The Moon
(Moon: Earth Natural satellite,internal structure,Atmosphere,presence of water,tidal effects,missions)
The
Moon is an astronomical body that orbits planet Earth, being Earth's
only permanent natural satellite. It is the fifth-largest natural
satellite in the Solar System, and the largest among
planetary satellites relative to the size of the planet that it orbits
(its primary). Following Jupiter's satellite Io, the Moon is
second-densest satellite among those whose densities are known.
The
Moon is thought to have formed about 4.51 billion years ago, not long
after Earth. There are several hypotheses for its origin; the most
widely accepted explanation is that the Moon formed from the debris left
over after a giant impact between Earth and a Mars-sized body called
Theia.
The
Moon is in synchronous rotation with Earth, always showing the same
face, with its near side marked by dark volcanic maria that fill the
spaces between the bright ancient crustal highlands and the prominent
impact craters. As seen from the Earth, it is the second-brightest
regularly visible celestial object in Earth's sky, after the Sun. Its
surface is actually dark, although compared to the night sky it appears
very bright, with a reflectance just slightly higher than that of worn
asphalt. Its gravitational influence produces the ocean tides, body
tides, and the slight lengthening of the day.
The
Moon's current orbital distance is 384,400 km (238,900 mi), or 1.28
light-seconds. This is about thirty times the diameter of Earth, with
its apparent size in the sky almost the same as that of the Sun,
resulting in the Moon covering the Sun nearly precisely in total solar
eclipse. This matching of apparent visual size will not continue in the
far future, because the Moon's distance from Earth is slowly increasing.
The
Soviet Union's Luna programme was the first to reach the Moon with
uncrewed spacecraft in 1959; the United States' NASA Apollo program
achieved the only crewed missions to date, beginning with the first
crewed lunar orbiting mission by Apollo 8 in 1968, and six crewed lunar
landings between 1969 and 1972, with the first being Apollo 11. These
missions returned lunar rocks which have been used to develop a
geological understanding of the Moon's origin, internal structure, and
later history. Since the Apollo 17 mission in 1972, the Moon has been
visited only by uncrewed spacecraft.
Within
human culture, both the Moon's natural prominence in the earthly sky,
and its regular cycle of phases as seen from the Earth have provided
cultural references and influences for human societies and cultures
since time immemorial. Such cultural influences can be found in
language, lunar based calendar sy
stems, art, and mythology.
Internal structure
The
Moon is a differentiated body: it has a geochemically distinct crust,
mantle, and core. The Moon has a solid iron-rich inner core with a
radius of 240 km (150 mi) and a fluid outer core primarily made of
liquid iron with a radius of roughly 300 km (190 mi). Around
the core is a partially molten boundary layer with a radius of about
500 km (310 mi). This structure is thought to have developed through the
fractional crystallization of a global magma ocean shortly after the
Moon's formation 4.5 billion years ago. Crystallization of this magma
ocean would have created a mafic mantle from the precipitation and
sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after
about three-quarters of the magma ocean had crystallised, lower-density
plagioclase minerals could form and float into a crust atop. The final
liquids to crystallise would have been initially sandwiched between the
crust and mantle, with a high abundance of incompatible and
heat-producing elements. Consistent with this perspective, geochemical
mapping made from orbit suggests the crust of mostly anorthosite. The
Moon rock samples of the flood lavas that erupted onto the surface from
partial melting in the mantle confirm the mafic mantle composition,
which is more iron rich than that of Earth. The crust is on average
about 50 km (31 mi) thick.
The
Moon is the second-densest satellite in the Solar System, after Io.
However, the inner core of the Moon is small, with a radius of about 350
km (220 mi) or less, around 20% of the radius of the Moon. Its
composition is not well defined, but is probably metallic iron alloyed
with a small amount of sulfur and nickel; analyses of the Moon's
time-variable rotation suggest that it is at least partly molten.
Volcanic features
The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for "seas"; singular mare), as they were once believed to be filled with water; they are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. The majority of these lavas erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria".
The
lighter-coloured regions of the Moon are called terrae, or more
commonly highlands, because they are higher than most maria. They have
been radiometrically dated to having formed 4.4 billion years ago, and
may represent plagioclase cumulates of the lunar magma ocean. In
contrast to Earth, no major lunar mountains are believed to have formed
as a result of tectonic events.
The
concentration of maria on the Near Side likely reflects the
substantially thicker crust of the highlands of the Far Side, which may
have formed in a slow-velocity impact of a second moon of Earth a few
tens of millions of years after their formation.
Presence of water
Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly survive in cold, permanently shadowed craters at either pole on the Moon. Computer simulations suggest that up to 14,000 km2 (5,400 sq mi) of the surface may be in permanent shadow. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan; the alternative of transporting water from Earth would be prohibitively expensive.
In
years since, signatures of water have been found to exist on the lunar
surface. In 1994, the bistatic radar experiment located on the
Clementine spacecraft, indicated the existence of small, frozen pockets
of water close to the surface. However, later radar observations by
Arecibo, suggest these findings may rather be rocks ejected from young
impact craters. In 1998, the neutron spectrometer on the Lunar
Prospector spacecraft showed that high concentrations of hydrogen are
present in the first meter of depth in the regolith near the polar
regions. Volcanic lava beads, brought back to Earth aboard Apollo 15,
showed small amounts of water in their interior.
The
2008 Chandrayaan-1 spacecraft has since confirmed the existence of
surface water ice, using the on-board Moon Mineralogy Mapper. The
spectrometer observed absorption lines common to hydroxyl, in reflected
sunlight, providing evidence of large quantities of water ice, on the
lunar surface. The spacecraft showed that concentrations may possibly be
as high as 1,000 ppm. In 2009, LCROSS sent a 2,300 kg (5,100 lb)
impactor into a permanently
shadowed polar crater, and detected at least 100 kg (220 lb) of water
in a plume of ejected material. Another examination of the LCROSS data
showed the amount of detected water to be closer to 155 ± 12 kg (342 ±
26 lb).
In
May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220
was reported, the famous high-titanium "orange glass soil" of volcanic
origin collected during the Apollo 17 mission in 1972. The inclusions
were formed during explosive eruptions on the Moon approximately 3.7
billion years ago. This concentration is comparable with that of magma
in Earth's upper mantle. Although of considerable selenological
interest, this announcement affords little comfort to would-be lunar
colonists—the sample originated many kilometers below the surface, and
the inclusions are so difficult to access that it took 39 years to find
them with a state-of-the-art ion microprobe instrument.
Atmosphere
Sketch by the Apollo 17 astronauts. The lunar atmosphere was later studied by LADEE.
The
Moon has an atmosphere so tenuous as to be nearly vacuum, with a total
mass of less than 10 metric tons (9.8 long tons; 11 short tons). The
surface pressure of this small mass is around 3 × 10−15 atm (0.3 nPa);
it varies with the lunar day. Its sources include outgassing and
sputtering, a product of the bombardment of lunar soil by solar wind
ions. Elements
that have been detected include sodium and potassium, produced by
sputtering (also found in the atmospheres of Mercury and Io); helium-4
and neon from the solar wind; and argon-40, radon-222, and polonium-210,
outgassed after their creation by radioactive decay within the crust
and mantle. The absence of such neutral species (atoms or molecules) as
oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in
the regolith, is not understood. Water vapour has been detected by
Chandrayaan-1 and found to vary with latitude, with a maximum at ~60–70
degrees; it is possibly generated from the sublimation of water ice in
the regolith. These gases either return into the regolith due to the
Moon's gravity or are lost to space, either through solar radiation
pressure or, if they are ionized, by being swept away by the solar
wind's magnetic field.Tidal effects
The libration of the Moon over a single lunar month. Also visible is the slight variation in the Moon's visual size from Earth.
The
gravitational attraction that masses have for one another decreases
inversely with the square of the distance of those masses from each
other. As a result, the slightly greater attraction that the Moon has
for the side of Earth closest to the Moon, as compared to the part of
the Earth opposite the Moon, results in tidal forces. Tidal forces
affect both the Earth's crust and oceans.
The
most obvious effect of tidal forces is to cause two bulges in the
Earth's oceans, one on the side facing the Moon and the other on the
side opposite. This results in elevated sea levels called ocean tides.
As the Earth spins on its axis, one of the ocean bulges (high tide) is
held in place "under" the Moon, while another such tide is opposite. As a
result, there are two high tides, and two low tides in about 24 hours.
Since the Moon is orbiting the Earth in the same direction of the
Earth's rotation, the high tides occur about every 12 hours and 25
minutes; the 25 minutes is due to the Moon's time to orbit the Earth.
The Sun has the same tidal effect on the Earth, but its forces of
attraction are only 40% that of the Moon's; the Sun's and Moon's
interplay is responsible for spring and neap tides. If the Earth were a
water world (one with no continents) it would produce a tide of only one
meter, and that tide would be very predictable, but the ocean tides are
greatly modified by other effects: the frictional coupling of water to
Earth's rotation through the ocean floors, the inertia of water's
movement, ocean basins that grow shallower near land, the sloshing of
water between different ocean basins. As a result, the timing of the
tides at most points on the Earth is a product of observations that are
explained, incidentally, by theory.
While
gravitation causes acceleration and movement of the Earth's fluid
oceans, gravitational coupling between the Moon and Earth's solid body
is mostly elastic and plastic. The result is a further tidal effect of
the Moon on the Earth that causes a bulge of the solid portion of the
Earth nearest the Moon that acts as a torque in opposition to the
Earth's rotation. This "drains" angular momentum and rotational kinetic
energy from Earth's spin, slowing the Earth's rotation. That angular
momentum, lost from the Earth, is transferred to the Moon in a process
(confusingly known as tidal acceleration), which lifts the Moon into a
higher orbit and results in its lower orbital speed about the Earth.
Thus the distance between Earth and Moon is increasing, and the Earth's
spin is slowing in reaction. Measurements from laser reflectors left
during the Apollo missions (lunar ranging experiments) have found that
the Moon's distance increases by 38 mm (1.5 in) per year (roughly the
rate at which human fingernails grow). Atomic clocks also show that
Earth's day lengthens by about 15 microseconds every year, slowly
increasing the rate at which UTC is adjusted by leap seconds. Left to
run its course, this tidal drag would continue until the spin of Earth
and the orbital period of the Moon matched, creating mutual tidal
locking between the two. As a result, the Moon would be suspended in the
sky over one meridian, as is already currently the case with Pluto and
its moon Charon. However, the Sun will become a red giant engulfing the
Earth-Moon system long before this occurrence.
In
a like manner, the lunar surface experiences tides of around 10 cm (4
in) amplitude over 27 days, with two components: a fixed one due to
Earth, because they are in synchronous rotation, and a varying component
from the Sun. The Earth-induced component arises from libration, a
result of the Moon's orbital eccentricity (if the Moon's orbit were
perfectly circular, there would only be solar tides). Libration also
changes the angle from which the Moon is seen, allowing a total of about
59% of its surface to be seen from Earth over time. The cumulative
effects of stress built up by these tidal forces produces moonquakes.
Moonquakes are much less common and weaker than are earthquakes,
although moon quakes can last for up to an hour—a significantly longer
time than terrestrial quakes—because of the absence of water to damp out
the seismic vibrations. The existence of moonquakes was an unexpected
discovery from seismometers placed on the Moon by Apollo astronauts from
1969 through 1972.
By spacecraft
20th century
Soviet missions
The
first spacecraft to perform a successful lunar soft landing was Luna 9
and the first uncrewed vehicle to orbit the Moon was Luna 10, both in
1966. Rock and soil samples were brought back to Earth by three Luna
sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in
1976), which returned 0.3 kg total. Two pioneering robotic rovers
landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod
programme.
United States missions
During
the late 1950s at the height of the Cold War, the United States Army
conducted a classified feasibility study that proposed the construction
of a manned military outpost on the Moon called Project Horizon with the
potential to conduct a wide range of missions from scientific research
to nuclear Earth bombardment. The study included the possibility of
conducting a lunar-based nuclear test. The Air Force, which at the time
was in competition with the Army for a leading role in the space
program, developed its own similar plan called Lunex. However, both
these proposals were ultimately passed over as the space program was
largely transferred from the military to the civilian agency NASA. 
Neil
Armstrong became the first person to walk on the Moon as the commander
of the American mission Apollo 11 by first setting foot on the Moon at
02:56 UTC on 21 July 1969. An estimated 500 million people worldwide
watched the transmission by the Apollo TV camera, the largest television
audience for a live broadcast at that time. The Apollo missions 11 to
17 (except Apollo 13, which aborted its planned lunar landing) returned
380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate
samples. The American Moon landing and return was enabled by
considerable technological advances in the early 1960s, in domains such
as ablation chemistry, software engineering and atmospheric re-entry
technology, and by highly competent management of the enormous technical
undertaking.
Scientific
instrument packages were installed on the lunar surface during all the
Apollo landings. Long-lived instrument stations, including heat flow
probes, seismometers, and magnetometers, were installed at the Apollo
12, 14, 15, 16, and 17 landing sites. Direct transmission of data to
Earth concluded in late 1977 due to budgetary considerations, but as the
stations' lunar laser ranging corner-cube retroreflector arrays are
passive instruments, they are still being used. Ranging to the stations
is routinely performed from Earth-based stations with an accuracy of a
few centimetres, and data from this experiment are being used to place
constraints on the size of the lunar core.
1980s–2000
An
artificially coloured mosaic constructed from a series of 53 images
taken through three spectral filters by Galileo' s imaging system as the
spacecraft flew over the northern regions of the Moon on 7 December
1992.
After
the first Moon race there were years of near quietude but starting in
the 1990s, many more countries have become involved in direct
exploration of the Moon. In 1990, Japan became the third country to
place a spacecraft into lunar orbit with its Hiten spacecraft. The
spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the
transmitter failed, preventing further scientific use of the mission. In
1994, the U.S. sent the joint Defense Department/NASA spacecraft
Clementine to lunar orbit. This mission obtained the first near-global
topographic map of the Moon, and the first global multispectral images
of the lunar surface. This was followed in 1998 by the Lunar Prospector
mission, whose instruments indicated the presence of excess hydrogen at
the lunar poles, which is likely to have been caused by the presence of
water ice in the upper few meters of the regolith within permanently
shadowed craters.
India,
Japan, China, the United States, and the European Space Agency each
sent lunar orbiters, and especially ISRO's Chandrayaan-1 has contributed
to confirming the discovery of lunar water ice in permanently shadowed
craters at the poles and bound into the lunar regolith. The post-Apollo
era has also seen two rover missions: the final Soviet Lunokhod mission
in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu
rover on 14 December 2013. The Moon remains, under the Outer Space
Treaty, free to all nations to explore for peaceful purposes.
21st century
The
European spacecraft SMART-1, the second ion-propelled spacecraft, was
in lunar orbit from 15 November 2004 until its lunar impact on 3
September 2006, and made the first detailed survey of chemical elements
on the lunar surface.
The
ambitious Chinese Lunar Exploration Program began with Chang'e 1, which
successfully orbited the Moon from 5 November 2007 until its controlled
lunar impact on 1 March 2009. It obtained a full image map of the Moon.
Chang'e 2, beginning in October 2010, reached the Moon more quickly,
mapped the Moon at a higher resolution over an
eight-month
period, then left lunar orbit for an extended stay at the Earth–Sun L2
Lagrangian point, before finally performing a flyby of asteroid 4179
Toutatis on 13 December
2012, and then heading off into deep space. On 14 December 2013,
Chang'e 3 landed a lunar lander onto the Moon's surface, which in turn
deployed a lunar rover, named Yutu (Chinese: 玉兔;
literally "Jade Rabbit"). This was the first lunar soft landing since
Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in
1973. China intends to launch another rover mission (Chang'e 4) before
2020, followed by a sample return mission (Chang'e 5) soon after.
Between
4 October 2007 and 10 June 2009, the Japan Aerospace Exploration
Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a
high-definition video camera, and two small radio-transmitter
satellites, obtained lunar geophysics data and took the first
high-definition movies from beyond Earth orbit. India's first lunar
mission, Chandrayaan I, orbited from 8 November 2008 until loss of
contact on 27 August 2009, creating a high resolution chemical,
mineralogical and photo-geological map of the lunar surface, and
confirming the presence of water molecules in lunar soil. The Indian
Space Research Organisation planned to launch Chandrayaan II in 2013,
which would have included a Russian robotic lunar rover. However, the
failure of Russia's Fobos-Grunt mission has delayed this project.
The
U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS
impactor and follow-up observation orbiter on 18 June 2009; LCROSS
completed its mission by making a planned and widely observed impact in
the crater Cabeus on 9 October 2009, whereas LRO is currently in
operation, obtaining precise lunar altimetry and high-resolution
imagery. In November 2011, the LRO passed over the large and bright
Aristarchus crater. NASA released photos of the crater on 25 December
2011.
Two
NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, on
a mission to learn more about the Moon's internal structure. NASA's
LADEE probe, designed to study the lunar exosphere, achieved orbit on 6
October 2013.
Upcoming
lunar missions include Russia's Luna-Glob: an uncrewed lander with a
set of seismometers, and an orbiter based on its failed Martian
Fobos-Grunt mission. Privately funded lunar exploration has been
promoted by the Google Lunar X Prize, announced 13 September 2007, which
offers US$20 million to anyone who can land a robotic rover on the Moon
and meet other specified criteria. Shackleton Energy Company is
building a program to establish operations on the south pole of the Moon
to harvest water and supply their Propellant Depots.
NASA
began to plan to resume crewed missions following the call by U.S.
President George W. Bush on 14 January 2004 for a crewed mission to the
Moon by 2019 and the construction of a lunar base by 2024. The
Constellation program was funded and construction and testing begun on a
crewed spacecraft and launch vehicle, and design studies for a lunar
base. However, that program has been cancelled in favor of a crewed
asteroid landing by 2025 and a crewed Mars orbit by 2035. India has also
expressed its hope to send a crewed mission to the Moon by 2020.
Planned commercial missions
In
2007, the X Prize Foundation together with Google launched the Google
Lunar X Prize to encourage commercial endeavors to the Moon. A prize of
$20 million will be awarded to the first private venture to get to the
moon with a robotic lander by the end of 2017. As of August 2016, 16
teams are participating in the competition.
In
August 2016, the US government granted permission to US-based start-up
Moon Express to land on the Moon. This marked the first time that a
private enterprise was given the right to do so. The decision is
regarded as a precedent helping to define regulatory standards for
deep-space commercial activity in the future, as thus far companies'
operation had been restricted to being on or around Earth.
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