The Moon At a distance of 384,400 km from the Earth, the Moon is our closest celestial neighbor and only natural satellite. Because of this fact, we have been able to gain more knowledge about it than any other body in the Solar System besides the Earth. Like the Earth itself, the Moon is unique in some ways and rather ordinary in others. The Moon is unique in that it is the only spherical satellite orbiting a terrestrial planet. The reason for its shape is a result of its mass being great enough so that gravity pulls all of the Moon's matter toward its center equally. Another distinct property the Moon possesses lies in its size compared to the Earth. At 3,475 km, the Moon's diameter is over one fourth that of the Earth's. In relation to its own size, no other planet has a moon as large. For its size, however, the Moon's mass is rather low. This means the Moon is not very dense. The explanation behind this lies in the formation of the Moon. It is believed that a large body, perhaps the size of Mars, struck the Earth early in its life. As a result of this collision a great deal of the young Earth's outer mantle and crust was ejected into space. This material then began orbiting Earth and over time joined together due to gravitational forces, forming what is now Earth's moon. Furthermore, since Earth's outer mantle and crust are significantly less dense than its interior explains why the Moon is so much less dense than the Earth. When viewed from Earth, the many impact craters found on the Moon's surface are visible. The reason for this is simple. Unlike the Earth, the Moon is not geologically active, and so it does not possess an atmosphere nor does it possess volcanic activity. Consequently, the Moon does not undergo resurfacing as does the Earth. The Moon (Latin: luna) is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its 1 primary, having 27% the diameter and 60% the density of Earth, resulting in ⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter. The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. The Moon's linear distance from the Earth is currently increasing at a rate of 3.82±0.07cm per year, however this rate is not constant. The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body. The Moon is the only celestial body other than Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history. After the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft, notably by the final Soviet Lunokhod rover. Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters. These spacecraft have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes. Lunar Water Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapour is quickly decomposed by sunlight and lost to outer space. However, scientists have since the 1960s conjectured that water ice could survive in cold, permanently shadowed craters at the Moon's poles. Water, and the chemically related hydroxyl group ( · OH), can also exist in forms chemically bound to lunar minerals (rather than as free water), and evidence strongly suggests that this is indeed the case in low concentrations over much of the Moon's surface. In fact, adsorbed water is calculated to exist at trace concentrations of 10 to 1000 parts per million. Inconclusive evidence of free water ice at the lunar poles was accumulated from a variety of observations suggesting the presence of bound hydrogen. In September 2009, India's Chandrayaan- 1 detected water on the Moon and hydroxyl absorption lines in reflected sunlight. In November 2009, NASA reported that its LCROSS space probe had detected a significant amount of hydroxyl group in the material thrown up from a south polar crater by an impactor; this may be attributed to water- bearing materials – what appears to be "near pure crystalline water-ice". In March 2010, it was reported that the Mini-RF on board the India's Chandrayaan-1 had discovered more than 40 permanently darkened craters near the Moon's north pole which are hypothesized to contain an estimated 600 million metric tonnes(1.3 trillion pounds) of water-ice. Water may have been delivered to the Moon over geological timescales by the regular bombardment of water-bearing comets, asteroids and meteoroids or continuously produced in situ by the hydrogen ions (protons) of the solar wind impacting oxygen-bearing minerals. The search for the presence of lunar water has attracted considerable attention and motivated several recent lunar missions, largely because of water's usefulness in rendering long-term lunar habitation feasible. On 24 September 2009 Science magazine reported that the Moon Mineralogy Mapper (M3) on India's Chandrayaan-1 had detected water on the Moon. M3 detected absorption features near 2.8-3.0 µm on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer H abundance data suggests that the formation and retention of OH and H2O is an ongoing surficial process. OH/H2O production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration. The Moon Mineralogy Mapper (M3), an imaging spectrometer, was one of the 11 instruments on board India's Chandrayaan-I that came to a premature end on 29 August 2009. M3 was aimed at providing the first mineral map of the entire lunar surface. Lunar scientists had discussed the possibility of water repositories for decades. They are now increasingly "confident that the decades-long debate is over" a report says. "The Moon, in fact, has water in all sorts of places; not just locked up in minerals, but scattered throughout the broken-up surface, and, potentially, in blocks or sheets of ice at depth." The results from the Chandrayaan mission are also "offering a wide array of watery signals." Atmosphere The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 metric tons. The surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa); it varies with the lunar day. Its sources include outgassing and sputtering, the release of atoms from the bombardment of lunar soil by solar wind ions. Elements that have been detected include sodium and potassium, produced by sputtering, which are also found in the atmospheres of Mercury and Io; helium-4 from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. Water vapour has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70 degrees; it is possibly generated from the sublimation of water ice in the regolith. These gases can either return into the regolith due to the Moon's gravity, or be lost to space: either through solar radiation pressure, or if they are ionised, by being swept away by the solar wind's magnetic field.