The Moon is Earth's only natural satellite and the only extraterrestrial body to have been visited by humans – 12 in all aboard six Apollo missions between July 1969 and December 1972. It has also been the target of numerous robotic probes including, most recently, Clementine (1994),Lunar Prospector (1999), SMART-1 (2003), Kaguya (2007), Chang'e 1 (2007), Chandrayaan 1 (2008), Lunar Reconnaissance Orbiter, and Lunar Crater Observation & Sensing Satellite (both 2009).
A total of 382kg of rock samples were returned to Earth by Apollo and the Russian Luna programs; in addition, a number of lunar meteorites have been found. Most of these samples have been dated at between 4.6 and 3 billion years (the one exception is a lunar meteorite dated at 2.8 billion years), and provide information about the early history of the Solar System which is missing on Earth due to a lack of rocks more than about 3.8 billion years old.
The Moon is unusually large relative to the size of its primary (being exceeded by Charon alone in this respect). Its crust averages 68km thick, is thinner on the near side, and varies from essentially zero thickness under Mare Crisium to a depth of 107km north of the crater Korolev on the lunar farside. Below the crust is a mantle and probably a small core, some 340km radius, containing about 2% of the lunar mass. Curiously, the Moon's center of mass is offset from its geometric center by about 2km in the direction toward the Earth.
Origin and early history
The outer layers of the Moon, initially molten and comprising a global "magma ocean", cooled to form the 4.5-billion-year-old rocks now found in the lunar highlands. These ancient igneous rocks known as anorthosites are rich in the silicate mineral plagioclase and impart to the lunar highlands their characteristic light color. A period of intense bombardment followed which caused extensive cratering and fragmentation of the crust. About 4 billion years ago, the Moon suffered a number of major impacts which excavated the basins referred to as maria. Subsequent volcanic activity, between about 4 and 2.5 billion years ago, flooded these basins with molten lava which then cooled and solidified to form dark basalt. Since that time, the Moon has changed little except for the occasional impact of a meteorite or comet.
Geological activity on the MoonHowever, the Moon is not completely inactive. Seismometers left on the lunar surface by the Apollo astronauts have recorded small seismic events, known as "moonquakes", occurring at depths of several hundred kilometers, which probably result from tidal stresses induced by the Earth's gravitational pull. In addition, there have been many reports over the years of unusual color changes and cloud-like obscurations associated with certain craters, including Aristarchus and Alphonsus, and referred to as transient lunar phenomena. Most unusually, Ken Mattingley, the Command Module pilot on Apollo 16, reported seeing flashes of light on the dark side of the Moon on two consecutive orbits of his craft.
Mass concentrations, or mascons, associated with the circular maria, and thought to be caused by layers of denser, basaltic lava, were discovered as a result of their gravitational effect on the orbital motions of Lunar Orbiter probes in the 1960s. Local magnetic areas have also been detected around some craters, although the Moon has no global magnetic field.
Air and waterIt was natural that philosophers and romantics alike should have long dreamt of traveling to the Moon (see Moon, voyages) and there finding intelligent life (see Moon, life). But the possibility of lunar life (except possibly for certain types of hardy microbes) died with the realization that the Moon has neither an atmosphere nor any liquid water (see Moon, water). Recent observations by Lunar Prospector, however, have confirmed the existence of substantial quantities of ice in deep craters at the lunar poles.
Earth-Moon interactionsThe gravitational forces between Earth and the Moon cause some interesting effects, the most obvious of which is the tides. The Moon's gravitational attraction is stronger on the side of Earth nearest to the Moon. Since Earth and its oceans are not perfectly rigid, they are stretched along the line toward the Moon. From our perspective, we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the oceans than in the solid crust so the water bulges are higher. Also, because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around Earth about once a day producing two high tides per day.
However, as Earth is not completely fluid, Earth's rotation carries our planet's bulges slightly ahead of the point directly beneath the Moon. This means that the force between Earth and the Moon isn't exactly along the line between their centers; the result is a torque on Earth and an accelerating force of the Moon. This causes a net transfer of rotational energy from Earth to the Moon, slowing down Earth's rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8cm per year.
The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward us. Just as Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the Solar System. Eventually, Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.
The Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until Luna 3 photographed it in 1959.
Related entries lunar phases
Related category• MOON TOPICS
• PLANETS AND MOONS
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