Geology of the Moon
Encyclopedia
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is quite different from that of the Earth
. The Moon
lacks a significant atmosphere and any bodies of water
, which eliminates erosion
due to weather
; it does not possess any form of plate tectonics
, it has a lower gravity
, and because of its small size, it cools more rapidly. The complex geomorphology
of the lunar surface has been formed by a combination of processes, chief among which are impact crater
ing and volcanism
. The Moon is a differentiated body, possessing a crust
, mantle
and core
.
Geological
studies of the Moon are based on a combination of Earth-based telescope
observations, measurements from orbiting spacecraft
, lunar samples
, and geophysical data. A few locations were sampled directly during the Apollo
missions in the late 1960s and early 1970s, which returned approximately 385 kilograms of lunar rock and soil to Earth, as well as several missions of the Soviet
Luna programme
. The Moon is the only extraterrestrial body for which we possess samples with a known geologic context. A handful of lunar meteorite
s have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.
(O), silicon
(Si), iron
(Fe), magnesium
(Mg), calcium
(Ca), aluminium
(Al), manganese
(Mn) and titanium
(Ti). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45%. Carbon
(C) and nitrogen
(N) appear to be present only in trace quantities from deposition by solar wind
.
Neutron spectrometry data from the Lunar Prospector
indicate the presence of hydrogen
(H) concentrated at the poles
.
For a long time, the fundamental question regarding the history of the Moon was of its origin
. Early hypotheses included fission from the Earth, capture, and co-accretion. Today, the giant impact hypothesis
is widely accepted by the scientific community.
(son of the famous biologist Charles Darwin
). It was commonly assumed that the Pacific Ocean
represented the scar of this event. However, today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old or less, whereas the Moon is much older. This hypothesis also cannot account for the angular momentum
of the Earth-Moon system.
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
. In addition, this hypothesis has difficulty explaining the essentially identical oxygen
isotope
ratios of the two worlds.
.
The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system, nor why the Moon has a relative small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).
, a Canadian professor at Harvard college, originated it in the 1940s), satisfies the orbital conditions of the Earth and Moon and can account for the relatively small metallic core of the Moon. Collisions between planetesimal
s are now recognized to lead to the growth of planetary bodies early in the evolution of the Solar System, and in this framework it is inevitable that large impacts will sometimes occur when the planets are nearly formed.
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. Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon.
Computer simulation
s of this event appear to show that the collision must occur with a somewhat 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 (18,000°F).
This formation theory helps explain why the Moon possesses only a small iron
core (roughly 25% of its radius, in comparison to about 50% for the Earth). Most of the iron core from the impacting body is predicted to have accreted to the core of the Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit about the Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma
ocean.
The newly formed moon orbited at about one-tenth the distance that it does today, and became tidally-locked
with the Earth, where one side continually faces toward the Earth. The geology of the Moon has since been independent of the Earth. While this theory explains many aspects of the Earth-Moon system, there are still a few unresolved problems facing this theory, such as the Moon's volatile element
s not being as depleted as expected from such an energetic impact.
. Starting about 4.5 billion 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 geologic evolution of the Moon was the crystallization
of the near global magma ocean. It is not known with certainty what its depth was, but several studies imply a depth of about 500 km or greater. The first minerals to form in this ocean were the iron and magnesium silicate
s olivine
and pyroxene
. Because these minerals were denser than the molten material around them, they sank. After crystallization was about 75% complete, less dense anorthositic
plagioclase
feldspar
crystallized and floated, forming an anorthositic crust about 50 km in thickness. The majority of the magma ocean crystallized quickly (within about 100 million years or less), though the final remaining KREEP
-rich magmas, which are highly enriched in incompatible and heat producing elements, could have remained partially molten for several hundred million (or perhaps 1 billion) years. It appears that the final KREEP-rich magmas of the magma ocean eventually became concentrated within the region of Oceanus Procellarum
and the Imbrium basin
, a unique geologic province that is now known as the Procellarum KREEP Terrane.
Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the Mg
-suite norite
s and troctolite
s began to form, although the exact depths at which this occurred are not known precisely. Recent theories suggest that Mg-suite plutonism
was largely confined to the region of the Procellarum KREEP Terrane, and that these magmas are genetically related to KREEP in some manner, though their origin is still highly debated in the scientific community. The oldest of the Mg-suite rocks have crystallization ages of about 3.85 Ga. However, the last large impact that could have excavated deep into the crust (the Imbrium basin
) also occurred at 3.85 Ga before present. Thus, it seems probable that Mg-suite plutonic activity continued for a much longer time, and that younger plutonic rocks exist deep below the surface.
Analysis of the lunar samples seem to imply that a significant percentage of the lunar impact basins formed within a very short period of time between about 4 and 3.85 Ga ago. This hypothesis is referred to as the lunar cataclysm or late heavy bombardment
. However, it is now recognized that ejecta from the Imbrium impact basin (one of the youngest large impact basins on the Moon) should be found at all of the Apollo landing sites. It is thus possible that ages for some impact basins (in particular Mare Nectaris
) could have been mistakenly assigned the same age as Imbrium.
The lunar maria
represent ancient flood basaltic eruptions. In comparison to terrestrial lavas, these contain higher iron abundances, have low viscosities, and some contain highly elevated abundances of the titanium
-rich mineral ilmenite
. The majority of basalt
ic eruptions occurred between about 3 and 3.5 Ga ago, though some mare samples have ages as old as 4.2 Ga, and the youngest (based on the method of crater counting) are believed to have erupted only 1 billion years ago. Along with mare volcanism came pyroclastic eruptions, which launched molten basaltic materials hundreds of kilometres away from the volcano
. A large portion of the mare formed, or flowed into, the low elevations associated with the nearside impact basins. However, Oceanus Procellarum
does not correspond to any known impact structure, and the lowest elevations of the Moon within the farside South Pole-Aitken basin
are only modestly covered by mare (see lunar mare
for a more detailed discussion).
Impacts by meteorite
s and comet
s are the only abrupt geologic force acting on the Moon today, though the variation of Earth tides on the scale of the Lunar anomalistic month causes small variations in stresses. Some of the most important crater
s used in lunar stratigraphy formed in this recent epoch. For example, the crater Copernicus
, which has a depth of 3.76 km and a radius of 93 km, is believed to have formed about 900 million years ago (though this is debatable). The Apollo 17
mission landed in an area in which the material coming from the crater Tycho
might have been sampled. The study of these rocks seem to indicate that this crater could have formed 100 million years ago, though this is debatable as well. The surface has also experienced space weathering
due to high energy particles, solar wind
implantation, and micrometeorite impacts. This process causes the ray system
s associated with young craters to darken until it matches the albedo of the surrounding surface. However, if the composition of the ray is different from the underlying crustal materials (as might occur when a "highland" ray is emplaced on the mare), the ray could be visible for much longer times.
After resumption of Lunar exploration in the 1990s, it was discovered there are scarps across the globe which are caused by the contraction due to cooling
of the Moon.
is very similar to the lunar case.
s, their ejecta, a few volcano
es, hills, lava flows and depressions filled by magma.
for Earth
), and the darker plains are called maria (singular mare, from the Latin for sea
), after Johannes Kepler
who introduced the name in the 17th century. The highlands are anorthositic
in composition, whereas the maria are basalt
ic. The maria often coincide with the "lowlands," but it is important to note that the lowlands (such as within the South Pole-Aitken basin
) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.
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 correspond to low-albedo
surfaces covering nearly a third of the near side. Only a few percent of the farside has been affected by mare volcanism. Even before the Apollo missions confirmed it, most scientists believed that the maria were lava-filled plains, since they possessed lava flow patterns and collapses attributed to lava tube
s.
The ages of the mare basalts have been determined both by direct radiometric dating
and by the technique of crater counting
. The oldest radiometric ages are about 4.2 Ga, whereas the youngest ages determined from crater counting are about 1 Ga (1 Ga = 1 billion years). Volumetrically, most of the mare formed between about 3 and 3.5 Ga before present. The youngest lavas erupted within Oceanus Procellarum
, whereas some of the oldest appear to be located on the farside. The maria are clearly younger than the surrounding highlands given their lower density of impact craters.
A large portion of maria erupted within, or flowed into, the low-lying impact basins on the lunar nearside. Nevertheless, it is unlikely that a causal relationship exists between the impact event and mare volcanism because the impact basins are much older (by about 500 million years) than the mare fill. Furthermore, Oceanus Procellarum
, which is the largest expanse of mare volcanism on the Moon, does not correspond to any known impact basin. It is commonly suggested that the reason the mare only erupted on the nearside is that the nearside crust is thinner than the farside. While crustal thickness variations might act to modulate the amount of magma that ultimately reaches the surface, this hypothesis does not explain why the farside South Pole-Aitken basin
, whose crust is thinner than Oceanus Procellarum, was only modestly filled by volcanic products. Finally, it should be noted that the Earth's gravity played no preferential role in causing mare volcanism to occur on the near side, as the Earth's gravitational attraction is exactly balanced by the centrifugal acceleration resulting from the Moon's rotation.
Another type of deposit associated with the maria, although it also covers the highland areas, are the "dark mantle" deposits. These deposits cannot be seen with the naked eye, but they can be seen in images taken from telescopes or orbiting spacecraft. Before the Apollo missions, scientists believed that they were deposits produced by pyroclastic eruptions. Some deposits appear to be associated with dark elongated ash cones, reinforcing the idea of pyroclasts. The existence of pyroclastic eruptions was later confirmed by the discovery of glass spherules similar to those found in pyroclastic eruptions here on Earth.
Many of the lunar basalts contain small holes called vesicle
s, which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface. It is not known with certainty which gases escaped these rocks, but carbon monoxide
is one candidate.
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).
s on the Moon sometimes resulted from the formation of localized lava channel
s. These generally fall into three categories, consisting of sinuous, arcuate, or linear shapes. By following these meandering rilles back to their source, they often lead to an old volcanic vent. One of the most notable sinuous rilles is the Vallis Schröteri
feature, located in the Aristarchus plateau along the eastern edge of Oceanus Procellarum
. An example of a sinuous rille exists at the Apollo 15
landing site, Rima Hadley, located on the rim of the Imbrium Basin. Based on observations from the mission, it is generally believed that this rille was formed by volcanic processes, a topic long debated before the mission took place.
es can be found in selected locations on the lunar surface, such as on Mons Rümker
. These are believed to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. 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 pit at their peak.
.
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 geologic principle of superposition
. That is, if a crater (or its ejecta) overlaid another, it must be the younger. The amount of erosion experienced by a crater was another clue to its age, though this is more subjective. Adopting this approach in the late 1950s, Gene Shoemaker
took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.
Impact crater
ing is the most notable geological process on the Moon. The craters are formed when a solid body, such as an asteroid
or comet
, collides with the surface at a high velocity (mean impact velocities for the Moon are about 17 km per second). 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 hydrodynamic rebound of the floor 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. In a very general sense, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters. The size frequency distribution (SFD) of crater diameters on a given surface (that is, the number of craters as a function of diameter) approximately follows a power law
with increasing number of craters with decreasing crater size. The vertical position of this curve can be used to estimate the age of the surface.
The most recent impacts are distinguished by well-defined features, including a sharp-edged rim. Small craters tend to form a bowl shape, while larger impacts can have a central peak with flat floors. Larger craters generally display slumping features along the inner walls that can form terraces and ledges. The largest impact basins, the multiring basins, can even have secondary concentric rings of raised material.
The impact process excavates high albedo
materials that initially gives the crater, ejecta, and ray system
a bright appearance. The process of space weathering
gradually decreases the albedo of this material such that the rays fade with time. Gradually the crater and its ejecta undergo impact erosion from micrometeorites and smaller impacts. This erosional process softens and rounds the features of the crater. 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 can include larges blocks of material that reimpact the surface to form secondary impact craters. These craters are sometimes formed in clearly discernible radial patterns, and generally have shallower depths than primary craters of the same size. 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. Laboratory experiments at NASA's Ames Research Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that elliptical craters start forming at impact angles below five degrees. However, a low angle impact can produce a central peak that is offset from the mid-point of the crater. Additionally, the ejecta from oblique impacts show distinctive patterns at different impact angles: asymmetry starting around 60˚ and a wedge-shaped "zone of avoidance" free of ejecta in the direction the projectile came from starting around 45˚.
Dark-halo craters are formed when an impact excavates lower albedo material from beneath the surface, then deposits this darker ejecta around the main crater. This can occur when an area of darker basalt
ic material, such as that found on the maria
, is later covered by lighter ejecta derived from more distant impacts in the highlands. This covering conceals the darker material below, which is later excavated by subsequent craters.
The largest impacts produced melt sheets of molten rock that covered portions of the surface which could be as thick as a kilometer. Examples of such impact melt can be seen in the northeastern part of the Mare Orientale
impact basin.
The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the 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 agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium
and silica, just as the rocks in those regions. The regolith in the maria is rich in iron
and magnesium
and is silica-poor, as the basalt
ic rocks from which it is formed.
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 oxygen, hydrogen
(water
), carbon
and nitrogen
are not only essential to sustain life, but are also potentially very useful in the production of fuel
. The composition of the lunar regolith can also be used to infer its source origin.
s form a potentially important location for constructing a future lunar base, which may be used for local exploration and development, or as a human outpost
to serve exploration beyond the Moon. A lunar lava cave
potential has long been suggested and discussed in literature and thesis. Any intact lava tube on the moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultraviolet radiation and energetic particles, and extreme diurnal temperature variations. Following the launch of the Lunar Reconnaissance Orbiter
, many lunar lava tubes have been imaged. These lunar pits are found in several locations across the moon, including Marius Hills
, Mare Ingenii
and Mare Tranquillitatis
.
were basalt
s. Although the mission landed on Mare Tranquillitatis
, 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 anorthositic plagioclase
. The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean.
A natural outcome of the giant impact event is that the materials that reaccreted to form the Moon must have been hot. Current models predict that a large portion of the Moon would have been molten shortly after the Moon formed, with estimates for the depth of this magma ocean ranging from about 500 km to full moon melting. Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.
As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a KREEP
-rich magma that initially should have been sandwiched between the crust and mantle. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.
, most of which is stored at the Lunar Receiving Laboratory
in Houston, Texas
and the unmanned Soviet Luna Program returned 0.3 kilograms of lunar material. These rocks have proved to be invaluable in deciphering the geologic evolution of the Moon. Lunar rocks are in large part made of the same common rock forming minerals as found on Earth, such as olivine
, pyroxene
, and plagioclase
feldspar
(anorthosite). Plagioclase feldspar is mostly found in the lunar crust, while pyroxene and olivine are typically seen in the lunar mantle. The mineral ilmenite
is highly abundant in some mare basalts, and a new mineral named armalcolite
(named for Armstrong, Aldrin, and Collins, the three members of the Apollo 11
crew) was first discovered in the lunar samples.
The maria are composed predominantly of basalt
, whereas the highland regions are iron-poor and composed primarily of anorthosite
, a rock composed primarily of calcium
-rich plagioclase feldspar. Another significant component of the crust are the igneous Mg-suite rocks, such as the troctolite
s, norite
s, and KREEP-basalts. These rocks are believed to be genetically related to the petrogenesis of KREEP
.
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 mafic
impact melt breccia
s, which are typified by the low-K Fra Mauro composition, have a higher proportion of iron and magnesium
than typical upper crust anorthositic rocks, as well as higher abundances of KREEP.
ic rocks with respect to the rocks of the lunar highlands is that the basalts contain higher abundances of olivine
and pyroxene
, and less plagioclase
. They are more rich in iron than terrestrial basalts, and also have lower viscosities. Some of them have high abundances of 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. The Apollo 12
mission returned to Earth with basalts of lower titanium concentrations, and these were dubbed "low titanium" basalts. Subsequent missions, including the Soviet
unmanned probes, returned with basalts with even lower concentrations, now called "very low titanium" basalts. The Clementine
space probe returned data showing that the mare basalts possess a continuum in titanium concentrations, with the highest concentration rocks being the least abundant.
s left behind during the manned Apollo program missions, as well as investigations of the Moon's gravity field and rotation.
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. Its low bulk density (~3346 kg m−3) indicates a low metal abundance. Mass and moment of inertia
constraints indicate that the Moon likely has an iron core that is less than about 450 km in radius. Studies of the Moon's physical librations (small perturbations to its rotation) furthermore indicate that the core is still molten. Most planetary bodies and moons have iron cores that are about half the size of the body. The Moon is thus anomalous in possessing a core whose size is only about one quarter of its radius.
The crust of the Moon is on average about 50 km thick (though this is uncertain by about ±15 km). It is widely believed that the far-side crust is on average thicker than the near side by about 15 km. Seismology has constrained the thickness of the crust only near the Apollo 12 and 14 landing sites. While the initial Apollo-era analyses suggested a crustal thickness of about 60 km at this site, recent reanalyses of this data set suggest a thinner value, somewhere between about 30 and 45 km.
Compared to that of Earth, the Moon has only a very weak external magnetic field. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo
in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on airless bodies such as the Moon, transient magnetic fields could be generated during impact processes. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the largest impact basins.
While the Moon does not possess a dipolar magnetic field
like the Earth does, some of the returned rocks possess strong magnetizations. Furthermore measurements from orbit show that some portions of the lunar surface are associated with strong magnetic fields.
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. The Moon
Moon
The Moon is Earth's only known natural satellite,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 . These are quasi-satellites and not true moons. For more...
lacks a significant atmosphere and any bodies of water
Body of water
A body of water or waterbody is any significant accumulation of water, usually covering the Earth or another planet. The term body of water most often refers to large accumulations of water, such as oceans, seas, and lakes, but it may also include smaller pools of water such as ponds, puddles or...
, which eliminates erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
due to weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
; it does not possess any form of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, it has a lower gravity
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
, and because of its small size, it cools more rapidly. The complex geomorphology
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
of the lunar surface has been formed by a combination of processes, chief among which are impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
ing and volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
. The Moon is a differentiated body, possessing a crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
, mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
and core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
.
Geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
studies of the Moon are based on a combination of Earth-based telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
observations, measurements from orbiting spacecraft
Exploration of the Moon
The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation. The invention of the optical telescope brought about the...
, lunar samples
Moon rock
Moon rock describes rock that formed on the Earth's moon. The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.The rocks collected from the Moon are measured by radiometric dating techniques...
, and geophysical data. A few locations were sampled directly during the Apollo
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...
missions in the late 1960s and early 1970s, which returned approximately 385 kilograms of lunar rock and soil to Earth, as well as several missions of the Soviet
Soviet space program
The Soviet space program is the rocketry and space exploration programs conducted by the former Union of Soviet Socialist Republics from the 1930s until its dissolution in 1991...
Luna programme
Luna programme
The Luna programme , occasionally called Lunik or Lunnik, was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976. Fifteen were successful, each designed as either an orbiter or lander, and accomplished many firsts in space exploration...
. The Moon is the only extraterrestrial body for which we possess samples with a known geologic context. A handful of lunar meteorite
Lunar meteorite
A Lunar meteorite is a meteorite that is known to have originated on the Moon.-Discovery:In January 1982, John Schutt, leading an expedition in Antarctica for the ANSMET program, found a meteorite that he recognized to be unusual...
s have been recognized on Earth, though their source craters on the Moon are unknown. A substantial portion of the lunar surface has not been explored, and a number of geological questions remain unanswered.
Elemental composition
Elements known to be present on the lunar surface include, among others, oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O), silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
(Si), iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(Fe), magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
(Mg), calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca), aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
(Al), manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
(Mn) and titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
(Ti). Among the more abundant are oxygen, iron and silicon. The oxygen content is estimated at 45%. Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
(C) and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N) appear to be present only in trace quantities from deposition by solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
.
Neutron spectrometry data from the Lunar Prospector
Lunar Prospector
The Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
indicate the presence of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H) concentrated at the poles
Poles of astronomical bodies
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere.-Geographic poles:...
.
Formation
Formation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...
. Early hypotheses included fission from the Earth, capture, and co-accretion. Today, the giant impact hypothesis
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
is widely accepted by the scientific community.
Fission hypothesis
The idea that the early Earth, with an accelerated rotation, expelled a piece of its mass was proposed by George DarwinGeorge Darwin
Sir George Howard Darwin, FRS was an English astronomer and mathematician.-Biography:Darwin was born at Down House, Kent, the second son and fifth child of Charles and Emma Darwin...
(son of the famous biologist Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
). It was commonly assumed that the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
represented the scar of this event. However, today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old or less, whereas the Moon is much older. This hypothesis also cannot account for the angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
of the Earth-Moon system.
Lunar capture
This hypothesis states that the Moon was captured, completely formed, by the gravitational fieldGravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...
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
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. In addition, this hypothesis has difficulty explaining the essentially identical oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
ratios of the two worlds.
Co-accretion hypothesis
This hypothesis states that the Earth and the Moon formed together as a double system from the primordial accretion disk of the Solar SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system, nor why the Moon has a relative small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).
Giant impact theory
At present the best explanation for the origin of the Moon involves a collision of two protoplanetary bodies during the early accretional period of Solar System evolution. This "giant impact theory", which became popular in 1984 (although Reginald Aldworth DalyReginald Aldworth Daly
-External links:* Daly′s Biography, American Geophysical Union* GSA Today, vol. 16, no. 2, 2006* National Academy of Sciences, Washington, DC, 36 pp., 1960...
, a Canadian professor at Harvard college, originated it in the 1940s), satisfies the orbital conditions of the Earth and Moon and can account for the relatively small metallic core of the Moon. Collisions between planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
s are now recognized to lead to the growth of planetary bodies early in the evolution of the Solar System, and in this framework it is inevitable that large impacts will sometimes occur when the planets are nearly formed.
The theory requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
(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
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....
, the Moon goddess
Goddess
A goddess is a female deity. In some cultures goddesses are associated with Earth, motherhood, love, and the household. In other cultures, goddesses also rule over war, death, and destruction as well as healing....
in Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
. This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon.
Computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
s of this event appear to show that the collision must occur with a somewhat 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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
would have risen to 10,000°C (18,000°F).
This formation theory helps explain why the Moon possesses only a small iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
core (roughly 25% of its radius, in comparison to about 50% for the Earth). Most of the iron core from the impacting body is predicted to have accreted to the core of the Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit about the Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
ocean.
The newly formed moon orbited at about one-tenth the distance that it does today, and became tidally-locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
with the Earth, where one side continually faces toward the Earth. The geology of the Moon has since been independent of the Earth. While this theory explains many aspects of the Earth-Moon system, there are still a few unresolved problems facing this theory, such as the Moon's volatile element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s not being as depleted as expected from such an energetic impact.
Geologic history
The geological history of the Moon has been defined into six major epochs, called the lunar geologic timescaleLunar geologic timescale
The lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian , Nectarian, and Pre-Nectarian...
. Starting about 4.5 billion years ago, the newly formed Moon was in a molten
Mölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
state and was orbiting much closer to the Earth. The resulting tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
s deformed the molten body into an ellipsoid, with the major axis pointed towards Earth.
The first important event in the geologic evolution of the Moon was the crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
of the near global magma ocean. It is not known with certainty what its depth was, but several studies imply a depth of about 500 km or greater. The first minerals to form in this ocean were the iron and magnesium silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
. Because these minerals were denser than the molten material around them, they sank. After crystallization was about 75% complete, less dense anorthositic
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
crystallized and floated, forming an anorthositic crust about 50 km in thickness. The majority of the magma ocean crystallized quickly (within about 100 million years or less), though the final remaining KREEP
KREEP
KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
-rich magmas, which are highly enriched in incompatible and heat producing elements, could have remained partially molten for several hundred million (or perhaps 1 billion) years. It appears that the final KREEP-rich magmas of the magma ocean eventually became concentrated within the region of Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
and the Imbrium basin
Mare Imbrium
Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon and one of the larger craters in the Solar System. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
, a unique geologic province that is now known as the Procellarum KREEP Terrane.
Quickly after the lunar crust formed, or even as it was forming, different types of magmas that would give rise to the Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
-suite norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
s and troctolite
Troctolite
Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a...
s began to form, although the exact depths at which this occurred are not known precisely. Recent theories suggest that Mg-suite plutonism
Plutonism
Plutonism is the geologic theory that the rocks forming the Earth were formed in fire by volcanic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on the sea bed, re-formed into layers of sedimentary rock by heat and pressure, and...
was largely confined to the region of the Procellarum KREEP Terrane, and that these magmas are genetically related to KREEP in some manner, though their origin is still highly debated in the scientific community. The oldest of the Mg-suite rocks have crystallization ages of about 3.85 Ga. However, the last large impact that could have excavated deep into the crust (the Imbrium basin
Mare Imbrium
Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon and one of the larger craters in the Solar System. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
) also occurred at 3.85 Ga before present. Thus, it seems probable that Mg-suite plutonic activity continued for a much longer time, and that younger plutonic rocks exist deep below the surface.
Analysis of the lunar samples seem to imply that a significant percentage of the lunar impact basins formed within a very short period of time between about 4 and 3.85 Ga ago. This hypothesis is referred to as the lunar cataclysm or late heavy bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
. However, it is now recognized that ejecta from the Imbrium impact basin (one of the youngest large impact basins on the Moon) should be found at all of the Apollo landing sites. It is thus possible that ages for some impact basins (in particular Mare Nectaris
Mare Nectaris
The Sea of Nectar is a small lunar mare or sea located between the Sea of Tranquillity and the Sea of Fecundity . Montes Pyrenaeus borders the mare to the west and the large crater near the south center of the mare is known as Rosse...
) could have been mistakenly assigned the same age as Imbrium.
The lunar maria
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
represent ancient flood basaltic eruptions. In comparison to terrestrial lavas, these contain higher iron abundances, have low viscosities, and some contain highly elevated abundances of the titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
-rich mineral ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
. The majority of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic eruptions occurred between about 3 and 3.5 Ga ago, though some mare samples have ages as old as 4.2 Ga, and the youngest (based on the method of crater counting) are believed to have erupted only 1 billion years ago. Along with mare volcanism came pyroclastic eruptions, which launched molten basaltic materials hundreds of kilometres away from the volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
. A large portion of the mare formed, or flowed into, the low elevations associated with the nearside impact basins. However, Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
does not correspond to any known impact structure, and the lowest elevations of the Moon within the farside South Pole-Aitken basin
South Pole-Aitken basin
The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest and deepest basin recognized on the Moon. This moon basin was named for two features on opposing sides; the...
are only modestly covered by mare (see lunar mare
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
for a more detailed discussion).
Impacts by meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s are the only abrupt geologic force acting on the Moon today, though the variation of Earth tides on the scale of the Lunar anomalistic month causes small variations in stresses. Some of the most important crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s used in lunar stratigraphy formed in this recent epoch. For example, the crater Copernicus
Copernicus (lunar crater)
Copernicus is a prominent lunar impact crater named after the astronomer Nicolaus Copernicus, located in eastern Oceanus Procellarum. It is estimated to be about 800 million years old, and typifies craters that formed during the Copernican period in that it has a prominent ray system.-...
, which has a depth of 3.76 km and a radius of 93 km, is believed to have formed about 900 million years ago (though this is debatable). The Apollo 17
Apollo 17
Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the...
mission landed in an area in which the material coming from the crater Tycho
Tycho (crater)
Tycho is a prominent lunar impact crater located in the southern lunar highlands, named after the Danish astronomer Tycho Brahe . To the south is the crater Street; to the east is Pictet, and to the north-northeast is Sasserides. The surface around Tycho is replete with craters of various sizes,...
might have been sampled. The study of these rocks seem to indicate that this crater could have formed 100 million years ago, though this is debatable as well. The surface has also experienced space weathering
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
due to high energy particles, solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
implantation, and micrometeorite impacts. This process causes the ray system
Ray system
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...
s associated with young craters to darken until it matches the albedo of the surrounding surface. However, if the composition of the ray is different from the underlying crustal materials (as might occur when a "highland" ray is emplaced on the mare), the ray could be visible for much longer times.
After resumption of Lunar exploration in the 1990s, it was discovered there are scarps across the globe which are caused by the contraction due to cooling
Geophysical global cooling
Before the concept of plate tectonics, global cooling was a reference to a geophysical theory by James Dwight Dana, also referred to as the contracting earth theory. It suggested that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank. As the...
of the Moon.
Strata and epochs
On the top of the lunar stratigraphical sequence rayed impact craters can be found. Such youngest craters belong to the Copernican unit. Below it can be found craters without the ray system, but with rather well developed impact crater morphology. This is the Eratosthenian unit. The two younger stratigraphical units can be found in crater sized spots on the Moon. Below them two extending strata can be found: mare units (earlier defined as Procellarian unit) and the Imbrium basin related ejecta and tectonic units (Imbrian units). Another impact basin related unit is the Nectarian unit, defined around the Nectarian Basin. At the bottom of the lunar stratigraphical sequence the pre-Nectarian unit of old crater plains can be found. The stratigraphy of MercuryMercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
is very similar to the lunar case.
Lunar landscape
The lunar landscape is characterized by impact craterImpact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s, their ejecta, a few volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es, hills, lava flows and depressions filled by magma.
The highlands
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 LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
for Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
), and the darker plains are called maria (singular mare, from the Latin for sea
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
), after Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...
who introduced the name in the 17th century. The highlands are anorthositic
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
in composition, whereas the maria are basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic. The maria often coincide with the "lowlands," but it is important to note that the lowlands (such as within the South Pole-Aitken basin
South Pole-Aitken basin
The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest and deepest basin recognized on the Moon. This moon basin was named for two features on opposing sides; the...
) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.
The maria
The major products of volcanic processesVolcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
on the Moon are evident to the Earth-bound observer in the form of the lunar maria
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
. These are large flows of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
that correspond to low-albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
surfaces covering nearly a third of the near side. Only a few percent of the farside has been affected by mare volcanism. Even before the Apollo missions confirmed it, most scientists believed that the maria were lava-filled plains, since they possessed lava flow patterns and collapses attributed to lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s.
The ages of the mare basalts have been determined both by direct radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
and by the technique of crater counting
Crater counting
Crater counting is a method for estimating the age of a planet's surface. The method is based upon the assumptions that a new surface forms with zero impact craters, and that impact craters accumulate at some constant rate...
. The oldest radiometric ages are about 4.2 Ga, whereas the youngest ages determined from crater counting are about 1 Ga (1 Ga = 1 billion years). Volumetrically, most of the mare formed between about 3 and 3.5 Ga before present. The youngest lavas erupted within Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
, whereas some of the oldest appear to be located on the farside. The maria are clearly younger than the surrounding highlands given their lower density of impact craters.
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A large portion of maria erupted within, or flowed into, the low-lying impact basins on the lunar nearside. Nevertheless, it is unlikely that a causal relationship exists between the impact event and mare volcanism because the impact basins are much older (by about 500 million years) than the mare fill. Furthermore, Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
, which is the largest expanse of mare volcanism on the Moon, does not correspond to any known impact basin. It is commonly suggested that the reason the mare only erupted on the nearside is that the nearside crust is thinner than the farside. While crustal thickness variations might act to modulate the amount of magma that ultimately reaches the surface, this hypothesis does not explain why the farside South Pole-Aitken basin
South Pole-Aitken basin
The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest and deepest basin recognized on the Moon. This moon basin was named for two features on opposing sides; the...
, whose crust is thinner than Oceanus Procellarum, was only modestly filled by volcanic products. Finally, it should be noted that the Earth's gravity played no preferential role in causing mare volcanism to occur on the near side, as the Earth's gravitational attraction is exactly balanced by the centrifugal acceleration resulting from the Moon's rotation.
Another type of deposit associated with the maria, although it also covers the highland areas, are the "dark mantle" deposits. These deposits cannot be seen with the naked eye, but they can be seen in images taken from telescopes or orbiting spacecraft. Before the Apollo missions, scientists believed that they were deposits produced by pyroclastic eruptions. Some deposits appear to be associated with dark elongated ash cones, reinforcing the idea of pyroclasts. The existence of pyroclastic eruptions was later confirmed by the discovery of glass spherules similar to those found in pyroclastic eruptions here on Earth.
Many of the lunar basalts contain small holes called vesicle
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
s, which were formed by gas bubbles exsolving from the magma at the vacuum conditions encountered at the surface. It is not known with certainty which gases escaped these rocks, but carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
is one candidate.
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).
Rilles
RilleRille
Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...
s on the Moon sometimes resulted from the formation of localized lava channel
Lava channel
A lava channel is a stream of fluid lava contained within marginal zones of static lava or levees. The initial channel may not contain levees per se, until the parental flow solidifies over what develops into the channel and creates simple levees. This initial levee allows for the building of a...
s. These generally fall into three categories, consisting of sinuous, arcuate, or linear shapes. By following these meandering rilles back to their source, they often lead to an old volcanic vent. One of the most notable sinuous rilles is the Vallis Schröteri
Vallis Schröteri
Schroter's Valley, frequently known by the Latinized name Vallis Schröteri, is a sinuous valley or rille on the surface of the near side of the Moon. It is located on a rise of continental ground, sometimes called the Aristarchus plateau, that is surrounded by the Oceanus Procellarum to the south...
feature, located in the Aristarchus plateau along the eastern edge of Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
. An example of a sinuous rille exists at the Apollo 15
Apollo 15
Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...
landing site, Rima Hadley, located on the rim of the Imbrium Basin. Based on observations from the mission, it is generally believed that this rille was formed by volcanic processes, a topic long debated before the mission took place.
Domes
A variety of shield volcanoShield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es can be found in selected locations on the lunar surface, such as on Mons Rümker
Mons Rümker
Mons Rümker is an isolated volcanic formation that is located in the northwest part of the Moon's near side, at selenographic coordinates 40.8° N, 58.1° W. The feature forms a large, elevated mound in the northern part of the Oceanus Procellarum. The mound has a diameter of 70 kilometres, and...
. These are believed to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. The resulting lunar dome
Lunar dome
A lunar dome is a type of shield volcano that is found on the surface of the Earth's Moon. They are typically formed by highly viscous, possibly silica-rich lava, erupting from localized vents followed by relatively slow cooling. Lunar domes are wide, rounded, circular features with a gentle slope...
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 pit at their peak.
Wrinkle ridges
Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Some of these ridges may outline buried craters or other features beneath the maria. A prime example of such an outlined feature is the crater LetronneLetronne (crater)
Letronne is the lava-flooded remnant of a lunar crater. The northern part of the rim is completely missing, and opens into the Oceanus Procellarum, forming a bay along the southwestern shore...
.
Grabens
Grabens are tectonic features that form under extension stresses. Structurally, they are composed of two normal faults, with a down-dropped block between them. Most grabens are found within the lunar maria near the edges of large impact basins.Impact craters
Law of superposition
The law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
. That is, if a crater (or its ejecta) overlaid another, it must be the younger. The amount of erosion experienced by a crater was another clue to its age, though this is more subjective. Adopting this approach in the late 1950s, Gene Shoemaker
Eugene Merle Shoemaker
Eugene Merle Shoemaker , American geologist, was one of the founders of the fields of planetary science....
took the systematic study of the Moon away from the astronomers and placed it firmly in the hands of the lunar geologists.
Impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
ing is the most notable geological process on the Moon. The craters are formed when a solid body, such as an asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
or comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
, collides with the surface at a high velocity (mean impact velocities for the Moon are about 17 km per second). 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
Rarefaction
Rarefaction is the reduction of a medium's density, or the opposite of compression.A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.Another...
wave, which is responsible for propelling most of the ejecta out of the crater. Finally there is a hydrodynamic rebound of the floor 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
South Pole-Aitken basin
The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest and deepest basin recognized on the Moon. This moon basin was named for two features on opposing sides; the...
with a diameter of nearly 2,500 km and a depth of 13 km. In a very general sense, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters. The size frequency distribution (SFD) of crater diameters on a given surface (that is, the number of craters as a function of diameter) approximately follows a power law
Power law
A power law is a special kind of mathematical relationship between two quantities. When the frequency of an event varies as a power of some attribute of that event , the frequency is said to follow a power law. For instance, the number of cities having a certain population size is found to vary...
with increasing number of craters with decreasing crater size. The vertical position of this curve can be used to estimate the age of the surface.
The impact process excavates high albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
materials that initially gives the crater, ejecta, and ray system
Ray system
A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking a bit like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by...
a bright appearance. The process of space weathering
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
gradually decreases the albedo of this material such that the rays fade with time. Gradually the crater and its ejecta undergo impact erosion from micrometeorites and smaller impacts. This erosional process softens and rounds the features of the crater. 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 can include larges blocks of material that reimpact the surface to form secondary impact craters. These craters are sometimes formed in clearly discernible radial patterns, and generally have shallower depths than primary craters of the same size. 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. Laboratory experiments at NASA's Ames Research Center have demonstrated that even very low-angle impacts tend to produce circular craters, and that elliptical craters start forming at impact angles below five degrees. However, a low angle impact can produce a central peak that is offset from the mid-point of the crater. Additionally, the ejecta from oblique impacts show distinctive patterns at different impact angles: asymmetry starting around 60˚ and a wedge-shaped "zone of avoidance" free of ejecta in the direction the projectile came from starting around 45˚.
Dark-halo craters are formed when an impact excavates lower albedo material from beneath the surface, then deposits this darker ejecta around the main crater. This can occur when an area of darker basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic material, such as that found on the maria
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
, is later covered by lighter ejecta derived from more distant impacts in the highlands. This covering conceals the darker material below, which is later excavated by subsequent craters.
The largest impacts produced melt sheets of molten rock that covered portions of the surface which could be as thick as a kilometer. Examples of such impact melt can be seen in the northeastern part of the Mare Orientale
Mare Orientale
Mare Orientale is one of the most striking large scale lunar features, resembling a target ring bull's-eye. Located on the extreme western edge of the lunar nearside, this impact basin is difficult to see from an Earthbound perspective.Material from this basin was not sampled by the Apollo program...
impact basin.
Regolith
The surface of the Moon has been subject to billions of years of collisions with both small and large asteroidal and cometary materials. Over time, these impact processes have pulverized and "gardened" the surface materials, forming a fine grained layer termed "regolith". The thickness of the regolith varies between 2 meters beneath the younger maria, to up to 20 meters beneath the oldest surfaces of the lunar highlands. The regolith is predominantly composed of materials found in the region, but also contains traces of materials ejected by distant impact craters. The term "mega-regolith" is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer.The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the 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 agglutinates. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
and silica, just as the rocks in those regions. The regolith in the maria is rich in iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and is silica-poor, as the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic rocks from which it is formed.
The lunar regolith is very important because it also stores information about the history of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. The atoms that compose the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
– mostly helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
– hit the lunar surface and insert themselves into the mineral grains. Upon analyzing the composition of the regolith, particularly its isotopic
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
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 oxygen, hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
), carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
are not only essential to sustain life, but are also potentially very useful in the production of fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
. The composition of the lunar regolith can also be used to infer its source origin.
Lunar lava tubes
Lunar lava tubeLava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s form a potentially important location for constructing a future lunar base, which may be used for local exploration and development, or as a human outpost
Human outpost
Human outposts are artificially-created, controlled human habitats located in environments inhospitable for humans, such as on the ocean floor, in space or on another planet....
to serve exploration beyond the Moon. A lunar lava cave
Lava cave
A lava cave is any cave formed in volcanic rock, although in typical usage it is reserved for caves formed by volcanic processes, which are more properly termed volcanic caves...
potential has long been suggested and discussed in literature and thesis. Any intact lava tube on the moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultraviolet radiation and energetic particles, and extreme diurnal temperature variations. Following the launch of the Lunar Reconnaissance Orbiter
Lunar Reconnaissance Orbiter
The Lunar Precursor Robotic Program is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite , were launched in June 2009...
, many lunar lava tubes have been imaged. These lunar pits are found in several locations across the moon, including Marius Hills
Marius (crater)
Marius is a lunar crater located on the Oceanus Procellarum. The surface to the west and north of this crater contains a large number of lunar domes spread across an area over a hundred kilometers in diameter that may be of volcanic origin dubbed the Marius Hills...
, Mare Ingenii
Mare Ingenii
Mare Ingenii is one of the few lunar mare features on the far side of the Moon. The mare sits in the Ingenii basin. This basin material is of the Pre-Nectarian epoch. The mare material located in Ingenii and the surrounding craters is of the Upper Imbrian epoch...
and Mare Tranquillitatis
Mare Tranquillitatis
Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but...
.
The lunar magma ocean
The first rocks brought back by Apollo 11Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
were basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s. Although the mission landed on Mare Tranquillitatis
Mare Tranquillitatis
Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but...
, a few millimetric fragments of rocks coming from the highlands were picked up. These are composed mainly of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
; some fragments were composed exclusively of anorthositic plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
. The identification of these mineral fragments led to the bold hypothesis that a large portion of the Moon was once molten, and that the crust formed by fractional crystallization of this magma ocean.
A natural outcome of the giant impact event is that the materials that reaccreted to form the Moon must have been hot. Current models predict that a large portion of the Moon would have been molten shortly after the Moon formed, with estimates for the depth of this magma ocean ranging from about 500 km to full moon melting. Crystallization of this magma ocean would have given rise to a differentiated body with a compositionally distinct crust and mantle and accounts for the major suites of lunar rocks.
As crystallization of the lunar magma ocean proceeded, minerals such as olivine and pyroxene would have precipitated and sank to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a KREEP
KREEP
KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
-rich magma that initially should have been sandwiched between the crust and mantle. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.
Surface materials
The Apollo program brought back 381.7 kg (841.5 lb) of lunar surface materialMoon rock
Moon rock describes rock that formed on the Earth's moon. The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.The rocks collected from the Moon are measured by radiometric dating techniques...
, most of which is stored at the Lunar Receiving Laboratory
Lunar Receiving Laboratory
The Lunar Receiving Laboratory was a facility at NASA's Lyndon B. Johnson Space Center that was constructed to quarantine astronauts and material brought back from the Moon during the Apollo program to mitigate the risk of back-contamination...
in Houston, Texas
Houston, Texas
Houston is the fourth-largest city in the United States, and the largest city in the state of Texas. According to the 2010 U.S. Census, the city had a population of 2.1 million people within an area of . Houston is the seat of Harris County and the economic center of , which is the ...
and the unmanned Soviet Luna Program returned 0.3 kilograms of lunar material. These rocks have proved to be invaluable in deciphering the geologic evolution of the Moon. Lunar rocks are in large part made of the same common rock forming minerals as found on Earth, such as olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, and plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
(anorthosite). Plagioclase feldspar is mostly found in the lunar crust, while pyroxene and olivine are typically seen in the lunar mantle. The mineral ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
is highly abundant in some mare basalts, and a new mineral named armalcolite
Armalcolite
Armalcolite is a titanium-rich mineral with the chemical formula Ti2O5. It was first found at Tranquility Base on the Moon in 1969 and named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Together with tranquillityite and pyroxferroite, it is one of the three minerals which...
(named for Armstrong, Aldrin, and Collins, the three members of the Apollo 11
Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
crew) was first discovered in the lunar samples.
The maria are composed predominantly of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
, whereas the highland regions are iron-poor and composed primarily of anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
, a rock composed primarily of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
-rich plagioclase feldspar. Another significant component of the crust are the igneous Mg-suite rocks, such as the troctolite
Troctolite
Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a...
s, norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
s, and KREEP-basalts. These rocks are believed to be genetically related to the petrogenesis of KREEP
KREEP
KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...
.
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 mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
impact melt breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s, which are typified by the low-K Fra Mauro composition, have a higher proportion of iron and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
than typical upper crust anorthositic rocks, as well as higher abundances of KREEP.
Composition of the maria
The main characteristics of the basaltBasalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic rocks with respect to the rocks of the lunar highlands is that the basalts contain higher abundances of olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, and less plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
. They are more rich in iron than terrestrial basalts, and also have lower viscosities. Some of them have high abundances of a ferro
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
-titanic
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
called ilmenite
Ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
. Since the first sampling of rocks contained a high content of ilmenite and other related minerals, they received the name of "high titanium" basalts. The Apollo 12
Apollo 12
Apollo 12 was the sixth manned flight in the American Apollo program and the second to land on the Moon . It was launched on November 14, 1969 from the Kennedy Space Center, Florida, four months after Apollo 11. Mission commander Charles "Pete" Conrad and Lunar Module Pilot Alan L...
mission returned to Earth with basalts of lower titanium concentrations, and these were dubbed "low titanium" basalts. Subsequent missions, including the Soviet
Soviet space program
The Soviet space program is the rocketry and space exploration programs conducted by the former Union of Soviet Socialist Republics from the 1930s until its dissolution in 1991...
unmanned probes, returned with basalts with even lower concentrations, now called "very low titanium" basalts. The Clementine
Clementine mission
Clementine was a joint space project between the Ballistic Missile Defense Organization and NASA...
space probe returned data showing that the mare basalts possess a continuum in titanium concentrations, with the highest concentration rocks being the least abundant.
Internal structure of the Moon
The current model of the interior of the Moon was derived using seismometerSeismometer
Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
s left behind during the manned Apollo program missions, as well as investigations of the Moon's gravity field and rotation.
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. Its low bulk density (~3346 kg m−3) indicates a low metal abundance. Mass and moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
constraints indicate that the Moon likely has an iron core that is less than about 450 km in radius. Studies of the Moon's physical librations (small perturbations to its rotation) furthermore indicate that the core is still molten. Most planetary bodies and moons have iron cores that are about half the size of the body. The Moon is thus anomalous in possessing a core whose size is only about one quarter of its radius.
The crust of the Moon is on average about 50 km thick (though this is uncertain by about ±15 km). It is widely believed that the far-side crust is on average thicker than the near side by about 15 km. Seismology has constrained the thickness of the crust only near the Apollo 12 and 14 landing sites. While the initial Apollo-era analyses suggested a crustal thickness of about 60 km at this site, recent reanalyses of this data set suggest a thinner value, somewhere between about 30 and 45 km.
Compared to that of Earth, the Moon has only a very weak external magnetic field. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo
Dynamo theory
In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time...
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 geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on airless bodies such as the Moon, transient magnetic fields could be generated during impact processes. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the largest impact basins.
While the Moon does not possess a dipolar magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
like the Earth does, some of the returned rocks possess strong magnetizations. Furthermore measurements from orbit show that some portions of the lunar surface are associated with strong magnetic fields.
See also
- Lunar geologic timescaleLunar geologic timescaleThe lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian , Nectarian, and Pre-Nectarian...
- SelenographySelenographySelenography is the study of the surface and physical features of the Moon. Historically, the principal concern of selenographists was the mapping and naming of the lunar maria, craters, mountain ranges, and other various features...
- Transient lunar phenomenonTransient lunar phenomenonA transient lunar phenomenon , or lunar transient phenomenon , is a short-lived light, color, or change in appearance on the lunar surface....
External links
- Apollo over the Moon: A View from Orbit, edited by Harold Masursky, G. W. Colton, and Farouk El-baz, NASA SP-362.
- Eric Douglass, Geologic Processes on the Moon
- Lunar Sample Information (JSC)
- The Apollo Lunar Surface Journal (NASA)
- Lunar and Planetary Institute: Exploring the Moon
- Clementine Lunar Image Browser
- Ralph Aeschliman Planetary Cartography and Graphics: Lunar Maps
- Lunar Gravity, Topography and Crustal Thickness Archive
- Lunar and Planetary Institute: Lunar Atlas and Photography Collection
- Moon Rocks through the Microscope Retrieved 22 August 2007
- Moon articles in Planetary Science Research Discoveries
- Another Hit to Hoax:Traces of Man on Lunar Surface