How life may have outlasted early blasts

updated 4:07 p.m. ET May 20, 2009

An asteroid bombardment of Earth nearly 4 billion years ago may have actually been a boon to early life on the planet, instead of wiping it out or preventing it from originating, a new study suggests.

Asteroids, comets and other impactors from space have been suggested as the causes behind some of the world's great mass extinctions, including the disappearance of the dinosaurs.

Impact evidence from lunar samples, meteorites and the pockmarked surfaces of the inner planets paints a picture of a violent environment in the solar system during the Hadean Eon 4.5 to 3.8 billion years ago, particularly through a cataclysmic event known as the Late Heavy Bombardment about 3.9 million years ago.

No such record exists for Earth because tectonic processes have folded ancient craters back into the interior, but scientists assume our planet took the same pummeling.

Although many believe the bombardment would have sterilized Earth, the new study uses a computer model to show it would have melted only a fraction of Earth's crust, and that microbes — if any existed in the first 500 million years or so of Earth's existence — could well have survived in subsurface habitats, insulated from the destruction.

"These new results push back the possible beginnings of life on Earth to well before the bombardment period 3.9 billion years ago," said CU-Boulder Research Associate Oleg Abramov. "It opens up the possibility that life emerged as far back as 4.4 billion years ago, about the time the first oceans are thought to have formed."

Modeling the bombardment
Because physical evidence of Earth's early bombardment has been erased by weathering and plate tectonics over the eons, Abramov and his colleagues used data from Apollo moon rocks, impact records from the moon, Mars and Mercury, and previous theoretical studies to build three-dimensional computer models that replicate the bombardment.

The researchers plugged in asteroid size, frequency and distribution estimates into their simulations to chart the damage to the Earth during the Late Heavy Bombardment, which is thought to have lasted for 20 million to 200 million years.

The 3-D models allowed the researchers to monitor temperatures beneath individual craters to assess heating and cooling of the crust following large impacts in order to evaluate habitability, said Abramov. The study, detailed in the May 21 issue of the journal Nature, indicated that less than 25 percent of Earth's crust would have melted during such a bombardment.

The team even cranked up the intensity of the asteroid barrage in their simulations by 10-fold — an event that could have vaporized Earth's oceans.

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