June 9, 2014

Material from the object whose impact with Earth some 4.5 billion years ago led to the formation of the moon has been found in lunar rocks, according to a team of German scientists. For several decades, the most widely accepted theory about the formation of the moon has focused on a cataclysmic collision between Earth and a Mars-sized object called Theia. According to this theory–known as the Giant Impact or the “Big Whack”–the crash caused Theia as well as some of Earth’s crust to melt and combine. During the impact, some of this melted rock vaporized, shot off Earth’s surface, and went into orbit around Earth. Over time, the cloud of vaporized rock cooled and condensed into a ring of small, solid bodies, which then gathered together to form the moon. However, no evidence of Theia had ever been found in moon rocks.

The rocks analyzed by the German researchers were brought back to Earth by NASA Apollo missions from 1969 to 1972. Earlier studies of the rocks indicated that the isotopes (forms) of oxygen in them were the same as those in Earth rocks. In other words, the moon had formed from Earth rock. This puzzled scientists because computer simulations of the Big Whack suggested that up to 70 percent of the moon should be material from Theia.

A tremendous collision between Earth and a smaller object led to the birth of the moon. Debris from the impact went into orbit in a ring around Earth, then gathered together to form the moon. (© William K. Hartmann)

For their research, the German scientists used a more advanced technique to analyze the rocks and so were able to measure smaller amounts of the isotopes. The new study indicated small but important differences in the isotopes between the two kinds of rocks. The scientists suggest that these differences can be explained by an alien origin for the lunar rocks. About 40 percent of the moon may be material from Theia, the scientists said. They noted that additional research comparing isotopes of other chemical elements could strengthen their conclusions. Because the tests are so difficult and the difference in the isotope levels are so small, however, other scientists questioned the results.

Additional World Book articles:

• Space exploration (Lunar probes)
• How the Moon Was Born (a Special Report)