Moon

The Moon imaged through an 11-inch Schmidt-Cassegrain (30mm eyepiece) and attached Nikon 4-megapixel camera. Courtesy: Jim Wentworth, "Fire in the Sky Observatory," Brainerd, Minnesota

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. The Moon has been the target of numerous robotic probes including, most recently, Clementine (1994) and Lunar Prospector (1999). 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 size of the Moon and the British Isles compared

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.

The Moon's origin remains a matter of debate, though the consensus view is that it formed over 4.5 billion years ago from debris resulting from the collision between the Earth and a Mars-sized body. 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.

The lunar farside

However, 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.

The impact hypothesis

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.

It 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.

The 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.

Lunar statistics
mean distance from center of Earth	384,400 km (238,906 miles)
diameter	3,476 km (2,160 miles)
mass (Earth=1)	0.0122
mean density	3.34 g/cm³
surface gravity (Earth=1)	0.165
escape velocity	2.38 km/s (8,568 km/h, 5,325 mph)
orbital period	27.3 days
orbital eccentricity	0.055
orbital inclination	5.1°
axial period	27.3 days (gravitational lock)
max. surface temperature	117 °C (243 °F)
min. surface temperature	-163 °C (-261 °F)
albedo	0.07

Some notable features on the Moon
Feature	Description
Aitken Basin	An impact basin in the south polar region. With a diameter of about 2,500km, a maximum depth of over 12km, and an average depth of about 10km, it is the biggest, deepest impact basin in the solar system
Apennines	A mountain range that rises to 4,572m at the southeastern edge of Mare Imbrium. The highest escarpment on the Moon, it is higher above the adjacent flatlands than the Himalayan front is above the plains of India and Nepal. The landing site of Apollo 15 was chosen to allow the astronauts to drive from the Lunar Module to the base of the Apennines during two excursions
Bailly	The largest crater on the nearside of the Moon with a diameter of 295km and maximum depth of 3.96km. A highly eroded structure, it is named after the French astronomer Jean Bailly
Copernicus	A 93km-wide crater that is one of the most prominent features on the lunar nearside. Made less than 1 billion years ago, and thus one of the Moon’s youngest major markings, it has a system of bright rays seen most clearly at full Moon
Imbrium Basin	The largest and youngest of the giant impact basins on the nearside of the Moon. The asteroid collision that formed it, about 3.9 billion years ago, went close to breaking the Moon apart; in the event, it threw ejecta over much of the lunar surface and created deep fissures. Through these cracks, lava poured out, filling much of the basin and leaving the 1,300km-wide dark feature known as Mare Imbrium
Orientale Basin	The youngest and best preserved impact basin on the Moon, visible from Earth only at the extreme western limb as a libration feature. Formed some 3.8 to 3.9 billion years ago, it is shows three concentric rings of mountains. Strong radial lineations made by the scouring flow of ejecta are also evident
Tycho	A spectacular, 85km-wide crater associated with the brightest and most extensive ray system on the Moon. In some cases, the rays extend for over 1,500km; their prominence suggests that Tycho was formed relatively recently, perhaps within the past 3 billion years

Related entries

• lunar phases
• lunar eclipse

Related category

• MOON TOPICS
• PLANETS AND MOONS

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