Encyclopedia
Moon
The Moon as seen from Earth |
| Orbital characteristics |
|---|
| Orbital circumference | 2,413,402 km
|
|---|
| Eccentricity | 0.0554 |
|---|
| Perigee | 363,104 km
|
|---|
| Apogee | 405,696 km
|
|---|
| Revolution period | 27.321 66155 d
|
|---|
| Synodic period | 29.530 588 d
|
|---|
| Avg. Orbital Speed | 1.022 km/s |
|---|
| Max. Orbital Speed | 1.082 km/s |
|---|
| Min. Orbital Speed | 0.968 km/s |
|---|
| Inclination | varies between
28.60° and 18.30°
see below |
|---|
Longitude of the
ascending node | regressing,
1 revolution in 18.6 years |
|---|
| Argument of perigee | progressing,
1 revolution in 8.85 years |
|---|
| Satellite of | Earth |
|---|
| Physical characteristics |
|---|
| Equatorial diameter | 3,476.2 km
|
|---|
| Polar diameter | 3,472.0 km
|
|---|
| Oblateness | 0.0012 |
|---|
| Surface area | 3.793 km²
|
|---|
| Volume | km³
|
|---|
| Mass | kg
|
|---|
| Mean density | 3,346.2 kg·m−3 |
|---|
| Equatorial gravity | 1.622 m·s−2,
|
|---|
| Escape velocity | 2.38 km·s−1 |
|---|
| Rotation period | 27.321 661 d
|
|---|
| Rotation velocity | 16.655 km·h−1
|
|---|
| Axial tilt | 1.5424° to ecliptic
see Orbit |
|---|
| Albedo | 0.12 |
|---|
| Magnitude | -12.74 |
|---|
| Surface temp. | | min | mean | max |
|---|
| 40 K | 250 K | 396 K |
|
|---|
Bulk composition of the moon's
mantle and crust |
|---|
| estimated, weight percent |
| Oxygen | 42.6 % |
| Magnesium | 20.8 % |
| Silicon | 20.5 % |
| Iron | 9.9 % |
| Calcium | 2.31 % |
| Aluminium | 2.04 % |
| Nickel | 0.472 % |
| Chromium | 0.314 % |
| Manganese | 0.131 % |
| Titanium | 0.122 % |
| Atmospheric characteristics |
|---|
| Atmospheric pressure | 3 × 10-13kPa |
|---|
| Helium | 25 % |
| Neon | 25 % |
| Hydrogen | 23 % |
| Argon | 20 % |
Methane
Ammonia
Carbon dioxide | trace |
The
Moon is
Earth's only
natural satellite. It has no formal
English name other than "the Moon", although it is occasionally called
Luna , or
Selene , to distinguish it from the generic term "moon" . Its symbol is a
crescent. The related adjective for the Moon is
lunar , but this is not found in combination, the combining forms
seleno-/-selene and
-cynthion being used in terms relating to the Moon in various other contexts .
The average distance from the Moon to the Earth is 384,401 kilometres . The Moon's diameter is 3,476 kilometres . Reflected sunlight from the Moon's surface reaches Earth in 1.3 seconds . The Moon is the
Solar System's fifth largest moon and is also the fifth most massive moon.
The first man-made object to land on the Moon was
Luna 2 in 1959; the first photographs of the otherwise occluded
far side of the Moon were made by
Luna 3 in the same year. The first manned mission to orbit the Moon was
Apollo 8, and the first people to land on the Moon came aboard
Apollo 11 was the first manned mission to land on the Moon [i]. ...
in 1969. It is the only celestial body other than the Earth upon which humans have set foot.
The two sides of the Moon
The Moon is in synchronous rotation, meaning that it keeps nearly the same face turned toward Earth at all times . The side of the Moon that faces Earth is called the
near side, and the opposite side is called the
far side. The far side is also sometimes called the "dark side", which means "unknown and hidden", and not "lacking light" as might seem to be implied by the name; in fact, the far side receives as much sunlight as the near side. Spacecraft are cut off from direct radio communication with Earth when on the far side of the Moon due to line of sight. One distinguishing feature of the far side is its almost complete lack of
maria , which are the dark albedo features.
| 90° W | Near side | |
|---|
| | |
| | |
| 90° E | Far side |
|---|
Orbit and relationship to Earth
The Moon makes a complete orbit about the Earth approximately once every 27.3 days; unlike most satellites of other planets, the Moon orbits near the
ecliptic and not the Earth's
equatorial plane.
The Earth and Moon have many physical effects upon one another, including the
tides. Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with a smaller contribution from the
Sun. Tidal effects result in an increase of the mean Earth-Moon distance, over long periods of time, of about 4
metres per century.
The Earth-Moon system may be considered to be a
double planet rather than a planet-moon system. This is due to the exceptionally large size of the Moon relative to its host planet; the Moon is one-fourth the diameter of Earth and 1/81 the mass. However, as the
barycenter is located within the Earth, the Earth-Moon system does not meet the official IAU definition of a double planet. With the exception of
Pluto and its satellite Charon, no other object in the
Solar System has an orbiting body with more than 1/4000 of the host object's mass.
Origin and history
The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate.
Early speculation proposed that the Moon broke off from the Earth's crust due to
centrifugal force, leaving an ocean basin behind as a scar. This concept requires too great an initial spin of the Earth and the presumption of a Pacific origin is not compatible with the geological standard model, the theory of
plate tectonics. Others speculated the Moon formed elsewhere and was captured into Earth's orbit. Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal.
The
coformation or
condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial
accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon.
Recently, the
giant impact hypothesis has been considered a more viable scientific hypothesis for the moon's origin than the coformation or condensation hypothesis. The Giant Impact hypothesis holds that the Moon formed from the ejecta resulting from a collision between a very early, semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia or
Orpheus. The material ejected from this impact would have gathered in orbit around Earth and formed the Moon. This hypothesis is bolstered by two main observations: First, the composition of the Moon resembles that of Earth's crust, though it has relatively few heavy elements that would have been present if it formed by itself out of the same material from which Earth formed. Second, through radiometric dating, it has been determined that the Moon's crust formed between 20 and 30 million years after that of Earth, despite its smallness and associated larger loss of internal heat, although it has been suggested that this hypothesis does not adequately address the abundance of volatile elements in the moon.
At that time the Moon was much closer to the Earth and strong tidal forces deformed the once molten sphere into an
ellipsoid, with the
major axis pointed towards Earth. When the Moon started to cool a solid crust was formed along its surface, but its molten interior remained displaced in the direction of the Earth. Due to this effect, the crust on the near side was much thinner than on the far side.
Especially during the late heavy bombardment, around 3.8 to 4 billion years ago, many large meteorites were able to penetrate the thin crust of the near side but only very few could do so on the far side. Where the crust was perforated the hot lavas from the interior oozed out and spread over the surface, only to cool later into the
maria as we know them today. This explains the lack of maria on the far side.
The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. The period of the late heavy bombardment is determined by analysis of craters and
Moon rocks. In 2005, a team of scientists from Germany, the United Kingdom, and Switzerland measured the Moon's age at 4527 ± 10 million years, which would imply that it was formed only 30 to 50 million years after the origin of the solar system.
Physical characteristics
Composition
More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, called KREEP , represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those that cannot fit into a
crystal structure and thus were left behind, floating to the surface of the magma. For the researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by
comets and other celestial bodies.
The lunar crust is composed of a variety of primary elements, including
uranium,
thorium, potassium,
oxygen,
silicon,
magnesium,
iron,
titanium, calcium,
aluminium and
hydrogen, as determined by
spectroscopy.
A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon;
Galileo did so when it flew by the Moon in 1992. The overall composition of the Moon is believed to be similar to that of the upper parts of the Earth other than a depletion of volatile elements and of iron.
Selenography
When observed with earth based telescopes, the moon can be seen to have some 30,000
craters having a
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; the lack of atmosphere, weather and recent geological processes ensures that most of them remain permanently preserved. In the lunar terrae, it is indeed impossible to add a crater of any size without obliterating another; this is termed
saturation.
The largest crater on the Moon, and indeed the largest known crater within the
solar system, forms the
South Pole-Aitken basin. This crater is located on the far side, near the
South Pole, and is some 2,240 kilometres in diameter, and 13 kilometres in depth.
The dark and relatively featureless lunar
plains are called
maria, Latin for seas, since they were believed by ancient
astronomers to be water-filled
seas. They are actually vast ancient
basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called
terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches.
Blanketed atop the Moon's crust is a dusty outer rock layer called regolith, the result of rocks shattered by billions of years of impacts. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 kilometres thick on the near side to 100 kilometres on the far side, and the regolith varies from 3 to 5 metres deep in the maria to 10 to 20 metres deep in the highlands.
In 2004, a team led by Dr. Ben Bussey of
Johns Hopkins University using images taken by the
Clementine mission determined that four mountainous regions on the rim of the 73-km-wide Peary crater at the Moon's
north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less mountainous
south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.
Dating of the lunar impact events through
40Ar/
39Ar isotope analysis of glass spherules, created during the impacts, showed a high impact number in early lunar history and in the last 400 million years.
Presence of water
Over time, comets and meteoroids continuously bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the Moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³.
Some water molecules, however, may have literally hopped along the surface and become trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped craters at the lunar south pole which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could 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 would be an important factor in rendering
lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive.
The equatorial Moon rock collected by Apollo astronauts contained no traces of water. Neither the
Lunar Prospector nor more recent surveys, such as those of the
Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapor.
Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.
Magnetic field
Compared to that of Earth, the Moon has a very weak
magnetic field. While some of the Moon's magnetism is thought to be intrinsic , collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements than Earth, then the impact theory loses some credibility .
Atmosphere
The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the
solar wind, which is briefly captured by the Moon's gravity.
Eclipses
Eclipses happen only if Sun, Earth, and Moon are lined up.
Solar eclipses can only occur near a
new moon;
lunar eclipses can only occur near a
full moon.
The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and
annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar
corona becomes
