Probes Map the Moon’s Violent Past
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)
