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)

December 10, 2012

Almost the entire surface of the moon was fractured by impacts billions of years ago, according to the most detailed map of the lunar gravity field ever created. The map is based on high-resolution data collected by NASA’s twin GRAIL orbiters. (GRAIL stands for Gravity Recovery and Interior Laboratory.) The map shows many of the moon’s structures in great detail, including features in the crust, volcanic formations, and numerous other landforms. The map has also revealed that the lunar crust is marked with many holes. Scientists believe this finding indicates that the surface was battered by a heavy bombardment of meteorites and other objects early in its history. The gravity map also indicates that the crust of the moon is only about 21 to 27 miles (34 to 43 kilometers) thick, about 6 to 12 miles (10 to 20 kilometers) thinner than scientists had thought.

A gravity map of the moon reveals mountains and other higher areas (red) and craters and other lower areas (blue). (NASA/ARC/MIT)

GRAIL was designed to measure changes in the moon’s gravitation as a way to reveal structures beneath the surface. The orbiters, which were launched in September 2011, travel around the moon on opposite sides at an altitude of about 34 miles (55 kilometers). They use radio signals to precisely measure the distance between them, which changes with the density of the rock and soil below. The system is so accurate that it can measure distances less than the width of a human hair. Scientists can link the satellites’ measurements to structural differences in the moon’s surface and underlying crust. The probes were named “Ebb” and “Flow” in January 2012 by elementary students in Bozeman, Montana, in a nationwide contest. The GRAIL mission is scheduled to end later this month.

Another interesting discovery by the orbiters is collections of long, linear dikes (thin,vertical bodies of solidified magma) beneath the surface. The dikes, which extend for hundreds of miles (kilometers), crisscross the surface. Scientists think that the dikes, which are among the oldest features on the moon, likely formed as the moon’s crust expanded. Such geologic events could have occurred if the moon formed as a result of a collision known as the Giant Impact or the “Big Whack.” According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. During 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 rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an “ocean” of magma. Over time, the magma ocean slowly cooled and solidified into rocks called basalts.

Additional World Book articles:

• Space exploration (Lunar probes)
• How the Moon Was Born (a special report)