MOON ROCKS MINERASL S AND VERY IMPORTENT FACTS

![](media/image1.jpeg){width="2.363888888888889in"
height="2.123611111111111in"}

dated samples are as old as 4.2 billion years
old[^\[10\]^](http://en.wikipedia.org/wiki/Moon#_note-6%23_note-6), and
the youngest eruptions, based on the method of [crater
counting](http://en.wikipedia.org/wiki/Crater_counting), are believed to
have occurred only 1.2 billion years
ago.[^\[11\]^](http://en.wikipedia.org/wiki/Moon#_note-7%23_note-7)
Recently, it has been suggested that a roughly 3 km diameter region of
the lunar surface was modified by a gas release event about a million
years
ago.^[\[12\]](http://en.wikipedia.org/wiki/Moon#_note-8%23_note-8)[\[13\]](http://en.wikipedia.org/wiki/Moon#_note-9%23_note-9)^

## \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=4)\] Physical characteristics

> *Main article: [Geology of the
> Moon](http://en.wikipedia.org/wiki/Geology_of_the_Moon)*

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=5)\] Structure

![](media/image2.jpeg){width="2.6041666666666665in"
height="1.7604166666666667in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Schematic illustration of the internal structure of the Moon.

The Moon is a
[differentiated](http://en.wikipedia.org/wiki/Planetary_differentiation)
body, being composed of a geochemically distinct
[crust](http://en.wikipedia.org/wiki/Crust_(geology)),
[mantle](http://en.wikipedia.org/wiki/Mantle_(geology)), and
[core](http://en.wikipedia.org/wiki/Planetary_core). This structure and
the compositional variations observed from orbit and among the samples
are believed to have resulted from the [fractional
crystallization](http://en.wikipedia.org/wiki/Fractional_crystallization)
of a [magma ocean](http://en.wikipedia.org/wiki/Lunar_magma_ocean) about
4.5 billion years ago.

A large amount of energy would have been liberated during both the giant
impact event that formed the Earth-Moon system and the reaccretion of
material in Earth orbit. The amount of energy generated would have been
sufficient to melt a large portion of the outer portion of the Moon,
with depths of this [magma
ocean](http://en.wikipedia.org/wiki/Lunar_magma_ocean) ranging from
about 500 km to the entire
Moon.[^\[1\]^](http://en.wikipedia.org/wiki/Moon#_note-S06%23_note-S06)
Fractional crystallization of this magma would have led to a mantle
composed largely of the minerals
[olivine](http://en.wikipedia.org/wiki/Olivine),
[clinopyroxene](http://en.wikipedia.org/wiki/Clinopyroxene), and
[orthopyroxene](http://en.wikipedia.org/wiki/Orthopyroxene), and after
about three-quarters of crystallization was complete, the mineral
[anorthosite](http://en.wikipedia.org/wiki/Anorthosite) would have
precipated and floated to the surface because of its low density.
Estimates for the average thickness of the crust are about 50 km, and
both lunar samples and geochemical mapping from orbit are consistent
with a large portion of the crust being anorthositic in
composition.^[\[3\]](http://en.wikipedia.org/wiki/Moon#_note-J2000%23_note-J2000)[\[14\]](http://en.wikipedia.org/wiki/Moon#_note-W06%23_note-W06)[\[15\]](http://en.wikipedia.org/wiki/Moon#_note-L06%23_note-L06)^.
The final liquids to crystallize from the magma ocean would have been
initially sandwiched between the crust and mantle, and would have
contained a high abundance of incompatible and heat-producing elements.
This geochemical component is referred to by the acronym
[KREEP](http://en.wikipedia.org/wiki/KREEP), for
[potasium](http://en.wikipedia.org/wiki/Potasium) (K), [rare earth
elements](http://en.wikipedia.org/wiki/Rare_earth_elements) (REE), and
[phosphorous](http://en.wikipedia.org/wiki/Phosphorous) (P). KREEP
appears to be concentrated within the [Procellarum KREEP
Terrane](http://en.wikipedia.org/wiki/Lunar_terranes), which is a small
geologic province that encompasses most of [Oceanus
Procellarum](http://en.wikipedia.org/wiki/Oceanus_Procellarum) and [Mare
Imbrium](http://en.wikipedia.org/wiki/Mare_Imbrium).

In terms of elements, the lunar crust is composed primarily of
[oxygen](http://en.wikipedia.org/wiki/Oxygen),
[silicon](http://en.wikipedia.org/wiki/Silicon),
[magnesium](http://en.wikipedia.org/wiki/Magnesium),
[iron](http://en.wikipedia.org/wiki/Iron),
[calcium](http://en.wikipedia.org/wiki/Calcium), and
[aluminium](http://en.wikipedia.org/wiki/Aluminium). Important minor and
trace elements include
[titanium](http://en.wikipedia.org/wiki/Titanium),
[uranium](http://en.wikipedia.org/wiki/Uranium),
[thorium](http://en.wikipedia.org/wiki/Thorium),
[potassium](http://en.wikipedia.org/wiki/Potassium), and
[hydrogen](http://en.wikipedia.org/wiki/Hydrogen). A complete global
mapping of the Moon for the abundance of the major and minor elements
has not yet been performed. However, some spacecraft have done so for
portions of the Moon, or for certain elements. In particular, a
gamma-ray spectrometer onboard the spacecraft [Lunar
Prospector](http://en.wikipedia.org/wiki/Lunar_Prospector) has
determined near-global abundances of
[iron](http://en.wikipedia.org/wiki/Iron),
[calcium](http://en.wikipedia.org/wiki/Calcium),
[aluminum](http://en.wikipedia.org/wiki/Aluminum),
[magnesium](http://en.wikipedia.org/wiki/Magnesium),
[titanium](http://en.wikipedia.org/wiki/Titanium),
[potassium](http://en.wikipedia.org/wiki/Potassium),
[thorium](http://en.wikipedia.org/wiki/Thorium),
[uranium](http://en.wikipedia.org/wiki/Uranium), and
[hydrogen](http://en.wikipedia.org/wiki/Hydrogen). The
[Clementine](http://en.wikipedia.org/wiki/Clementine) spacecraft has
obtained near-global abundances for
[iron](http://en.wikipedia.org/wiki/Iron) and
[titanium](http://en.wikipedia.org/wiki/Titanium), but at a much higher
spatial resolution.

The Moon has a mean density of 3,346.4 kg/m³, making it the second
densest moon in the Solar System after
[Io](http://en.wikipedia.org/wiki/Io_(moon)). Nevertheless, several
lines of evidence (which include the Moon\'s mean density, [moment of
inertia](http://en.wikipedia.org/wiki/Moment_of_inertia), rotation, and
magnetic induction) imply that the lunar core is small, with a radius of
about 350
km.[^\[14\]^](http://en.wikipedia.org/wiki/Moon#_note-W06%23_note-W06)
The radius of the lunar core is only about 20% its surface radius, in
contrast to most other terrestrial bodies that have a core radius close
to 50% of their size. The composition of the lunar core is not well
constrained, but most believe that it is composed of metallic iron with
a small amount of [sulfur](http://en.wikipedia.org/wiki/Sulfur), though
a dense titanium-rich silicate magma is also permissible.

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=6)\] Landscape

![](media/image4.jpeg){width="1.875in" height="1.8333333333333333in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Lunar crater [Daedalus](http://en.wikipedia.org/wiki/Daedalus_(crater)).

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height="1.6340277777777779in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

The Copernicus impact crater.

When observed with Earth based telescopes, the Moon can be seen to have
some 30,000 [craters](http://en.wikipedia.org/wiki/Impact_crater) having
a [diameter](http://en.wikipedia.org/wiki/Diameter) of at least 1 km,
but close up observation from lunar orbit reveals a multitude of ever
smaller craters. Most are hundreds of millions or billions of years old,
and the lack of an atmosphere, weather and recent geological processes
ensures that many of them have remained relatively well preserved in
comparison to their terrestrial counterparts. In many places, it is
indeed impossible to form a crater without obliterating portions of
another. The largest crater on the Moon, which also has the distinction
of being the largest known crater in the [solar
system](http://en.wikipedia.org/wiki/Solar_system), is the [South
Pole-Aitken
basin](http://en.wikipedia.org/wiki/South_Pole-Aitken_basin). This
impact basin is located on the [far
side](http://en.wikipedia.org/wiki/Far_side_(Moon)), between the South
Pole and equator and is some 2,240 kilometres in diameter and
13 kilometres in
depth.[^\[16\]^](http://en.wikipedia.org/wiki/Moon#_note-10%23_note-10)

The dark and relatively featureless lunar plains are called
[*maria*](http://en.wikipedia.org/wiki/Lunar_mare), Latin for seas,
since they were believed by ancient
[astronomers](http://en.wikipedia.org/wiki/Astronomer) to be filled by
water. They are actually vast ancient
[basaltic](http://en.wikipedia.org/wiki/Basalt) lava flows, many of
which filled the topographic depressions associated with large impact
basins ([Oceanus
Procellarum](http://en.wikipedia.org/wiki/Oceanus_Procellarum) is a
major exception in that it does not correspond to any known impact
basin). The lighter-colored highlands are called *terrae*. Maria are
found almost exclusively on the lunar nearside, with the lunar far side
having only a few scattered patches (see [lunar
mare](http://en.wikipedia.org/wiki/Lunar_mare) for a discussion of the
distribution of mare basalts).

Blanketed atop the Moon\'s crust is a highly comminuted and \"impact
gardened\" surficial layer called
[regolith](http://en.wikipedia.org/wiki/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 he regolith varies in thickness from about 3 to 5 metres (10 to
16 ft) in the maria to about 10 to 20 metres (33 to 66 ft) in the
highlands.[^\[17\]^](http://en.wikipedia.org/wiki/Moon#_note-11%23_note-11)
Beneath the finely comminuted regolith layer is what is generally
referred to as the \"megaregolith.\" This layer is much thicker (on the
order or tens of kilometers) and consists of highly fractured bedrock.

Using images taken by the [Clementine
mission](http://en.wikipedia.org/wiki/Clementine_mission), it appears
that four mountainous regions on the rim of the 73 km-wide [Peary
crater](http://en.wikipedia.org/wiki/Peary_(crater)) at the Moon\'s
north pole remain illuminated for the entire lunar day. These unnamed
[mountains of eternal
light](http://en.wikipedia.org/wiki/Peak_of_Eternal_Light) are possible
due to the Moon\'s extremely small axial tilt to the
[ecliptic](http://en.wikipedia.org/wiki/Ecliptic) plane. Since
Clementine\'s images were taken during the northern lunar hemisphere\'s
summer season, it remains unknown whether these mountains are shaded at
any point during their local winter season. No similar regions of
eternal light exist at the south pole, although the rim of [Shackleton
crater](http://en.wikipedia.org/wiki/Shackleton_(crater)) is illuminated
for 80% of the lunar day. Another consequence of the Moon\'s small axial
tilt is that there are many regions that remain in permanent shadow at
the bottoms of many polar
craters.[^\[18\]^](http://en.wikipedia.org/wiki/Moon#_note-M03%23_note-M03)

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=7)\] Topography

![](media/image6.jpeg){width="3.125in" height="2.3229166666666665in"}

![](media/image3.png){width="0.2111111111111111in"
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Topography of the Moon, referenced to the lunar
[geoid](http://en.wikipedia.org/wiki/Geoid).

The topography of the Moon has been measured by the methods of laser
altimetry and stereo image analysis, most recently during the
[Clementine mission](http://en.wikipedia.org/wiki/Clementine_mission).
The most visible feature is the giant far side [South Pole-Aitken
basin](http://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/Mare_Imbrium),
[Serenitatis](http://en.wikipedia.org/wiki/Mare_Serenitatis),
[Crisium](http://en.wikipedia.org/wiki/Mare_Crisium),
[Smythii](http://en.wikipedia.org/wiki/Mare_Smythii), and
[Orientale](http://en.wikipedia.org/wiki/Mare_Orientale), also possess
regionally low elevations and elevated rims.

Another distinguishing feature of the Moon\'s shape is that the
elevations are on average 1.9 km higher on the far side than the near
side. If it is assumed that the crust is in [isostatic
equilibrium](http://en.wikipedia.org/wiki/Isostasy), and that the
density of the crust is everywhere the same, then the higher elevations
would be associated with a thicker crust. Using gravity, topography, and
seismic data, the crust is thought to be on average about 50±15 km
thick, with the far-side crust being on average thicker than the near
side by about 15 km.
[^\[14\]^](http://en.wikipedia.org/wiki/Moon#_note-W06%23_note-W06).

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=8)\] Gravity field

> *See also: [Mascon](http://en.wikipedia.org/wiki/Mascon)*

![](media/image7.jpeg){width="3.125in" height="2.3229166666666665in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Radial gravitational anomaly at the surface of the Moon.

The gravitational field of the Moon has been determined by the tracking
of radio signals emitted by orbiting spacecraft. The principle used is
based on that of the [doppler
effect](http://en.wikipedia.org/wiki/Doppler_effect), whereby the
line-of-sight spacecraft acceleration can be measured by small shifts in
frequency of the radio signal, as well as by measuring the distance to
the spacecraft from the travel time of the signal between the spacecraft
and a station on Earth. Since the gravitational field of the body
affects the orbit of the spacecraft, it is possible to use these
tracking data to invert for gravitational anomalies. Unforntunately,
because of the Moon\'s [synchronous
rotation](http://en.wikipedia.org/wiki/Synchronous_rotation) it is not
possible to track spacecraft much over the limbs of the Moon.
Nevertheless, it is possible to make inferences about the farside
gravity field (though with a lower precision) as their existence does
influence the spacecraft orbit.

The major characteristic of the Moon\'s gravitational field is the
presence of [mascons](http://en.wikipedia.org/wiki/Mascon), which are
large positive gravitational anomalies associated with some of the giant
impact basins. These anomalies greatly influence the orbit of spacecraft
about the Moon, and an accurate gravitational model is neccesary in the
planning of both manned and unmanned missions. They were initially
discoved by the analysis of [Lunar
Orbiter](http://en.wikipedia.org/wiki/Lunar_Orbiter) tracking
data,[^\[19\]^](http://en.wikipedia.org/wiki/Moon#_note-12%23_note-12),
since pre-Apollo navigational tests were experiencing landing position
errors much larger than mission specifications.

The origin of mascons are in part due to the presence of dense mare
basaltic lava flows that fill some of the impact basins. However, lava
flows by themselves can not explain the entirety of the gravitional
signature, and uplift of the crust-mantle interface is required as well.
Based on [Lunar
Prospector](http://en.wikipedia.org/wiki/Lunar_Prospector) gravitational
models, it has been suggested that some mascons exist that do not show
evidence for mare basaltic
volcanism.[^\[20\]^](http://en.wikipedia.org/wiki/Moon#_note-13%23_note-13)
It should be noted that the huge expanse of mare basaltic volcanism
associated with [Oceanus
Procellarum](http://en.wikipedia.org/wiki/Oceanus_Procellarum) does not
possess a positive gravitational anomaly.

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=9)\] Magnetic field

![](media/image8.jpeg){width="3.125in" height="2.2604166666666665in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Total magnetic field strength at the surface of the Moon as derived from
the Lunar Prospector electron reflectometer experiment.

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](http://en.wikipedia.org/wiki/Geodynamo) in its core), and
the magnetizations that are present are almost entirely crustal in
orgin. One hypothesis holds that the crustal magnetizations were
acquired early in lunar history when a
[geodynamo](http://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/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.[^\[21\]^](http://en.wikipedia.org/wiki/Moon#_note-14%23_note-14)

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=10)\] Presence of water

Over time, comets and
[meteoroids](http://en.wikipedia.org/wiki/Meteoroid) continuously
bombard the Moon, some of which contain a significant component of
water. Energy from sunlight usually splits much of this water into its
constituent elements hydrogen and oxygen, both of which generally escape
to space. Attesting to the dryness of lunar rocks, it is noted that the
samples collected by Apollo astronauts near the equator contained only
traces of water. However, because of the very slight axial tilt of the
Moon\'s spin axis to the ecliptic plane (only 1.5°), some deep craters
near the poles never receive any light from the Sun, and are permanently
shadowed. Thus, any water molecules that eventually ended up in these
craters would be stable for long periods of time.

Clementine has mapped craters at the lunar south
pole[^\[22\]^](http://en.wikipedia.org/wiki/Moon#_note-15%23_note-15)
which are shadowed in this way, and computer simulations suggest that up
to 14,000 km^2^ might be in permanent
shadow.[^\[18\]^](http://en.wikipedia.org/wiki/Moon#_note-M03%23_note-M03)
Results from the [Clementine
mission](http://en.wikipedia.org/wiki/Clementine_mission) bistatic radar
experiment are consistent with small, frozen pockets of water close to
the surface, and data from the [Lunar
Prospector](http://en.wikipedia.org/wiki/Lunar_Prospector) neutron
spectrometer indicate that anomalously high concentrations of hydrogen
are present in the upper meter of the regolith near the polar regions
[\[1\]](http://lunar.arc.nasa.gov/results/ice/eureka.htm). Estimates for
the total quantity of water ice are close to one cubic kilometer. Recent
radar observations with the
[Arecibo](http://en.wikipedia.org/wiki/Arecibo_Observatory) planetary
radar, however, showed that some of the Clementine radar returns might
instead be associated with rocks ejected from young craters, but this
interpretation is not uniformly agreed
upon.[^\[23\]^](http://en.wikipedia.org/wiki/Moon#_note-16%23_note-16)
If true, this would indicate that the neutron results are primarily from
hydrogen in forms other than ice, such as trapped hydrogen molecules or
organics.

Water ice can be mined and then split into hydrogen and oxygen by solar
panel-equipped electric power stations or a nuclear generator. The
presence of usable quantities of water on the Moon is an important
factor in rendering [lunar
habitation](http://en.wikipedia.org/wiki/Lunar_habitation)
cost-effective, since transporting water (or hydrogen and oxygen) from
Earth would be prohibitively expensive.

### \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=11)\] Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere. One
source of this atmosphere is
[outgassing](http://en.wikipedia.org/wiki/Outgassing) --- the release of
gases such as [radon](http://en.wikipedia.org/wiki/Radon) that originate
by radioactive decay processes within the crust and mantle. Another
important source is generated through the process of
[sputtering](http://en.wikipedia.org/wiki/Sputtering), which involves
the bombardment of micrometeorites, solar wind ions, electrons, and
sunlight.[^\[15\]^](http://en.wikipedia.org/wiki/Moon#_note-L06%23_note-L06)
Gasses that are released by sputtering can either reimplant into the
regolith as a cause of the Moon\'s gravity, or can be lost to space
either by solar radiation pressure or by being swept away by the solar
wind magnetic field (if they are ionized). The elements
[sodium](http://en.wikipedia.org/wiki/Sodium) (Na) and
[potassium](http://en.wikipedia.org/wiki/Potassium) (K) have been
detected using earth-based spectroscopic methods, whereas the element
radon has been inferred from data obtained from the [Lunar
Prospector](http://en.wikipedia.org/wiki/Lunar_Prospector) [alpha
particle](http://en.wikipedia.org/wiki/Alpha_particle)
spectrometer.[^\[24\]^](http://en.wikipedia.org/wiki/Moon#_note-17%23_note-17)

## \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=12)\] Eclipses

> *Main article: [Eclipse](http://en.wikipedia.org/wiki/Eclipse)*
>
> *See also: [Occultation](http://en.wikipedia.org/wiki/Occultation)*

![](media/image9.jpeg){width="2.0833333333333335in"
height="2.0520833333333335in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

The [French](http://en.wikipedia.org/wiki/France)
[1999](http://en.wikipedia.org/wiki/1999) solar eclipse

Eclipses happen only if Sun, Earth, and Moon are all in a straight line.
[Solar eclipses](http://en.wikipedia.org/wiki/Solar_eclipse) can only
occur near a [new moon](http://en.wikipedia.org/wiki/New_moon), whereas
[lunar eclipses](http://en.wikipedia.org/wiki/Lunar_eclipse) can only
occur near a [full moon](http://en.wikipedia.org/wiki/Full_moon). The
angular diameters of the Moon and the Sun as seen from Earth overlap in
their variation, so that both
[total](http://en.wikipedia.org/wiki/Total_eclipse) and
[annular](http://en.wikipedia.org/wiki/Annular_eclipse) solar eclipses
are possible. In a total eclipse, the Moon completely covers the disc of
the Sun and the solar [corona](http://en.wikipedia.org/wiki/Corona)
becomes visible to the [naked
eye](http://en.wikipedia.org/wiki/Naked_eye).

Since the distance between the Moon and the Earth is very slightly
increasing over time, the angular diameter of the Moon is decreasing.
This means that hundreds of millions of years ago the Moon could always
completely cover the Sun on solar eclipses so that no annular eclipses
were possible. Likewise, about 600 million years from now (assuming that
the angular diameter of the Sun will not change), the Moon will no
longer cover the Sun completely and total eclipses will not occur.

A phenomenon related to eclipse is
[occultation](http://en.wikipedia.org/wiki/Occultation). The Moon is
continuously blocking our view of the sky by a 1/2 degree wide circular
area. When a bright star or planet *passes behind* the Moon it is
*occulted* or hidden from view. A solar eclipse is an occultation of the
Sun. Because the Moon is close to Earth, occultations of individual
stars are not visible everywhere, nor at the same time. Because of the
precession of the lunar orbit, each year different stars are occulted.

## \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=13)\] Observation of the Moon

> *Main article: [Observing the
> Moon](http://en.wikipedia.org/wiki/Observing_the_Moon)*
>
> *See also: [Lunar phase](http://en.wikipedia.org/wiki/Lunar_phase),
> [New moon](http://en.wikipedia.org/wiki/New_moon), [Full
> moon](http://en.wikipedia.org/wiki/Full_moon),
> [Earthshine](http://en.wikipedia.org/wiki/Earthshine),
> [Moonlight](http://en.wikipedia.org/wiki/Moonlight), and [Halo
> (optical
> phenomenon)](http://en.wikipedia.org/wiki/Halo_(optical_phenomenon))*

![](media/image10.jpeg){width="2.0833333333333335in" height="1.5625in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

The Moon as illuminated by
[Earthshine](http://en.wikipedia.org/wiki/Earthshine). The brightest
crescent is in direct sunlight; the upper portion is lit by light
reflected from Earth.

![](media/image11.jpeg){width="1.5625in" height="1.5625in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

[Halo](http://en.wikipedia.org/wiki/Halo_(optical_phenomenon)) around
Moon

During the brightest full moons, the Moon can have an [apparent
magnitude](http://en.wikipedia.org/wiki/Apparent_magnitude) of about
−12.6. For comparison, the Sun has an apparent magnitude of −26.8. When
the Moon is in a quarter phase, its brightness is not one half of a full
moon, but instead is only about 1/10. This is because the lunar surface
is not a perfect [Lambertian
reflector](http://en.wikipedia.org/wiki/Lambertian_reflectance) and
because shadows projected onto the surface also diminish the amount of
reflected light.

The Moon appears larger when close to the horizon. This is a purely
psychological effect (see [Moon
illusion](http://en.wikipedia.org/wiki/Moon_illusion)). The angular
diameter of the Moon from Earth is about one half of one degree, and is
actually about 1.5% smaller when the Moon is near the horizon than when
it is high in the sky (because it is farther away by up to 1 Earth
radius).

Another quirk of the visual system causes us to see the Moon as almost
pure white, when in fact it reflects only about 7% of the light falling
on it (about as dark as a lump of coal). It has a very low
[albedo](http://en.wikipedia.org/wiki/Albedo). [Color
constancy](http://en.wikipedia.org/wiki/Color_constancy) in the [visual
system](http://en.wikipedia.org/wiki/Visual_system) recalibrates the
relations between colors of an object and its surroundings; however,
there is nothing next to the Moon to reflect the light falling on the
Moon, therefore it is perceived as the brightest object visible. We have
no standard to compare it to. An example of this is that, if you used a
narrow beam of light to illuminate a lump of coal in a dark room, it
would look white. If you then broadened the beam of the light source to
illuminate the surroundings, it would revert to black.

The highest
[altitude](http://en.wikipedia.org/wiki/Altitude_(astronomy)) of the
Moon on a day varies and has the same limits as the Sun. It also depends
on season and lunar phase. This means that in the winter the Moon is
highest in the sky when it is full, and the full moon is highest in
winter. The orientation of the Moon\'s crescent side also depends on the
latitude of the observing site. Close to the equator an observer can see
a *boat*
Moon.[^\[25\]^](http://en.wikipedia.org/wiki/Moon#_note-18%23_note-18)

Like the Sun, the Moon can also give rise to the atmospheric effects
including a 22 degree
[halo](http://en.wikipedia.org/wiki/Halo_(optical_phenomenon)) ring and
the smaller [coronal
rings](http://en.wikipedia.org/wiki/Corona_(meteorology)) seen more
often through thin clouds. For more information on how the Moon appears
in Earth\'s sky, see [Lunar
phase](http://en.wikipedia.org/wiki/Lunar_phase).

## \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=14)\] Exploration of the Moon

> *Main articles: [Exploration of the
> Moon](http://en.wikipedia.org/wiki/Exploration_of_the_Moon), [Project
> Apollo](http://en.wikipedia.org/wiki/Project_Apollo), and [Apollo Moon
> Landing hoax
> accusations](http://en.wikipedia.org/wiki/Apollo_Moon_Landing_hoax_accusations)*
>
> *See also: [Moon landing](http://en.wikipedia.org/wiki/Moon_landing),
> [Robotic exploration of the
> Moon](http://en.wikipedia.org/wiki/Robotic_exploration_of_the_Moon),
> [Future lunar
> missions](http://en.wikipedia.org/wiki/Future_lunar_missions), and
> [Project
> Constellation](http://en.wikipedia.org/wiki/Project_Constellation)*

![](media/image12.jpeg){width="1.875in" height="1.9375in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

[*Apollo 17*](http://en.wikipedia.org/wiki/Apollo_17) astronaut
[Harrison Schmitt](http://en.wikipedia.org/wiki/Harrison_Schmitt)
standing next to boulder at Taurus-Littrow during third EVA
(extravehicular activity).
*[NASA](http://en.wikipedia.org/wiki/NASA) photo.*

![](media/image13.jpeg){width="1.875in" height="1.84375in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

The first time an \"Earth-rise\" was seen from the moon.

The first leap in lunar observation was caused by the invention of the
telescope. [Galileo
Galilei](http://en.wikipedia.org/wiki/Galileo_Galilei) made especially
good use of this new instrument and observed mountains and craters on
the Moon\'s surface.

The Cold War-inspired [space
race](http://en.wikipedia.org/wiki/Space_race) between the [Soviet
Union](http://en.wikipedia.org/wiki/Soviet_Union) and the [United States
of America](http://en.wikipedia.org/wiki/United_States) led to an
acceleration of interest in the Moon. Unmanned probes, both flyby and
impact/lander missions, were sent almost as soon as launcher
capabilities would allow. What was the next big step depends on the
political viewpoint: In the US (and the West in general) the landing of
the first humans on the Moon in 1969 is seen as the culmination of the
space race. [Neil
Armstrong](http://en.wikipedia.org/wiki/Neil_Armstrong) became the first
person to walk on the Moon as the commander of the American mission
[*Apollo 11*](http://en.wikipedia.org/wiki/Apollo_11) by first setting
foot on the Moon at 02:56 UTC on July 21, 1969. The last person (as of
2006) to stand on the Moon was [Eugene
Cernan](http://en.wikipedia.org/wiki/Eugene_Cernan), who as part of the
mission [*Apollo 17*](http://en.wikipedia.org/wiki/Apollo_17) walked on
the Moon in December 1972. The [USA](http://en.wikipedia.org/wiki/USA)
Moon landing and return was enabled by several technologies where the US
surpassed the Russians; for example, the US achieved considerable
advances in [ablation](http://en.wikipedia.org/wiki/Ablation) chemistry
and [atmospheric
re-entry](http://en.wikipedia.org/wiki/Atmospheric_re-entry) technology
in the early 1960s. On the other hand, many scientifically important
steps, such as the first photographs of the until then unseen far side
of the Moon in 1959, were first achieved by the Soviet Union. Moon
samples have been brought back to Earth by three Luna missions ([*Luna
16*](http://en.wikipedia.org/wiki/Luna_16),
[*20*](http://en.wikipedia.org/wiki/Luna_20), and
[*24*](http://en.wikipedia.org/wiki/Luna_24)) and the *Apollo* missions
*11* through *17* (excepting [*Apollo
13*](http://en.wikipedia.org/wiki/Apollo_13), which aborted its planned
lunar landing).

![](media/image14.jpeg){width="3.125in" height="2.34375in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

[Astronaut](http://en.wikipedia.org/wiki/Astronaut) [Alan
Shepard](http://en.wikipedia.org/wiki/Alan_Shepard) raises the [Flag of
the United
States](http://en.wikipedia.org/wiki/Flag_of_the_United_States) on the
surface of the Moon.

Scientific instrument packages were installed on the lunar surface
during all of the *Apollo* missions. Long-lived
[ALSEP](http://en.wikipedia.org/wiki/ALSEP) stations (Apollo lunar
surface experiment package) were installed at the Apollo 12, 14, 15, 16,
and 17 landing sites, whereas a temporary station referred to as EASEP
(Early Apollo Scientific Experiments Package) was installed during the
Apollo 11 mission. The ALSEP stations contained, among others, heat flow
probes, seismometers, magnetometers, and corner-cube retroreflectors.
Transmission of data to Earth was terminated on September 30 1977
because of budgetary considerations. Since the [lunar laser
ranging](http://en.wikipedia.org/wiki/Lunar_laser_ranging_experiment)
corner-cube arrays are passive instruments, they are still being used to
today. Ranging to the LLR stations is routinely performed with an
accuracy of a few centimeters, and data from this experiment are being
used to place constraints on the size of the lunar core.

From the mid-1960s to the mid-1970s, there were a total of 65 Moon
landings (both manned and robotic, with 10 in 1971 alone), but after
[Luna 24](http://en.wikipedia.org/wiki/Luna_24) in 1976 they stopped.
The Soviet Union started focusing on
[Venus](http://en.wikipedia.org/wiki/Venus) and [space
stations](http://en.wikipedia.org/wiki/Space_station) and the US on
[Mars](http://en.wikipedia.org/wiki/Mars) and beyond. In 1990
[Japan](http://en.wikipedia.org/wiki/Japan) orbited the Moon with the
[*Hiten*](http://en.wikipedia.org/wiki/Hiten) spacecraft, becoming the
third country to place a spacecraft into lunar orbit. The spacecraft
released a smaller probe,
[*Hagormo*](http://en.wikipedia.org/wiki/Hiten), in lunar orbit, but the
transmitter failed rendering the mission scientifically useless.

In 1994, the US finally returned to the Moon, robotically at least,
sending the Joint Defense Department/NASA spacecraft
[Clementine](http://en.wikipedia.org/wiki/Clementine_mission). This
mission obtained the first near global topographic map of the Moon, as
well as the first global
[multispectral](http://en.wikipedia.org/wiki/Multispectral) images of
the lunar surface. This was followed by the [*Lunar
Prospector*](http://en.wikipedia.org/wiki/Lunar_Prospector) mission in
1998. The [neutron](http://en.wikipedia.org/wiki/Neutron)
[spectrometer](http://en.wikipedia.org/wiki/Spectrometer) on *Lunar
Prospector* indicated the presence of excess hydrogen at the lunar
poles, which is likely due to the presence of water ice in the upper few
meters of the regolith within permanently shadowed craters. The European
spacecraft [*Smart 1*](http://en.wikipedia.org/wiki/Smart_1) was
launched [September 27](http://en.wikipedia.org/wiki/September_27),
[2003](http://en.wikipedia.org/wiki/2003) and was in lunar orbit from
[November 15](http://en.wikipedia.org/wiki/November_15),
[2004](http://en.wikipedia.org/wiki/2004) to [September
3](http://en.wikipedia.org/wiki/September_3),
[2006](http://en.wikipedia.org/wiki/2006).

On [January 14](http://en.wikipedia.org/wiki/January_14),
[2004](http://en.wikipedia.org/wiki/2004), US President George W. Bush
called for a plan to return manned missions to the Moon by
2020.[^\[26\]^](http://en.wikipedia.org/wiki/Moon#_note-19%23_note-19)
The [People\'s Republic of
China](http://en.wikipedia.org/wiki/People's_Republic_of_China) has
expressed ambitious plans for exploring the Moon and has started the
[Chang\'e program](http://en.wikipedia.org/wiki/Chang'e_program) for
lunar exploration. [Japan](http://en.wikipedia.org/wiki/Japan) has two
planned lunar missions,
[*LUNAR-A*](http://en.wikipedia.org/wiki/LUNAR-A) and
[*Selene*](http://en.wikipedia.org/wiki/SELENE). India is to launch an
unmanned mission
[*Chandrayaan-1*](http://en.wikipedia.org/wiki/Chandrayaan-1) in
February 2008. The US will launch the [Lunar Reconnaissance
Orbiter](http://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter) in
2008.

## \[[edit](http://en.wikipedia.org/w/index.php?title=Moon&action=edit&section=15)\] Human understanding of the Moon

![](media/image15.jpeg){width="1.875in" height="1.7083333333333333in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Map of the Moon by Johannes Hevelius (1647).

> *See also: [Moon in art and
> literature](http://en.wikipedia.org/wiki/Moon_in_art_and_literature) and
> [Lunar effect](http://en.wikipedia.org/wiki/Lunar_effect)*

The Moon has been the subject of many works of art and literature and
the inspiration for countless others. It is a motif in the visual arts,
the performing arts, poetry, prose and music. A 5,000 year old rock
carving at [Knowth](http://en.wikipedia.org/wiki/Knowth),
[Ireland](http://en.wikipedia.org/wiki/Ireland) may represent the Moon,
which would be the earliest depiction
discovered.[^\[27\]^](http://en.wikipedia.org/wiki/Moon#_note-spacetoday%23_note-spacetoday)
In many prehistoric and ancient cultures, the Moon was thought to be a
[deity](http://en.wikipedia.org/wiki/Lunar_deity) or other
[supernatural](http://en.wikipedia.org/wiki/Supernatural) phenomenon,
and [astrological views](http://en.wikipedia.org/wiki/Moon_(astrology))
of the Moon continue to be propagated today. For further details, see
[The Moon in
mythology](http://en.wikipedia.org/wiki/The_Moon_in_mythology),
[Chang\'e](http://en.wikipedia.org/wiki/Chang'e_(mythology)), and
[Selene](http://en.wikipedia.org/wiki/Selene).

![](media/image16.jpeg){width="1.875in" height="1.40625in"}

![](media/image3.png){width="0.2111111111111111in"
height="0.15486111111111112in"}

Moon over red and blue haze

Among the first in the Western world to offer a scientific explanation
for the Moon was the
[Greek](http://en.wikipedia.org/wiki/Ancient_Greece)
[philosopher](http://en.wikipedia.org/wiki/Philosopher)
[Anaxagoras](http://en.wikipedia.org/wiki/Anaxagoras), who reasoned that
the Sun and Moon were both giant
[spherical](http://en.wikipedia.org/wiki/Spherical) rocks, and that the
latter reflected the light of the former. His atheistic view of the
heavens was one cause for his imprisonment and eventual exile. By the
[Middle Ages](http://en.wikipedia.org/wiki/Middle_Ages), before the
invention of the [telescope](http://en.wikipedia.org/wiki/Telescope),
more and more people began to recognize the Moon as a
[sphere](http://en.wikipedia.org/wiki/Sphere), though they believed that
it was \"perfectly smooth\". In 1609, [Galileo
Galilei](http://en.wikipedia.org/wiki/Galileo_Galilei) drew one of the
first telescopic drawings of the Moon in his book [*Sidereus
Nuncius*](http://en.wikipedia.org/wiki/Sidereus_Nuncius) and noted that
it was not smooth but had mountains and
[craters](http://en.wikipedia.org/wiki/Impact_crater). Later in the 17th
century, [Giovanni Battista
Riccioli](http://en.wikipedia.org/wiki/Giovanni_Battista_Riccioli) and
[Francesco Maria
Grimaldi](http://en.wikipedia.org/wiki/Francesco_Maria_Grimaldi) drew a
map of the Moon and gave many craters the names they still have today.

![](media/image17.jpeg){width="2.0833333333333335in"
height="1.4784722222222222in"}

![](media/image3.png){width="0.15555555555555556in"
height="0.11388888888888889in"}

Still from silent movie \"Le voyage dans la lune\" (1902) by Georges
Méliès.

On maps, the dark parts of the Moon\'s surface were called *maria*
(singular *mare*) or seas, and the light parts were called *terrae* or
continents. The possibility that the Moon could contain vegetation and
be inhabited by selenites was seriously considered by some major
astronomers even into the first decades of the 19th century. The
contrast between the brighter highlands and darker maria create the
patterns seen by different cultures as the [Man in the
Moon](http://en.wikipedia.org/wiki/Man_in_the_Moon), the rabbit and the
buffalo, amongst others.

In 1835, the [Great Moon
Hoax](http://en.wikipedia.org/wiki/Great_Moon_Hoax) fooled some people
into thinking that there were exotic animals living on the Moon. Almost
at the same time however (during 1834--1836), [Wilhelm
Beer](http://en.wikipedia.org/wiki/Wilhelm_Beer) and [Johann Heinrich
Mädler](http://en.wikipedia.org/wiki/Johann_Heinrich_Mädler) were
publishing their four-volume *Mappa Selenographica* and the book *Der
Mond* in 1837, which firmly established the conclusion that the Moon has
no bodies of water nor any appreciable
atmosphere.[^\[28\]^](http://en.wikipedia.org/wiki/Moon#_note-20%23_note-20)

### \[[edit](http://en.wikipedia.org/w/index.php?title=Geology_of_the_Moon&action=edit&section=20)\] Study of lunar rocks

Most of the rocks brought from the Moon are stored in the [Lunar
Curatorial
Facility](http://en.wikipedia.org/wiki/Lunar_Curatorial_Facility) in the
[Johnson Space
Center](http://en.wikipedia.org/wiki/Johnson_Space_Center) in
[Houston](http://en.wikipedia.org/wiki/Houston),
[Texas](http://en.wikipedia.org/wiki/Texas). A small percentage is
distributed in auxiliary installations at [Brooks Air Force
Base](http://en.wikipedia.org/wiki/Brooks_Air_Force_Base), near [San
Antonio, Texas](http://en.wikipedia.org/wiki/San_Antonio%2C_Texas). Many
lunar samples are found in laboratories of researchers worldwide. A
small number of these rocks is available for public display in museums,
and only three pieces can be touched by the public. These are the
\"touchable rocks\", cut from basaltic rocks obtained by [*Apollo
17*](http://en.wikipedia.org/wiki/Apollo_17). One of these rocks is
located at the [Smithsonian
Institution](http://en.wikipedia.org/wiki/Smithsonian_Institution)\'s
[National Air and Space
Museum](http://en.wikipedia.org/wiki/National_Air_and_Space_Museum) in
[Washington, D.C.](http://en.wikipedia.org/wiki/Washington%2C_D.C.)
Another piece is located in the Houston Space Center, located near the
Johnson Space Center. The third rock can be found inside the Museum of
the Sciences in the [National Autonomous University of
Mexico](http://en.wikipedia.org/wiki/National_Autonomous_University_of_Mexico).

### \[[edit](http://en.wikipedia.org/w/index.php?title=Geology_of_the_Moon&action=edit&section=21)\] Mineral composition of lunar rocks

  --------------------------------------------------------- ------------------------------------------------------------- ------------------------------------------------------- ----------------------------------------------------- -------------------------------------------------------
  ** **                                                     [**Plagioclase**](http://en.wikipedia.org/wiki/Plagioclase)   [**Pyroxene**](http://en.wikipedia.org/wiki/Pyroxene)   [**Olivine**](http://en.wikipedia.org/wiki/Olivine)   [**Ilmenite**](http://en.wikipedia.org/wiki/Ilmenite)

  **Highland rocks**                                                                                                                                                                                                                    

  [Anorthosite](http://en.wikipedia.org/wiki/Anorthosite)   90%                                                           5%                                                      5%                                                    0%

  [Norite](http://en.wikipedia.org/wiki/Norite)             60%                                                           35%                                                     5%                                                    0%

  [Troctolite](http://en.wikipedia.org/wiki/Troctolite)     60%                                                           5%                                                      35%                                                   0%

  **Mare [basalts](http://en.wikipedia.org/wiki/Basalt)**                                                                                                                                                                               

  High [titanium](http://en.wikipedia.org/wiki/Titanium)    30%                                                           54%                                                     3%                                                    18%
  content                                                                                                                                                                                                                               

  Low [titanium](http://en.wikipedia.org/wiki/Titanium)     30%                                                           60%                                                     5%                                                    5%
  content                                                                                                                                                                                                                               

  Very low                                                  35%                                                           55%                                                     8%                                                    2%
  [titanium](http://en.wikipedia.org/wiki/Titanium) content                                                                                                                                                                             
  --------------------------------------------------------- ------------------------------------------------------------- ------------------------------------------------------- ----------------------------------------------------- -------------------------------------------------------

### \[[edit](http://en.wikipedia.org/w/index.php?title=Geology_of_the_Moon&action=edit&section=22)\] Common lunar minerals

  ----------------------------------------------------------- ------------------------------------------------------------ -------------------------------------------------------------------
  **Mineral**                                                 **Elements**                                                 **Lunar rock appearance**

  **[Plagioclase](http://en.wikipedia.org/wiki/Plagioclase)   [Calcium](http://en.wikipedia.org/wiki/Calcium) (Ca),\       White to
  [feldspar](http://en.wikipedia.org/wiki/Feldspar)**         [Aluminium](http://en.wikipedia.org/wiki/Aluminium) (Al),\   [transparent](http://en.wikipedia.org/wiki/Transparency_(optics))
                                                              [Silicon](http://en.wikipedia.org/wiki/Silicon) (Si),\       gray; usually as elongated grains.
                                                              [Oxygen](http://en.wikipedia.org/wiki/Oxygen) (O)            

  [**Pyroxene**](http://en.wikipedia.org/wiki/Pyroxene)       [Iron](http://en.wikipedia.org/wiki/Iron) (Fe),\             Maroon to black; the grains appear more elongated in the maria and
                                                              [Magnesium](http://en.wikipedia.org/wiki/Magnesium) (Mg),\   more square in the highlands.
                                                              [Calcium](http://en.wikipedia.org/wiki/Calcium) (Ca),\       
                                                              [Silicon](http://en.wikipedia.org/wiki/Silicon) (Si),\       
                                                              [Oxygen](http://en.wikipedia.org/wiki/Oxygen) (O)            

  [**Olivine**](http://en.wikipedia.org/wiki/Olivine)         [Iron](http://en.wikipedia.org/wiki/Iron) (Fe),\             Greenish color; generally, it appears in a rounded shape.
                                                              [Magnesium](http://en.wikipedia.org/wiki/Magnesium) (Mg),\   
                                                              [Silicon](http://en.wikipedia.org/wiki/Silicon) (Si),\       
                                                              [Oxygen](http://en.wikipedia.org/wiki/Oxygen) (O)            

  [**Ilmenite**](http://en.wikipedia.org/wiki/Ilmenite)       [Iron](http://en.wikipedia.org/wiki/Iron) (Fe),\             Black, elongated square crystals.
                                                              [Titanium](http://en.wikipedia.org/wiki/Titanium) (Ti),\     
                                                              [Oxygen](http://en.wikipedia.org/wiki/Oxygen) (O)            
  ----------------------------------------------------------- ------------------------------------------------------------ -------------------------------------------------------------------

## \[[edit](http://en.wikipedia.org/w/index.php?title=Geology_of_the_Moon&action=edit&section=23)\] Interior structure of the Moon

The current model of the interior of the Moon was derived using
[seismometers](http://en.wikipedia.org/wiki/Seismometer) left behind
during the manned [Apollo
program](http://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/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.
[^\[4\]^](http://en.wikipedia.org/wiki/Geology_of_the_Moon#_note-3%23_note-3).
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 orgin. 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](http://en.wikipedia.org/wiki/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.