Early life probably fell victim to massive space rocks

Space rocks larger in diameter than Utah bombarded the early Earth, probably repeatedly eradicating emerging life on the planet’s surface. The last of these death rocks struck around 4.3 billion years ago, scientists estimate in the July 31 Nature, providing an upper limit to when life first took hold on Earth.

From Earth’s origin around 4.6 billion years ago until 3.8 billion years ago, the planet was such a hellish place that geologists call this eon the Hadean after Hades, the Greek god of the underworld. Debris left over from the solar system’s creation regularly slammed into Earth, boiling away the early ocean and coating the planet with molten rock.

But it was during this chaotic period that scientists think life arose on Earth.

“If life on Earth emerged before [a] final sterilizing impact, it may have been completely erased,” says planetary scientist and lead author Simone Marchi of the Southwest Research Institute in Boulder, Colo. “Life would have had to start all over again.”

Enough material struck Earth during the Hadean to extend the planet’s surface by the height of Mount Everest. These impacts shaped the emergence of plate tectonics (SN: 5/17/14, p. 14); however, few rocks older than around 3.8 billion years remain to provide a natural record of Earth’s early impact history.

To reconstruct the barrage of rocks that assaulted early Earth, Marchi and colleagues looked to the relatively stagnant moon. Because the moon lacks the recycling action of plate tectonics, it still shows scars from early asteroid impacts. Scientists determine the ages of ancient lunar asteroid impacts using a method called crater counting. As a crater ages, falling meteors gradually blemish the impact site. Using the ages of moon rocks collected from lunar craters during the Apollo missions as calibration, scientists approximate the age of large lunar craters by counting the number of smaller, fresher craters within it. Marchi’s team used such information about the moon to estimate the number, frequency and size of asteroids that impacted early Earth, assuming the two had a similar impact history.

The team then ran a computer simulation of Earth’s early bombardment and observed that asteroid impacts became smaller and less frequent with time. The team also found that every bit of Earth’s surface was at some point covered in a magma-oozing crater created by an impact.

The researchers found that between three and seven asteroids larger than 500 kilometers across probably struck Earth during this time, any of which could have  vaporized all water and destroyed any life on Earth’s surface. The last of these life-sterilizing impacts took place 4.27 billion years ago, the researchers estimate. The oldest evidence of life on Earth is 3.8 billion years old, although that evidence is disputed (SN: 5/19/12, p. 22).

Geochemist Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, Calif., believes that a better understanding of early asteroid bombardment will help researchers studying the origins of life. “The window of when life appeared on Earth is sometime after these really traumatic impacts,” he says. “Life could not have started prior to that and survived.”

Editor’s Note: This article was updated August 8, 2014 to correct the size of the asteroids that caused the life-sterilizing impacts that ended 4.27 billion years ago.