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Solar System
The Moon
Chandra has been used to prospect for elements on the Moon. X-rays from the Moon are caused by "fluorescence" due to the impact of solar X-rays on the surface of the Moon. When a solar X-ray is absorbed by an atom on the lunar surface, the X-ray knocks an electron out of the inner part of the atom and excites the atom to a higher energy level. The atom almost immediately returns to its lower energy state with the emission of a fluorescent X-ray. In a similar way, ultraviolet light produces the visible light of fluorescent lamps. The energy of a fluorescent X-ray is unique to the particular type of atom, so fluorescent X-rays give a direct measurement of elements present, independent of assumptions about the type of mineral or other complications.
Oxygen, magnesium, aluminum and silicon were detected over a large area of the lunar surface. Longer observations of the Moon with Chandra, should help to determine if the Moon was formed by a giant impact of a planetoid with the Earth about 4.5 billion years ago, or by some other process.
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