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JumpStart - Space Science
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 Earth’s only natural satellite, the Moon is unusually large in relation to its planet, having a diameter roughly 1/4 that of Earth’s. Thus, the two bodies are sometimes referred to as a double-planet system. This situation suggests an unusual origin for the Moon. Some proposed origin theories include separation from Earth, independent formation, and capture from elsewhere in the solar system. The theory that seems to explain most of our observations, however, is that a Mars-sized body once hit Earth and the resulting debris (from both Earth and the impacting body) accumulated to form the Moon. Whatever the origin, we know the Moon was formed over 4.5 billion years ago (the age of the oldest collected lunar rocks). During the Moon’s formation, very high temperatures caused extensive melting of its outer layers. The melting resulted in the formation of the lunar crust, probably form a planet-wide “magma ocean.” The rocks found on the Moon’s highlands are at least 4.5 billion years old, and are rich in light-colored minerals, called feldspar. These rocks, called anorthosites, give the lunar highlands their bright color. In the years since they were formed, innumerable meteorites have hit the Moon, producing a crust that is intensely cratered and fragmented.About 4 billion years ago, a series of major impacts occurred, forming huge craters. These craters are now the sites of basins call maria (e.g., Mare Imbrium, Mare Serenitatis). Between 4 and 2.5 billion years ago, volcanic activity filled these basins with dark-colored lavas, called basalts. After this time of volcanism, the Moon cooled down, and has since been relatively inactive, except for the occasional impacts of meteorites and comets. The Moon has not undergone the continual mountain-building and volcanic activity that characterize Earth; it is a fossil planet on which the earliest stages of geologic evolution are preserved.
 The Moon, however, is not completely dead. Seismometers emplaced by Apollo astronauts have recorded small earthquakes (more properly called “moonquakes”) at depths of several hundred kilometers. The quakes are problaby triggered by tides caused by earth. Small eruptions of gas from some craters, such as Aristarchus, have also been reported. We know the deep interior of the Moon is still hot, and perhaps partially molten. Although there are local magnetic areas in the lunar crust associated with some craters, there is no planet-wide magnetic field; the Moon lacks Earth’s molten core. The Moon’s shape is unusual. It is slightly egg-shaped, with the small end of the “egg” pointing toward Earth. This position causes the Moon to keep the same face toward Earth at all times. The far side, which cannot be observed from Earth, has days and nights just like those on the near side. The lunar gravity field is also unusual. A surprising discovery form the tracking of the Lunar Orbiter photographic spacecraft revealed strong areas of high gravitational acceleration located over the circular maria. These “mascons” (mass concentrations) are thought to be caused by layers of denser, basaltic lavas that fill the mare basins.Much remains to be learned about our Moon, beginning with its origin. Active research still continues to yield information about our nearest neighbor in space using the samples and data returned by Apollo and other missions. Speculation has begun on how the Moon might be used to support lunar bases and other human activities in the next century. Material on this web page courtesy of NASA |
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