Friday, March 10, 2000
The Ups and Downs of Impacts

Left: This tiny glass bead, 0.25 millimeter across, was returned from the Moon by Apollo 11 astronauts in 1969. The pit at upper left contains a glassy area melted during a microscopic impact that created the pit and the fractured spall zone that surrounds it. Courtesy Tim Culler (University of California, Berkeley). Click on image for larger view.

A team of scientists led by Timothy S. Culler (University of California, Berkeley) concludes that the rate of impacts on the Moon -- and therefore Earth -- has taken some dramatic downs and ups over the past 4 billion years. The team used argon isotopes to determine the formation age of 155 impact-generated glass spherules gleaned from a sample of lunar soil. They found that the collision rates on the Moon dropped to an all-time low about 500 to 600 million years ago and have quadrupled since then. The team's findings confirm longstanding hunches by astronomers who, using other lines of evidence, suspected that the Earth and Moon have become more frequent cosmic targets over the past billion years. Curiously, this period of intensifying bombardment parallels the blossoming of life on Earth since Cambrian times. That might not be mere coincidence, notes team member Paul R. Renne, because all those infalling asteroids and comets might have fortified Earth's ecosystem with an abundance of water and organic-rich dust. Details of the study appear in the March 10th issue of Science.