Lunar History
Much is still unknown about the Moon’s origin.Until recently,the most widely held hypothesis argued that the formation of the Moon paralleled that of Earth and the other planets.That is,the Moon formed from minute rock fragments and gases that composed a disk-shaped structure (the solar nebula)that orbited the early Sun. Debris from this disk collided and accumulated into larger masses that,in turn, accreted into planetary-sized bodies.
A new hypothesis,which has recently gained support from many scientists, suggests that a giant body collided with Earth to produce the Moon.The explosion caused by the impact of a Mars-sized body with a semi-molten Earth is thought to have ejected huge quantities of mantle rock from the primordial Earth.A portion of this ejected material remained in orbit around Earth,while the remainder either escaped or impacted upon Earth’s surface.In a manner similar to that proposed in the earlier hypothesis,the material orbiting Earth then began to accumulate, eventually producing the Moon.Though the giant impact hypothesis provides a plausible mechanism for the Moon’s formation,many questions must be answered before this proposal can be considered viable.
Despite the fact that the origin of the Moon is still debated,planetary geologists have been able to work out some of the basic details of the Moon’s history,using among other things variations in crater density (quantity per unit area).Simply stated,the higher the crater density,the longer the topographic feature has existed. During its early history,the Moon was continually impacted as it swept up debris from the solar nebula.This continuous bombardment and perhaps radioactive decay generated enough heat to melt the Moon’s outer shell and quite possibly the rest of the Moon as well.
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