A battering. Outer planets may have rattled the asteroid belt, showering the moon with impactors.

Where Did Those Craters Come From?

Dick writes about Earth and planetary science for Science magazine.

As scientists accumulate evidence that something battered the inner planets 3.9 billion years ago, some say they are homing in on what did the pummeling. New findings indicate that the massive cratering seen on Earth and its neighbors originated in the asteroid belt. That in turn would point to a reshuffling of the outer planets as the ultimate cause of a cataclysmic bombardment that may have short-circuited the rise of life on Earth.

The most obvious clues to the source of the so-called late heavy bombardment are the many craters left behind and the hulking impactors responsible, as derived from crater size. So Robert Strom, a professor emeritus at the University of Arizona, Tucson, and co-workers compiled Strom's published and unpublished crater counts from the most pockmarked planetary surfaces, such as the highlands of the moon. They did the same for younger, more lightly cratered areas, such as certain volcanic plains on Mars.

An unusual preponderance of the objects that hit the younger surfaces were small, they found, a size distribution that matches that of the near-Earth asteroids that have drifted in from the main belt more recently. By contrast, a greater proportion of large impactors had cratered older terrains. That indicated that a different mechanism must have driven the ancient bombardment--one that did not discriminate between large and small asteroids.

The group argues that the asteroids must have pummeled the inner solar system after a rearrangement of the outer planets. Perhaps Jupiter and Saturn teamed up to scatter asteroids gravitationally (Science, 3 December 2004, p. 1676), or Neptune and Uranus formed long after the rest of the planets, the team proposes in the 16 September Science. Cratering specialist William Bottke of the Southwest Research Institute in Boulder, Colorado, suspects that Strom and his colleagues are on to something, but says the case remains open.

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