Magazine article Science News

Impacts Probably Stifled Early Life: Giant Asteroids Hit Earth until about 4.3 Billion Years Ago

Magazine article Science News

Impacts Probably Stifled Early Life: Giant Asteroids Hit Earth until about 4.3 Billion Years Ago

Article excerpt

Space rocks larger in diameter than Utah bombarded the early Earth, probably repeatedly eradicating any 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 could have taken hold on Earth.

From Earth's origin some 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 birth regularly slammed into Earth, boiling away the early ocean and coating the planet with molten rock (SN: 5/19/12, p. 22).

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 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 that it could have extended the planet's entire 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. The moon lacks the recycling action of plate tectonics, so it still shows scars from early asteroid impacts. Scientists can determine the ages of ancient lunar impacts using a method called crater counting. As a crater ages, falling meteorites gradually blemish the impact site. Using the ages of moon rocks collected from lunar craters during the Apollo missions as calibration, scientists can approximate the age of large lunar craters by counting the number of smaller, fresher craters within them. …

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