THE Man in the Moon was sculpted by asteroids rather than comets, say planetary scientists. If true, life on Earth probably began in meteor impact craters.
Around 3.9 billion years ago, something bombarded the Moon. Researchers thought it was icy comets, flung in from the outskirts of the Solar System by the antics of the outer planets changing their orbits (麻豆传媒, 7 April 2001, p 26). Now David Kring at the University of Arizona in Tucson and Barbara Cohen at the University of Tennessee in Knoxville say they have definite evidence that meteorites from the asteroid belt were the guilty party.
Although Earth would also have been in the firing line, there are no surviving craters here from that bombardment. So Kring took another look at samples brought back by the Apollo missions from the craters that form the Man in the Moon. He and Cohen plotted the ratios of trace elements such as iridium, germanium and gold in the Apollo samples and compared them to the amounts in asteroids and comets.
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No one knows exactly what comets are made of, but spacecraft observations of the dust in their halos and tails enabled Kring to make a good guess. He says there鈥檚 no way the traces from the lunar craters came from comets. Instead they match the composition of meteorites found on Earth.
But the man who proposed the comet hypothesis, Hal Levison from the Southwest Research Institute in Boulder, Colorado, isn鈥檛 convinced. 鈥淲e don鈥檛 know a lot about the composition of comets,鈥� says Levison. We鈥檒l have to wait another two years until the Stardust spacecraft completes its mission to land on a comet, he says, before we can be sure they didn鈥檛 cause the lunar craters.
But in this month鈥檚 Journal of Geophysical Research, Kring says that asteroids which have hit Earth since were themselves severely cratered 3.9 billion years ago. That fits the idea of a sudden event bombarding Earth so heavily that the crust totally melted, sweeping away traces of whatever hit it. That would explain why the oldest rocks on Earth are only 3.9 billion years old, 1.1 billion years younger than the planet itself.
The new results have fundamental implications for the origin of life on Earth. Analysis of DNA from ancient bacteria shows that life first evolved in hot water, like that found near deep-sea volcanic vents. But that could not have happened if asteroids were raining down, since such a heavy bombardment would have evaporated the oceans. Kring has carried out excavations on a much younger meteor crater on the Yucat谩n Peninsula in Mexico, and says that the impact created a system of hydrothermal vents when groundwater in the rocks was heated by energy from the collision.
If asteroids did bombard the Earth, the planet鈥檚 surface would have been covered with such vents, forming the perfect environment for life to emerge. The earliest evidence of life on Earth is 3.85 billion years ago, just a few millennia after the asteroid attacks. 鈥淚t was impact cratering, not volcanic areas, that dominated the geology of Earth at that time,鈥� says Kring.