The Giant-impact Theory

Influenced by the information from the moon rocks, in the mid-1970's two teams of planetary scientists separately proposed what is now called the “giant-impact” theory of the origin of the moon. The groups were led by A. G. W. “Al" Cameron and William R. Ward at Harvard University in Cambridge, Massachusetts, and William K. Hartmann and Donald R. Davis at the Planetary Science Institute in Tucson, Arizona. The researchers suggested that a huge Mars-sized object struck the Earth, blasting away enough of the planet's crust and mantle to create the moon from its debris. According to the theory, the object struck the Earth a glancing blow, rather than hitting it head-on. The blow set the Earth spinning faster than before. This may explain why the angular momentum of the Earth-moon system (a measure of the rate at which both bodies are rotating and orbiting) is unusually high, compared with other planets and moons in the solar system.

At first, many scientists were reluctant to accept the giant-impact theory, because the theory was based on a single event—a catastrophic collision—that most researchers believed had occurred very rarely in the solar system. However, the more that lunar and planetary scientists thought about the theory, the better they liked it. Surveys of lunar geography made with both Earth-based telescopes and telescopes and cameras on spacecraft showed that strikes on the moon by asteroids and meteoroids (metal or rocky objects smaller than asteroids) after it had formed and solidified had produced many huge craters, called impact basins. Why couldn't the impact of an even more massive object on the Earth have made the moon itself?