April 13, 2015

One problem with the theory of the moon’s origin may have been solved. Alessandra Mastrobuono-Battisti and Hagai B. Perets from the Israel Institute of Technology in Haifa and Sean N. Raymond from the Laboratoire d’Astrophysique de Bordeaux in France published their findings on the formation of the moon last week in the journal Nature.

An artist’s impression of a collision between Earth and a planet-sized object. The Moon was likely formed by such a collision. Scientists now think the object was very similar to Earth in composition. (Credit: NASA/JPL-Caltech)

Most scientists think that the moon formed as a result of a collision known as the Giant Impact. According to this hypothesis (a possible explanation for what is observed), Earth collided with a planet-sized object about 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off Earth’s surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon.

The Giant Impact hypothesis predicts that the moon would be formed mostly of the remains of the impactor (the object that struck Earth), but the makeup of the moon is actually very similar to that of Earth. Studies had suggested that the likelihood of an impactor being so similar to the planet it hits to be about 1 in 100. The unlikelihood of such an event led some scientists to look for other hypotheses to explain the formation of the moon. Whenever new data was found, however, it usually still supported the Giant Impact hypothesis.

Mastrobuono-Battisti, Perets, and Raymond created a computer model of the early solar system to study the formation of planets and impactors. In their analysis, they found that impactors had a composition similar the planets they crashed into about 20% to 40% of the time, not 1% of the time as had been previously thought. Thus, the Great Impact hypothesis is not as improbable or unlikely as scientists once thought, and scientists can now accept it is the most likely explanation for how our moon was made.

Other World Book articles:

• How the Moon Was Born (a Special report)
• Satellite

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)