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The geology of the Moon (sometimes called '''selenology''', although the latter term can refer more generally to "lunar science"), has a number of similarities to that of the Earth , particularly in terms of composition, but there are some substantial differences. The Moon lacks a significant Atmosphere , eliminating erosion due to Weather , it has a lower gravity, and is cooled more rapidly than the Earth. The complex morphology of the Lunar surface has been formed by a combination of processes, chief among which are Impact Crater ing, Volcanism , and Tectonics . Thanks to its proximity to the Earth, the Moon is the only extraterrestrial body for which there is detailed knowledge of its geology and from which samples from different regions were obtained. Geological studies of the Moon are based on Telescope observations and measurements from orbiting Spacecraft , and a few locations were sampled directly during the Project Apollo missions in the 1960's, which brought approximately 385 Kilogram s of Lunar Rock and soil, clarifying some details and adding some new questions. However, a substantial portion of the lunar surface has not been explored and a number of geological questions remain unanswered. These questions may only be answered with the placing of permanent bases on the lunar surface and a broader study of the surface. FORMATION For a long time, the fundamental question regarding the history of the moon was of its origin. The hypotheses that have been created regarding it are as numerous as they are different from each other. The most important ones are: Lunar capture The moon was captured, completely formed, by the Gravitational Field of the Earth . This is unlikely, since a close encounter with the Earth would have produced either a collision or an alteration of the trajectory of the body in question, so if it had indeed happened, the Moon probably would never return to meet again with the Earth. For this hypothesis to function, there would have to be a large atmosphere extended around the primitive Earth, which would be able to slow the movement of the Moon before it could escape. This hypothesis is considered to explain the irregular satellite orbits of Jupiter and Saturn ; nevertheless, it is very difficult to believe that this would explain the origin of our moon. In addition, this hypothesis has difficulty explaining the similar Oxygen Isotope ratio of the two worlds. Fission hypothesis The idea that a primitive Earth, with an accelerated rotation, expelled a piece of its mass was proposed by George Darwin (son of the famous biologist Charles Darwin ). This hypothesis does not explain why the Earth rotated once every 2.5 hours early in its geologic history, nor why the Moon and the Earth do not continue to rotate at an accelerated rate in the present. If this theory were true, the Angular Momentum of the Moon would be quite different. Accretion hypothesis This hypothesis states that the Earth and the Moon formed at the together in a double system. The problem with this hypothesis is that it does not explain the rotational periods of the Earth and the Moon, nor gives an answer to the absence of material orbiting the two bodies of the proposed double system, a phenomenon that only can be explained if they consider the terrestrial rotation and the lunar revolution through a physical property called Angular Momentum . Giant impact theory See Also: Giant impact theory At present the best explanation for the formation of the Moon involves a collision of two protoplanetary bodies during the early accretion period of the Solar System 's formation. This "giant impact theory", which was proposed in 1984 (although it originated in the mid- 1970s ) satisfies the orbital conditions of the Earth and Moon and the reasons that the Earth has a larger metallic core than the Moon. The modern theories of how the planets formed from smaller bodies, which were formed from still smaller bodies, predicts that when the formation of the Earth was almost finished it would have had a body the size of Mars and about a tenth of the mass of the Earth in close proximity. Because of this, the theorized giant impact is a plausible, perhaps inevitable, event. The theory requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth of its mass). The colliding body has sometimes been referred to as Theia , the mother of Selene , the Moon Goddess in Greek Mythology . This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital configuration. This impact would have expelled enough amounts of hot material around the Earth's orbit, that the Moon would have formed through the accumulation of this material. Computer simulations of this event appear to show that the collision must occur with a glancing blow. This will cause a small portion of the colliding body to form a long arm of material that will then shear off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: trillions of tons of material would have been vaporized and melted. In parts of the Earth the Temperature would have risen to 10,000 ° C . This formation theory helps explain why the Moon is Iron -poor and lacks a solid iron core. The iron from the impacting body had been absorbed into the core of the Earth, while the lighter crust materials produced the resulting moon. The collision also helps explain why the lunar rocks are so similar to the rocks present in the Earth's Mantle . The lack of volatiles was also explained by the energy of the collision, as were the early Magma oceans on the Moon. If this event had never happened, not only would the Earth not have a moon but its days might presumably be about a year long, although Mars has achieved a day about equal to ours without such an event. Following the impact, the orbiting debris coalesced into a body about the size of the Moon. The newly formed moon orbits about one-tenth the distance as it does today, and it became tidally-locked with the Earth. That is, one side of the Moon is always facing toward the Earth. The geology of the Moon then forms independently of the Earth, with the surface cooling more rapidly and forming a crust. GEOLOGIC HISTORY The geological history of the Moon has been defined into six major Epoch s, called the Lunar Geologic Timescale . Starting about 4,600 million years ago, the newly formed Moon was in a molten state and was orbiting much closer to the Earth. The resulting Tidal Force s deformed the molten body into an Ellipsoid , with the major axis pointed towards Earth. The first important event in the formation of the moon was the Crystallization of oceanic Magma . It is not known with certainty what its depth was, but according to different studies, the magma ocean was located at a depth of 500 Km . The first minerals to form in this ocean were the iron and magnesium silicates olivine and piroxene. Since these minerals were denser than the material around them, they sank. Less dense feldspar located in the upper part of the magma formed mountains, giving the moon its first crust. The magma ocean stage ended 4.4 billion years ago. As quickly as the lunar crust formed, or even as it was forming, different types of magmas that would become s for these magmas began to rise through the surface, through the Anorthosite crust, forming large rocks and even causing volcanic eruptions at the surface. Some of these magmatic bodies reacted chemically with the remnants of the magma ocean ( KREEP ), and others may have dissolved the anorthosites. This period of lunar history ended about 4 billion years ago. The period from the formation until about 4,000 million years ago was an era of heavy bombardment when the major impact basins and larger craters were formed. Several of these continued to modify the surface to depths that ranged between a few and up to 20 km. The strong tidal forces from the Earth also caused the orbit to expand, resulting in a longer orbital period. (The tidal forces also had the effect of increasing the length of a day on the Earth.) Analysis of craters and Moon rocks show that there was an additional, more intense Late Heavy Bombardment by Asteroid s around the period 4.000 to 3.800 billion years ago, corresponding to the Nectarian epoch. The mature Lunar Mare s were formed starting around 3,900 million years ago by upwellings of Pluton s of low viscosity Basalt ic Magma . This continued up to the end of the Upper Imbrian epoch, about 3,200 million years ago. Along with the basalt came Pyroclastic Eruptions , which launched blocks of molten basalt hundreds of kilometers away from the Volcano . Thereafter most of the significant volcanism ceased, although there remain traces of activity to this day. The mares typically formed in the areas of impact basins where the crust was relatively thin and fractured. The flows of magma submerged much of the basin terrain, leaving only projecting features that formed arcing mountain ranges or solitary mounts. Subsequently the surface has seen occasional impacts up to the present day, and impacts by Meteorites are the only substantial Geologic force acting on the Moon nowadays. Some of the most important Crater s on the Moon formed in this recent epoch. For example, the Copernicus Crater , which has a depth of 3.76 km and a radius of 93 km, formed about 900 million years ago. The '' Apollo 17 '' mission landed in an area in which the material coming from the Tycho Crater , which has a diameter of 85 km, had been distributed; the study of these rocks helped come to the conclusion that this crater had formed 100 million years ago. The surface has also experienced space weathering due to high energy particles and micrometeorites. This process causes the Ray System s produced by impacts to darken until it matches the albedo of the surrounding surface. LUNAR LANDSCAPE The lunar landscape is characterized by Impact Crater s, their ejecta, a few Volcano es, hills, Lava Flow s and depressions filled by magma. Lunar highlands and lowlands The most distinctive aspect of the Moon is the contrast between its light and dark zones. Lighter surfaces are the lunar highlands, which receive the name of ''terrae'' (singular ''terra'', from the Latin for Earth ) and darker plains which are called ''maria'' (singular ''mare'', from the latin for Sea ), after Johannes Kepler , who introduced the name in the 1600's. Impact cratering and the Copernicus Crater . '' NASA photo.'']] It may be surprising to learn that the origin of the Moon's craters as impact features became widely accepted only in the 1940s . This realization allowed the impact history of the Moon to be gradually worked out by means of the principle of Superposition . That is, if a crater overlaid another, it must be the younger. The amount of wear experienced by a crater was another clue to its age, as also is its diameter. Impact Crater ing is the most notable geological process on the Moon. The craters are formed when a solid body, such as a Comet or Asteroid , collides with the surface at a high velocity. The kinetic energy of the impact creates a compression shock wave that radiates away from the point of entry. This is succeeded by a Rarefaction wave, which is responsible for propelling most of the ''ejecta'' out of the crater. Finally there is a rebound of the floor from the compression that can create a central peak. These craters appear in a continuum of diameters across the surface of the Moon, ranging in size from tiny pits to the immense South Pole-Aitken Basin with a diameter of nearly 2,500 km and a depth of 13 km. Generally speaking, the lunar history of impact cratering follows a trend of decreasing size with the passage of time. This is apparent from studying the overlapping of craters, with the larger craters almost always lying underneath smaller impacts The largest impact basins were formed during the early periods, and these were successively overlaid by smaller and smaller craters. The Size Frequency Distribution (SFD) of crater diameters also approximately follows a Power Law , with increasing numbers at decreasing diameter. displays the characteristic features of a large impact formation, with a raised rim, slumped edges, terraced inner walls, a relatively flat floor with some hills, and a central ridge. The Y-shaped central ridge is unusually complex in form. '' NASA photo.'']] The most recent impacts are distinguished by well-defined features, including a sharp-edged rim. Larger craters display slumping features along the inner walls that can form and ledges. The interior floor is generally somewhat level, often due to fall-back material that has formed a flat surface. Small craters tend to form a bowl or cup shape, while larger impacts can have a central peak. The largest impact basins can even have a secondary, concentric ring of raised material. The impact process produces high Albedo particles that gives the crater appear bright. This material is also found in the ''ejecta'' and can form skirts of bright material or Ray System s. Gradually the crater and its ''ejecta'' undergoes impact erosion from micrometeorites and smaller impacts. This wear process softens and rounds the features of the crater, as well as leaving impact craters along the sides and interior. The erosion also darkens the previously bright ''ejecta'', and with time it gradually fades away. The crater can also be covered in ''ejecta'' from other impacts, which can submerge features and even bury the central peak. The ''ejecta'' from large impacts includes larges blocks of stone that can land to form their own secondary impact craters. These craters are formed in clearly discernable radial patterns. In some cases an entire line of these blocks can impact to form a valley. These are distinguished from ''catena'', or crater chains, which are linear strings of craters that are formed when the impact body breaks up prior to impact. Generally speaking a lunar crater is roughly circular in form, with a depth about one-tenth the diameter. It has been demonstrated in laboratory experiments that even low-angle impacts tend to produce circular craters, and there are few lunar craters that have formed naturally elliptical outlines. However a low angle impact can produce a central peak that is offset from the mid-point of the crater. In addition, such an impact will produce an asymmetrical ray system. ''Dark-halo craters'' are formed when an impact excavates lower Albedo material from beneath the surface, then deposits this darker ''ejecta'' in a skirt around the main crater. This can occur when an area of darker Basalt ic-lava material, such as that found on the Lunar Mare s, is later covered by lighter ''ejecta'' from more distant impacts in highland areas. This covering conceals the darker material below. The largest impacts produced melt sheets of molten rock that covered portions of the surface. These melts could be as thick as a kilometer. Examples of such melts can be seen in the northeastern part of the Mare Orientale impact basin. In many cases, older melts have since been concealed by layers of ''ejecta'' or mare formations. Highlands and craters , with rays extending from its rim]] The lunar highlands present the largest amount of Impact Crater s, which range in diameter from about a Meter to about 1,000 Kilometer s. Before any robotic probe could reach the Moon, scientists thought that the origin of some of these craters was volcanic, an idea that changed radically when soil and rock samples were returned by the ''Apollo'' missions, which clearly showed the important role of the impact process in the formation of the terrain. The impacts occurred at a velocity of about 20 km/s (70,000 km/h). In each impact, waves of high pressure bounce the Projectile and the impacted object, which destroys the projectile (usually a Meteorite ) as the Shock Wave Vaporizes it almost in its entirety. The material of the impacted object is strongly Compressed and quickly uncompressed a short time later. A portion of this material is vaporized, and another portion is Melted , but the majority of the mass (about 10,000 times the mass of the meteorite) is expelled out of the crater as ejecta, forming the ring that surrounds it. The central part of the crater is an area slightly more depressed than the rest of the terrain. The difference with Volcanic Calderas or ash cones are that they do not have rings of accumulated material, and their summits are above the level of the surface. A small portion of the impacted body is expelled large distances, creating Rays , which are shapes that resemble straight lines. Volcanism The major product of Volcanic Processes on the Moon are evident to the Earth-bound observer in the form of the Lunar Maria . These are large flows of Basalt ic- Lava that have left behind low- Albedo surfaces covering nearly a third of the near side. Measurements of samples brought back during the Apollo missions show the basalts melt at temperatures of about 1200° C, and have a lower Viscosity compared to basaltic magmas on the Earth. Current models of the lunar surface history place the mare volcanism primarily between 4 and 3.2 billion years ago. This began toward the end of the heavy bombardment phase, at about 3.8 billion years. The impacts had the effect of fissuring and Breccia ting the crust, increasing its insulation properties of the surface layers. Radioactive heat within the interior began to build up, resulting in a molten ocean beneath the solidified and crystalized surface. The cratering of the surface, especially in the case of the major impact basin events such as the Mare Imbrium formation, resulted in deep fissures below the surface. These cracks made convenient paths for upwelling magma, which breached the surface in repeated flows. Large volumes of high temperature, low viscosity magma spread into broad sheets even in areas with a low gradient. Multiple layers of magma covered the areas of the impact basins, and broke into neighbouring craters. Measurements of gravitational perturbations of orbiting spacecraft have determined that some lava-flooded impact basins form mass concentrations, or '' Mascon s''. The denser rock of the ''maria'' produced the gravitational anomalies. The amount of perturbation can be used to estimate the depth of the mares. The Mare Imbrium, for example, is particularly deep, displaying a depth in the range of 5-6 km with the central portion as deep as 8 km. In certain cases the upwelling of magma has been contained to the interior of an impact crater. These are known as ''floor-fractured craters'', as the energy of their impact crated a fracture in the surface through which magma could pass to the surface. Some of these floor-fractured craters have a convex floor, most likely due to the build-up of magma beneath the floor that resulted in a bulge. Perhaps the most extreme example of a flooded crater is Wargentin . The interior of this crater has completely filled with lava up to the edge of the rim. Lunar Orbiter spacecraft images revealed fields of volcanic domes that may indicate deep-seated, high-silica eruptions on the Moon, possible sources of some Tektite s found on Earth. These domes are similar to the Mona Lake craters of California; also, Mono obsidians resemble some layered tektites. Maria The lunar Maria cover about 16% of the lunar surface, and they were formed by Lava floods that covered large Impact Crater s. Although it is thought that the Moon does not have any Volcanic activity in the present, it did have activity in the past. Volcanic activity on the Moon began after the highlands were formed and theafter most of the impacts on the surface happened. Due to this, lunar maria are geologically younger than the lunar highlands. Even before the patterns and collapses attributed to Lava Tube s. The samples brought back form the missions in the 1960's and 1970's confirmed the suspicion and demonstrated that the basins are made out of a Basalt , a volcanic rock. The maria fill the most part of the impact basins on the visible side of the Moon. A few scientists suggested that this was a (''Sea of Serenity'') resides. As a result, the ''Mare Serenitatis'' is older. The most visible characteristic of the relative youth of the maria relative to the surrounding terrain is the mare are less cratered, which implies that these have been present for a shorter period of time. With the data compiled in the lunar missions, it is known that the mare could have formed billions of years after the formation of the basins. Another type of deposit associated with the mare, although it also covers the highland areas, are the deposits of ''dark mantle'' . These deposits cannot be seen with the naked eye, but they can be seen with Telescope s or when Spacecraft are near the lunar surface. Before the ''Apollo'' missions, scientists believed that they were deposits produced by Pyroclastic eruptions. Some depesits appear to be associated with dark elongated Ash Cone s, reinforcing the idea of pyroclasts, which were later confirmed by the discovery of Glass Pearl s similar to those found in pyroclastic eruptions here on Earth. Geologic composition of maria The main characteristics of the Basalt ic rocks with respect with the rocks of the lunar highlands is that the basalts contain higher amounts of Olivine and Pyroxene , with less Plagioclase . Many of them show a Ferro - Titanic Oxide called Ilmenite . Since the first sampling of rocks contained a high content of ilmenite and other related minerals, they received the name of "''high titanium''" basalts, in reference to the exceptionally high concentrations of the Metal . The '' Apollo 12 '' returned to Earth with basalts of lower concentrations, which were dubbed "''low titanium''" basalts. Subsequent missions and Soviet unmanned probes returned with basalts with even lower concentrations, now called "''very low titanium''" basalts. The '' Clementine '' space probe returned data that showed a wide range of titanium concentrations in basaltic rocks, with the high concentration rocks being the least abundant. The shape of the mineral grains of the basalts found in the maria indicate that these rocks formed when lava flowed through cracks in the surface. Some of these Dikes were narrow (about one Meter in thickness), while others reached up to 30 m. Many of the lunar basalts contain small holes called Vesicles , which were formed by gas bubbles trapped within the lava as it solidified. It is not known with certainty which gases escaped these rocks, but on Earth, vesicles form when Carbon Dioxide and Water Vapor mixed with Sulphur and Chlorine escape the rock. In the Moon, there is no evidence for the existence of Water . However, it is likely that the gases were carbon dioxide, and Carbon Monoxide with a bit of sulphur. The samples of Pyroclastic glasses are of green, yellow, and red tints. The difference in color indicates the concentration of titanium that the rock possesses, with the green particles having the lowest concentrations (about 1%), and red particles having the highest concentrations (up to 14%, much more than the basalts with the highest concentrations). The experiments conducted on the basaltic rocks and pyroclastic glasses show that these were formed when the interior of the Moon was partially molten. The rocks do not have a specific temperature of fusion, since they can melt inside a range of temperatures (for example, the basalts melt at temperatures between 1,000 and 1,200°C). The experiments demonstrated that the melting in the Moon took place at depths between 100 and 500 km, and that the rocks that fused partially contained mainly olivine and pyroxene with some ilmenite in the regions that formed the high titanium basalts. Unanswered questions about maria There's still a few mysteries surrounding maria:
In some cases, it is visible that the lava came from the impact basins, or perhaps along Fissure s concentric to the basin, but in the majority of cases, it is not seen from where it erupted. Another curious characteristic of the Moon is that almost all of the mare are present on the side facing Earth. The majority of scientists believe that this is due to the Asymmetry of the lunar crust, since the crust of the highlands on the opposite side is thicker, making it more difficult for the basalt to reach the surface. Rilles The surface of the moon includes a number of these long, slender features that form a channel-like depression in the surface. Rille s generally fall into three categories, consisting of sinuous, arcuate, or linear shapes. When the ''maria'' were formed by widespread flows of magma, the material gradually cooled and contracted. This produced cracks in the surface along the edges, resulting in arc-shaped arcuate rilles. These can be found along the edges of many of the lunar mares. . NASA photo taken during Apollo 10 mission.]] Lunar volcanoes emitted streams of lava that proceeded to form a path through the surface, often creating a Lava Tube . The paths followed by these lava flows were generally sinuous in shape, presumably for the same reason that rivers meander on a plain. By following these meandering rilles back to their source, they often lead to an old volcano vent in the lunar surface. One of the most notable sinuous rilles is the Vallis Schröteri feature, located on a continental rise along the east edge of Oceanus Procellarum . At the terminus of this flow is a 1 km high precipice where the lava dropped to the mare. The final form is the linear rille, which are thought to be Graben s created by shifts in the lunar surface. As such they may represent evidence of fault lines. These may be caused by major impacts or stresses induced by tidal forces. Wrinkle-ridges Complementary to the arcuate rills created by the shrinking mare crusts are the Wrinkle Ridge s. These surface features resemble a buckling of the surface, forming long ridges across parts of the mare. They often form curving shapes, and pass through peaks that protrude through the surface. Some of these ridges may outline buried craters or other features beneath the mares. A prime example of such an outlined feature is the Letronne Crater . Lunar domes A variety of Shield Volcano can be found in selected locations on the lunar surface, such as on Mons Rümker . These are believed to be formed by highly viscous, possibly silica-rich lava, erupting from localized vents followed by relatively slow cooling. The resulting Lunar Dome s are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the mid-point. They are typically 8-12 km in diameter, but can be up to 20 km across. Some of the domes contain a small craterlet at the peak. COMPOSITION More than 4.5 structure and thus were left behind, floating to the surface of the magma. The lunar crust is composed of a variety of primary elements, including Uranium , Thorium , Potassium , Oxygen , Silicon , Magnesium , Iron , Titanium , Calcium , Aluminum and Hydrogen . The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron. Surface materials The Apollo Program brought back 381.7 kg (841.5 lb) of Lunar Surface Material , most of which is stored at the Lunar Receiving Laboratory in Houston, Texas . These rocks have produced additional insights into the geological processes on the Moon. Material scattered by formation of the impact basins was collected among the soil samples, producing data for other portions of the surface. Due to the impact history of the Moon, particularly the largest basins, the upper crust to a depth of several kilometers is composed primarily of deposits of ''ejecta'' blankets from these impacts. This broken and intermixed rock is called Breccia , and the surface layer of this rock is the ''megaregolith''. The Regolith is the name given to such unconsolidated debris overlaying the bedrock in the highland regions and covering parts of the mares. Hypervelocity impacts of micrometeorites on the lunar surface release energy, which has the effect of fusing rock together to form larger, glassy bodies called Agglutinate s. (The accumulation of these bodies darkens the soil, causing high Albedo features such as Ray System s to fade.) The impacts also grind down materials into ever finer particles, until a fine powder is formed. Over time an equilibrium is reached between the formation of the larger glassy rocks and the powder, resulting in the regolith soil that covers much of the surface. Among the minerals found on the surface are Armalcolite , Ilmenite , Olivine , Plagioclase Feldspar , Pyroxene , and Quartz . Armalcolite, a mineral composed of iron and Titanium Oxide , was named for ''Arm''strong, ''Al''drin, and ''Col''lins, the three members of the Apollo 11 crew. The mares are composed primarily of Basalt s that are relatively rich in Iron . The highland regions are iron-poor, and are thought to be composed primarily of Ferroan Anorthosite , a Plagioclase Feldspar that is rich in Aluminum and Calcium . Another significant component of the crust are the Igneous Rock s called the ''Mg-suite'', of which Troctolite is made of equal portions of Olivine and Plagioclase . The Norite is an igneous rock found in the lunar crust, consisting of plagioclase feldspar and pyroxene. Composite rocks on the lunar surface often appear in the form of breccias. Of these, the subcategories are called fragmental, granulitic, and impact-melt breccias, depending on how they were formed. The aluminous melt group of breccias are impact-melt rocks with a high proportion of aluminum. Conglomerates are lunar rocks made from other rocks that have previously become rounded from impact wear. Finally there is the LKFM, or ''low-K Fra Mauro'' rocks. These were formed by impact-melt, but have a higher proportion of iron and Magnesium than is normal for upper crust rocks. Regolith The surface of the Moon is colored Grey and presents a large amount of fine Sediment as a result of innumerable impacts by meteorites. This "dust" receives the name of lunar Regolith , a term coined to describe layers of sediments produced by mechanical effects on the surfaces of the planets. The thickness of the regolith varies between 2 Meter s, on the younger maria, up to 20 meters in the oldest surfaces of the lunar highlands. The lunar regolith is composed of the material of the rocks found in the region, but also contains traces of materials expelled by distant impacts, which makes the regolith a rock of high scientific value. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles; these objects are called Agglutinate s. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in Aluminium , just as the rocks in those regions. The regolith in the maria is rich in Iron and Magnesium , as the Basalt ic rocks from which it is made of. The lunar regolith is very important because it also stores information about the history of the Sun . The atoms that compose the Solar Wind —mostly Helium , Neon , Carbon and Nitrogen —hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularly its Isotopic composition, it is possible to determine if the activity of the Sun has changed with time. The gases of the solar wind could be useful for future lunar bases, since the oxygen, Hydrogen ( Water ), Carbon and Nitrogen are not only essential to sustain life, but are also very useful in the production of Fuel . There is a large quantity of oxygen stored in Silicate s, which compose almost 50% of lunar minerals by volume, and the solar wind provides the rest. LUNAR ROCKS Highlands and lunar magma The first rocks brought back by '' Apollo 11 '' were Basalt s. In spite that the mission landed on Mare Tranquillitatis ( Latin for "''Sea of Tranquility''"), a few millimetric fragments of rocks coming from the highlands were picked up. These are composed mainly of Plagioclase Feldspar ; some fragments were composed exclusively of plagioclase. These rocks are called Anorthosite s. The rocks of the highlands are made mainly of plagioclase since this mineral started floating and accumulating on top of the primeval ocean of magma, giving rise to the hypothesis that the Moon was once covered by such an ocean. crust]] The concept of the magma ocean was proved in 1994 when Clementine , an American Space Probe , took pictures of the surface of the Moon in different Wavelength s while in Polar Orbit . Scientists analyzed the Iron content of the surface by measuring the variations in the intensity of reflected Electromagnetic Radiation . The magma ocean hypothesis predicts that the lunar highlands should have a low content of iron—which is found as an Oxide , FeO —corresponding to approximately less than 5% by weight. According to ''Clementine'''s measurements, the average presence of FeO is less than 5% by weight. This data was later confirmed in 1998 when the '' Lunar Prospector '' probe orbited the Moon. The lunar highlands contain another kind of s and Troctolite s, which have equal amounts of plagioclase and Olivine or Pyroxene (both being Silicate -based minerals containing iron and Magnesium ). The Radiometric Dating of these rocks suggests that they are younger than the anorthosites that formed after the magma ocean had crystallized. The rocks of the highlands are also very complex, due to the craterization process. The majority of these rocks are complex Mixture s of others. The original rocks were molten, mixed, and impacted during the first 500 million years of the Moon. The resulting rocks, called Breccia s, are so mixed that they contain breccias inside breccias. Most of the Anorthosite s, Norite s and Troctolite s are in fact fragments of rocks inside breccias. What is interesting about the breccias of the lunar higlands, especially the impact breccias (rocks partially molten by an impact event) is that most of them were formed between 3.85 and 4.0 billion years ago. This carries the idea that the Moon experienced a very intense bombardment of Meteorites during this period. However, it must be taken into account that the samples brought back by the ''Apollo'' missions is very reduced and corresponds only to a small region of the Moon. Many breccias and a few igneous rocks are enriched with Element s that are not common on Earth. These elements do not tend to be a fundamental part of the minerals present in the rocks. Their presence originates when the magma crystallizes, and the part that remains liquid is progressively enriched by these special elements. The rocks that contain them are called KREEP , which stands for Potassium (K) , Rare Earth Elements (REE) and Phosphorus (P) . Currently, it is believed that KREEPs represent the final remnants of the crystallization of the magma ocean. Large impacts dug through the crust, expelling the underlying material and mixing it with other debris to form KREEP breccias. Mineral composition of lunar rocks Lunar minerals STUDY OF LUNAR ROCKS Most of the rocks brought from the Moon are stored in the Lunar Curatorial Facility in the Johnson Space Center in Houston , Texas . A small percentage is distributed in auxiliary installations at Brooks Air Force Base , near San Antonio, Texas . Many lunar samples are found in laboratories of researchers worldwide. A small number of these rocks is available for public display in museums, and only three pieces can be touched by the public. These are the "touchable rocks", cut from basaltic rocks obtained by '' Apollo 17 ''. One of these rocks is located at the Smithsonian Institution 's National Air And Space Museum in Washington, D.C. Another piece is located in the Houston Space Center, located near the Johnson Space Center. The third rock can be found inside the Museum of the Sciences in the National Autonomous University Of Mexico . Interior The current model of the interior of the Moon was derived using Seismometer s left behind during the manned Apollo Program missions. These instruments were used to measure the propagation of seismic waves through the interior due to quakes and impacts. The mass of the moon is sufficient to eliminate any voids within the interior, so it is believed to be composed of solid rock throughout. The low density indicates a low metal abundance. The crust layer of the moon varies in depth, but has an average thickness of about 68 km. The crust is thicker in the highland regions and thinnest in the ''maria''. The Far Side is generally thicker than the near side, which could explain the paucity of lunar mares on side hidden from the Earth. The lunar mantle is roughly 1000 km in depth, with a small core having a radius of about 300 to 425 km. The outer portion of the core may still be molten, but the remainder of the Moon is geologically inactive. Most of the current seismic activity appears to be triggered by Tidal interaction with the Earth. The center of gravity of the Moon is offset from the center of figure by about 2 km in the direction of the Earth. Scientists currently do not have good measurements of the interior temperatures of the Moon, even within the upper crust. INTERIOR AND MOONQUAKES The Moon does not have Tectonic Plate s, and as a result, its crust is not renewed constantly as Earth's surface is. Earthquakes on the Moon, called Moonquake s, are minimal, and the largest (of Magnitude 5), only occur about once a year. The interior of the Moon is very different from the Interior of the Earth; the lunar Crust only has a thickness of about 70 km in the side facing Earth, and of about 150 km on the side opposite. Maria have a thickness of about 1 km (data derived from Photogeologic studies). The samples returned to Earth and Space Probe data suggest that the lower part of the crust contains less Plagioclase than the upper half. Under the crust, the lunar Mantle is found, which is the Moon's thickest layer. There may be a difference in the composition of the rocks above and below a depth of 500 km, representing the depth of the ocean of Magma . Below the mantle, the lunar core can be found. Its size is uncertain, but estimates range between 100 and 400 km. While the Moon does not possess a Magnetic Field , like the Earth does, it did have it in the past. Lunar rocks are magnetized, especially older ones, which possess the most Magnetism . This may mean than in the ancient past, the magnetic field was stronger than in the moons recent past. While the reasons behind the field's weakening are unknown, they are useful for theorizing the absence of a liquid Iron core, as the Earth has. The Earth's Liquid Core produces Electric Currents necessary for the Creation Of The Field . Another difference between the Moon and Earth derived this way is that the average Density of the Moon is about 3.3 g/cm³, while the Earth's density is about 5.5 g/cm³. In some regions of the Moon, the density of its Gravitational Field is more intense. This mystery was solved by the '' Lunar Prospector '' Space Probe , which associated them with regions of higher mass concentrations ( Mascon s), present in the maria of the basins. photo.'']] Near the south pole, at about 80°, there exist the remnants of the Aitken Basin , the largest in the Solar System , with a diameter of about 2,500 km. The largest of these areas, of about 15,000 km&2, does not receive solar Radiation due to the shadows of the high elevations that surrounds them. Images from the Clementine space probe and the Neutron Spectrometer of the ''Lunar Prospector'' indicate that the region contains deposits of frozen Water . Up to that moment, the presence of a water reserve between 10 and 300 million metric Ton nes was suspected. Also, the ''Lunar Prospector'' discovered that the north pole contains about twice the amount of Ice than the south pole does. SEE ALSO REFERENCES
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