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The Moon was heavily bombarded early in its history, which caused many of the original rocks of the ancient crust to be thoroughly mixed, melted, buried, or obliterated. Meteoritic impacts brought a variety of "exotic" rocks to the Moon so that samples obtained from only 9 locations produced many different rock types for study. The impacts also exposed Moon rocks of great depth and distributed their fragments laterally away from their places of origin, making them more accessible. The underlying crust was also thinned and cracked, allowing molten basalt from the interior to reach the surface. Because the Moon has neither an atmosphere nor any water, the components in the soils do not weather chemically as they would on Earth. Rocks more than 4 billion years old still exist there, yielding information about the early history of the solar system that is unavailable on Earth. Geological activity on the Moon consists of occasional large impacts and the continued formation of the regolith. It is thus considered geologically dead. With such an active early history of bombardment and a relatively abrupt end of heavy impact activity, the Moon is considered fossilized in time. |
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Analysis of data from NASA's Lunar Prospector spacecraft has confirmed that the Moon has a small core, supporting
the theory that the bulk of the Moon was ripped away from the early Earth when an object the size of Mars collided
with the Earth.
Scientists presented these findings on March 16, 1999 in Houston, Texas. Their data show that the lunar core contains less than four percent of the Moon's total mass, with the probable value being two percent or slightly less. This is very small when compared with the Earth, whose iron core contains approximately 30 percent of the planet's mass. |