Astronomers have long agreed that debris from an object impacting Earth formed the moon. But exactly what hit Earth and when is still up for debate. To accommodate the size and angular momentum of the Earth-Moon system, various models have been suggested. One holds that the impact occurred early in Earth's history; another posits that two later collisions in sequence were responsible'or else a planetary body much larger than Mars must have been involved. Now new research suggests that a single collision with a Mars-size object near the end of Earth's formation gave rise to our satellite. The findings appear today in the journal Nature.

Robin Canup of the Southwest Research Institute and Erik Asphaug of the University of California at Santa Cruz used a highly detailed computer modeling system—one that divided Earth and its impactor into more than 20,000 tiny particles whose interactions under stress both with each other and with gravity were simulated—to investigate their scenario. "The model we propose is the least restrictive impact scenario," Canup says, "since it involves only a single impact and requires little or no modification of the Earth-Moon system after the Moon-forming event." It is also strikingly similar to the first proposed impact theory 25 years ago, before computer simulations were available.

The new model does not close the moon formation debate, however. As Jay Melosh of the University of Arizona points out in commentary accompanying the report, computer limitations required Canup and Asphaug to use a primitive version of the equation of state in their calculations. That equation does not distinguish between solid, melt and vapor phases of material—a limitation, Melosh asserts, because "vapor pressure seems to have been involved in a collision that spewed vaporized Earth material into orbit."