Science/Medicine : Small, Tailless Comets May Have Dug Craters

Data from Voyager II indicates that the craters on the moon, Earth, Mercury and Mars may have been caused by small, ice-coated comets previously undetected because they have no tail, scientists say.

The shower of comets streaming through the solar system had enough force and frequency to cause all craters up to 600 feet in diameter and some up to 2 miles in diameter on the moon during the past 3.2 billion years, researchers from the University of Michigan said.

Thomas M. Donahue and Tamas I. Gombosi of the school’s College of Engineering and the University of Arizona’s B. R. Sandel based their findings on data from the Voyager II spacecraft, which was launched in 1977 and has explored the outer reaches of the solar system.

The small comets, or cometesimals, had eluded detection because of their size and their coats of pure ice, which prevent them from forming the comet’s characteristic tail of dust and debris, the researchers said.