Far side crater count yields surprises.

Asteroids and comets have pummeled the inner solar system since its infancy, but most places haven't retained a good record of the bombardment. For example, many small projectiles burned up in Venus' thick atmosphere. Others gouged craters that were erased when widespread volcanic eruptions gave the planet a facelift several hundred million years ago. The cratering record on Earth isn't much better: Erosion and faulting obliterate traces of impacts.

In contrast, Earth's moon-geologically inactive and airless-provides a well-preserved record. Indeed, planetary scientists have used the distribution of craters on the moon to calibrate the time elapsed since the surface of other bodies in the inner solar system got a complete or partial makeover. New data-the first complete counts of craters on the side of the moon facing away from Earth-suggest, however, that researchers haven't been counting lunar craters correctly.

Like previous lunar crater counts, the new ones focus on craters that have bright rays of material emanating from them. In most cases, the rays represent freshly excavated material that hasn't yet been darkened by exposure to sunlight and cosmic radiation. The rays were thought to disappear in about a billion years, thus their presence indicates recent cratering. However, compositional differences between areas on the near side of the moon could have created persistent, raylike markings that might disguise an old crater as a young one.

High-resolution images taken by the Clementine spacecraft have for the first time allowed researchers to count accurately the rayed craters on the moon's far side. This region of the moon has a more uniform composition, offering scientists the opportunity to identify young craters more clearly. Jeffrey M. Moore of NASA's Ames Research Center in Mountain View, Calif., and Alfred S. McEwen of the U.S. Geological Survey in Flagstaff, Ariz., found that the far side has about twice as many rayed craters under 10 kilometers in diameter as predicted by the crater counts from the near side. They reported these results last month at the Lunar and Planetary Science Conference in Houston.

James W. Head of Brown University in Providence, R.I., calls the results intriguing but would like to see more studies.

The researchers note that their findings indicate that Earth's vicinity may harbor a much larger than expected population of asteroids and comets 300 meters to 1 km in diameter. Such objects pose a major threat to Earth. If an object 1 km in diameter landed in the middle of the Atlantic Ocean, for instance, the resulting tidal wave would wipe out coastal Europe and eastern North America.

The lunar researchers also counted fewer rayed craters over 20 km in diameter than expected. The relatively low number of such craters makes this finding less statistically significant, Moore notes. However, if fewer large craters did pepper the moon and the terrestrial planets over the last billion years, then Venus' surface is probably older than the 300 million to 500 million years previously calculated, Moore says. It would have taken longer, perhaps 700 million years, for Venus to acquire the number of large craters now seen. An independent calculation by Kevin Zahnle of Ames and William B. McKinnon of Washington University in St. Louis, based solely on the estimated distribution of comets and asteroids near Venus, suggests a similar age.