, Anbar and his team--Rochester graduate student Gail Arnold; Kevin Zahnle of the NASA Ames Research Center in Palo Alto, California; and Steve Mojzsis of the University of California at Los Angeles--determined that perhaps once every 30 to 100 million years--relatively rarely in geological terms--was the interstellar pounding so severe that microbes would have had difficulty time surviving.

Even under the worst deluge, scientists say, hardy bacteria and viruses could have found sheltered places, such as beneath the earth's crust, deep in the ocean near thermal vents, and other hiding spots, to ride out the storm.

Comparing their findings to other evidence of asteroid and comet impacts, Anbar's team estimates that every several hundred thousand years or so, an asteroid a few miles wide--the size of the object that likely wiped out the dinosaurs--would have plunked into Earth.

The most severe bombardment happened in very distinct episodes, with conditions in between quite livable for microbes, the team argues.

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