MOON

MOON

 

The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about 1,700 kilometres (1,100 mi)—a quarter the Earth's radius—beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days^{[nb 3]} (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the phases of the Moon, which repeat every 29.5 days^{[nb 4]} (the synodic period).

The Moon's diameter is 3,474 kilometres (2,159 mi),^[4] a little more than a quarter of that of the Earth. Thus, the Moon's surface area is less than a tenth of the Earth (about a quarter of Earth's land area, approximately as large as Russia, Canada, and the United States combined), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth's surface.

The Moon is the only celestial body on which human beings have made a manned landing. While the Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft, the NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972 – the first being Apollo 11 in 1969. Human exploration of the Moon temporarily ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. The U.S. has committed to return to the Moon by 2018.^[5][6][7] on November 13, 2009, NASA announced the discovery of proof that water exists on the Moon.^[8]

Name and etymology

The proper English name for Earth's natural satellite is, simply, the Moon (capitalized).^[9][10] Moon is a Germanic word, related to the Latin mensis (month). It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure^[11] (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, as well as an adjectival prefix seleno- and suffix -selene (from selēnē, σελήνη, the Ancient Greek word for the Moon). In English, the word moon exclusively meant "the Moon" until 1665, when it was extended to refer to the recently discovered natural satellites of other planets.^[11] Subsequently, these objects were given distinct names in order to avoid confusion.^[10] The Moon is occasionally referred to by its Latin name Luna, primarily in science fiction.

Lunar surface

Main article: Geology of the Moon

Two sides of the Moon

The Moon is in synchronous rotation, which means it rotates about its axis in about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.^[12]

Small variations (libration) in the angle from which the Moon is seen allow about 59% of its surface to be seen from the Earth (but only half at any instant).^[4]

Near side of the Moon		Far side of the Moon

The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the "dark side," but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark. The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. one distinguishing feature of the far side is its almost complete lack of maria.

Lunar libration

Maria

Main article: Lunar mare

The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. These are now known to be vast solidified pools of ancient basaltic lava. The majority of these lavas erupted or flowed into the depressions associated with impact basins that formed by the collisions of meteors and comets with the lunar surface. (Oceanus Procellarum is a major exception in that it does not correspond to a known impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering about 2% of its surface,^[13] compared with about 31% on the near side.^[4] The most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer.^[14][15] Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.^[16]

Terrae

The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are hypothesized to be the surviving remnants of the impact basin's outer rims.^[17] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.^[18]

From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day. These peaks of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Other consequences of the Moon's small axial tilt are regions that remain in permanent shadow at the bottoms of many polar craters.^[19]

Impact craters

Lunar crater Daedalus on the Moon's far side

Main article: List of craters on the Moon

The surface of Earth's Moon is marked by impact craters^[20] which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the Moon.^{[citation needed]} Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.

The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System,^[21] is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth.^[22] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.

Regolith

Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands.^[23] Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.^[24]

Astronauts have reported that the dust from the surface felt like snow and smelled like spent gunpowder.^[25] The dust is mostly made of silicon dioxide glass (SiO₂), most likely created from the meteors that have crashed into the Moon's surface. It also contains calcium and magnesium.

Presence of water

Main article: Lunar water

The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole^[26] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.^[19] Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions.^[27] Estimate for the quantity of water on the Moon is 32 ounces per one ton of top layer of Moon's surface.

Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters.^[28] The question of how much water there is on the Moon has not been resolved.

In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth in 1971 by the Apollo 15 astronauts).^[29][30]

On September 24, 2009, the Indian Space Research Organisation (ISRO) reported that their first lunar mission, Chandrayaan-1 using NASA's Moon Mineralogy Mapper, found evidence of large quantities of water on the Moon's surface, and that water is still presently being formed.^[31][32] The instrument observed an absorption line in the spectrum of sunlight reflected from the Moon, indicating that light of a particular wavelength (around 2.8 microns) is being absorbed more readily than other nearby wavelengths. The position and shape of the line indicate the absorption is due to water. A nearby line also revealed the presence of the closely-related molecule hydroxyl, which consists of an oxygen atom with a single hydrogen atom. The exact abundance of water was not determined, but the team believed it could be as high as 1,000 parts per million in the top layer of Lunar soil.

On November 13, 2009, NASA announced the results of the Lunar Crater Observation and Sensing Satellite, saying that "not just water, but lots of water" had been found by the mission near the southern pole.^[33]

Physical characteristics

Internal structure

Main article: Internal structure of the Moon

Schematic illustration of the internal structure of the Moon

The Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and core. This structure is hypothesized to have resulted from the fractional crystallization of a magma ocean shortly after its formation, at about 4.4 billion years ago^[34]. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust (see Origin and geologic evolution below).

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition,^[35] consistent with the magma ocean hypothesis. In terms of elements, the crust is composed primarily of oxygen (41% to 46% by mass), silicon (21%), magnesium (6%), iron (13%), calcium (8%), and aluminium (7%).^[36][37] Based on geophysical techniques, its thickness is estimated to be on average about 50 km.^[1]

Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals olivine, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about a thousand kilometres below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.^[1]

The Moon has a mean density of 3 346.4 kg/m³, making it the second densest moon in the Solar System after Io. Nevertheless, several lines of evidence imply that the core of the Moon is small, with a radius of about 350 km or less.^[1] This corresponds to only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.^[38]

Topography

Main article: Topography of the Moon

Topography of the Moon, referenced to the lunar geoid

The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side.^[1]

Gravity field

Main article: Gravity of the Moon

The gravitational field of the Moon has been determined through tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the spacecraft acceleration in the line-of-sight direction can be determined by means of small shifts in frequency of the radio signal, and the distance from the spacecraft to a station on Earth. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterised.^[39]

Radial gravitational anomaly at the surface of the Moon

The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins.^[40] These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. The mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitational signature, and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism.^[41] The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.

Magnetic field

Main article: Magnetic field of the Moon

Total magnetic field strength at the surface of the Moon as derived from the Lunar Prospector electron reflectometer experiment

The Moon has an external magnetic field of the order of one to a hundred nanotesla—less than one hundredth that of the Earth, which is 30–60 microtesla. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin.^[42] one hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.^[43]

Atmosphere

Main article: Atmosphere of the Moon

The Moon has an atmosphere so thin as to be almost negligible, with a total atmospheric mass of less than 10⁴ kg.^[44] The effective surface pressure of this small mass is around 3  × 10^-15 atm.^[45] This pressure varies, of course, with the diurnal moon cycle. one source of its atmosphere is outgassing—the release of gases such as radon that originate by radioactive decay processes within the crust and mantle.^[46] Another important source is generated through the process of sputtering, which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight.^[35] Gases that are released by sputtering can either reimplant into the regolith as a result of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionised. The elements sodium (Na) and potassium (K) have been detected using earth-based spectroscopic methods, whereas the element radon–222 (²²²Rn) and polonium-210 (²¹⁰Po) have been inferred from data obtained from the Lunar Prospector alpha particle spectrometer.^[47] Argon–40 (⁴⁰Ar), helium-4 (⁴He), oxygen (O₂) and/or methane (CH₄), nitrogen (N₂) and/or carbon monoxide (CO), and carbon dioxide (CO₂) were detected by in-situ detectors placed by the Apollo astronauts.^[48]

Surface temperature

During the lunar day, the surface temperature averages 107 °C, and during the lunar night, it averages −153 °C.^[49]

Origin and geologic evolution

Formation

Several mechanisms have been suggested for the Moon's formation. The formation of the Moon is hypothesized to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System.^[50]

Fission hypothesis 

Early speculation proposed that the Moon broke off from the Earth's crust because of centrifugal forces, leaving a basin – presumed to be the Pacific Ocean – behind as a scar.^[51] This idea, however, would require too great an initial spin of the Earth and also would have resulted in the Moon's orbit following Earth's equatorial plane rather than its current path.^[52]

Capture hypothesis 

Other speculation has centered on the Moon being formed elsewhere and subsequently being captured by Earth's gravity.^[53] However, the conditions conjectured necessary for such a mechanism to work, such as an extended atmosphere of the Earth in order to dissipate the energy of the passing Moon, are improbable.^[52]

Co-formation hypothesis 

The co-formation hypothesis proposes that the Earth and the Moon formed together at the same time and place from the primordial accretion disk. The Moon would have formed from material surrounding the proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon.^[52]

A major deficiency in all these hypotheses is that they cannot readily account for the high angular momentum of the Earth–Moon system.^[54]

Giant impact hypothesis

Main article: Giant impact hypothesis

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact. A Mars-sized body (labelled "Theia") is hypothesized to have hit the proto-Earth, blasting sufficient material into orbit around the proto-Earth to form the Moon through accretion.^[4] As accretion is the process by which all planetary bodies are therorized to have formed, giant impacts are thought to have affected most if not all planets. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system, as well as the small size of the lunar core.^[55] Unresolved questions regarding this theory concern the determination of the relative sizes of the proto-Earth and Theia and of how much material from these two bodies formed the Moon. Recent oxygen isotope composition analysis of the Moon shows its oxygen isotope composition is more similar to the Earth's than this hypothesis would suggest.^[56]

Lunar magma ocean

As a result of the large amount of energy converted during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly hypothesized that a large portion of the Moon was once initially molten. The molten outer portion of the Moon at this time is referred to as a magma ocean, and estimates for its depth range from about 500 km to the entire radius of the Moon.^[14]

As the magma ocean cooled, it fractionally crystallised and differentiated, giving rise to a geochemically distinct crust and mantle. The mantle is inferred to have formed largely by the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallisation was complete, the mineral anorthite is inferred to have precipitated and floated to the surface because of its low density, forming the crust.^[14]

The final liquids to crystallise from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym KREEP, for potassium (K), rare earth elements (REE), and phosphorus (P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of Oceanus Procellarum and Mare Imbrium on the near side of the Moon.^[1]

Geologic evolution

See also: Geology of the Moon

A large portion of the Moon's post–magma-ocean geologic evolution was dominated by impact cratering. The lunar geologic timescale is largely divided in time on the basis of prominent basin-forming impact events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on the basis of stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.

The other major geologic process that affected the Moon's surface was mare volcanism. The enhancement of heat-producing elements within the Procellarum KREEP Terrane is thought to have caused the underlying mantle to heat up, and eventually, to partially melt. A portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon.^[14] Most of the Moon's mare basalts erupted during the Imbrian period in this geologic province 3.0–3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years,^[57] and the youngest eruptions, based on the method of crater counting, are hypothesized to have occurred only 1.2 billion years ago.^[58]

There has been controversy over whether features on the Moon's surface undergo changes over time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of outgassing does occasionally occur, and these events could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.^[59][60]

Moon rocks

Main article: Moon rocks

Moon rocks fall into two main categories, based on whether they underlie the lunar highlands (terrae) or the maria. The lunar highlands rocks are composed of three suites: the ferroan anorthosite suite, the magnesian suite, and the alkali suite (some consider the alkali suite to be a subset of the mg-suite). The ferroan anorthosite suite rocks are composed almost exclusively of the mineral anorthite (a calic plagioclase feldspar), and are hypothesized to represent plagioclase flotation cumulates of the lunar magma ocean. The ferroan anorthosites have been dated using radiometric methods to have formed about 4.4 billion years ago.^[57][58]

The mg- and alkali-suite rocks are predominantly mafic plutonic rocks. Typical rocks are dunites, troctolites, gabbros, alkali anorthosites, and more rarely, granite. In contrast to the ferroan anorthosite suite, these rocks all have relatively high Mg/Fe ratios in their mafic minerals. In general, these rocks represent intrusions into the already-formed highlands crust (though a few rare samples appear to represent extrusive lavas), and they have been dated to have formed about 4.4–3.9 billion years ago. Many of these rocks have high abundances of, or are genetically related to, the geochemical component KREEP.

The lunar maria consist entirely of mare basalts. While similar to terrestrial basalts, they have much higher abundances of iron, are completely lacking in hydrous alteration products, and have a large range of titanium abundances.^[61][62]