# Moon

Last updated

Moon
The near side of the Moon (north at top) as seen from Earth
Designations
Designation
Earth I
Orbital characteristics
Epoch J2000
Perigee 362600 km
(356400370400 km)
Apogee 405400 km
(404000406700 km)
384399 km  (1.28  ls , 0.00257  AU ) [1]
Eccentricity 0.0549 [1]
27.321661  d
(27d 7h 43min 11.5s [1] )
29.530589 d
(29d 12h 44min 2.9s)
Average orbital speed
1.022  km/s
Inclination 5.145° to the ecliptic [2] [lower-alpha 1]
Regressing by one revolution in 18.61 years
Progressing by one
revolution in 8.85 years
Satellite of Earth [lower-alpha 2] [3]
Physical characteristics
1737.4 km
(0.2727 of Earth's)
[1] [4] [5]
1738.1 km
(0.2725 of Earth's)
[4]
1736.0 km
(0.2731 of Earth's)
[4]
Flattening 0.0012 [4]
Circumference10921 km
3.793×107 km2
(0.074 of Earth's)
Volume 2.1958×1010 km3
(0.02 of Earth's) [4]
Mass 7.342×1022 kg
(0.0123 of Earth's) [1] [4]
[6]
Mean density
3.344  g/cm3 [1] [4]
0.606 × Earth
1.622  m/s2   (0.1654  g ; 5.318  ft/s2 ) [4]
0.3929±0.0009 [7]
2.38 km/s
(8600 km/h; 5300 mph)
29.530589 d
(29d 12h 44min 2.9s; synodic; solar day) (spin-orbit locked)
27.321661 d  (spin-orbit locked)
Equatorial rotation velocity
4.627 m/s
North pole
• 17h 47m 26s
• 266.86° [10]
North pole declination
65.64° [10]
Albedo 0.136 [11]
Surface temp. minmeanmax
Equator100  K [12] 250 K390 K [12]
85°N 150 K230 K [13]
Surface absorbed dose rate 13.2 μGy/h [14]
Surface equivalent dose rate 57.0 μSv/h [14]
29.3 to 34.1 arcminutes [4] [lower-alpha 4]
Atmosphere [15]
Surface pressure
Composition by volume

The Moon is Earth's only natural satellite. Together with Earth it forms the Earth–Moon satellite system. It is about one-quarter of Earth in diameter (comparable to the width of Australia). [16] In the Solar System it is the fifth largest satellite, larger than any of the known dwarf planets and the largest (and most massive) satellite of a planet relative to the planet. [lower-alpha 6] The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. [17] It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654  g ). Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

## Contents

Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence very slowly lengthens Earth's day and is the main driver of Earth's tides. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. However, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.

The most widely accepted origin explanation posits that the Moon formed 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in Earth's sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

## Names and etymology

The usual English proper name for Earth's natural satellite is simply Moon, with a capital M. [18] [19] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn, [20] which in turn comes from Proto-Indo-European *mēnsis "month" [21] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time). [22]

Occasionally, the name Luna is used in scientific writing [23] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. [24] Cynthia is another poetic name, though rare, for the Moon personified as a goddess, [25] while Selene (literally "Moon") is the Greek goddess of the Moon.

The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian, [26] derived from the Greek word for the Moon, σελήνηselēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare, [27] while its cognate selenic was originally a rare synonym [28] but now nearly always refers to the chemical element selenium. [29] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography , the study of the physical features of the Moon, as well as the element name selenium. [30] [31]

The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus. [32] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.

The astronomical symbol for the Moon is a crescent, , for example in M 'lunar mass' (also ML).

## Natural history

### Lunar geologic timescale

Millions of years before present

### Formation

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. [33] [34] Historically, several formation mechanisms have been proposed, [35] but none satisfactorily explains the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force [36] would require too great an initial rotation rate of Earth. [37] Gravitational capture of a pre-formed Moon [38] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. [37] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. [37] None of these hypotheses can account for the high angular momentum of the Earth–Moon system. [39]

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia ) with the proto-Earth. The impact blasted material into orbit about the Earth and the material accreted and formed the Moon [40] [41] just beyond the Earth's Roche limit of ~2.56  R🜨 . [42]

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations show that most of the Moon derived from the impactor, rather than the proto-Earth. [43] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. [44] [45] [46] [47] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, [48] although this is debated. [49]

The impact would have released enough energy to liquefy both the ejecta and the Earth's crust, forming a magma ocean. The liquefied ejecta could have then re-accreted into the Earth–Moon system. [50] [51] Similarly, the newly formed Moon would have had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles). [50]

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. [52] [ example needed ]

### Natural development

After the Moon's formation the Moon settled in orbit around Earth much closer than today, making both bodies appear much larger in each's sky and causing on both more frequent and stronger eclipses and tidal effects. [53] Since then, due to tidal acceleration, the Moon's orbit around Earth has become significantly larger as well as longer, tidally locking the so-called lunar near side, always facing Earth with this same side.

The post formation cooled lunar surface has been shaped by large and many small impact events, retaining a broadly cratered landscape of all ages, as well as by volcanic activity, producing the prominent lunar mares. Volcanically active until 1.2 billion years ago, most of the Moon's mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years [54] and some as old as 4.2 billion years. [55] The causes for the eruption of mare basalts, particularly their uneven occurrence on mainly the near-side, like the lunar highlands on the far side, has been an unresolved issue due to differing explanations. One explanation suggests that large meteorites were hitting the Moon in its early history leaving large craters which then were filled with lava. Other explanations suggest processes of lunar volcanism. [56]

## Physical characteristics

The Moon

The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit. [57]

### Size and mass

The Moon is by size and mass the fifth largest natural satellite of the Solar System, categorizeable as one of its planetary-mass moons, making it a satellite planet under the geophysical definitions of the term. [17] It is smaller than Mercury and considerably larger than the largest dwarf planet of the Solar System, Pluto. While the minor-planet moon Charon of the Pluto-Charon system is larger relative to Pluto, [lower-alpha 6] [58] the Moon is the largest natural satellite of the Solar System relative to their primary planets. [lower-alpha 7]

The Moon's diameter is about 3,500 km, more than a quarter of Earth's, with the face of the Moon comparable to the width of Australia. [16] The whole surface area of the Moon is about 38 million square kilometers, slightly less than the area of the Americas (North and South America).

The Moon's mass is 1/81 of Earth's, [59] being the second densest among the planetary moons, and having the second highest surface gravity, after Io, at 0.1654  g and an escape velocity of 2.38 km/s (8600 km/h; 5300 mph).

### Internal structure

The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. [60] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). [61] [62] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. [63]

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop. [64] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. [1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite. [15] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. [1] The crust is on average about 50 kilometres (31 mi) thick. [1]

The Moon is the second-densest satellite in the Solar System, after Io. [65] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, [1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel; analyses of the Moon's time-variable rotation suggest that it is at least partly molten. [66] The pressure at the lunar core is estimated to be 5 GPa (49,000 atm). [67]

#### Magnetic field

The Moon has an external magnetic field of generally less than 0.2 nanoteslas, [68] or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. [69] [70] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. [68] This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. [68] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins. [71]

#### Gravitational field

The Moon's gravitational field is not uniform, but on average, at the Moon's surface, it is 1.622  m/s2   (0.1654  g ; 5.318  ft/s2 ) [4] strong. Therefore, the surface gravity of the Moon is about half of the surface gravity of Mars and about a sixth of Earth's.

The details of the gravitational field have been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. [72] [73] The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism. [74]

### Surface conditions

The surface of the Moon, having a surface pressure of 10−10 Pa, lacks an atmosphere which can regulate the resulting extreme conditions of the surface. The surface is exposed to drastic temperature differences ranging from 140 °C to −171 °C depending on the solar irradiance. Ionizing radiation from cosmic rays, the Sun and the resulting neutron radiation [75] produce radiation levels on average of 1,369 microsieverts per day, which is about 2-3 times more than on the International Space Station at about 400 km above Earth in orbit, [76] 5-10 times more than during a trans-Atlantic flight, [77] 200 times more than on Earth's surface. [76] For further comparison radiation on a flight to Mars is about 1.84 millisieverts per day and on Mars 0.64 millisieverts per day. [78] These extreme conditions for example are considered making it unlikely for spacecrafts to harbor bacterial spores at the Moon longer than just one lunar orbit. [79]

The surface gravity of the Moon is approximately 1.625 m/s2, about a sixth or 0.166 of that on Earth's surface [4] and about half on Mars'.

#### Surface temperature

The Moon's axial tilt with respect to the ecliptic is only 1.5427°, [8] [80] much less than the 23.44° of Earth. Because of this small tilt, the Moon's solar illumination varies much less with season than on Earth and it allows for the existence of some peaks of eternal light at the Moon's north pole, at the rim of the crater Peary.

Because of the lack of atmosphere, temperatures of different areas vary particularly upon whether they are in sunlight or shadow, [81] making topographical details play a decisive role on local surface temperatures. [82] Parts of many craters, particularly the bottoms of many polar craters, [83] are permanently shadowed, these "craters of eternal darkness" have extremely low temperatures. The Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C; −397 °F) [84] and just 26 K (−247 °C; −413 °F) close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. [82]

#### Atmosphere

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes (9.8 long tons; 11 short tons). [89] 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, a product of the bombardment of lunar soil by solar wind ions. [15] [90] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io); helium-4 and neon [91] from the solar wind; and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. [92] [93] 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. [92] Water vapor 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. [94] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. [92]

Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. [95]

#### Dust cloud

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside. [96] [97]

### Surface features

The topography of the Moon has been measured with laser altimetry and stereo image analysis. [98] Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. [99] [100] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. [99] [101] The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. [102] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims. [99] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side. [1]

The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. [103] Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat. [104]

#### Volcanic features

The main features visible from Earth by the naked eye are dark and relatively featureless lunar plains called maria (singular mare; Latin for "seas", as they were once believed to be filled with water) [105] are vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. [106] The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria". [107]

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side [59] compared with 2% of the far side. [108] This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. [64] [109] [110] Most of the Moon's mare basalts erupted during the Imbrian period, 3.3–3.7 billion years ago, though some being as young as 1.2 billion years [54] and as old as 4.2 billion years. [55]

In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. [111] Moonquakes and releases of gas indicate continued lunar activity. [111] Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. [112] [113] [114] [115] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, [116] [117] inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin. [118] [119]

The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. [55] [54] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events. [120]

The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. [121] [122] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth. [123]

#### Impact craters

A major geologic process that has affected the Moon's surface is impact cratering, [124] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side. [125] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale; structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. [126] The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. [126] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts. [127]

High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. [128] [129] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts. [130]

#### Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and often have a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering. [131]

### Surface composition

#### Regolith

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened mostly gray surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. [132] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–15 m (33–49 ft) in the highlands and 4–5 m (13–16 ft) in the maria. [133] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick. [134]

Relative molecular composition of the lunar surface [135]
CompoundFormulaComposition
MariaHighlands
silica SiO245.4%45.5%
alumina Al2O314.9%24.0%
lime CaO11.8%15.9%
iron(II) oxide FeO14.1%5.9%
magnesia MgO9.2%7.5%
titanium dioxide TiO23.9%0.6%
sodium oxide Na2O0.6%0.6%
99.9%100.0%

#### Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. [136] [137] Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow. [83] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive. [138]

In years since, signatures of water have been found to exist on the lunar surface. [139] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters. [140] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. [141] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. [142]

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000  ppm. [143] Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. [144] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material. [145] [146] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb). [147]

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, [148] the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this insight does not mean that water is easily available since the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. [149] [150] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. [151] The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun. [149] [151]

In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA). [152] [153] [154] [155]

## Earth–Moon system

### Orbit

The Earth and the Moon form the Earth-Moon satellite system with a shared center of mass, or barycentre. This barycentre stays located at all times 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface, making the Moon seemingly orbit the Earth.

The orbital eccentricity, giving ovalness of the orbit, is 0.055. [1] The Lunar distance, or the semi-major axis of the geocentric lunar orbit, is approximately 400,000 km, which is a quarter of a million miles or 1.28 light-seconds, and a unit of measure in astronomy. This is not to be confused with the instantaneous Earth–Moon distance, or distance to the Moon, the momentanous distance from the center of Earth to the center of the Moon.

The Moon makes a complete orbit around Earth with respect to the fixed stars, its sidereal period, about once every 27.3 days, [lower-alpha 8] . However, because the Earth-Moon system moves at the same time in its orbit around the Sun, it takes slightly longer, 29.5 days; [lower-alpha 9] , [59] to return at the same lunar phase, completing a full cycle, as seen from Earth. This synodic period or synodic month is commonly known as the lunar month and is equal to the length of the solar day on the Moon. [156]

Due to tidal locking, the Moon has a 1:1 spin–orbit resonance. This rotationorbit ratio makes the Moon's orbital periods around Earth equal to its corresponding rotation periods. This is the reason for only one side of the Moon, its so-called near side, being visible from Earth. That said, while the movement of the Moon is in resonance, it still is not without nuances such as libration, resulting in slightly changing perspectives, making over time and location on Earth about 59% of the Moon's surface visible from Earth. [157]

Unlike most satellites of other planets, the Moon's orbital plane is closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 years, [158] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws. [159]

### Tidal effects

The gravitational attraction that Earth and the Moon (as well as the Sun) exert on each other manifests in a slightly greater attraction on the sides of closest to each other, resulting in tidal forces. Ocean tides are the most widely experienced result of this, but tidal forces considerably affect also other mechanics of Earth, as well as the Moon and their system.

#### Tides of the Moon

The lunar solid crust experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. [160] The Earth-induced variable component arises from changing distance and libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). [160]

The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. [161]

#### Ocean tides

The most commonly known effect of tidal forces are elevated sea levels called ocean tides. [162] While the Moon exerts most of the tidal forces, the Sun also exerts tidal forces and therefore contributes to the tides as much as 40% of the Moon's tidal force; producing in interplay the spring and neap tides. [162]

The tides are two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. [162] Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes; the 25 minutes is due to the Moon's time to orbit the Earth.

If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects:

• the frictional coupling of water to Earth's rotation through the ocean floors
• the inertia of water's movement
• ocean basins that grow shallower near land
• the sloshing of water between different ocean basins [163]

As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

#### Influence on Earth's magnetic field

According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field. [164]

#### Influence on orbit and rotation

Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. [162] [160] That angular momentum, lost from the Earth, is transferred to the Moon in a process known as tidal acceleration, which lifts the Moon into a higher orbit while lowering orbital speed around the Earth.

Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction. [160] Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow). [165] [166] [167] Atomic clocks show that Earth's day lengthens by about 17  microseconds every year, [168] [169] [170] slowly increasing the rate at which UTC is adjusted by leap seconds.

This tidal drag makes the rotation of Earth and the orbital period of the Moon very slowly match. This matching first results in tidally locking the lighter body of the orbital system, as already the case with the Moon. Eventually, after 50 billion years, [171] also the Earth would be made to always face the Moon with the same side. This would complete the mutual tidal locking of Earth and the Moon, matching the length of Earth's day to the then also significantly increased lunar month and the Moon's day, and suspending the Moon over one meridian (comparable to the Pluto-Charon system). However, the Sun will become a red giant engulfing the Earth-Moon system long before the latter occurs. [172] [173]

## Position and appearance

### Rotation

The tidally locked synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During dark moon to new moon, the near side is dark. [174]

The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. [175] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth. [176]

### Libration

Despite the Moon's tidal locking the effect of libration makes about 59% of the Moon's surface visible from Earth. [157] [59]

### Movement across Earth's skies

The Moon's highest altitude at culmination varies by its lunar phase, or more correctly its orbital position, and time of the year, or more correctly the position of the Earth's axis. The full moon is highest in the sky during winter and lowest during summer (for each hemisphere respectively), with its altitude changing towards dark moon to the opposite.

The Moon's highest altitude at culmination varies by its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The Moon is above the horizon for two weeks every tropical month (about 27.3 days) at the North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end. [177]

#### Apparent orientation

The apparent orientation of the Moon depends on its position in the sky and the hemisphere of the Earth from which it is being viewed.

In the northern hemisphere it is seen upside down compared to the view in the southern hemisphere. [178]

Sometimes the "horns" of a crescent moon appear to be pointing more upwards than sideways. This phenomenon is called a wet moon and occurs more frequently in the tropics. [179]

### Albedo and colour

The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. [59] [lower-alpha 10] This is due partly to the brightness enhancement of the opposition surge; the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. [180] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon's color depends on the light the Moon reflects, which in turn depends on the Moon's surface and its features, having for example large darker regions. In general the lunar surface reflects a brown-tinged gray light. [181]

#### Apparent colour

Viewed from Earth the air filters the reflected light, at times giving it a red colour depending on the angle of the Moon in the sky and thickness of the atmosphere, or a blue tinge depending on the particles in the air, [181] as in cases of volcanic particles. [182]

#### Cultural references

The terms blood moon and blue moon do not necessarily refer to circumstances of red or blue moonlight, but are rather particular cultural references such as particular full moons of a year.

### Phases

The Moon is always illuminated the same amount by the Sun, but the illuminated area of the visible sphere (degree of illumination) is given by ${\displaystyle (1-\cos e)/2=\sin ^{2}(e/2)}$, where ${\displaystyle e}$ is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).

The monthly changes in the angle between the direction of sunlight and view from Earth, and the phases of the Moon that result, as viewed from the Northern Hemisphere. The Earth–Moon distance is not to scale.

### Apparent size

The Moon's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses).

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (closest) and apogee (farthest), respectively, making the Moon's apparent size fluctuate.

#### Moon illusion

The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. [183]

### Illumination and brightness

Half of the Moon's surface is always illuminated by the Sun (except during a lunar eclipse). Earth also reflects light onto the Moon, observable at times as Earthlight when it is again reflected back to Earth from areas of the near side of the Moon that are not illuminated by the Sun.

On 14 November 2016, the Moon was at full phase closer to Earth than it had been since 1948. It was 14% closer and larger than its farthest position in apogee. [185] This closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because of its increased apparent diameter, which made it a particularly notable example of a "supermoon". [186] [187] [188]

At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula: [189] [190]

${\displaystyle {\text{perceived reduction}}\%=100\times {\sqrt {{\text{actual reduction}}\% \over 100}}}$

When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase. [191]

### Eclipses

From Earth, the Moon and the Sun appear the same size, as seen in the 1999 solar eclipse (left), whereas from the STEREO-B spacecraft in an Earth-trailing orbit, the Moon appears much smaller than the Sun (right). [192]

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. [193] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, [162] the angular diameter of the Moon is decreasing. As it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. [lower-alpha 11] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. [194]

Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. [195] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. [196]

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, [lower-alpha 12] [197] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted. [198]

### Transient lunar phenomena

There has been historical controversy over whether observed features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago. [199] [200]

## History of exploration and human presence

### Before spaceflight

#### Pre-telescopic observation (until 1609)

Since pre-historic times people have taken note of the Moon's phases, its waxing and waning, and used it to keep record of time. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. [201] One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth, Ireland. [202] [203]

The ancient Greek philosopher Anaxagoras (d.428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. [204] [205] :227 Elsewhere in the 5th century BC to 4th century BC, Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, [206] and Indian astronomers had described the Moon's monthly elongation. [207] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. [205] :411

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. [208] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. [209] In the 2nd century BC, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. [210] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance.

Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi , their 'radiating influence' theory recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon. [205] :413–414 Ptolemy (90–168 AD) greatly improved on the numbers of Aristarchus, calculating the values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. [211] In the 2nd century AD, Lucian wrote the novel A True Story , in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. [212] The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. [213] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. [205] :415–416

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth". [214]

#### Telescopic exploration (1609-1959)

In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book Sidereus Nuncius , and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier.

Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. [215] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. [59] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, [216] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. [59]

### First missions to the Moon (1959–1990)

After World War II the first launch systems were developed and by the end of the 1950s they reached capabilities that allowed the Soviet Union and the United States to launch spacecrafts into space. The Cold War fueled a closely followed development of the launch systems by the two states, resulting in the so-called Space Race and its later phase the Moon Race, accelerating efforts and interest in exploration of the Moon.

#### First robotic missions (Soviet lunar program 1959-1976)

Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, [217] the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1 in 1959; the first human-made object to impact the lunar surface was Luna 2 , and the first photographs of the normally occluded far side of the Moon were made by Luna 3 , all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first vehicle to orbit the Moon was Luna 10 , both in 1966. [59] Rock and soil samples were brought back to Earth by three Luna sample return missions ( Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. [218] Luna 17 deployed on the Moon the first remote controlled rover on an extraterrestrial surface, Lunokhod 1, in 1970.

#### First crewed missions (United States lunar program 1962-1973)

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a staffed military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. [219] [220] The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. [221] [222] [219] However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. [222]

Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures; the Lunar Orbiter program produced maps of the entire Moon; the Surveyor program landed its first spacecraft four months after Luna 9. The crewed Apollo program was developed in parallel; after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. [223]

#### Moon Treaty and explorational absence (1976–1990)

A near lunar quietude followed the 24th and last Luna as well as Soviet mission to the Moon in 1976 until 1990, for fourteen years. Astronautics had shifted its focus towards the exploration of the inner (e.g. Venera program) and outer (e.g. Pioneer 10, 1972) Solar System planets, but also towards Earth orbit, developing and continuously operating, beside communication satellites, Earth observation satellites (e.g. Landsat program, 1972) space telescopes and particularly space stations (e.g. Salyut program, 1971).

The until 1979 negotiated Moon treaty, with its ratification in 1984 by its few signatories was about the only major activity regarding the Moon until 1990.

### Renewed exploration (1990-present)

In 1990 Hiten -Hagoromo, [233] the first dedicated lunar mission since 1976, reached the Moon. Sent by Japan, it became the first mission that was not a Soviet Union or U.S. mission to the Moon.

In 1994, the U.S. dedicated a mission to fly a spacecraft ( Clementine ) to the Moon again for the first time since 1973. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. [234] In 1998 this was followed by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. [235]

The next years saw a row of first missions to the Moon by a new group of states actively exploring the Moon. Between 2004 and 2006 the first spacecraft by the European Space Agency (ESA) ( SMART-1 ) reached the Moon, recording the first detailed survey of chemical elements on the lunar surface. [236] The Chinese Lunar Exploration Program began with Chang'e 1 between 2007 and 2009, [237] obtaining a full image map of the Moon. India reached the Moon in 2008 for the first time with its Chandrayaan-1 , creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. [238]

The U.S. launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor on 18 June 2009. LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, [239] whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery.

China continued its lunar program in 2010 with Chang'e 2 , mapping the surface at a higher resolution over an eight-month period, and in 2013 with Chang'e 3 , a lunar lander along with a lunar rover named Yutu (Chinese: 玉兔; literally "Jade Rabbit"). This was the first lunar rover mission since Lunokhod 2 in 1973 and the first lunar soft landing since Luna 24 in 1976.

In 2014 the first privately funded probe, the Manfred Memorial Moon Mission, reached the Moon.

Another Chinese rover mission, Chang'e 4 , achieved the first landing on the Moon's far side in early 2019. [240]

Also in 2019, India successfully sent its second probe, Chandrayaan-2 to the Moon.

In 2020, China carried out its first robotic sample return mission ( Chang'e 5 ), bringing back 1,731 grams of lunar material to Earth. [241]

With the signing of the U.S.-led Artemis Accords in 2020, the Artemis program aims to return in the astronauts to the Moon in the 2020s. [242] The Accords have been joined by a growing number of countries. The introduction of the Artemis Accords has fueled a renewed discussion about the international framework and cooperation of lunar activity, building on the Moon Treaty and the ESA-led Moon Village concept. [243] [244] [245] The U.S. developed plans for returning to the Moon beginning in 2004, [246] which resulted in several programs. The Artemis program has advanced the farthest, and includes plans to send the first woman to the Moon [247] as well as build an international lunar space station called Lunar Gateway.

### Future

Upcoming lunar missions include Artemis 1 and Russia's first lunar mission, Luna-Glob : an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. [248]

China has announced in 2021 the plan to develop and construct with Russia an International Lunar Research Station towards and into the 2030s. India in 2006 had among others expressed its hope to send people to the Moon by 2020. [249]

## Human presence

### Human impact

#### Pollution, contamination and sustainability

While the Moon has the lowest planetary protection target-categorization, its degradation as a pristine body and scientific place has been discussed [250] and particularly understood regarding keeping the Shielded Zone of the Moon (SZM) on the far side, of value for astronomy from the Moon, free from any radio spectrum pollution, as well as conserving the special and scientifically interesting nature of the Moon, in face of prospecting commercial and national projects to claim and exploit the Moon. [251] While the Moon has no significant atmosphere, traffic and impacts on the Moon causes clouds of dust that can spread far and possibly contaminate the original state of the Moon and its special scientific content. [252] Scholar Alice Gorman asserts that, although the Moon is inhospitable, it is not dead, and that sustainable human activity would require treating the Moon's ecology as a co-participant. [253]

The so-called "Tardigrade affair" of the 2019 crashed Beresheet lander and its carrying of tardigrades has been discussed as an example for lacking measures and lacking international regulation for planetary protection. [251]

Space debris beyond Earth around the Moon has been considered as a future challenge with increasing numbers of missions to the Moon, particularly as a danger for such missions. [254] [255] As such lunar waste management has been raised as an issue which future lunar missions, particularly on the surface, need to tackle. [256] [257]

#### Intended remains

Beside the remains of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six Lunar plaques, the Fallen Astronaut memorial, and other artifacts. [258]

#### Infrastructure

Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational. [259] Five retroreflectors have been installed on the Moon since the 1970s and since used for accurate measurements of the physical librations through laser ranging to the Moon.

There are several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.

### Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes. [260] It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. [261] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter. [262] A lunar zenith telescope can be made cheaply with an ionic liquid. [263]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. [264]

The Moon has been also a sight of Earth observation, particularly culturally as in the imagery called Earthrise.

### Living on the Moon

The only instances of humans living on the Moon have taken place in an Apollo Lunar Module (for example, during the Apollo 17 mission) for several days at a time. [265] One challenge to astronauts during their stay on the surface is that lunar dust sticks to their suits and is carried into their quarters. Astronauts could taste and smell the dust, calling it the "Apollo aroma". [266] This fine lunar dust can cause health issues. [266]

In 2019 at least one plant seed sprouted in an experiment on the Chang'e 4 lander. It was carried from Earth along with other small life in its Lunar Micro Ecosystem. [267]

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S.A. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. [268] Likewise no private ownership of parts of the Moon, or as a whole, is considered credible. [269] [270] [271]

The 1967 Outer Space Treaty defines the Moon and all outer space as the "province of all mankind". [268] It restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. [272] A majority of countries are parties of this treaty. [273] The 1979 Moon Agreement was created to elaborate, and restrict the exploitation of the Moon's resources by any single nation, leaving it to a yet unspecified international regulatory regime. [274] As of January 2020, it has been signed and ratified by 18 nations, [275] none of which have human spaceflight capabilities.

Since 2020 countries have joined the U.S.A. in their Artemis Accords, which are challenging the treaty. The U.S.A. has furthermore emphasized in an presidential executive order ("Encouraging International Support for the Recovery and Use of Space Resources.") that "the United States does not view outer space as a 'global commons'" and calls the Moon Agreement "a failed attempt at constraining free enterprise." [276] [277]

With Australia signing and ratifying both the Moon Treaty in 1986 as well as the Artemis Accords in 2020, there has been a discussion if they can be harmonized. [244] In this light an Implementation Agreement for the Moon Treaty has been advocated for, as a way to compensate for the shortcomings of the Moon Treaty and to harmonize it with other laws, allowing it to be more widely accepted. [243] [245]

In the face of such increasing commercial and national interest, particularly prospecting territories, U.S.A. lawmakers have introduced in late 2020 specific regulation for the conservation of historic landing sites [278] and interest groups have argued for making such sites World Heritage Sites [279] and zones of scientific value protected zones, all of which add to the legal availability and territorialization of the Moon. [251]

In 2021 the Declaration of the Rights of the Moon [280] was created by a group of "lawyers, space archaeologists and concerned citizens", drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space. [281] [282]

### Coordination

In light of future development on the Moon some international and multi-space agency organizations have been created:

## In culture and life

### Calendar

Since pre-historic times people have taken note of the Moon's phases, its waxing and waning, and used it to keep record of time. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. [201] [285] [286] The counting of the days between the Moon's phases gave eventually rise to generalized time periods of the full lunar cycle as months, and possibly of its phases as weeks. [287]

The words for the month in a range of different languages carry this relation between the period of the month and the Moon etymologically. The English month as well as moon, and its cognates in other Indo-European languages (e.g. the Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month") [288] [289] [290] [291] stem from the Proto-Indo-European (PIE) root of moon, *méh1nōt, derived from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual). [292] [293] [294] To give another example from a different language family, the Chinese language uses the same word () for moon as well as for month, which furthermore can be found in the symbols for the word week ().

This lunar timekeeping gave rise to the historically dominant, but varied, lunisolar calendars. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. [295]

Of particular significance has been for a range of cultures and calendars the occasion of the Full Moon, to use or celebrate, particularly around the autumnal equinox, the so-called Harvest Moon.

Furthermore, association of time with the Moon can also be found in religion, such as the ancient Egyptian temporal and lunar deity Khonsu.

### Cultural representation

From top: examples of lunar deities featuring around the world recurring aspects, like the crescent (Nanna/Sîn, c.2100 BC), crescent headgear and chariot (Luna, 2nd–5th century), as well as the Moon rabbit (Mayan moon goddess, 6th–9th century). [296]

Since prehistoric and ancient times humans have depicted and interpreted the Moon, particularly for astrology and religion, as lunar deity.

For the representation of the Moon, especially its lunar phases, the crescent symbol has been particularly used by many cultures. In writing systems such as Chinese the crescent has developed into the symbol , the word for Moon, and in ancient Egyptian it was the symbol 𓇹, which is spelled like the ancient Egyptian lunar deity Iah , meaning Moon. [297]

Iconographically the crescent was used in Mesopotamia as the primary symbol of Nanna/Sîn, [298] the ancient Sumerian lunar deity, [299] [298] who was the father of Innana/Ishtar, the goddess of the planet Venus (symbolized as the eight pointed Star of Ishtar), [299] [298] and Utu/Shamash, the god of the Sun (symbolized as a disc, optionally with eight rays), [299] [298] all three often depicted next to each other. Nanna was later known as Sîn, [298] [299] and was particularly associated with magic and sorcery. [299]

The crescent was further used as an element of lunar deities wearing headgears or crowns in an arrangement reminiscent of horns, as in the case of the ancient Greek Selene [300] [301] or the ancient Egyptian Khonsu. Selene is associated with Artemis and paralleled by the Roman Luna, which both are occasionally depicted driving a chariot, like the Hindu lunar deity Chandra. The different or sharing aspects of deities within pantheons has been observed in many cultures, especially by later or contemporary culture, particularly forming triple deities. The Moon in Roman mythology for example has been associated with Juno and Diana, while Luna being identified as their byname and as part of a triplet (diva triformis) with Diana and Proserpina, Hecate being identified as their binding manifestation as trimorphos.

The star and crescent (☪️) arrangement goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium, possibly influencing the development of the Ottoman flag, specifically the combination of the Turkish crescent with a star. [302] Since then the heraldric use of the star and crescent proliferated becoming a popular symbol for Islam (as the hilal of the Islamic calendar) and for a range of nations. [303]

In Roman Catholic Marian veneration, the Virgin Mary (Queen of Heaven) has been depicted since the late middle ages on a crescent and adorned with stars. In Islam Muhammad is particularly attributed with the Moon through the so-called splitting of the Moon (Arabic : انشقاق القمر) miracle. [304]

The contrast between the brighter highlands and the darker maria have been seen by different cultures forming abstract shapes, which are among others the Man in the Moon or the Moon Rabbit (e.g. the Chinese Tu'er Ye or in Indigenous American mythologies, as with the aspect of the Mayan Moon goddess). [296]

In Western alchemy silver is associated with the Moon, and gold with the Sun. [305]

#### Modern representation

The perception of the Moon in modern times has been informed by telescope enabled modern astronomy and later by spaceflight enabled actual human activity at the Moon, particularly the culturally impactful lunar landings. These new insights inspired cultural references, connecting romantic reflections about the Moon [306] and speculative fiction such as science-fiction dealing with the Moon. [307] [308]

Contemporarily the Moon has been seen as a place for economic expansion into space, with missions prospecting for lunar resources. This has been accompanied with renewed public and critical reflection on humanity's cultural and legal relation to the celestial body, especially regarding colonialism, [251] as in the 1970 poem "Whitey on the Moon". In this light the Moon's nature has been invoked, [280] particularly for lunar conservation [255] and as a common. [309] [274] [282]

A song titled "Moon Anthem" by Abhay Kumar, paralleling the proposals for an Earth Anthem, was released 2019 on the occasion of India's lunar probe Chandrayaan-2. [310] [311]

The Moon is prominently featured in Vincent van Gogh's 1889 painting, The Starry Night (left). An iconic image of the Man in the Moon from the first science-fiction film set in space, A Trip to the Moon (1902), inspired by a history of literature about going to the Moon (right).

### Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. The Moon has long been associated with insanity and irrationality; the words lunacy and lunatic are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. [312] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims. [312] [313] [314] [315] [316]

## Explanatory notes

1. Between 18.29° and 28.58° to Earth's equator. [1]
2. There are a number of near-Earth asteroids, including 3753 Cruithne, that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term (Morais et al, 2002). These are quasi-satellites  – they are not moons as they do not orbit Earth. For more information, see Other moons of Earth.
3. The maximum value is given based on scaling of the brightness from the value of −12.74 given for an equator to Moon-centre distance of 378 000 km in the NASA factsheet reference to the minimum Earth–Moon distance given there, after the latter is corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km. The minimum value (for a distant new moon) is based on a similar scaling using the maximum Earth–Moon distance of 407 000 km (given in the factsheet) and by calculating the brightness of the earthshine onto such a new moon. The brightness of the earthshine is [ Earth albedo × Radius of Moon's orbit)2 ] relative to the direct solar illumination that occurs for a full moon. (Earth albedo = 0.367; Earth radius = (polar radius × equatorial radius)½ = 6 367 km.)
4. The range of angular size values given are based on simple scaling of the following values given in the fact sheet reference: at an Earth-equator to Moon-centre distance of 378 000 km, the angular size is 1896  arcseconds. The same fact sheet gives extreme Earth–Moon distances of 407 000 km and 357 000 km. For the maximum angular size, the minimum distance has to be corrected for Earth's equatorial radius of 6 378 km, giving 350 600 km.
5. Lucey et al. (2006) give 107 particles cm−3 by day and 105 particles cm−3 by night. Along with equatorial surface temperatures of 390  K by day and 100 K by night, the ideal gas law yields the pressures given in the infobox (rounded to the nearest order of magnitude): 10−7  Pa by day and 10−10 Pa by night.
6. With 27% the diameter and 60% the density of Earth, the Moon has 1.23% of the mass of Earth. The moon Charon is larger relative to its primary Pluto, but Earth and the Moon are different since Pluto is considered a dwarf planet and not a planet, unlike Earth.
7. There is no strong correlation between the sizes of planets and the sizes of their satellites. Larger planets tend to have more satellites, both large and small, than smaller planets.
8. More accurately, the Moon's mean sidereal period (fixed star to fixed star) is 27.321661 days (27 d 07 h 43 min 11.5 s), and its mean tropical orbital period (from equinox to equinox) is 27.321582 days (27 d 07 h 43 min 04.7 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
9. More accurately, the Moon's mean synodic period (between mean solar conjunctions) is 29.530589 days (29 d 12 h 44 min 02.9 s) (Explanatory Supplement to the Astronomical Ephemeris, 1961, at p.107).
10. The Sun's apparent magnitude is −26.7, while the full moon's apparent magnitude is −12.7.
11. See graph in Sun#Life phases. At present, the diameter of the Sun is increasing at a rate of about five percent per billion years. This is very similar to the rate at which the apparent angular diameter of the Moon is decreasing as it recedes from Earth.
12. On average, the Moon covers an area of 0.21078 square degrees on the night sky.

## Related Research Articles

Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god Mercurius (Mercury), god of commerce, messenger of the gods, and mediator between gods and mortals, corresponding to the Greek god Hermes (Ἑρμῆς). Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, and its apparent distance from the Sun as viewed from Earth never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight. At this time, it may appear as a bright star-like object, but is more difficult to observe than Venus. From Earth, the planet telescopically displays the complete range of phases, similar to Venus and the Moon, which recurs over its synodic period of approximately 116 days.

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System that may not be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. With a diameter of 4821 km, Callisto is about 99% the diameter of the planet Mercury, but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward. Because of this, there is a sub-Jovian point on Callisto's surface, from which Jupiter would appear to hang directly overhead. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.

Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 80 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus.

Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt.

Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere, and is the only known object in space other than Earth on which clear evidence of stable bodies of surface liquid has been found.

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is named after the Greek deity Phobos, a son of Ares (Mars) and twin brother of Deimos.

Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth-largest object of the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi), making it 26 percent larger than the planet Mercury by volume, although it is only 45 percent as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Deimos is the smaller and outermost of the two natural satellites of Mars, the other being Phobos. Of similar composition to C and D-type asteroids, Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much farther than Mars's other moon, Phobos. It is named after Deimos, the Ancient Greek god and personification of dread and terror.

Enceladus is the sixth-largest moon of Saturn. It is about 500 kilometers in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon only reaches −198 °C, far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains.

Mimas, also designated Saturn I, is a moon of Saturn discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology.

Ariel is the fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus and so has an extreme seasonal cycle.

Hermite is a lunar impact crater located along the northern lunar limb, close to the north pole of the Moon. Named for Charles Hermite, the crater was formed roughly 3.91 billion years ago. Hermite's southwestern edge is the coldest place currently known in the Solar System.

The far side of the Moon is the lunar hemisphere that always faces away from Earth, opposite to the near side, because of synchronous rotation in the moon's orbit. Compared to the near side, the far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the solar system such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. The hemisphere is sometimes called the "dark side of the Moon", where "dark" means "unknown" instead of "lacking sunlight" – both sides of the Moon experience two weeks of sunlight while the opposite side experiences two weeks of night.

Shackleton is an impact crater that lies at the lunar south pole. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.

Chandrayaan-1 was the first Indian lunar probe under the Chandrayaan program. It was launched by the Indian Space Research Organisation in October 2008, and operated until August 2009. The mission included a lunar orbiter and an impactor. India launched the spacecraft using a PSLV-XL rocket on 22 October 2008 at 00:52 UTC from Satish Dhawan Space Centre, at Sriharikota, Andhra Pradesh. The mission was a major boost to India's space program, as India researched and developed indigenous technology to explore the Moon. The vehicle was inserted into lunar orbit on 8 November 2008.

Lunar water is water that is present on the Moon. Diffuse water molecules can persist at the Moon's sunlit surface, as discovered by NASA's SOFIA observatory in 2020. Gradually water vapor is decomposed by sunlight, leaving hydrogen and oxygen lost to outer space. Scientists have found water ice in the cold, permanently shadowed craters at the Moon's poles. Water molecules are also present in the extremely thin lunar atmosphere.

The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a situation known as tidal locking.

The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

The lunar south pole is the southernmost point on the Moon, at 90°S. It is of special interest to scientists because of the occurrence of water ice in permanently shadowed areas around it. The lunar south pole region features craters that are unique in that the near-constant sunlight does not reach their interior. Such craters are cold traps that contain a fossil record of hydrogen, water ice, and other volatiles dating from the early Solar System. In contrast, the lunar north pole region exhibits a much lower quantity of similarly sheltered craters.

A permanently shadowed crater is a depression on a body in the Solar System within which lies a point that is always in darkness.

## References

1. Wieczorek, Mark A.; Jolliff, Bradley L.; Khan, Amir; Pritchard, Matthew E.; Weiss, Benjamin P.; Williams, James G.; Hood, Lon L.; Righter, Kevin; Neal, Clive R.; Shearer, Charles K.; McCallum, I. Stewart; Tompkins, Stephanie; Hawke, B. Ray; Peterson, Chris; Gillis, Jeffrey J.; Bussey, Ben (2006). "The constitution and structure of the lunar interior". Reviews in Mineralogy and Geochemistry . 60 (1): 221–364. Bibcode:2006RvMG...60..221W. doi:10.2138/rmg.2006.60.3. S2CID   130734866. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
2. Lang, Kenneth R. (2011). The Cambridge Guide to the Solar System (2nd ed.). Cambridge University Press. ISBN   9781139494175. Archived from the original on 1 January 2016.
3. Morais, M. H. M.; Morbidelli, A. (2002). "The Population of Near-Earth Asteroids in Coorbital Motion with the Earth". Icarus . 160 (1): 1–9. Bibcode:2002Icar..160....1M. doi:10.1006/icar.2002.6937. hdl:. S2CID   55214551. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
4. Williams, David R. (2 February 2006). "Moon Fact Sheet". NASA/National Space Science Data Center. Archived from the original on 23 March 2010. Retrieved 31 December 2008.
5. Smith, David E.; Zuber, Maria T.; Neumann, Gregory A.; Lemoine, Frank G. (1 January 1997). "Topography of the Moon from the Clementine lidar". Journal of Geophysical Research . 102 (E1): 1601. Bibcode:1997JGR...102.1591S. doi:10.1029/96JE02940. hdl:. S2CID   17475023. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
6. Terry, Paul (2013). Top 10 of Everything. Octopus Publishing Group Ltd. p. 226. ISBN   978-0-600-62887-3.
7. Williams, James G.; Newhall, XX; Dickey, Jean O. (1996). "Lunar moments, tides, orientation, and coordinate frames". Planetary and Space Science . 44 (10): 1077–1080. Bibcode:1996P&SS...44.1077W. doi:10.1016/0032-0633(95)00154-9.
8. Hamilton, Calvin J.; Hamilton, Rosanna L., The Moon, Views of the Solar System Archived 4 February 2016 at the Wayback Machine , 1995–2011.
9. Makemson, Maud W. (1971). "Determination of selenographic positions". The Moon. 2 (3): 293–308. Bibcode:1971Moon....2..293M. doi:10.1007/BF00561882. S2CID   119603394.
10. Archinal, Brent A.; A'Hearn, Michael F.; Bowell, Edward G.; Conrad, Albert R.; Consolmagno, Guy J.; Courtin, Régis; Fukushima, Toshio; Hestroffer, Daniel; Hilton, James L.; Krasinsky, George A.; Neumann, Gregory A.; Oberst, Jürgen; Seidelmann, P. Kenneth; Stooke, Philip J.; Tholen, David J.; Thomas, Paul C.; Williams, Iwan P. (2010). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (PDF). Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4. S2CID   189842666. Archived from the original (PDF) on 4 March 2016. Retrieved 24 September 2018. also available "via usgs.gov" (PDF). Archived (PDF) from the original on 27 April 2019. Retrieved 26 September 2018.
11. Matthews, Grant (2008). "Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES". Applied Optics . 47 (27): 4981–4993. Bibcode:2008ApOpt..47.4981M. doi:10.1364/AO.47.004981. PMID   18806861.
12. Bugby, D. C.; Farmer, J. T.; O’Connor, B. F.; Wirzburger, M. J.; C. J. Stouffer, E. D. Abel (January 2010). Two‐Phase Thermal Switching System for a Small, Extended Duration Lunar Surface Science Platform. AIP Conference Proceedings. Vol. 1208. pp. 76–83. Bibcode:2010AIPC.1208...76B. doi:10.1063/1.3326291. hdl:.
13. Vasavada, A. R.; Paige, D. A.; Wood, S. E. (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus . 141 (2): 179–193. Bibcode:1999Icar..141..179V. doi:10.1006/icar.1999.6175. S2CID   37706412. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
14. Zhang S, Wimmer-Schweingruber RF, Yu J, Wang C, Fu Q, Zou Y, et al. (2020). "First measurements of the radiation dose on the lunar surface". Science Advances. 6 (39). Bibcode:2020SciA....6.1334Z. doi:10.1126/sciadv.aaz1334. PMC  . PMID   32978156. We measured an average total absorbed dose rate in silicon of 13.2 ± 1 μGy/hour ... LND measured an average dose equivalent of 1369 μSv/day on the surface of the Moon
15. Lucey, Paul; Korotev, Randy L.; Gillis, Jeffrey J.; Taylor, Larry A.; Lawrence, David; Campbell, Bruce A.; Elphic, Rick; Feldman, Bill; Hood, Lon L.; Hunten, Donald; Mendillo, Michael; Noble, Sarah; Papike, James J.; Reedy, Robert C.; Lawson, Stefanie; Prettyman, Tom; Gasnault, Olivier; Maurice, Sylvestre (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry . 60 (1): 83–219. Bibcode:2006RvMG...60...83L. doi:10.2138/rmg.2006.60.2.
16. Horner, Jonti (18 July 2019). "How big is the Moon?". Archived from the original on 7 November 2020. Retrieved 15 November 2020.
17. Metzger, Philip; Grundy, Will; Sykes, Mark; Stern, Alan; Bell, James; Detelich, Charlene; Runyon, Kirby; Summers, Michael (2021), "Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science", Icarus , 374: 114768, doi:10.1016/j.icarus.2021.114768, S2CID   240071005
18. "Naming Astronomical Objects: Spelling of Names". International Astronomical Union. Archived from the original on 16 December 2008. Retrieved 6 April 2020.
19. "Gazetteer of Planetary Nomenclature: Planetary Nomenclature FAQ". USGS Astrogeology Research Program. Archived from the original on 27 May 2010. Retrieved 6 April 2020.
20. Orel, Vladimir (2003). A Handbook of Germanic Etymology. Brill. Archived from the original on 17 June 2020. Retrieved 5 March 2020.
21. López-Menchero, Fernando (22 May 2020). "Late Proto-Indo-European Etymological Lexicon". Archived from the original on 22 May 2020. Retrieved 30 July 2022.
22. Barnhart, Robert K. (1995). The Barnhart Concise Dictionary of Etymology. HarperCollins. p. 487. ISBN   978-0-06-270084-1.
23. E.g.: Hall III, James A. (2016). Moons of the Solar System. Springer International. ISBN   978-3-319-20636-3.
24. . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
25. . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
26. . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
27. . Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
28. "Oxford English Dictionary: lunar, a. and n." Oxford English Dictionary: Second Edition 1989. Oxford University Press. Archived from the original on 19 August 2020. Retrieved 23 March 2010.
29. Pannen, Imke (2010). When the Bad Bleeds: Mantic Elements in English Renaissance Revenge Tragedy. V&R unipress GmbH. pp. 96–. ISBN   978-3-89971-640-5. Archived from the original on 4 September 2016.
30. Thiemens, Maxwell M.; Sprung, Peter; Fonseca, Raúl O. C.; Leitzke, Felipe P.; Münker, Carsten (July 2019). "Early Moon formation inferred from hafnium-tungsten systematics". Nature Geoscience. 12 (9): 696–700. Bibcode:2019NatGe..12..696T. doi:10.1038/s41561-019-0398-3. S2CID   198997377.
31. "The Moon is older than scientists thought". Universe Today. Archived from the original on 3 August 2019. Retrieved 3 August 2019.
32. Barboni, M.; Boehnke, P.; Keller, C.B.; Kohl, I.E.; Schoene, B.; Young, E.D.; McKeegan, K.D. (2017). "Early formation of the Moon 4.51 billion years ago". Science Advances. 3 (1): e1602365. Bibcode:2017SciA....3E2365B. doi:10.1126/sciadv.1602365. PMC  . PMID   28097222.
33. Binder, A. B. (1974). "On the origin of the Moon by rotational fission". The Moon . 11 (2): 53–76. Bibcode:1974Moon...11...53B. doi:10.1007/BF01877794. S2CID   122622374.
34. Stroud, Rick (2009). . Walken and Company. pp.  24–27. ISBN   978-0-8027-1734-4. Archived from the original on 17 June 2020. Retrieved 11 November 2019.
35. Mitler, H. E. (1975). "Formation of an iron-poor moon by partial capture, or: Yet another exotic theory of lunar origin". Icarus . 24 (2): 256–268. Bibcode:1975Icar...24..256M. doi:10.1016/0019-1035(75)90102-5.
36. Stevenson, D.J. (1987). "Origin of the moon–The collision hypothesis". Annual Review of Earth and Planetary Sciences . 15 (1): 271–315. Bibcode:1987AREPS..15..271S. doi:10.1146/annurev.ea.15.050187.001415. S2CID   53516498. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
37. Taylor, G. Jeffrey (31 December 1998). "Origin of the Earth and Moon". Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology. Archived from the original on 10 June 2010. Retrieved 7 April 2010.
38. "Asteroids Bear Scars of Moon's Violent Formation". 16 April 2015. Archived from the original on 8 October 2016.
39. van Putten, Maurice H. P. M. (July 2017). "Scaling in global tidal dissipation of the Earth-Moon system". New Astronomy. 54: 115–121. arXiv:. Bibcode:2017NewA...54..115V. doi:10.1016/j.newast.2017.01.012. S2CID   119285032.
40. Canup, R.; Asphaug, E. (2001). "Origin of the Moon in a giant impact near the end of Earth's formation". Nature . 412 (6848): 708–712. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID   11507633. S2CID   4413525.
41. "Earth-Asteroid Collision Formed Moon Later Than Thought". National Geographic . 28 October 2010. Archived from the original on 18 April 2009. Retrieved 7 May 2012.
42. Kleine, Thorsten (2008). "2008 Pellas-Ryder Award for Mathieu Touboul" (PDF). Meteoritics and Planetary Science. 43 (S7): A11–A12. Bibcode:2008M&PS...43...11K. doi:10.1111/j.1945-5100.2008.tb00709.x. S2CID   128609987. Archived from the original (PDF) on 27 July 2018. Retrieved 8 April 2020.
43. Touboul, M.; Kleine, T.; Bourdon, B.; Palme, H.; Wieler, R. (2007). "Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals". Nature . 450 (7173): 1206–1209. Bibcode:2007Natur.450.1206T. doi:10.1038/nature06428. PMID   18097403. S2CID   4416259.
44. "Flying Oceans of Magma Help Demystify the Moon's Creation". National Geographic . 8 April 2015. Archived from the original on 9 April 2015.
45. Pahlevan, Kaveh; Stevenson, David J. (2007). "Equilibration in the aftermath of the lunar-forming giant impact". Earth and Planetary Science Letters . 262 (3–4): 438–449. arXiv:. Bibcode:2007E&PSL.262..438P. doi:10.1016/j.epsl.2007.07.055. S2CID   53064179.
46. Nield, Ted (2009). "Moonwalk (summary of meeting at Meteoritical Society's 72nd Annual Meeting, Nancy, France)". Geoscientist . Vol. 19. p. 8. Archived from the original on 27 September 2012.
47. Warren, P. H. (1985). "The magma ocean concept and lunar evolution". Annual Review of Earth and Planetary Sciences . 13 (1): 201–240. Bibcode:1985AREPS..13..201W. doi:10.1146/annurev.ea.13.050185.001221.
48. Tonks, W. Brian; Melosh, H. Jay (1993). "Magma ocean formation due to giant impacts". Journal of Geophysical Research . 98 (E3): 5319–5333. Bibcode:1993JGR....98.5319T. doi:10.1029/92JE02726.
49. Daniel Clery (11 October 2013). "Impact Theory Gets Whacked". Science . 342 (6155): 183–185. Bibcode:2013Sci...342..183C. doi:10.1126/science.342.6155.183. PMID   24115419.
50. "Earth-Moon Dynamics". Lunar and Planetary Institute. Retrieved 2 September 2022.
51. Hiesinger, H.; Head, J. W.; Wolf, U.; Jaumann, R.; Neukum, G. (2003). "Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Numbium, Mare Cognitum, and Mare Insularum". Journal of Geophysical Research . 108 (E7): 1029. Bibcode:2003JGRE..108.5065H. doi:. S2CID   9570915.
52. Papike, J.; Ryder, G.; Shearer, C. (1998). "Lunar Samples". Reviews in Mineralogy and Geochemistry . 36: 5.1–5.234.
53. "Lunar Far Side Highlands". ESA Science & Technology. 14 July 2006. Retrieved 2 September 2022.
54. Garrick-Bethell, Ian; Perera, Viranga; Nimmo, Francis; Zuber, Maria T. (2014). "The tidal-rotational shape of the Moon and evidence for polar wander" (PDF). Nature. 512 (7513): 181–184. Bibcode:2014Natur.512..181G. doi:10.1038/nature13639. PMID   25079322. S2CID   4452886. Archived (PDF) from the original on 4 August 2020. Retrieved 12 April 2020.
55. "Space Topics: Pluto and Charon". The Planetary Society. Archived from the original on 18 February 2012. Retrieved 6 April 2010.
56. Spudis, P. D. (2004). "Moon". World Book Online Reference Center, NASA. Archived from the original on 3 July 2013. Retrieved 12 April 2007.
57. Runcorn, Stanley Keith (31 March 1977). "Interpretation of lunar potential fields". Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences. 285 (1327): 507–516. Bibcode:1977RSPTA.285..507R. doi:10.1098/rsta.1977.0094. S2CID   124703189.
58. Brown, D.; Anderson, J. (6 January 2011). "NASA Research Team Reveals Moon Has Earth-Like Core". NASA. NASA. Archived from the original on 11 January 2012.
59. Weber, R.C.; Lin, P.-Y.; Garnero, E.J.; Williams, Q.; Lognonne, P. (21 January 2011). "Seismic Detection of the Lunar Core" (PDF). Science. 331 (6015): 309–312. Bibcode:2011Sci...331..309W. doi:10.1126/science.1199375. PMID   21212323. S2CID   206530647. Archived from the original (PDF) on 15 October 2015. Retrieved 10 April 2017.
60. Nemchin, A.; Timms, N.; Pidgeon, R.; Geisler, T.; Reddy, S.; Meyer, C. (2009). "Timing of crystallization of the lunar magma ocean constrained by the oldest zircon". Nature Geoscience . 2 (2): 133–136. Bibcode:2009NatGe...2..133N. doi:10.1038/ngeo417. hdl:.
61. Shearer, Charles K.; Hess, Paul C.; Wieczorek, Mark A.; Pritchard, Matt E.; Parmentier, E. Mark; Borg, Lars E.; Longhi, John; Elkins-Tanton, Linda T.; Neal, Clive R.; Antonenko, Irene; Canup, Robin M.; Halliday, Alex N.; Grove, Tim L.; Hager, Bradford H.; Lee, D.-C.; Wiechert, Uwe (2006). "Thermal and magmatic evolution of the Moon". Reviews in Mineralogy and Geochemistry . 60 (1): 365–518. Bibcode:2006RvMG...60..365S. doi:10.2138/rmg.2006.60.4. S2CID   129184748. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
62. Schubert, J. (2004). "Interior composition, structure, and dynamics of the Galilean satellites.". In F. Bagenal; et al. (eds.). Jupiter: The Planet, Satellites, and Magnetosphere. Cambridge University Press. pp. 281–306. ISBN   978-0-521-81808-7.
63. Williams, J.G.; Turyshev, S.G.; Boggs, D.H.; Ratcliff, J.T. (2006). "Lunar laser ranging science: Gravitational physics and lunar interior and geodesy". Advances in Space Research . 37 (1): 67–71. arXiv:. Bibcode:2006AdSpR..37...67W. doi:10.1016/j.asr.2005.05.013. S2CID   14801321.
64. Evans, Alexander J.; Tikoo, Sonia M.; Jeffrey C., Andrews-Hanna (January 2018). "The Case Against an Early Lunar Dynamo Powered by Core Convection". Geophysical Research Letters. 45 (1): 98–107. Bibcode:2018GeoRL..45...98E. doi:.
65. Mighani, S.; Wang, H.; Shuster, D.L.; Borlina, C.S.; Nichols, C.I.O.; Weiss, B.P. (2020). "The end of the lunar dynamo". Science Advances. 6 (1): eaax0883. Bibcode:2020SciA....6..883M. doi:10.1126/sciadv.aax0883. PMC  . PMID   31911941.
66. Garrick-Bethell, Ian; Weiss, iBenjamin P.; Shuster, David L.; Buz, Jennifer (2009). "Early Lunar Magnetism". Science . 323 (5912): 356–359. Bibcode:2009Sci...323..356G. doi:10.1126/science.1166804. PMID   19150839. S2CID   23227936. Archived from the original on 19 August 2020. Retrieved 2 December 2019.
67. "Magnetometer / Electron Reflectometer Results". Lunar Prospector (NASA). 2001. Archived from the original on 27 May 2010. Retrieved 17 March 2010.
68. Hood, L.L.; Huang, Z. (1991). "Formation of magnetic anomalies antipodal to lunar impact basins: Two-dimensional model calculations". Journal of Geophysical Research . 96 (B6): 9837–9846. Bibcode:1991JGR....96.9837H. doi:10.1029/91JB00308.
69. Muller, P.; Sjogren, W. (1968). "Mascons: lunar mass concentrations". Science . 161 (3842): 680–684. Bibcode:1968Sci...161..680M. doi:10.1126/science.161.3842.680. PMID   17801458. S2CID   40110502.
70. Richard A. Kerr (12 April 2013). "The Mystery of Our Moon's Gravitational Bumps Solved?". Science . 340 (6129): 138–139. doi:10.1126/science.340.6129.138-a. PMID   23580504.
71. Konopliv, A.; Asmar, S.; Carranza, E.; Sjogren, W.; Yuan, D. (2001). "Recent gravity models as a result of the Lunar Prospector mission" (PDF). Icarus . 50 (1): 1–18. Bibcode:2001Icar..150....1K. CiteSeerX  . doi:10.1006/icar.2000.6573. Archived from the original (PDF) on 13 November 2004.
72. "Radioactive Moon". Science Mission Directorate. 8 September 2005. Archived from the original on 2 November 2019. Retrieved 28 July 2022.
73. "We Finally Know How Much Radiation There Is on The Moon, And It's Not Great News". ScienceAlert. 26 September 2020. Archived from the original on 28 July 2022. Retrieved 28 July 2022.
74. Wall, Mike (28 September 2020). "We now know exactly how much radiation astronauts will face on the moon". Space.com. Retrieved 7 August 2022.
75. Wall, Mike (9 December 2013). "Radiation on Mars 'Manageable' for Manned Mission, Curiosity Rover Reveals". Space.com. Retrieved 7 August 2022.
76. Schuerger, Andrew C.; Moores, John E.; Smith, David J.; Reitz, Günther (June 2019). "A Lunar Microbial Survival Model for Predicting the Forward Contamination of the Moon". Astrobiology. 19 (6): 730–756. Bibcode:2019AsBio..19..730S. doi:10.1089/ast.2018.1952. PMID   30810338. S2CID   73491587.
77. Rambaux, N.; Williams, J. G. (2011). "The Moon's physical librations and determination of their free modes". Celestial Mechanics and Dynamical Astronomy. 109 (1): 85–100. Bibcode:2011CeMDA.109...85R. doi:10.1007/s10569-010-9314-2. S2CID   45209988. Archived from the original on 30 July 2022. Retrieved 30 July 2022.
78. Rocheleau, Jake (21 May 2012). "Temperature on the Moon – Surface Temperature of the Moon". PlanetFacts.org. Archived from the original on 27 May 2015.
79. Amos, Jonathan (16 December 2009). "'Coldest place' found on the Moon". BBC News. Archived from the original on 11 August 2017. Retrieved 20 March 2010.
80. Martel, L. M. V. (4 June 2003). "The Moon's Dark, Icy Poles". Planetary Science Research Discoveries: 73. Bibcode:2003psrd.reptE..73M. Archived from the original on 1 March 2012. Retrieved 12 April 2007.
81. "Diviner News". UCLA. 17 September 2009. Archived from the original on 7 March 2010. Retrieved 17 March 2010.
82. "Lunar horizon glow from Surveyor 7". The Planetary Society. 6 May 2016. Retrieved 8 August 2022.
83. "NASA Mission To Study Mysterious Lunar Twilight Rays". Science Mission Directorate. 3 September 2013. Retrieved 8 August 2022.
84. Colwell, Joshua E.; Robertson, Scott R.; Horányi, Mihály; Wang, Xu; Poppe, Andrew; Wheeler, Patrick (1 January 2009). "Lunar Dust Levitation - Journal of Aerospace Engineering - Vol 22, No 1". Journal of Aerospace Engineering. 22 (1): 2–9. doi:10.1061/(ASCE)0893-1321(2009)22:1(2) . Retrieved 8 August 2022.
85. Deborah Byrd (24 April 2014). "The zodiacal light, seen from the moon". EarthSky. Retrieved 8 August 2022.
86. Globus, Ruth (1977). "Chapter 5, Appendix J: Impact Upon Lunar Atmosphere". In Richard D. Johnson & Charles Holbrow (ed.). Space Settlements: A Design Study. NASA. Archived from the original on 31 May 2010. Retrieved 17 March 2010.
87. Crotts, Arlin P.S. (2008). "Lunar Outgassing, Transient Phenomena and The Return to The Moon, I: Existing Data" (PDF). The Astrophysical Journal . 687 (1): 692–705. arXiv:. Bibcode:2008ApJ...687..692C. doi:10.1086/591634. S2CID   16821394. Archived from the original (PDF) on 20 February 2009. Retrieved 29 September 2009.
88. Steigerwald, William (17 August 2015). "NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere". NASA. Archived from the original on 19 August 2015. Retrieved 18 August 2015.
89. Stern, S.A. (1999). "The Lunar atmosphere: History, status, current problems, and context". Reviews of Geophysics . 37 (4): 453–491. Bibcode:1999RvGeo..37..453S. CiteSeerX  . doi:10.1029/1999RG900005. S2CID   10406165.
90. Lawson, S.; Feldman, W.; Lawrence, D.; Moore, K.; Elphic, R.; Belian, R. (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". Journal of Geophysical Research . 110 (E9): 1029. Bibcode:2005JGRE..11009009L. doi:.
91. R. Sridharan; S.M. Ahmed; Tirtha Pratim Dasa; P. Sreelathaa; P. Pradeepkumara; Neha Naika; Gogulapati Supriya (2010). "'Direct' evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I". Planetary and Space Science . 58 (6): 947–950. Bibcode:2010P&SS...58..947S. doi:10.1016/j.pss.2010.02.013.
92. "NASA: The Moon Once Had an Atmosphere That Faded Away". Time. Archived from the original on 14 October 2017. Retrieved 14 October 2017.
93. Drake, Nadia (17 June 2015). "Lopsided Cloud of Dust Discovered Around the Moon". National Geographic News. Archived from the original on 19 June 2015. Retrieved 20 June 2015.
94. Horányi, M.; Szalay, J.R.; Kempf, S.; Schmidt, J.; Grün, E.; Srama, R.; Sternovsky, Z. (18 June 2015). "A permanent, asymmetric dust cloud around the Moon". Nature . 522 (7556): 324–326. Bibcode:2015Natur.522..324H. doi:10.1038/nature14479. PMID   26085272. S2CID   4453018.
95. Spudis, Paul D.; Cook, A.; Robinson, M.; Bussey, B.; Fessler, B. (January 1998). "Topography of the South Polar Region from Clementine Stereo Imaging". Workshop on New Views of the Moon: Integrated Remotely Sensed, Geophysical, and Sample Datasets: 69. Bibcode:1998nvmi.conf...69S.
96. Spudis, Paul D.; Reisse, Robert A.; Gillis, Jeffrey J. (1994). "Ancient Multiring Basins on the Moon Revealed by Clementine Laser Altimetry". Science . 266 (5192): 1848–1851. Bibcode:1994Sci...266.1848S. doi:10.1126/science.266.5192.1848. PMID   17737079. S2CID   41861312.
97. Pieters, C. M.; Tompkins, S.; Head, J. W.; Hess, P. C. (1997). "Mineralogy of the Mafic Anomaly in the South Pole‐Aitken Basin: Implications for excavation of the lunar mantle". Geophysical Research Letters . 24 (15): 1903–1906. Bibcode:1997GeoRL..24.1903P. doi:10.1029/97GL01718. hdl:. S2CID   128767066.
98. Taylor, G. J. (17 July 1998). "The Biggest Hole in the Solar System". Planetary Science Research Discoveries: 20. Bibcode:1998psrd.reptE..20T. Archived from the original on 20 August 2007. Retrieved 12 April 2007.
99. Schultz, P.H. (March 1997). "Forming the south-pole Aitken basin – The extreme games". Conference Paper, 28th Annual Lunar and Planetary Science Conference. 28: 1259. Bibcode:1997LPI....28.1259S.
100. "NASA's LRO Reveals 'Incredible Shrinking Moon'". NASA. 19 August 2010. Archived from the original on 21 August 2010.
101. Watters, Thomas R.; Weber, Renee C.; Collins, Geoffrey C.; Howley, Ian J.; Schmerr, Nicholas C.; Johnson, Catherine L. (June 2019). "Shallow seismic activity and young thrust faults on the Moon". Nature Geoscience (published 13 May 2019). 12 (6): 411–417. Bibcode:2019NatGe..12..411W. doi:10.1038/s41561-019-0362-2. ISSN   1752-0894. S2CID   182137223.
102. Wlasuk, Peter (2000). Observing the Moon. Springer. p. 19. ISBN   978-1-85233-193-1.
103. Norman, M. (21 April 2004). "The Oldest Moon Rocks". Planetary Science Research Discoveries. Hawai'i Institute of Geophysics and Planetology. Archived from the original on 18 April 2007. Retrieved 12 April 2007.
104. Wilson, Lionel; Head, James W. (2003). "Lunar Gruithuisen and Mairan domes: Rheology and mode of emplacement". Journal of Geophysical Research . 108 (E2): 5012. Bibcode:2003JGRE..108.5012W. CiteSeerX  . doi:10.1029/2002JE001909. Archived from the original on 12 March 2007. Retrieved 12 April 2007.
105. Gillis, J. J.; Spudis, P. D. (1996). "The Composition and Geologic Setting of Lunar Far Side Maria". Lunar and Planetary Science . 27: 413. Bibcode:1996LPI....27..413G.
106. Lawrence, D. J.; Feldman, W. C.; Barraclough, B. L.; Binder, A. B.; Elphic, R. C.; Maurice, S.; Thomsen, D. R. (11 August 1998). "Global Elemental Maps of the Moon: The Lunar Prospector Gamma-Ray Spectrometer". Science . 281 (5382): 1484–1489. Bibcode:1998Sci...281.1484L. doi:. PMID   9727970.
107. Taylor, G. J. (31 August 2000). "A New Moon for the Twenty-First Century". Planetary Science Research Discoveries: 41. Bibcode:2000psrd.reptE..41T. Archived from the original on 1 March 2012. Retrieved 12 April 2007.
108. Phil Berardelli (9 November 2006). "Long Live the Moon!". Science . Archived from the original on 18 October 2014. Retrieved 14 October 2014.
109. Jason Major (14 October 2014). "Volcanoes Erupted 'Recently' on the Moon". Discovery News. Archived from the original on 16 October 2014.
110. "NASA Mission Finds Widespread Evidence of Young Lunar Volcanism". NASA. 12 October 2014. Archived from the original on 3 January 2015.
111. Eric Hand (12 October 2014). "Recent volcanic eruptions on the moon". Science . Archived from the original on 14 October 2014.
112. Braden, S.E.; Stopar, J.D.; Robinson, M.S.; Lawrence, S.J.; van der Bogert, C.H.; Hiesinger, H. (2014). "Evidence for basaltic volcanism on the Moon within the past 100 million years". Nature Geoscience . 7 (11): 787–791. Bibcode:2014NatGe...7..787B. doi:10.1038/ngeo2252.
113. Srivastava, N.; Gupta, R.P. (2013). "Young viscous flows in the Lowell crater of Orientale basin, Moon: Impact melts or volcanic eruptions?". Planetary and Space Science . 87: 37–45. Bibcode:2013P&SS...87...37S. doi:10.1016/j.pss.2013.09.001.
114. Gupta, R.P.; Srivastava, N.; Tiwari, R.K. (2014). "Evidences of relatively new volcanic flows on the Moon". Current Science . 107 (3): 454–460. JSTOR   24103498.
115. Whitten, Jennifer; Head, James W.; Staid, Matthew; Pieters, Carle M.; Mustard, John; Clark, Roger; Nettles, Jeff; Klima, Rachel L.; Taylor, Larry (2011). "Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, and relation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan-1". Journal of Geophysical Research . 116: E00G09. Bibcode:2011JGRE..116.0G09W. doi:. S2CID   7234547.
116. Cho, Y.; et al. (2012). "Young mare volcanism in the Orientale region contemporary with the Procellarum KREEP Terrane (PKT) volcanism peak period 2 b.y. ago". Geophysical Research Letters . 39 (11): L11203. Bibcode:2012GeoRL..3911203C. doi:10.1029/2012GL051838. S2CID   134074700.
117. Munsell, K. (4 December 2006). "Majestic Mountains". Solar System Exploration. NASA. Archived from the original on 17 September 2008. Retrieved 12 April 2007.
118. Richard Lovett (2011). "Early Earth may have had two moons : Nature News". Nature. doi:10.1038/news.2011.456. Archived from the original on 3 November 2012. Retrieved 1 November 2012.
119. "Was our two-faced moon in a small collision?". Theconversation.edu.au. Archived from the original on 30 January 2013. Retrieved 1 November 2012.
120. Quillen, Alice C.; Martini, Larkin; Nakajima, Miki (September 2019). "Near/far side asymmetry in the tidally heated Moon". Icarus. 329: 182–196. arXiv:. Bibcode:2019Icar..329..182Q. doi:10.1016/j.icarus.2019.04.010. PMC  . PMID   32934397.