Earth's Moon is an astronomical body that orbits the planet and acts as its only permanent natural satellite. It is the fifth-largest satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). The Moon is, after Jupiter's satellite Io, the second-densest satellite in the Solar System among those whose densities are known.
The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. 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. New research of moon rocks, although not rejecting the Theia hypothesis, suggests that the moon may be older than previously thought.[13]
The Moon is in synchronous rotation with Earth, and thus always shows the same side to Earth, the near side. The near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest regularly visible celestial object in Earth's sky. 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 average orbital distance is 384,402 km (238,856 mi),[14][15] or 1.28 light-seconds. This is about thirty times the diameter of Earth. The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly precisely during a total solar eclipse. This matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is gradually increasing.
The Moon was first reached in September 1959 by the Soviet Union's Luna 2, an unmanned spacecraft, followed by the first successful soft landing by Luna 9 in 1966. The United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, and six manned landings between 1969 and 1972, with the first being Apollo 11 in July 1969. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, and the Moon's later history. Since the 1972 Apollo 17 mission the Moon has been visited only by unmanned spacecraft.
Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems, art, and mythology.
The usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is usually not capitalized.[16][17][18][19][20] The noun moon is derived from Old English mōna, which (like all Germanic language cognates) stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon.[21][22] Occasionally, the name "Luna" is used. In literature, especially science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon.[23]
The modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic (usually only used to refer to the chemical element selenium) is so rarely used to refer to the Moon that this meaning is not recorded in most major dictionaries.[24][25][26] It is derived from the Ancient Greek word for the Moon, σελήνη (selḗnē), from which is however also derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium.[27][28] Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia.[29] The names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky, heaven, and god" and is also the origin of Latin dies, "day".
The Moon formed 4.51 billion years ago,[f] some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed,[30] including the fission of the Moon from Earth's crust through centrifugal force[31] (which would require too great an initial spin of Earth),[32] the gravitational capture of a pre-formed Moon[33] (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon),[32] and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon).[32] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system
The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body (named Theia) with the proto-Earth (giant impact). The impact blasted material into Earth's orbit and then the material accreted and formed the Moon.[35][36]
The Moon's far side has a crust that is 50 km (31 mi) thicker than that of the near side. This is thought to be because the Moon fused from two different bodies.
This hypothesis, although not perfect, perhaps best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most consensual theory.
Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn’t think the debate would ever be resolved. Afterward, there were essentially only two groups: the giant impact camp and the agnostics.[37]
Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth.[38] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth.[39][40][41][42] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two,[43] although this is debated.[44]
The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean.[45][46] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean; its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles).[45]
While the giant impact hypothesis might explain many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition.[47]
In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks.[48] To their surprise, the rocks from the Apollo program had the same isotopic signature as rocks from Earth, however they differed from almost all other bodies in the Solar System. Indeed, this observation was unexpected, because most of the material that formed the Moon was thought to come from Theia and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures.[49] Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth,[50] which conflicts with what is expected if the Moon formed far from Earth or is derived from Theia. These discrepancies may be explained by variations of the giant impact hypothesis.
Physical characteristics
Internal structure
Main article: Internal structure of the Moon
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 possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi).[52][53] 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.[54]
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.[55] 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.[1]
Consistent with this perspective, geochemical mapping made from orbit suggests the crust of mostly anorthosite.[12] 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.[1] The crust is on average about 50 kilometres (31 mi) thick.[1]
The Moon is the second-densest satellite in the Solar System, after Io.[56] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less,[1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulphur and nickel; analyzes of the Moon's time-variable rotation suggest that it is at least partly molten.[57]
Surface geology
Main articles: Geology of the Moon, Moon rocks, and List of lunar features
The topography of the Moon has been measured with laser altimetry and stereo image analysis.[58] Its most visible topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System.[59][60] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon.[59][61] The highest elevations of the surface are located directly to the northeast, and it has been suggested might have been thickened by the oblique formation impact of the South Pole–Aitken basin.[62] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale also possess regionally low elevations and elevated rims.[59] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side.[1]
The discovery of fault scarp cliffs by the Lunar Reconnaissance Orbiter suggest that the Moon has shrunk within the past billion years, by about 90 metres (300 ft).[63] Similar shrinkage features exist on Mercury. A recent study of over 12000 images from the orbiter has observed that Mare Frigoris near the north pole, a vast basin assumed to be geologically dead, has been cracking and shifting. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat over the years.[64]
Volcanic features
Main article: Lunar mare
Lunar nearside with major maria and craters labeled
Lunar nearside with major maria and craters labeled
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;[65] 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.[66] 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
