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An early cosmic wallop for life on Earth?

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8:04pm, October 18, 2004
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Life on Earth either started with a bang or it suffered an unusually rocky childhood, a new study suggests.

Analyzing lunar meteorites, researchers have found new evidence that a swarm of debris bombarded the moon some 3.9 billion years ago. That's about the same time that life may have formed on Earth. If our planet suffered a similarly catastrophic bombardment, as scientists claim, it could have dramatically affected the evolution of life here.

"This makes it more real that there could have been substantial editing of the biosphere and organisms...3.9 billion years ago," comments astrobiologist David J. Des Marais of NASA's Ames Research Center at Moffett Field, Calif.

Indications that the moon underwent a heavy bombardment date to the early 1970s. That's when researchers analyzed age-revealing isotopes in moon rocks collected by spacecraft. The results of that work suggested that a huge number of asteroid or comet fragments had pummeled the moon's crust roughly 3.9 billion years ago.

To many scientists, the data indicated that the moon's entire surface had been battered. But critics noted that the rocks were all collected from the equator on the moon's near side. Perhaps the catastrophic event was limited to that region, they argued.

To settle the argument, Barbara A. Cohen, now at the University of Tennessee in Knoxville, and Timothy D. Swindle and David A. Kring of the University of Arizona in Tucson analyzed four meteorites of lunar material. These differ in composition or texture from each other and from the rocks collected by the spacecraft, and so they probably constitute a more global sampling of the moon.

Within each meteorite, the team zeroed in on glassy material—so-called impact melts—formed by the rapid melting and cooling of rock that occurs during and after an impact. The impact melts range in age from 2.76 billion to 3.92 billion years, the team reports in the Dec. 1 Science.

Finding nothing older "supports the concept of an early, intense period of bombardment in the Earth-moon system at about 3.9 billion years," the researchers say. The most violent era may have lasted half a billion years, they propose.

Given Earth's proximity to the moon, it's "inescapable" that our planet also took a beating, says Des Marais. Indeed, because of its stronger gravity, Earth may have been pummeled by 10 to 20 times as much debris, Cohen notes.

The largest of the ancient space debris would have boiled Earth's oceans, scorched the crust, and filled the atmosphere with vaporized rock, Cohen's team notes. Because Earth's surface has since been torn apart by volcanic activity and the slip-sliding of tectonic plates, signs of this cataclysm would be hard to spot today. The moon, in comparison, has been geologically dead for billions of years, and its rocks represent a pristine relic of its early history.

Traces of carbon in some of the oldest rocks on Earth suggest life on our planet dates to 3.85 billion years ago (SN: 11/9/96, p. 292: http://www.sciencenews.org/pages/sn_arch/11_9_96/fob1.htm). That puts primitive organisms smack in the middle of a cosmic bombardment.

A mere coincidence or not, the timing suggests three possibilities for the origin and evolution of life, says Christopher P. McKay of Ames. The bombardment may have delivered heat and organic compounds to Earth, sparking the rapid formation of primitive life. Alternatively, the pummeling could have wiped out existing life on Earth's surface but enabled other organisms to flourish—particularly those at the ocean bottom near heat-generating vents (SN: 1/9/99, p. 24: http://www.sciencenews.org/pages/sn_arc99/1_9_99/bob1.htm).

Or life may have been delivered to Earth by the very projectiles that smashed up the surface. "In this case, life may have originated on Mars or come from outside the solar system," McKay notes.

Although different theorists may favor one scenario over another, at present "we have no real basis for choosing between [these ideas]," he says.

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