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Giant impact hypothesis

Giant impact hypothesis

Overview
The giant impact hypothesis states that the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 was created out of the debris left over from a collision between the young Earth and a Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

-sized body
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

. The colliding body is sometimes called Theia for the mythical Greek Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 who was the mother of Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

, the goddess of the moon.

The giant impact hypothesis is the currently favoured scientific hypothesis for the formation of the Moon.
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Encyclopedia
The giant impact hypothesis states that the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 was created out of the debris left over from a collision between the young Earth and a Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

-sized body
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

. The colliding body is sometimes called Theia for the mythical Greek Titan
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 who was the mother of Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

, the goddess of the moon.

The giant impact hypothesis is the currently favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes the same direction of motion of the Earth's spin and the Moon's orbit, Moon samples which indicate the surface of the Moon was once molten, the Moon's relatively small iron core and lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...

s). Further, giant collisions are consistent with the leading computer models of the formation of the solar system.

There remain several unanswered questions concerning the best current models of the giant impact hypothesis. The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

 of the heavier material sinking into Earth's mantle. Further, lunar samples do not have expected ratios of oxygen isotopic ratios, volatile elements, iron oxide, or siderophile elements compared to those of Earth.

Origins


In 1898, George Darwin
George Darwin
Sir George Howard Darwin, FRS was an English astronomer and mathematician.-Biography:Darwin was born at Down House, Kent, the second son and fifth child of Charles and Emma Darwin...

 made the suggestion that the Earth and Moon had once been one body. Darwin's hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation. Using Newtonian mechanics, he calculated that the Moon had actually orbited much closer in the past and was drifting away from the Earth. This drifting was later confirmed by American and Soviet
Russian Federal Space Agency
The Russian Federal Space Agency , commonly called Roscosmos and abbreviated as FKA and RKA , is the government agency responsible for the Russian space science program and general aerospace research. It was previously the Russian Aviation and Space Agency .Headquarters of Roscosmos are located...

 experiments using laser ranging targets placed on the Moon.

However, Darwin's calculations could not resolve the mechanics required to trace the Moon backwards to the surface of the Earth. In 1946, Reginald Aldworth Daly
Reginald Aldworth Daly
-External links:* Daly′s Biography, American Geophysical Union* GSA Today, vol. 16, no. 2, 2006* National Academy of Sciences, Washington, DC, 36 pp., 1960...

 of Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...

 challenged Darwin's explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces. Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974 where the idea was reintroduced and later published and discussed in Icarus
Icarus (journal)
Icarus is a premier scientific journal dedicated to the field of planetary science. It is published under the auspices of the American Astronomical Society's Division for Planetary Sciences . The longtime publisher was Academic Press, which is now part of Elsevier...

in 1975 by Drs. William K. Hartmann
William Kenneth Hartmann
William K. Hartmann is a noted planetary scientist, author, and writer. He was the first to convince the scientific mainstream that the Earth had once been hit by a planet sized body , creating both the moon and the Earth's 23° tilt....

 and Donald R. Davis. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects may have collided with the Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. This collision could help explain the unique geological properties of the Moon.

A similar approach was taken by Alfred G. W. Cameron and William Ward, who suggested that the Moon was formed by the tangential impact of a body the size of Mars. The outer silicates of the colliding body would mostly be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision would probably escape the Solar System, whereas silicates would tend to coalesce.

Theia


The name of the hypothesized protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

 is derived from the mythical Greek titan
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

 Theia
Theia
In Greek mythology, Theia "goddess" or "divine" , also called Euryphaessa "wide-shining," was a Titan...

, who gave birth to the Moon goddess Selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

. According to the giant impact hypothesis, Theia formed alongside the other planet size bodies in the Solar System about 4.6 Ga
Gya
In astronomy, geology, and paleontology, Gya or Ga is a year multiplied by one of the SI prefix multipliers. It is often used as a unit of time to denote length of time before the present...

 (4.6 billion years ago), and was approximately the size of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

.


One idea is that Theia coalesced at the or Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

 relative to Earth (in about the same orbit and about 60° ahead or behind), similar to a trojan asteroid
Trojan asteroid
The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...

. The stability of Theia's orbit was affected when its growing mass exceeded a threshold of about 10% of the Earth's mass. Gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth caused the two bodies to collide. However, this idea is based on two-dimensional computer models. Fully three-dimensional calculations must be done to scrutinize this idea further.

Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the rest of the Solar System formed. However, evidence presented in 2008 suggests that the collision may have occurred later, at about 4.48 Ga.

Impact


In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle
Oblique Angle
Oblique angle can refer to:*An angle which is not a multiple of 90°*Another word for "Dutch angle" in cinematography...

 when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s. Theia's iron core sank into the young Earth's core, as most of Theia's mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

 and a significant portion of the Earth's mantle and crust were ejected
Ejecta
Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...

 into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulation
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

s of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. The Earth would have gained significant amounts of angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 and mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 from such a collision. Regardless of the rotation and inclination the Earth had before the impact, it would have had a day some five hours long after the impact, and the Earth's equator would have shifted closer to the plane of the Moon's orbit.

It has been suggested that other significant objects may have been created by the impact, which could have remained in orbit between the Earth and Moon, stuck in Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...

s. Such objects may have stayed within the Earth-Moon system for up to 100 million years, until the gravitational tugs of other planets destabilized the system enough to free the objects. A study published in 2011 suggested that a subsequent collision between the Moon and one of these smaller bodies caused the notable differences in physical characteristics between the two hemispheres of the Moon. This collision, simulations have supported, would have been at a low enough velocity so as not to form a crater; the material from the smaller body would instead have spread out across the Moon (in what would become its far side
Far side of the Moon
The far side of the Moon is the lunar hemisphere that is permanently turned away, and is not visible from the surface of the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon...

), adding a thick layer of highlands crust. The resulting mass irregularities would subsequently produce a gravity gradient that helped to rotationally lock
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

 the Moon so that today only the near side remains visible from Earth.

Evidence


Indirect evidence for this impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 ratios identical to those of Earth. The highly anorthositic composition of the lunar crust, as well as the existence of KREEP
KREEP
KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...

-rich samples, gave rise to the idea that a large portion of the Moon was once molten, and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean
Lunar magma ocean
According to the giant impact hypothesis a large amount of energy was liberated in the formation of the Moon and it is predicted that as a result a large portion of the Moon was once completely molten, forming a lunar magma ocean...

. Several lines of evidence show that if the Moon has an iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-rich core, it must be small. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the radius of the core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other terrestrial
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

 bodies. Impact conditions can be found that give rise to a Moon that formed mostly from the mantles of the Earth and impactor, with the core of the impactor accreting to the Earth, and which satisfy the angular momentum constraints of the Earth-Moon system.

Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/sec) impacts between rocky bodies has been detected around the nearby (29 pc distant) young (~12 My old) Beta Pic Moving Group star HD172555 by the Spitzer Space Telescope. A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

 cluster appears similar to the predicted results of Theia's collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other. This is similar to another belt of warm dust detected around the star BD +20°307 (HIP 8920, SAO 75016).

Difficulties


This lunar origin hypothesis has some difficulties which have yet to be resolved.
These difficulties include:
  • The ratios of the Moon's volatile elements are not explained by the giant impact hypothesis. If the giant impact hypothesis is correct, they must be due to some other cause.
  • There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis), and it is likely there exists material which has never been processed by a magma ocean.
  • The iron oxide (FeO) content (13%) of the Moon, which is intermediate between Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from the Earth's mantle.
  • If the bulk of the proto-lunar material had come from the impactor, the Moon should be enriched in siderophilic elements, when it is actually deficient in those.
  • The presence of volatiles such as water trapped in lunar basalt
    Basalt
    Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

    s is more difficult to explain if the impact caused a catastrophic heating event.
  • The Moon's oxygen isotopic ratios are essentially identical to those of Earth. Oxygen isotopic ratios, which can be measured very precisely, yield a unique and distinct signature for each solar system body. If Theia had been a separate proto-planet, it would probably have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.

Alternative hypotheses


Other mechanisms which have been suggested at various times for the Moon's origin are that the Moon was spun off from the Earth's molten, blobular surface by centrifugal force
Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

, that it was formed elsewhere and later captured by the Earth's gravitational field, or that the Moon formed at the same time and place as the Earth from the same accretion disk. Each of these hypotheses is claimed to lack a mechanism to account for the high angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 of the Earth–Moon system.

See also


  • Geology of the Moon
    Geology of the Moon
    The geology of the Moon is quite different from that of the Earth...

  • Lunar geologic timescale
    Lunar geologic timescale
    The lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian , Nectarian, and Pre-Nectarian...

  • History of Earth
    History of Earth
    The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation 4.578 billion years ago to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's...

  • Geologic time scale
    Geologic time scale
    The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

  • Roche limit
    Roche limit
    The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...


Further reading


Academic articles
  • William K. Hartmann and Donald R. Davis, Satellite-sized planetesimals and lunar origin, (International Astronomical Union
    International Astronomical Union
    The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

    , Colloquium on Planetary Satellites, Cornell University
    Cornell University
    Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

    , Ithaca, N.Y., Aug. 18-21, 1974) Icarus
    Icarus
    -Space and astronomy:* Icarus , on the Moon* Icarus , a planetary science journal* 1566 Icarus, an asteroid* IKAROS, a interplanetary unmanned spacecraft...

    , vol. 24, April 1975, p. 504-515
  • Alastair G. W. Cameron
    Alastair GW Cameron
    Alastair G. W. Cameron was a Canadian astrophysicist and space scientist who was an eminent staff member of the Astronomy department of Harvard University. Cameron, the son of a Canadian biochemist, was born in Winnipeg...

     and William R. Ward, The Origin of the Moon, Abstracts of the Lunar and Planetary Science Conference, volume 7, page 120, 1976


Non-academic books
  • Dana Mackenzie, The Big Splat, or How Our Moon Came to Be, 2003, John Wiley & Sons
    John Wiley & Sons
    John Wiley & Sons, Inc., also referred to as Wiley, is a global publishing company that specializes in academic publishing and markets its products to professionals and consumers, students and instructors in higher education, and researchers and practitioners in scientific, technical, medical, and...

    , ISBN 0-471-15057-6.


External links