The Moon

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Astronomers have offered a wide range of theories to explain the origin of the Moon. Many of these hypotheses, including the capture, fission and accretion theories, were based on conjecture and observation rather than empirical geologic evidence. In 1969, however, the mission of Apollo 11 to the Moon began the process of acquiring geologic data on the bulk chemistry of the Moon. When this data proved inconsistent with the assumptions and required conditions that supported the earlier theories, most scientists came to accept the theory that the Moon was formed by the impact with the Earth of a celestial body about the size of Mars.

Testing any hypothesis of the Moon's origin requires comparing the bulk composition of the Moon with the Earth's mantle. The proportionate quantity of iron, refractory elements and silicates all play a role in determining the likely method of lunar origin. If the Moon originated from a process of fission or co-accretion, then both bodies should have approximately the same silicate proportion and element abundance. The capture and impact theories, however, do not require the bulk composition of the bodies to be the same.

The geo-chemical data gathered by various lunar missions provides a picture of the early record of the Moon. The crust is older than three billion years. There have been no techtonic activity, and the only crust deformation has been caused by comet and asteroid impact. These impacts have excavated material from the lower crust and deposited it on the surface, thus providing a window to the Moon's interior. The Apollo and Luna landing missions identified major lunar terrains, ages, rock type, but had little polar coverage. The Clementine mission provided multi-spectral data that led to a map of iron and titanium concentrations. It also included radar data, which suggested that the polar regions might contain ice. The recent Lunar Prospector mission tested and extended these results by obtaining gravity, magnetic and compositional data of the entire Moon at high resolution.

Some of the most pertinent data for lunar origin models comes from the Lunar Prospector, which was designed to gather data on lunar composition by measuring the energy level of lunar neutrons. These neutrons are created by the interaction of cosmic rays with nuclei on the lunar surface. The neutrons lose energy by inelastic scattering off other soil nuclei until they approach the energy corresponding to the ambient temperature. Then they are captured by the most abundant elements with large absorption cross sections. The Lunar Prospector measured neutrons at thermal, epithermal and fast energies. The impact of cosmic rays on iron and titanium produces more fast neutrons. At the same time, these elements absorb more thermal neutrons.