By Sid Perkins
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: An early cosmic wallop for life on Earth?). 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.