Magazine article Science News

Space Rocks' Demo Job: Asteroids, Not Comets, Pummeled Early Earth

Magazine article Science News

Space Rocks' Demo Job: Asteroids, Not Comets, Pummeled Early Earth

Article excerpt

It's no secret that the inner solar system took a beating from swarms of speeding objects soon after the planets and moons formed. What's been uncertain is the nature of those extraterrestrial bullets. A new analysis of meteorites suggests that it was asteroids rather than comets that rained hell on Earth about 3.9 billion years ago.

Evidence that the moon suffered several massive strikes soon after it formed comes, in part, from a recent study of space rocks that fell to Earth after being sputtered from the moon by impacts (SN: 12/2/00, p. 357). That information backs up evidence of such impacts culled from moon rocks obtained during the Apollo missions 3 decades ago, says David A. Kring, a planetary geologist at the University of Arizona in Tucson. Similar analyses of meteorites traced back to other celestial bodies show there was much bumping and scraping in the asteroid belt--between the orbits of Mars and Jupiter--about 3.9 billion years ago as well.

During a period that lasted between 20 million and 200 million years, space rocks blasted more than 1,700 lunar craters that were at least 20 kilometers wide, says Kring. He and Barbara A. Cohen, a planetary scientist now at the University of Hawaii at Manoa, estimate that more than 22,000 similar objects slammed into Earth during that era. Other planets took a beating, too: Mars probably suffered more than 6,400 impacts, and Mercury took about half that many hits. Kring and Cohen report their analysis in the Feb. 28 Journal of Geophysical Research-Planets.

When space debris strikes planets and moons, minerals melt and fuse into so-called impact melts that retain chemical fingerprints of the impactor, says Kring. …

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