• Dick writes about Earth and planetary science for Science magazine.

Earth's Hellish Era Not So Bad for Life

20 May 2009 (All day)

Oleg Abramov

Not so hot. According to a new simulation, even at the end of Earth's last heavy battering by asteroid impacts, cool, habitable crust (blue) predominated over impact-heated crust (red).

Four billion years ago, asteroids and comets rained down on our planet with such ferocity that scientists have labeled the era the "Hadean"--literally, hell on Earth. Yet despite these infernal conditions, early life could have survived--and may even have thrived in the warm, wet spots left in the crust by impacters--according to a new study.

Earth was born into violence. Shortly after its formation 4.6 billion years ago, a Mars-size body slammed into it, throwing off enough debris to create our moon. As Earth was cooling from this event, debris from the solar system's own formation was shelling it on a regular basis. This hell storm eventually waned, but about 3.9 billion years ago it briefly surged again, a period geologists refer to as the late heavy bombardment.

The bombardment could have spelled the end for any nascent life on Earth. In fact, early calculations suggested that some of the largest impacters, ranging up to several hundred kilometers in diameter, could have vaporized the oceans and sterilized the planet down to a kilometer or so beneath the surface. But in recent years, analyses of chemical and isotopic records preserved in tiny rock crystals have shown that conditions as early as a couple of hundred million years after the moon's formation were relatively mild and possibly conducive to life.

The new simulations back this up. Geoscientists Oleg Abramov and Stephen Mojzsis of the University of Colorado, Boulder, calculated what happens to the heat from large impacters, which vaporize themselves and melt the crust where they hit. In the simulations, these impacts don't generate as much sustained heat as initially thought. Even during the late heavy bombardment, enough water could have percolated through the newly heated crust to quickly cool it, the researchers report tomorrow in Nature. They also assume, from studies of modern deep subsurface microbes, that life could have thrived in the rock down to a depth of 4 kilometers, putting it out of the reach of impact heating.

But Abramov and Mojzsis don't stop there. Not only would life have survived the late heavy bombardment, they say, but it could have arisen as early as 4.3 billion years ago--hundreds of millions of years earlier than the geologic record suggests. That may explain why the earliest ancestor of modern life is believed to be a heat-loving organism: Such a life form may have relied on the hot springs created by impacts long before the late heavy bombardment, the duo says.

"I think we're in basic agreement on" the habitability of the Hadean, says geophysicist Norman Sleep of Stanford University in Stanford, California, who has done similar calculations. But he still thinks that the largest impacter could have devastated most life on Earth and that it may have wiped out all but the heat-loving organisms.

Posted In: