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| There Is No Such Thing as The Moon Its Not Real No Body Ever Walked On It We Will Never See it ITS NOT REAL GET THAT IN YOUR HEAD OK | |||
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| {{dablink|This article is about Earth's moon. For moons in general, see ]. For other uses, see ].}} | |||
| {{Planet Infobox/Moon}} | |||
| The '''Moon''' is ]'s only ]. It has no formal ] name other than "the Moon", although it is occasionally called ''']''' ({{lang-la|moon}}) to distinguish it from the generic term "]" (referring to any of the various natural satellites of other planets). Its symbol is a ] (☽). The related adjective for the Moon is ''lunar'' (from the Latin root), but this is not found in combination with words using the prefix ''seleno-'' or suffix ''-selene'' (from the Greek deity ]). | |||
| The average distance from the Earth to the Moon is 384,399 ]s (238,854 miles), which is about 30 times the diameter of the Earth. At this distance, it takes sunlight reflected from the lunar surface approximately 1.3 seconds to reach Earth. The Moon's diameter is 3,474 kilometres (2,159 miles),<ref name="worldbook">{{cite web | last = Spudis | first = Paul D. | year = 2004 | url = http://www.nasa.gov/worldbook/moon_worldbook.html | title = Moon | publisher = World Book Online Reference Center, NASA | accessdate = 2006-12-23 }}</ref> which is about 3.7 times smaller than the Earth, making it the ]'s fifth largest moon, both by diameter and mass, ranking behind ], ], ], and ]. The gravitational attraction of the Moon is responsible for the ]s on Earth. The Moon makes one complete orbit about the Earth every 27.3 days, and the periodic variations in the geometry of the Earth-Moon-Sun system are responsible for the ]s that repeat every 29.5 days. | |||
| The Moon is the only celestial body (other than the Earth) which human beings have orbited and landed on. The ]'s (]) ] was the first to reach the Moon with unmanned ]. The first man-made object to escape Earth's gravity and pass near the Moon was ], the first man-made object to impact the lunar surface was ], and the first photographs of the normally occluded ] of the Moon were made by ], all in 1959. The first spacecraft to perform a successful lunar soft landing was ] and the first unmanned vehicle to orbit the Moon was ], both in 1966.<ref name="worldbook" /> The ]' ] achieved the first (and only) manned missions to the Moon. The first manned mission to orbit the Moon was ] in 1968, and the first people to land and walk on the Moon came aboard ] in 1969.<ref name="worldbook" /> | |||
| Manned exploration of the moon ceased with the conclusion of the Apollo program, although several countries have recently announced plans to send either spacecraft or humans to the Moon in the near future. On December 4, 2006, NASA outlined plans for a ] on the Moon as part of preparation for a voyage to ]. Construction of the base is scheduled to take approximately five years, with the first preliminary missions beginning by 2020.<ref>{{cite web | author=M. Braukus, B. Dickey, K. Humphries | date = December 4, 2006 | url = http://www.nasa.gov/home/hqnews/2006/dec/HQ_06361_ESMD_Lunar_Architecture.html | title = NASA Unveils Global Exploration Strategy and Lunar Architecture | publisher = NASA | accessdate = 2006-08-10 }}</ref> | |||
| ==The Lunar Surface== | |||
| {{main|Geology of the Moon}} | |||
| ===The two sides of the Moon=== | |||
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| The Moon is in ], meaning that it keeps nearly the same face turned toward Earth at all times. Early in the Moon's history, its rotation slowed and became ] in this configuration as a result of frictional effects associated with tidal deformations caused by the Earth.<ref>{{cite journal | last = Alexander | first = M. E. | title=The Weak Friction Approximation and Tidal Evolution in Close Binary Systems | journal=Astrophysics and Space Science | year=1973 | volume=23 | pages=459–508 | url=http://adsabs.harvard.edu/abs/1973Ap&SS..23..459A }}</ref> Nevertheless, small variations resulting from the finite eccentricity of the lunar orbit, termed optical ],<ref>{{cite journal | author=J. D. Mulhollan, E. C. Silverberg | title=Measurement of Physical Librations Using Laser Retroreflectors | journal=Earth, Moon, and Planets | year=1972 | volume=4 | pages=155-159 | url=http://adsabs.harvard.edu/abs/1972Moon....4..155M }}</ref> allow up to about 59% of the lunar surface to be visible from Earth.<ref name="worldbook" /> | |||
| The side of the Moon that faces Earth is called the ], and the opposite side is called the ]. The far side should not be confused with the dark side, which is the hemisphere that is not illuminated by the ]. Spacecraft are cut off from direct radio communication with Earth when behind the Moon (see ]). The far side of the Moon was first photographed by the ] probe ] in ]. One distinguishing feature of the far side is its almost complete lack of '']'' (singular: ''mare''), which are the dark ]s. | |||
| ===Maria=== | |||
| The dark and relatively featureless lunar plains are called '']'', Latin for seas, since they were believed by ancient ]s to be filled by water. They are actually vast ancient ]ic lava flows, many of which filled the topographic depressions associated with large impact basins (] is a major exception in that it does not correspond to any known impact basin). Maria are found almost exclusively on the lunar nearside, with the lunar far side having only a few scattered patches. Only about 2% of the surface of the far side is covered by maria,<ref>{{cite journal | author=J. J. Gillis, P. D. Spudis | title=The Composition and Geologic Setting of Lunar Far Side Maria | journal=Lunar and Planetary Science | year=1996 | volume=27 | pages=413–404 | url=http://adsabs.harvard.edu/abs/1996LPI....27..413G }}</ref> compared to about 31% on the near side.<ref name="worldbook" /> The most likely explanation for this difference is related to a higher concentration of heat producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the ] gamma-ray spectrometer.<ref name="S06">{{cite journal | last = Charles Shearer and 15 coauthors | title = Thermal and magmatic evolution of the Moon | journal = Reviews in Mineralogy and Geochemistry | volume = 60 | pages = 365-518 | date = 2006 }}</ref><ref>{{cite web | url = http://www.psrd.hawaii.edu/Aug00/newMoon.html | title = A New Moon for the Twenty-First Century | last = G. Jeffrey Taylor | date = August 31, 2000}}</ref> Several provinces containing ]es and volcanic ]s are found within the near side maria.<ref>{{cite journal | author = Lionel Wilson James W. Head | title=Lunar Gruithuisen and Mairan domes: Rheology and mode of emplacement | journal=Journal of Geophysical Research | year=2003 | volume=108 | url=http://www.agu.org/pubs/crossref/2003/2002JE001909.shtml }}</ref> | |||
| ===Terrae=== | |||
| The lighter-colored regions of the Moon are called ''terrae'', or more commonly just ''highlands'' since they are topographically higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are believed to be the surviving remnants of the impact basin's outer rims.<ref>{{cite web | date = October 3, 2000 | url = http://www.lpi.usra.edu/expmoon/orbiter/orbiter-basins.html | title = Lunar Orbiter: Impact Basin Geology | publisher = Lunar and Planetary Institute | language = English | accessdate = 2006-12-24 }}</ref> In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events. | |||
| ] on the Moon's far side.]] | |||
| Using images taken by the ], it appears that four mountainous regions on the rim of the 73 km-wide ] at the Moon's north pole remain illuminated for the entire lunar day, at least during the hemisphere's summer season. These unnamed ] are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of ] is illuminated for 80% of the lunar day. Another consequence of the Moon's small axial tilt is that there are regions that remain in permanent shadow at the bottoms of many polar craters.<ref name="M03">{{cite web | url = http://www.psrd.hawaii.edu/June03/lunarShadows.html | title=The Moon's Dark, Icy Poles| last = Linda | first=Martel | date = June 4, 2003}}</ref> | |||
| ===Impact craters=== | |||
| The Moon's surface everywhere shows evidence of having been affected by the process of ].<ref>{{cite book | title = Impact cratering: A geologic process | last = H. J. Melosh | publisher = Oxford Univ. Press | date = 1989}}</ref> Most impact craters formed when asteroids and comets collided with the lunar surface over the past billion or so years, and globally, about half a million craters with diameters greater than 1 km can be found on the lunar surface. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area on a geologic unit can be used to estimate the age of its surface (see ]). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those found on Earth. The largest crater on the Moon, which also has the distinction of being the largest known crater in the ], is the ]. This impact basin is located on the ], between the South Pole and equator, and is some 2,240 kilometres in diameter and 13 kilometres in depth.<ref>{{cite web|url =http://www.psrd.hawaii.edu/July98/spa.html| title = The biggest hole in the Solar System|author =G. Jeffrey Taylor| year =1998}}</ref> Prominent impact basins on the near side include ], ], ], and ]. | |||
| ===Regolith=== | |||
| Blanketed atop the Moon's crust is a highly ] (meaning it has been broken into ever smaller particles) and "impact gardened" surficial layer called ]. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3 to 5 metres (10 to 16 ft) in the maria to about 10 to 20 metres (33 to 66 ft) in the highlands.<ref>{{cite book| last = G. Heiken, D. Vaniman, and B. French (editors)|date= 1991 |title=Lunar Sourcebook, a user's guide to the Moon | publisher = Cambridge University Press, New York | pages= 736 pp}}</ref> Beneath the finely comminuted regolith layer is what is generally referred to as the "megaregolith." This layer is much thicker (on the order of tens of kilometers) and consists of highly fractured bedrock.<ref>{{cite journal | author=K. L. Rasmussen, P. H. Warren | title=Megaregolith thickness, heat flow, and the bulk composition of the moon | journal=Nature | year=1985 | volume=313 | pages=121-124 | url=http://adsabs.harvard.edu/abs/1985Natur.313..121R }}</ref> | |||
| ===Presence of water=== | |||
| The continuous bombardment of the Moon by ]s and ]s have added some amount of water to the lunar surface. Energy from sunlight usually splits much of this water into its constituent elements hydrogen and oxygen, which generally escape to space. Attesting to the dryness of lunar rocks, the samples collected by Apollo astronauts near the equator have been found to contain only traces of water. However, because of the very slight axial tilt of the Moon's spin axis to the ecliptic plane (only 1.5°), some deep craters near the poles never receive any light from the Sun, and are permanently shadowed (see ]). Thus, any water molecules that eventually ended up in these craters could be stable for long periods of time. | |||
| Clementine has mapped craters at the lunar south pole<ref>{{cite web| url=http://www.lpi.usra.edu/research/clemen/2polar.gif| title=Lunar Polar Composites| format=GIF| accessdate=2006-03-20}}</ref> that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.<ref name="M03"/> Results from the ] bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the ] neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper meter of the regolith near the polar regions. <ref>{{cite web |url =http://lunar.arc.nasa.gov/results/ice/eureka.htm |title = Eureka! Ice found at lunar poles |date = August 31, 2001}}</ref>. Estimates for the total quantity of water ice are close to one cubic kilometer. Recently, observations with the ] planetary radar showed that some of the near polar Clementine radar returns might instead be associated with rocks ejected from young craters. If true, this would indicate that the neutron results are primarily from hydrogen in forms other than ice, such as trapped hydrogen molecules or organics. Nevertheless, the interprettation of these data are non unique (ice or surface roughness could give rise to the observed signature), and it appears that these results do not exclude the possibility of water ice in permanently shadowed craters.<ref>{{cite web| last=Paul Spudis | title = Ice on the Moon | url = http://www.thespacereview.com/article/740/1 | date = 2006}}</ref> | |||
| Water ice can be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon is an important factor in rendering ] cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. | |||
| ==Physical characteristics== | |||
| ===Internal Structure=== | |||
| ] | |||
| The Moon is a ] body, being composed of a geochemically distinct ], ], and ]. This structure is believed to have resulted from the ] of a ] shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a ] event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust (see ''Origin and geologic evolution'' below). | |||
| Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition,<ref name="L06">{{cite journal | last =P. Lucey and 12 coauthors |title = Understanding the lunar surface and space-Moon interactions | journal = Reviews in Mineralogy and Geochemistry | volume = 60 | pages = 83-219 | date = 2006}}</ref> consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of ], ], ], ], ], and ], but important minor and trace elements such as ], ], ], ], and ] are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.<ref name="W06">{{cite journal | last = Mark Wieczorek and 15 coauthors | title = The constitution and structure of the lunar interior | journal = Reviews in Mineralogy and Geochemistry | volume = 60 | pages = 221-364 | date = 2006 }}</ref> | |||
| Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals ], ] and ], and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ]), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about 1000 km below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth. A few shallow moonquakes with hypocenters located about 100 km below the surface have also been detected, but these occur more infrequently and appear to be unrelated to the lunar tides.<ref name="W06" /> | |||
| The Moon has a mean density of 3,346.4 kg/m³, making it the second densest moon in the Solar System after ]. Nevertheless, several lines of evidence imply that the lunar core is small, with a radius of about 350 km or less.<ref name="W06"/> The size of the lunar core is only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of ] and ]. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.<ref>{{cite journal | author = J. G. Williams, S. G. Turyshev, D. H. Boggs, J. T. Ratcliff | title=Lunar laser ranging science: Gravitational physics and lunar interior and geodesy | journal=Advances in Space Research | year=2006 | volume=37 | issue=1 | pages=67-71 | url=http://adsabs.harvard.edu/abs/1987AREPS..15..271S }}</ref> | |||
| ===Topography=== | |||
| ].]] | |||
| The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the ]. The most visible topographic feature is the giant far side ], which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ] deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as ], ], ], ], and ], also possess regionally low elevations and elevated rims. | |||
| Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side. If it is assumed that the crust is in ], and that the density of the crust is everywhere the same, then the higher elevations would be associated with a thicker crust. Using gravity, topography and seismic data, the crust is thought to be on average about 50±15 km thick, with the far-side crust being on average thicker than the near side by about 15 km. <ref name="W06"/>. | |||
| ===Gravity field === | |||
| {{see also|Mascon}} | |||
| ] | |||
| The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the ], whereby the line-of-sight spacecraft acceleration can be measured by small shifts in frequency of the radio signal, and the measurement of the distance from the spacecraft to a station on Earth. Since the gravitational field of the Moon affects the orbit of a spacecraft, it is possible to use these tracking data to invert for gravitational anomalies. However, because of the Moon's ] it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterized. | |||
| The major characteristic of the Moon's gravitational field is the presence of ]s, which are large positive gravitational anomalies associated with some of the giant impact basins. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. They were initially discoved by the analysis of ] tracking data,<ref>{{cite journal | author = Paul Muller and William Sjogren | title = Masons: lunar mass concentrations | journal = Science | volume = 161 |pages = 680-684 | year = 1968}}</ref>, since pre-Apollo navigational tests were experiencing landing position errors much larger than mission specifications. | |||
| The origin of mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitional signature, and uplift of the crust-mantle interface is required as well. Based on ] gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism.<ref>{{cite journal | author = A. Konopliv, S. Asmar, E. Carranza, W. Sjogren, and D. Yuan | title = Recent gravity models as a result of the Lunar Prospector mission | journal = Icarus | volume = 50 | pages = 1-18 | year = 2001}}</ref> It should be noted that the huge expanse of mare basaltic volcanism associated with ] does not possess a positive gravitational anomaly. | |||
| ===Magnetic field=== | |||
| ] | |||
| The Moon has only a very weak external magnetic field in comparison to the Earth. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a ] in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a ] was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the ] of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.<ref>{{cite journal |last = Hood, L. L., and Z. Huang | title = Formation of magnetic anomalies antipodal to lunar impact basins: Two-dimensional model calculations | journal = J. Geophys. Res.| volume = 96 | pages = 9837-9846 | date=1991}}</ref> | |||
| ===Atmosphere=== | |||
| The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is ]—the release of gases such as ] that originate by radioactive decay processes within the crust and mantle. Another important source is generated through the process of ], which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight.<ref name="L06"/> Gasses that are released by sputtering can either reimplant into the regolith as a cause of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionized. The elements ] (Na) and ] (K) have been detected using earth-based spectroscopic methods, whereas the element ]-222 and ]-210 have been inferred from data obtained from the ] ] spectrometer.<ref>{{cite journal|last = S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian | title=Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer | journal=J. Geophys. Res. | volume = 110 | pages=doi:10.1029/2005JE002433 | date =2005}}</ref> Argon-40, He-4, O and/or CH<sub>4</sub>, N<sub>2</sub> and/or CO, and CO<sub>2</sub> were detected by in-situ detectors emplaced by the Apollo astronauts.<ref>{{cite journal | last = S. Alan Stern | title= The Lunar atmosphere: History, status, current problems, and context | journal = Rev. Geophys. | volume = 37 | year = 1999 | pages = 453-491}}</ref> | |||
| == Origin and geologic evolution== | |||
| {{main|Giant impact hypothesis|Geology of the Moon}} | |||
| ===Formation of the Moon=== | |||
| Several mechanisms have been suggested for the Moon's formation. Early speculation proposed that the Moon broke off from the Earth's crust because of ]s, leaving a basin (presumed to be the Pacific Ocean) behind as a scar.<ref>{{cite journal | last = Binder | first = A. B. | title=On the origin of the moon by rotational fission | journal=The Moon | year=1974 | volume=11 | issue=2 | pages=53-76 | url=http://adsabs.harvard.edu/abs/1974Moon...11...53B }}</ref> This '''fission''' concept, however, requires too great an initial spin of the Earth. Others speculated that the Moon formed elsewhere and was captured into Earth's orbit.<ref>{{cite journal | last = Mitler | first = H. E. | title=Formation of an iron-poor moon by partial capture, or: Yet another exotic theory of lunar origin | journal=Icarus | year=1975 | volume=24 | pages=256-268 | url=http://adsabs.harvard.edu/abs/1975Icar...24..256M }}</ref> However, the conditions required for this '''capture''' mechanism to work (such as an extended atmosphere of the Earth for dissipating energy) are not too probable. The '''coformation''' hypothesis posits that the Earth and the Moon formed together at the same time and place from the primordial ]. In this theory, the Moon forms from material surrounding the proto-Earth, similar to the way in which the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of metallic iron in the Moon. A major deficiency with all of these hypotheses is that they cannot easily account for the high angular momentum of the Earth-Moon system.<ref>{{cite journal | last = Stevenson | first = D. J. | title=Origin of the moon - The collision hypothesis | journal=Annual review of earth and planetary sciences | year=1987 | volume=15 | pages=271-315 | url=http://adsabs.harvard.edu/abs/1987AREPS..15..271S }}</ref> | |||
| Today, the '''giant impact hypothesis''' for forming the Earth-Moon system is widely accepted by the scientific community. In this theory, the impact of a Mars-sized body (which has been referred to as ] or ]) into the proto-Earth is postulated to have put enough material into circumterrestrial orbit to form the Moon.<ref name="worldbook" /> Given that planetary bodies are believed to have formed by the hierarchical accretion of smaller to larger sized bodies, it is now recognized that giant impact events such as this should be expected to have occurred for some planets. Computer simulations modeling this impact can account for the angular momentum of the Earth-Moon system, as well as the small size of the lunar core.<ref>{{cite journal | last = R. Canup and E. Asphaug | title = Origin of the Moon in a giant impact near the end of the Earth's formation | journal = Nature | volume = 412 | pages = 708-712 | date = 2001 }}</ref> Unresolved questions concerning this theory are the relative sizes of the proto-Earth and impactor, and the proportion of material from the proto-Earth and impactor that contribute to making the Moon. The formation of the Moon is believed to have occurred at 4.527 ± 0.01 billion years, about 30 to 50 million years after the origin of the solar system.<ref>{{cite journal| url=http://www.sciencemag.org/cgi/content/abstract/310/5754/1671| journal=Science| year=2005| volume=310| issue=5754| pages=1671 - 1674| id={{DOI|10.1126/science.1118842}}| title=Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon| last=Thorsten Kleine| coauthors=Herbert Palme, Klaus Mezger, Alex N. Halliday}}</ref> | |||
| ===The Lunar Magma Ocean=== | |||
| As a result of the large amount of energy liberated during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly believed that a large portion of the Moon was once initially molten. The molten outer portion of the Moon is referred to as a ], and estimates for its depth range from about 500 km to the entire radius of the Moon.<ref name="S06"/> As the magma ocean cooled, it ] and ], giving rise to a geochemically distinct crust and mantle. The mantle is predicted to have formed largely by the precipitation and sinking of the minerals ], ], and ]. After about three-quarters of magma ocean crystallization was complete, the mineral ] is predicted to have precipated and floated to the surface because of its low density, forming the crust. The final liquids to crystallize from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym ], for ] (K), ] (REE), and ] (P), and appears to be concentrated within the ], which is a small geologic province that encompasses most of ] and ] on the near side of the Moon.<ref name="W06"/>. | |||
| ===Geologic Evolution=== | |||
| A large portion of the Moon's post-magma-ocean geologic evolution was dominated by impact cratering. The ] is divided in time based on a few prominent impact events, such as ], ], ], and ]. While not all of these craters have been definitively dated (and some ages are still being debated), they are useful for assigning relative ages based on ] grounds. The continuous effects of impact cratering are responsible for forming the ]. | |||
| The other major geologic process that affected the Moon's surface was ]. The enhancement of heat producing elements within the ] would have caused the underlying mantle to heat up, and eventually, to partially melt. Some portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon.<ref name="S06"/> Most of the Moon's ] erupted during the Imbrian period within this geologic province, around 3 to 3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years<ref>{{cite journal | last = James Papike, Grahm Ryder, and Charles Shearer | title = Lunar Samples | journal = Reviews in Mineralogy and Geochemistry | volume = 36 | pages = 5.1-5.234 | date = 1998 }}</ref>, and the youngest eruptions, based on the method of ], are believed to have occurred only 1.2 billion years ago.<ref>{{cite journal | last = H. Hiesinger, J. W. Head, U. Wolf, R. Jaumanm, and G. Neukum | title = Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Numbium, Mare Cognitum, and Mare Insularum | journal = J. Geophys. Res. | volume = 108 | pages = doi:10.1029/2002JE001985 | date = 2003 }}</ref> Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.<ref>{{cite web |url =http://www.psrd.hawaii.edu/Nov06/MoonGas.html |last = G. Jeffrey Taylor |title = Recent Gas Escape from the Moon |date=2006}}</ref><ref>{{cite journal | last = P. H. Schultz, M. I. Staid, and C. M. Pieters |date = 2006 | title= Lunar activity from recent gas release | journal= Nature |volume = 444 |pages = 184-186}}</ref> | |||
| ==Orbit and relationship to Earth== | |||
| {{main|Orbit of the Moon}} | |||
| {{see also|Libration|Tidal acceleration}} | |||
| The Moon makes a complete orbit about the Earth with respect to the fixed stars (its ]) approximately once every 27.3 days. However, since the Earth is moving in its orbit about the ] at the same time, it takes slightly longer for the Moon to show its same ] to Earth, which is about 29.5 days (its ]).<ref name="worldbook" /> Unlike most satellites of other planets, the Moon orbits near the ] and not the Earth's ]. | |||
| The Earth and Moon have many physical effects upon one another, including the ]s. Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with the ] making only a small contribution. Tidal effects result in an increase of the mean Earth-Moon distance of about 4 meters per century, or 4 centimetres per year.<ref>{{cite web | last = Phillips | first = Tony | date = July 20, 2004 | url = http://science.nasa.gov/headlines/y2004/21jul_llr.htm | title = What Neil & Buzz Left on the Moon | publisher = Science @ NASA | accessdate = 2006-12-23 }}</ref> As a result of the ], the increasing semimajor axis of the Moon is accompanied by a gradual slowing of the Earth's rotation by about 0.002 seconds per day.<ref>{{cite web | last = Ray | first = Richard | date = May 15, 2001 | url = http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html | title = Ocean Tides and the Earth's Rotation | publisher = IERS Special Bureau for Tides | accessdate = 2006-12-23 }}</ref> | |||
| The Earth-Moon system is sometimes considered to be a ] rather than a planet-moon system. This is due to the exceptionally large size of the Moon relative to its host planet; the Moon is one-fourth the diameter of Earth and 1/81 its mass. However, this definition is criticized by some since the common center of mass of the system (the ]) is located beneath the surface of the Earth. The surface of the Moon is less than 1/10th that of the Earth, and only about one quarter the size of Earth's land area (about as large as ], ], and the ] combined). | |||
| In 1997 the asteroid ] was found to have an unusual Earth-associated ]. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term.<ref>{{cite web|url =http://www.captaincosmos.clara.co.uk/cruithne.html| title =No, it's not our "second" moon!!!| accessdate =2006-10-10|language =English}}</ref> Three other ]s, (54509) 2000 PH5, (85770) 1998 UP1 and ], which exist in orbits similar to Cruithne's, have since been discovered.<ref>{{cite journal | author=M. H. M. Morais, A. Morbidelli | title=The Population of Near-Earth Asteroids in Coorbital Motion with the Earth | journal=Icarus | year=2002 | volume=160 | pages=1-9 | url=http://adsabs.harvard.edu/abs/2002Icar..160....1M }}</ref> | |||
| ] | |||
| ==Eclipses== | |||
| {{main|Eclipse|Solar eclipse|lunar eclipse}} | |||
| {{see also|Occultation}} | |||
| ] solar eclipse]] | |||
| Eclipses can occur only when the Sun, Earth, and Moon are all in a straight line. ]s can occur near a ], when the Moon is between the ] and ]. In contrast, ]s can occur near a ], when the Earth is between the Sun and Moon. Because the Moon's orbit around the Earth is inclined by about 5° with respect to the ], 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. | |||
| The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both ] and ] solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar ] becomes visible to the ]. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon could always completely cover the Sun on solar eclipses so that no annular eclipses were possible. Likewise, about 600 million years from now (assuming that the angular diameter of the Sun will not change), the Moon will no longer cover the Sun completely and total eclipses will not occur. | |||
| A phenomenon related to eclipse is ]. The Moon is continuously blocking our view of the sky by a 1/2 degree wide circular area. When a bright star or planet ''passes behind'' the Moon it is ''occulted'' or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of individual stars are not visible everywhere, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted. | |||
| ==Observation of the Moon== | |||
| {{see also|Lunar phase|Earthshine|Observing the Moon}} | |||
| ]. The brightest crescent is in direct sunlight; the upper portion is lit by light reflected from Earth.]] | |||
| ] around Moon]] | |||
| During the brightest full moons, the Moon can have an ] of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8. When the Moon is in a quarter phase, its brightness is not one half of a full moon, but instead is only about 1/10. This is because the lunar surface is not a perfect ] and because shadows projected onto the surface also diminish the amount of reflected light. | |||
| The Moon appears larger when close to the horizon. This is a purely psychological effect (see ]). The angular diameter of the Moon from Earth is about one half of one degree, and is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is farther away by up to 1 Earth radius). | |||
| Another quirk of the visual system causes us to see the Moon as almost pure white, when in fact it reflects only about 7% of the light falling on it (about as dark as a lump of coal). It has a very low ]. ] in the ] recalibrates the relations between colors of an object and its surroundings; however, there is nothing next to the Moon to reflect the light falling on the Moon, therefore it is perceived as the brightest object visible. We have no standard to compare it to. An example of this is that, if you used a narrow beam of light to illuminate a lump of coal in a dark room, it would look white. If you then broadened the beam of the light source to illuminate the surroundings, it would revert to black. | |||
| The highest ] of the Moon on a day varies and has nearly the same limits as the Sun. It also depends on season and lunar phase with the full moon being highest in winter. The orientation of the Moon's crescent also depends on the latitude of the observing site. Close to the equator an observer can see a ''boat'' Moon.<ref>{{cite web| url=http://curious.astro.cornell.edu/question.php?number=393| publisher=Curious About Astronomy| title=Is the Moon seen as a crescent (and not a "boat") all over the world?| accessdate=2006-03-20| month=October| year=2002| first=Kristine| last=Spekkens}}</ref> | |||
| Like the Sun, the Moon can also give rise to the atmospheric effects including a 22 degree ] ring and the smaller ] seen more often through thin clouds. For more information on how the Moon appears in Earth's sky, see ]. | |||
| == Exploration of the Moon == | |||
| {{main|Exploration of the Moon|Project Apollo}} | |||
| {{see also|Robotic exploration of the Moon|Future lunar missions|Colonization of the Moon}} | |||
| ]'' astronaut ] standing next to boulder at Taurus-Littrow during third EVA (extravehicular activity). ''] photo.'']] | |||
| ] | |||
| The first leap in lunar observation was caused by the invention of the telescope. ] made especially good use of this new instrument and observed mountains and craters on the Moon's surface. | |||
| The Cold War-inspired ] between the ] and the ] led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent almost as soon as launcher capabilities would allow. What was the next big step depends on the political viewpoint: In the US (and the West in general) the landing of the first humans on the Moon in 1969 is seen as the culmination of the space race. ] became the first person to walk on the Moon as the commander of the American mission '']'' by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The last person (as of 2006) to stand on the Moon was ], who as part of the mission '']'' walked on the Moon in December 1972. The ] Moon landing and return was enabled by several technologies where the US surpassed the Russians; for example, the US achieved considerable advances in ] chemistry and ] technology in the early 1960s. On the other hand, many scientifically important steps, such as the first photographs of the until then unseen far side of the Moon in 1959, were first achieved by the Soviet Union. Moon samples have been brought back to Earth by three Luna missions ('']'', '']'', and '']'') and the ''Apollo'' missions ''11'' through ''17'' (excepting '']'', which aborted its planned lunar landing). | |||
| ] ] raises the ] on the surface of the Moon.]] | |||
| Scientific instrument packages were installed on the lunar surface during all of the ''Apollo'' missions. Long-lived ] stations (Apollo lunar surface experiment package) were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, whereas a temporary station referred to as EASEP (Early Apollo Scientific Experiments Package) was installed during the Apollo 11 mission. The ALSEP stations contained, among others, heat flow probes, seismometers, magnetometers, and corner-cube retroreflectors. Transmission of data to Earth was terminated on September 30 1977 because of budgetary considerations. Since the ] (LLR) corner-cube arrays are passive instruments, they are still being used to today. Ranging to the LLR stations is routinely performed from earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core.<ref>{{cite journal | last = J. Dickey, P. Bender, J. Faller, X. Newhall, R. Ricklefs, J. Ries, P. Shelus, C. Veillet, A. Whipple, J. Wiant, J. Williams, and C. Yoder | date=1994 | title= Lunar laser ranging: a continuing legacy of the Apollo program | journal= Science | volume=265 |pages=482-490}}</ref>. | |||
| From the mid-1960s to the mid-1970s, there were a total of 65 Moon landings (both manned and robotic, with 10 in 1971 alone), but after ] in 1976 they stopped. The Soviet Union started focusing on ] and ]s and the US on ] and beyond. In 1990 ] orbited the Moon with the '']'' spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe, '']'', in lunar orbit, but the transmitter failed rendering the mission scientifically useless. | |||
| In 1994, the US finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft ]. This mission obtained the first near global topographic map of the Moon, as well as the first global ] images of the lunar surface. This was followed by the '']'' mission in 1998. The ] ] on ''Lunar Prospector'' indicated the presence of excess hydrogen at the lunar poles, which is likely due to the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. The European spacecraft '']'' was launched ] ] and was in lunar orbit from ] ] to ] ]. | |||
| On ] ], US President George W. Bush called for a plan to return manned missions to the Moon by 2020 (see ]).<ref>{{cite press release|url =http://www.nasa.gov/missions/solarsystem/bush_vision.html| title = President Bush Offers New Vision For NASA| date =Dec. 14, 2004|publisher =NASA}}</ref> NASA is now planning for the construction of a permanent outposts at one of the lunar poles.<ref>{{cite press release |title= | |||
| NASA Unveils Global Exploration Strategy and Lunar Architecture |publisher= NASA |date=Dec. 4, 2006 |url=http://www.nasa.gov/home/hqnews/2006/dec/HQ_06361_ESMD_Lunar_Architecture.html |format= |language= |accessdate=Dec. 4, 2006}}</ref> The ] has expressed ambitious plans for exploring the Moon and has started the ] for lunar exploration. ] has two planned lunar missions, '']'' and '']''. India is to launch an unmanned mission '']'' in February 2008. The US will launch the ] in 2008. ] also announced to resume its previously frozen project ], consisting of an unmanned lander and orbiter, which is slated to land in ]. <ref>{{cite web | url = http://www.aviationnow.com/avnow/news/channel_awst_story.jsp?id=news/aw060506p2.xml | title = Russia Plans Ambitious Robotic Lunar Mission | last = Craig Covault | date = June 4, 2006}}</ref> | |||
| ==Human understanding of the Moon== | |||
| ] (1647).]] | |||
| {{see also|Moon in art and literature|Lunar effect}} | |||
| The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5,000 year old rock carving at ], ] may represent the Moon, which would be the earliest depiction discovered.<ref name=spacetoday>{{cite web|url=http://www.spacetoday.org/SolSys/Earth/OldStarCharts.html |title = Carved and Drawn Prehistoric Maps of the Cosmos | |||
| | publisher=Space Today Online |accessdate=2006-10-08}}</ref> In many prehistoric and ancient cultures, the Moon was thought to be a ] or other ] phenomenon, and ] of the Moon continue to be propagated today. For further details, see ]. | |||
| ] | |||
| Among the first in the Western world to offer a scientific explanation for the Moon was the ] ] ], who reasoned that the Sun and Moon were both giant ] rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile. By the ], before the invention of the ], more and more people began to recognize the Moon as a ], though they believed that it was "perfectly smooth". In 1609, ] drew one of the first telescopic drawings of the Moon in his book '']'' and noted that it was not smooth but had mountains and ]s. Later in the 17th century, ] and ] drew a map of the Moon and gave many craters the names they still have today. | |||
| ]" (1902) by ].]] | |||
| On maps, the dark parts of the Moon's surface were called ''maria'' (singular ''mare'') or seas, and the light parts were called ''terrae'' or continents. | |||
| The possibility that the Moon could contain vegetation and be inhabited by selenites was seriously considered by some major astronomers even into the first decades of the 19th century. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the ], the rabbit and the buffalo, amongst others. | |||
| In 1835, the ] fooled some people into thinking that there were exotic animals living on the Moon.<ref>{{cite web|url =http://www.museumofhoaxes.com/moonhoax.html| title = The Great Moon Hoax |author =Alex Boese | language =English|date=2002}}</ref> Almost at the same time however (during 1834–1836), ] and ] were publishing their four-volume ''Mappa Selenographica'' and the book ''Der Mond'' in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere. | |||
| There remained some controversy over whether features on the Moon could undergo changes. Some observers claimed that some small craters had appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observing under different lighting conditions, poor ], or the inadequacy of earlier drawings. It is however known that the phenomenon of ] occasionally occurs, and these might be responsible for a minor percentage of the reported ]. | |||
| The ] of the Moon remained completely unknown until the ] probe was launched in 1959, and was extensively mapped by the ] in the 1960s. | |||
| == Legal status == | |||
| {{see also|Space law}} | |||
| Though several flags of the Soviet Union<ref>Whilst no Soviet flags have been hand-placed, Soviet coats of arms were scattered by ] in 1959, and similar devices by later landing missions</ref> and the United States have been symbolically planted on the Moon, the Russian and U.S. governments make no claims to any part of the Moon's surface. Russia and the U.S. are party to the ], which places the Moon under the same jurisdiction as ] (]). This treaty also restricts use of the Moon to peaceful purposes, explicitly banning ] (including ]) and military installations of any kind. | |||
| A second treaty, the ], was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the ]. Several individuals have made claims to the Moon in whole or in part, though none of these claims are generally considered credible (see ]). | |||
| == See also == | |||
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| ==References== | |||
| <!-- ---------------------------------------------------------- | |||
| See http://en.wikipedia.org/Wikipedia:Footnotes for a | |||
| discussion of different citation methods and how to generate | |||
| footnotes using the<ref>, </ref> and <reference /> tags | |||
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| <div class="references-2column"> | |||
| '''Cited references''' | |||
| <references /> | |||
| '''Scientific references''' | |||
| * Ben Bussey and ], ''The Clementine Atlas of the Moon'', Cambridge University Press, 2004, ISBN 0-521-81528-2. | |||
| * {{cite journal | last = Don Wilhelms | title = , | journal = U.S. Geological Survey Professional paper| date = 1987 |volume = 1348}} | |||
| * , Brad Jolliff, Mark Wieczorek, Charles Shearer and Clive Neal (editors), Rev. Mineral. Geochem., 60, Min. Soc. Amer., Chantilly, Virginia, 721 pp., 2006. | |||
| * {{cite book | last = Stuart R. Taylor | title = Solar system evolution | publisher = Cambridge Univ. Press | pages = 307 pp | date = 1992 }} | |||
| '''General references''' | |||
| * {{cite book| title=To a Rocky Moon: A Geologist's History of Lunar Exploration | last = D.E. Wilhelms | publisher = University of Arizona Press, Tucson | date = 1993}} | |||
| * ], ''On the Moon'', Sterling Publishing Co., ], ISBN 0-304-35469-4. | |||
| * Paul D. Spudis, ''The Once and Future Moon'', Smithsonian Institution Press, 1996, ISBN 1-56098-634-4. | |||
| * | |||
| </div> | |||
| ==External links== | |||
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Revision as of 07:52, 2 January 2007
There Is No Such Thing as The Moon Its Not Real No Body Ever Walked On It We Will Never See it ITS NOT REAL GET THAT IN YOUR HEAD OK