Earth as seen by astronauts on Apollo 17. Photo: NASA

Even during an extraordinarily violent era in Earth's early history, when our young planet was being whacked by asteroids and comets so frequently that scientists refer to it as "Late Heavy Bombardment," conditions most of the time at the Earth's surface were quite hospitable for the microbes that lived here, according to research presented recently at the annual meeting of the American Geophysical Union in San Francisco. The work has also been accepted by the Journal of Geophysical Research.

By making fine measurements of a kind of stardust carried to Earth by asteroids and comets some four billion years ago, a team of scientists led by University of Rochester geochemist Ariel Anbar has determined that only rarely -- perhaps once every 30 to 100 million years -- was the bombardment from the skies so severe that microbes would have had a difficult time surviving at the Earth's surface. Even in such instances, scientists say that 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.

Many scientists have assumed that no life could have survived the Late Heavy Bombardment period, but the team's research indicates that most of the time, conditions on Earth during this time weren't so bad. 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 the Earth's surface wasn't habitable," says Anbar, assistant professor of earth and environmental sciences and of chemistry. "It was a violent period in Earth history. Sure, every 30 million years or so, life would have been really challenging. 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. We're continually finding that life is incredibly hardy and adaptable, and it's plausible that bacteria could have survived these high-impact environments."

Anbar's team included Rochester graduate student Gail Arnold, who made sensitive measurements of the metal iridium, which is rare on Earth but abundant in space-borne objects; Kevin Zahnle of the NASA Ames Research Center in Palo Alto, Calif., who developed a computer model to simulate Earth under bombardment; and Steve Mojzsis of the University of California at Los Angeles.

Mojzsis had previously discovered evidence of ancient microbial life in the oldest sediments ever found on Earth -- 3.85 billion years old -- from Akilia Island in southern Greenland. He and Anbar decided to study the same sediments for signs of bombardment. In several rocks from Akilia Island Arnold measured the level of iridium: If asteroids and comets struck the Earth continually, the sediments should have shown high levels of iridium. Instead, the rocks contained little iridium, indicating a break in the bombardment.

"We looked in these sediments fully expecting to find high levels of iridium. If Earth was getting pummeled, we thought we should see it. But these rocks were quite clean of iridium," says Anbar.

Since other evidence that Earth was heavily bombarded during this time is very strong, the finding initially puzzled the researchers. The team decided to carefully estimate the rate of bombardment at that time. Their estimates showed that during most of the bombardment period, large asteroid or comet impacts would have been rare enough that their traces wouldn't be seen in sediments like the ones they studied. They estimate 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. They say that much larger objects, those capable of killing off most microbial life on Earth, slammed into Earth much less frequently, probably only once every 30 to 100 million years. The most severe bombardment happened in very distinct episodes, with conditions in between quite livable for microbes.

"Sure, conditions at this time were nastier than today, with objects constantly hitting the Earth, but it was really quite rare for an object big enough to vaporize the upper part of the ocean and do a tremendous amount of damage to hit the Earth," says Anbar, whose work is funded by NASA's Astrobiology Institute and the National Science Foundation. "It's been estimated that it would only take a few tens of thousands of years for the planet to recover from such an event. So it would be just very short periods of time millions of years apart when it would have been difficult to maintain life."