Space rocks' demo job: asteroids, not comets, pummeled early Earth.

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. He and Cohen have found that the proportions of metallic elements in lunar impact melts and in meteorites blasted from the moon's surface are similar to those found in meteorites that have fallen directly to Earth. If comets had struck the moon, the concentrations of metals would have been much lower.

"This [research] makes a good case that most of the objects that struck the moon 3.9 billion years ago were asteroids," says Randy L. Korotev, a lunar geochemist at Washington University in St. Louis.

By extrapolation, the same types of objects were smacking Earth directly about once every millennium. Each of these impacts would have lofted enough dust and water vapor into the atmosphere to affect climate worldwide.

The impacts could have played a crucial role in the early development of life by creating hydrothermal systems in the shattered rock, says Kring. The first evidence for life on Earth is locked in rocks about 3.85 billion years old (SN: 11/9/96, p. 292). Because most older rocks were probably pulverized during the cataclysmic pummeling, it's not clear whether the resulting hydrothermal systems served as refuges for life that existed before the extraterrestrial blitz or as crucibles within which life began.

Most signs suggest that the bombardment of the inner solar system began suddenly 500 million to 600 million years after the formation of the solar system. However, it's possible that the intense barrage of Earth wiped out evidence of earlier impacts, says William K. Hartmann of the Planetary Science Institute in Tucson, Ariz. Only when the extraterrestrial pummeling began to subside, he suggests, could impact melts survive to tell their tale.