In the decades since Apollo astronauts collected moon rocks, one theory has emerged over competing explanations of lunar birth � the moon formed when a big object whacked Earth.
Scientists believe the solar system formed 4.56 billion years ago from a cloud or nebula of gas and dust surrounding the newborn sun. The nebula condensed into pebbles, rocks and larger "planetesimals," which collided and fused to create the planets.
A theory that the moon and Earth both condensed or accreted from the same material was rendered doubtful by a major difference in the moon�s composition � the lack of a significant iron core like Earth�s.
Another theory said the moon formed elsewhere and was somehow captured by Earth�s gravity. But that seemed physically improbable and did not account for similarities in compositions of the moon and Earth.
The moon�s composition was enough like Earth�s mantle and crust to make some believe the moon formed from material hurled off of a rapidly spinning Earth. But hard evidence could not be found.
And none of the old theories explained how the moon�s oldest rocks solidified from molten rock about 4.44 billion years ago, roughly 100 million years after the solar system formed.
The giant impact theory proposed that at least 50 million years after the solar system formed, a large protoplanet whacked a perhaps still-molten Earth, heating and ejecting debris from both objects. Part of the debris then clumped together to form the moon, which was covered by molten rock from the heat of the collision.
Supercomputer simulations of the early solar system indicated in 1985 that the moon was not formed from huge solid chunks that were ripped off Earth. Instead, a planet-sized object hit a still-molten Earth, hurling into orbit a thick ring of vapor, dust and rocks, part of which clumped together to form the moon.
Data from the Clementine spacecraft, analyzed in 1995, showed the moon contained less iron than previously thought, and thus had a different bulk composition than Earth. That meant Earth and moon could not have accreted from the same nebula in the early solar system. Researchers argued the moon congealed from debris hurled into space by a giant impact.
In July 1997, University of Colorado, Boulder scientists said their computer simulations showed the object striking early Earth must have been three times more massive than Mars. Harvard University work in the 1980s had concluded the object was as massive as Mars.
More Colorado computer simulations, released in September 1997, mimicked conditions in a disk of debris blasted off early Earth by a giant impact. The simulations reveal the moon must have formed within a year after the collision, and at a distance of only 14,000 miles (22,500 kilometers) from Earth, a fraction of its present mean distance of 239,000 miles (384,600 kilometers).
University of Michigan chemists said in November 1997 that the decay of hafnium 182 to tungsten 182 in lunar samples indicated the moon-forming collision happened at least 50 million years after the solar system was born � findings confirmed by a new study of other isotopes.
NASA reported in 1999 that gravity and magnetic-field measurements by the Lunar Prospector orbiter show the moon�s core is roughly 420 miles (680 kilometers) wide, or less than 4 percent of the moon�s mass. Earth�s iron core contains about 30 percent of the planet�s mass. NASA said the fact the cores are so different indicates the moon was made of material blasted off of Earth�s outer shell by a huge impact.
The early moon�s orbit was inclined about 10 degrees from Earth�s equator. Scientists at the Southwest Research Institute in Boulder, Colorado ran computer simulations to show how an object hitting Earth would throw debris into orbit, and some debris would start accreting to form the moon.
Gravitational interactions between the newborn moon and the remaining debris would have tilted the moon�s orbit, they found. The young moon�s inclined orbit had been a stumbling block for the impact theory because scientists thought an impact would produce the moon in equatorial orbit.
http://www.space.com/scienceastronomy/planetearth/moonwhack_side_000901.html