Meteorites date moon-forming impact.

• Position: Planetary Science

Not too long after the planets began forming, a Mars-sized object slammed into Earth, creating the debris that later would coalesce into the moon. Some of the debris from this giant impact escaped all the way out to the asteroid belt. Collisions there left shock-heating signatures--a permanent record of the impact event--that still can be detected billions of years later in meteorites that have fallen to Earth.

Planetary scientists, including Ed Scott at the University of Hawaii, Manoa's Institute for Geophysics and Planetology, have found that a significant number of these altered meteorites have ages clustering at 100,000,000 years after the solar system's birth--the true age of the moon-forming impact, they maintain. The result is an independent check on other estimates for the moon's age, and it suggests that the asteroid belt can provide important clues to the timing and nature of major events in the inner solar system when planets were forming.

"Meteorites provide an extraordinary record of major events in the formation of the planets and the solar system," says Scott. "We study meteorites and asteroids as they provide the key to understanding how and when planets formed."

The team of NASA-funded...