Moon 
The Moon as seen by an observer on Earth |
| Orbital characteristics |
| Perigee: |
363,104 km
0.0024 AU |
| Apogee |
405,696 km
0.0027 AU |
| Semi-major axis: |
384,399 km
0.00257 AU [1] |
| Eccentricity: |
0.0549 [1] |
| Orbital period: |
27.321582 d
27 d 7 h 43.1 min [1] |
| Synodic period: |
29.530588 d
29 d 12 h 44.0 min |
| Avg. orbital speed: |
1.022 km/s |
| Inclination: |
5.145° to ecliptic [1]
(between 18.29° and 28.58° to Earth's equator) |
| Longitude of ascending node: |
regressing,
1 revolution in 18.6 years |
| Argument of perigee: |
progressing,
1 revolution in 8.85 years |
| Satellite of: |
Earth |
| Physical characteristics |
| Mean radius: |
1,737.10 km
0.273 Earths [1] |
| Equatorial radius: |
1,738.14 km
0.273 Earths |
| Polar radius: |
1,735.97 km
0.273 Earths |
| Flattening: |
0.00125 |
| Circumference: |
10,921 km (equatorial) |
| Surface area: |
3.793×107 km²
0.074 Earths |
| Volume: |
2.1958×1010 km³
0.020 Earths |
| Mass: |
7.3477×1022 kg
0.0123 Earths [1] |
| Mean density: |
3,346.4 kg/m³ [1] |
| Equatorial surface gravity: |
1.622 m/s²
0.1654 g |
| Escape velocity: |
2.38 km/s |
| Sidereal rotation period: |
27.321582 d (synchronous) |
| Rotation velocity at equator: |
4.627 m/s |
| Axial tilt: |
1.5424° (to ecliptic)
6.687° (to orbit plane) |
| Albedo: |
0.12 |
Surface temp.:
equator
85°N [5] |
| min |
mean |
max |
| 100 K |
220 K |
390 K |
| 70 K |
130 K |
230 K |
|
| Apparent magnitude: |
–2.5 to –12.9 [2]
–12.74 (mean full moon)[3] |
| Angular size: |
29.3′ — 34.1' [3][4] |
| Adjectives: |
lunar |
| Atmosphere [6][7] |
| Surface pressure: |
10-7 Pa (day)
10-10 Pa (night) |
The Moon (Latin: Luna) is Earth's only natural satellite, and the
fifth largest moon in the Solar System.
The average centre-to-centre distance from the Earth to the Moon is 384,403 km which is about thirty times the diameter of the
Earth. The Moon has a diameter of 3,474 km [8]—slightly more than a quarter that of the Earth and a little bit smaller than the length
across the United States. This means that the volume of the Moon is close to 1/50th that of Earth. The
gravitational pull at its surface is about 1/6th of Earth's. The Moon makes a
complete orbit around the Earth every 27.3 days, and the periodic variations in the geometry of
the Earth–Moon–Sun system are responsible for the lunar phases
that repeat every 29.5 days.
The Moon is the only celestial body to which humans have travelled and upon which
humans have landed. The first artificial object to escape Earth's gravity and pass near the Moon was the Soviet Union's Luna 1, the first artificial object to impact the lunar
surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first
spacecraft to perform a successful lunar soft landing was Luna 9, and the first unmanned vehicle
to orbit the Moon was Luna 10, both in 1966.[8] The United States (U.S.)
Apollo program achieved the only manned missions to date, resulting in six landings
between 1969 and 1972. Human exploration of the Moon ceased with the conclusion of the Apollo program, although several countries
have announced plans to send people or robotic spacecraft to the Moon.
Name and etymology
Unlike the moons of other planets, the moon of the Earth has no proper English name
other than "the Moon" (capitalized).
The word moon is a Germanic word, related to Latin mensis; it is ultimately a derivative of the Proto-Indo-European
root me-, also represented in measure[9] (time), with reminders of its importance in measuring time in words derived from it like
Monday, month and menstrual. In English, the word moon exclusively meant "the Moon" until
1665, when it was extended to refer to the recently-discovered natural satellites of
other planets.[9] The Moon is
occasionally referred to by its Latin name, Luna, in order to distinguish it from other natural
satellites, with a related adjective lunar, and an adjectival prefix seleno- or suffix -selene (from the
Greek deity Selene).
Ocean tides
Earth’s ocean tides are initiated by the tidal
force (a gradient in intensity) of Moon’s gravity and are magnified by a host of effects in Earth’s oceans. The
gravitational tidal force arises because the side of Earth facing the Moon (nearest it) is attracted more strongly by the Moon’s
gravity than is the center of the Earth and—even less so—the Earth’s far side. The gravitational tide stretches the Earth’s
oceans into an ellipse—with the Earth in the center. The effect takes the form of two bulges—elevated sea level—relative
to the Earth: one nearest the Moon and one farthest from it. Since these two bulges rotate around the Earth once a day as it
spins on its axis, ocean water is continuously rushing towards the ever-moving bulges. The effects of the two bulges and the
massive ocean currents chasing them are magnified by an interplay of other effects; namely frictional coupling of water to
Earth’s rotation through the ocean floors, inertia of water’s movement, ocean basins that get shallower near land, and
oscillations between different ocean basins. The magnifying effect is a bit like water sloshing high up the sloped end of a
bathtub after a relatively small disturbance of one’s body in the deep part of the tub.
Gravitational coupling between the Moon and the oceans affects the orbit of the
Moon. From the Moon's point of view, the tidal bulges are carried ahead by the rotation of the Earth, so that they don't
point directly toward the Moon. The gravitational coupling drains kinetic energy and
angular momentum from the Earth’s rotation. In turn, angular momentum is added to the
Moon's orbit. Somewhat counterintuitively, this moves the Moon to a higher orbit with a longer period. This results in a
3.8 cm yearly increase in the distance between the two bodies.[10] The Moon will continue to move slowly away from the Earth until the tidal effects between the two
are no longer of significance, whereupon the Moon's orbit will stabilize.
Lunar surface
-
Two sides of the Moon
The Moon is in synchronous rotation, meaning that it keeps nearly the same face
turned towards the Earth at all times. Early in the Moon's history, its rotation slowed and became locked in this configuration as a result of frictional effects
associated with tidal deformations caused by the Earth.[11]
Long ago when the Moon spun much faster, the Moon's tidal bulge preceded the Earth-Moon line because the Moon couldn't "snap
back" its bulges quickly enough to keep its bulges in line with Earth.[12] The rotation swept the bulge beyond the Earth-Moon line. This out-of-line bulge caused a torque,
slowing the Moon spin, like a wrench tightening a nut. When the Moon's spin slowed enough to match its orbital rate, then the
bulge always faced Earth, the bulge was in line with Earth, and the torque disappeared. That's why the Moon rotates at the same
rate as it orbits and we always see the same side of the Moon.
Small variations (libration) in the angle from which the moon is seen allow about 59% of
its surface to be seen from the earth (but only half at any instant).[8]
The side of the Moon that faces Earth is called the near side, and the opposite
side the far side. The far side should not be confused with the dark side, which is
the hemisphere that is not being illuminated by the Sun at a given moment (this may be the side
facing the Earth, as it is once a month during the New Moon phase). The far side of the Moon was first photographed by the Soviet
probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of
maria.
Maria
-
The dark and relatively featureless lunar plains humans can clearly see when the Moon is full are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. These are now known to be vast solidified pools of ancient
basaltic lava. The majority of these lavas erupted or flowed into the depressions associated with
impact basins that formed by the collisions of meteors and comets with the lunar surface.
(Oceanus Procellarum is a major exception in that it does not correspond to a known
impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered
patches covering only about 2% of its surface,[13] compared
with about 31% on the near side.[8] The
most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side
hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar
Prospector gamma-ray spectrometer.[14][15] Several provinces containing shield volcanoes and volcanic domes are found within the near side
maria.[16]
Terrae
The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are
higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant
impact basins, many of which have been filled by mare basalt. These are believed to be the
surviving remnants of the impact basin's outer rims.[17] In
contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[18]
From images taken by the Clementine mission, it appears that four mountainous
regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain
illuminated for the entire lunar day. These peaks of eternal light are possible
because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of
eternal light were found at the south pole, although the rim of Shackleton crater is
illuminated for about 80% of the lunar day. Another consequence of the Moon's small axial tilt is regions that remain in
permanent shadow at the bottoms of many polar craters.[19]
Impact craters
Lunar crater
Daedalus on the Moon's far side
The Moon's surface shows obvious evidence of having been affected by impact
cratering.[20] Impact craters form when asteroids
and comets collide with the lunar surface, and globally about half a million craters with diameters greater than 1 km can be
found. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic
unit can be used to estimate the age of the surface (see crater counting). The lack of
an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved
in comparison to those found on Earth.
The largest crater on the Moon, which also has the distinction of being the largest known crater in the Solar System, is the
South Pole-Aitken basin. This impact basin is located on the far side, between
the South Pole and equator, and is some 2240 km in diameter and 13 km in depth.[21] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.
Regolith
Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller
particles) and "impact gardened" surface layer called regolith. Since the regolith forms by
impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been
estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the
highlands.[22] Beneath the finely comminuted regolith
layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of
kilometres) and comprises highly fractured bedrock.[23]
Presence of water
-
The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much
of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time,
because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic
plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see
Shackleton crater). Water molecules that ended up in these craters could be stable
for long periods of time.
Clementine has mapped craters at the lunar south pole[24] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be
in permanent shadow.[19] Results from the
Clementine mission bistatic radar experiment are consistent with small, frozen pockets
of water close to the surface, and data from the Lunar Prospector neutron spectrometer
indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar
regions.[25] Estimates for the total quantity of water
ice are close to one cubic kilometre.
Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or
electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor
in rendering lunar habitation cost-effective, since transporting water from
Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were
previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact
craters.[26] The question of how much water there is on
the Moon has not been resolved.
Physical characteristics
Internal structure
-
Schematic illustration of the internal structure of the Moon
The Moon is a differentiated body, being composed of a geochemically
distinct crust, mantle, and core. This structure is believed to have resulted from the fractional crystallization of a magma
ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon
is commonly attributed to a giant impact event that is postulated to have formed
the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have
given rise to a mafic mantle and a plagioclase-rich crust
(see Origin and geologic evolution below).
Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic
in composition,[6] consistent with the magma ocean
hypothesis. In terms of elements, the crust is composed primarily of oxygen, silicon, magnesium, iron,
calcium, and aluminium. Based on geophysical techniques, its
thickness is estimated to be on average about 50 km.[1]
Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of
these basalts indicate that the mantle is composed predominantly of the minerals olivine,
orthopyroxene and clinopyroxene, and that the lunar mantle is
more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium
(present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in
composition. Moonquakes have been found to occur deep within the mantle of the Moon about 1,000 km below the surface. These occur
with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.[1]
The Moon has a mean density of 3,346.4 kg/m³, making it the second densest moon in the Solar System after
Io. Nevertheless, several lines of evidence imply that the core of the Moon is small, with a
radius of about 350 km or less.[1] This
corresponds to only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies.
The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a
small amount of sulfur and nickel. Analyses of the Moon's
time-variable rotation indicate that the core is at least partly molten.[27]
Topography
-
Topography of the Moon, referenced to the lunar geoid
The topography of the Moon has been measured by the methods of laser altimetry and stereo
image analysis, most recently from data obtained during the Clementine mission. The
most visible topographic feature is the giant far side South Pole-Aitken basin,
which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it
has been suggested that this area might represent thick ejecta deposits that were emplaced during
an oblique South Pole-Aitken basin impact event. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. Another distinguishing feature of
the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side.[1]
Gravity field
-
The gravitational field of the Moon has been determined through tracking of radio signals emitted by orbiting spacecraft. The
principle used depends on the Doppler effect, whereby the spacecraft acceleration in the
line-of-sight direction can be determined by means of small shifts in frequency of the radio signal, and the distance from the
spacecraft to a station on Earth. However, because of the Moon's synchronous
rotation it is not possible to track spacecraft much over the limbs of the Moon, and
the farside gravity field is thus only poorly characterised.[28]
Radial gravitational anomaly at the surface of the Moon
The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant
impact basins.[29]
These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the
planning of both manned and unmanned missions. The mascons are in part due to the presence of dense mare basaltic lava flows that
fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitational signature,
and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector
gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic
volcanism.[30] It should be noted that the huge expanse
of mare basaltic volcanism associated with Oceanus Procellarum does not possess a
positive gravitational anomaly.
Magnetic field
-
Total magnetic field strength at the surface of the Moon as derived from the
Lunar
Prospector electron reflectometer experiment
The Moon has an external magnetic field of the order of one to a hundred
nanotesla—more than 100 times smaller than the Earth's, which is 30-60 microtesla. Other major differences are that the Moon does not
currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in
origin.[31] One hypothesis holds that the crustal
magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core,
however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon,
transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest
crustal magnetizations appear to be located near the antipodes of the giant impact basins. It
has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon
in the presence of an ambient magnetic field.[32]
Atmosphere
-
The Moon has an atmosphere so thin as to be almost negligible, with a total atmospheric mass of less than 104
kg.[33] One source of its atmosphere is outgassing—the release of gases such as radon that originate by
radioactive decay processes within the crust and mantle. Another important source is
generated through the process of sputtering, which involves the bombardment of
micrometeorites, solar wind ions, electrons, and sunlight.[6] Gases that are released by sputtering can either reimplant into the regolith as a result of the Moon's gravity, or can be lost to space either by solar radiation pressure or by
being swept away by the solar wind magnetic field if they are ionised. The elements sodium (Na)
and potassium (K) have been detected using earth-based spectroscopic methods, whereas the
element radon–222 and polonium–210 have been inferred from data
obtained from the Lunar Prospector alpha
particle spectrometer.[34] Argon–40, He–4, O
and/or CH4, N2 and/or CO, and CO2 were detected by in-situ detectors placed by the Apollo
astronauts.[35]
Origin and geologic evolution
Formation
Several mechanisms have been suggested for the Moon's formation. Early speculation proposed that the Moon broke off from the
Earth's crust because of centrifugal forces, leaving a basin (presumed to be the
Pacific Ocean) behind as a scar.[36] This fission
concept, however, requires too great an initial spin of the Earth. Furthermore, it would have resulted in an orbit following
Earth's equatorial plane, which is not the case. Others speculated that the Moon formed elsewhere and was captured into Earth's
orbit.[37] However, the conditions required for this
capture mechanism to work (such as an extended atmosphere of the Earth for dissipating energy) are improbable. The
coformation hypothesis posits that the Earth and the Moon formed together at the same time and place from the primordial
accretion disk. In this hypothesis, the Moon formed from material surrounding the
proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails to adequately
explain the depletion of metallic iron in the Moon. A major deficiency with all of these hypotheses is that they cannot easily
account for the high angular momentum of the Earth–Moon system.[38]
Today, the giant impact hypothesis for forming the Earth–Moon system is
widely accepted by the scientific community. In this hypothesis, the impact of a Mars-sized body (Theia) on the proto-Earth is postulated to have put enough material into circumterrestrial orbit
to form the Moon.[8] Given that planetary
bodies are believed to have formed by the hierarchical accretion of smaller bodies to larger ones, giant impact events such as
this are thought to have affected most planets. Computer simulations modelling this impact are consistent with measurements of
the angular momentum of the Earth–Moon system, as well as the small size of the lunar core.[39] Unresolved questions regarding this theory have to do with determining the
relative sizes of the proto-Earth and impactor, and with determining how much material from the proto-Earth and impactor ended up
in the Moon. The formation of the Moon is believed to have occurred 4.527 ± 0.01 billion years ago, a
Herodotus and Schroeter's Valley, the largest sinuous rille on the Moon. The impact crater Aristarchus, about 25 mi (40 km) in diameter and more than 2.5 mi (4 km) deep, lies at the edge of a mountainous region that shows evidence of volcanic activity. (NASA)">
