Origin of the Moon

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The Moon's heavily cratered far-side

The origin of the Moon is usually explained by a

debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be formed.[1][2] Other proposed scenarios include captured body, fission, formed together (condensation theory, synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories.[3]

The standard giant-impact hypothesis suggests that a Mars-sized body, called

47Ti) appears so close to the Earth's (within 4 parts per million) that little if any of the colliding body's mass could likely have been part of the Moon.[7]

Formation

Lunar sample 61016, better known as "Big Muley"

Some theories have been stated that presume the proto-Earth had no large moons early in the formation of the Solar System, 4.425 billion years ago, Earth being basically rock and lava. Theia, an early protoplanet the size of Mars, hit Earth in such a way that it ejected a considerable amount of material away from Earth. Some proportion of these ejecta escaped into space, but the rest consolidated into a single spherical body in orbit about Earth, creating the Moon.

The hypothesis requires a collision between a proto-Earth about 90% of the diameter of present Earth, and another body the diameter of Mars (half of the terrestrial diameter and a tenth of its mass). The latter has sometimes been referred to as Theia, the name of the mother of Selene, the Moon goddess in Greek mythology. This size ratio is needed in order for the resulting system to have sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit around Earth to have eventually accumulated to form the Moon.

Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: possibly trillions of tonnes of material would have been vaporized and melted. In parts of the Earth, the temperature would have risen to 10,000 °C (18,000 °F).

The Moon's relatively small

rocky planets and moons in the Solar System) is explained by Theia's core mostly merging into that of Earth. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccretion of material in orbit around Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma ocean
.

The newly formed Moon orbited at about one-tenth the distance that it does today, and spiraled outward because of

tidal friction transferring angular momentum from the rotations of both bodies to the Moon's orbital motion. Along the way, the Moon's rotation became tidally locked
to Earth, so that one side of the Moon continually faces toward Earth. Also, the Moon would have collided with and incorporated any small preexisting satellites of Earth, which would have shared the Earth's composition, including isotopic abundances. The geology of the Moon has since been more independent of the Earth.

A 2012 study on the depletion of zinc isotopes on the Moon found evidence for volatile depletion consistent with the giant-impact origin for Earth and the Moon.[8] In 2013, a study was released that indicated that water in lunar magma is indistinguishable from that in carbonaceous chondrites and nearly the same as that of Earth in isotopic composition.[9][10][11]

Derivatives of the hypothesis

Although the giant-impact hypothesis explains many aspects of the Earth–Moon system, there are still a few unresolved problems, such as the Moon's volatile elements not being as depleted as expected from such an energetic impact.[12]

Another issue is lunar and Earth isotope comparisons. In 2001, the most precise measurement yet of the

conflicts
with the Moon forming far from Earth's orbit.

Merger of two planets

To help resolve these problems, a theory published in 2012 posits that two bodies—each five times the size of Mars—collided, then recollided, forming a large disc of mixed debris that eventually formed Earth and the Moon.[1]

Immediate origin of the Moon as a post-impact satellite

Simulation of the formation of the moon caused by a giant impact.

The Moon is traditionally thought to have coalesced from the debris ejected by a giant impact onto the early Earth. However, such models struggle to explain the similar isotopic compositions of Earth and lunar rocks at the same time as the system's angular momentum, and the details of potential impact scenarios are hotly debated. Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon's Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon's early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.[15]

Multiple impacts

In 2004, Russian astrophysicist Nikolai Gorkavyi proposed a novel model titled the multiple large asteroid impacts model,[16][17] which found support from a notable group of Russian astronomers in 2013[18] and later, in 2017, by planetary researchers at Weizmann Institute of Science in Rehovot, Israel.[19] In general terms, the main idea of the model suggests that the Moon was formed as a result of a violent rain of large asteroids (1–100 km) that repeatedly hammered the fledgling Earth over millions of years. Such a series of smaller impacts, which were likely more common in the early Solar System, could blast enough rocky Earth debris into orbit to form a protosatellite disk which later forms into a small moonlet.[17][19] As repeated impacts created more balls of debris, the moonlets could merge over time into one large moon.[17][19]

Synestia hypothesis

In 2018 researchers at Harvard and the UC Davis developed computer models demonstrating that one possible outcome of a planetary collision is that it creates a synestia, a mass of vaporized rock and metal which forms a biconcave disc extending beyond the lunar orbit. The synestia will eventually shrink and cool to accrete the satellite and reform the impacted planet.[20]

Moon –
GRAIL gravity gradients) (October 1, 2014).
Ancient rift valleys – context.

Other hypotheses

Density[21]
Body Density
g/cm3
Mercury 5.4
Venus 5.2
Earth 5.5
Moon 3.3

Capture

This hypothesis states that the Moon was captured by the Earth.[22] This model was popular until the 1980s, and some points in its favor are the Moon's size, orbit, and tidal locking.[22]

One problem is understanding the capture mechanism.

irregular satellite orbits of Jupiter and Saturn.[23] However, this hypothesis does not adequately explain the essentially identical oxygen isotope ratios of the two bodies.[4]

Earth and Moon to scale, 500 km per pixel

Fission

This is the now discredited hypothesis that an ancient, rapidly spinning Earth expelled a piece of its mass.[22][24] This was first proposed by George Darwin (son of the famous biologist Charles Darwin) in 1879[25] and retained some popularity until Apollo.[22] The Austrian geologist Otto Ampferer in 1925 also suggested the emerging of the Moon as cause for continental drift.[26]

It was proposed that the Pacific Ocean represented the scar of this event.[22] Today it is known that the oceanic crust that makes up this ocean basin is relatively young, about 200 million years old or less, whereas the Moon is much older. The Moon does not consist of oceanic crust but of mantle material, which originated inside the proto-Earth in the Precambrian.[7]

Thorium concentrations on the Moon, as mapped by Lunar Prospector

Accretion

The hypothesis of accretion suggests that the Earth and the Moon formed together as a double system from the primordial accretion disk of the Solar System[27] or even a black hole.[28] The problem with this hypothesis is that it does not explain the angular momentum of the Earth-Moon system or why the Moon has a relatively small iron core compared to the Earth (25% of its radius compared to 50% for the Earth).[27]

Nuclear explosion

Dutch scientists Rob de Meijer and Wim van Westrenen suggested in 2010 that the Moon may have formed from a nuclear explosion caused by the centrifugal force of an earlier, spinning proto-Earth. The centrifugal force would have concentrated heavy elements such as

outer core and mantle. If the concentrations of these radioactive elements were high enough, this could have led to a nuclear chain reaction that became supercritical, causing a nuclear explosion ejecting the Moon into orbit.[29][30][31] This natural nuclear fission reactor
has been observed on Earth at a much smaller scale.

Additional theories and studies

Evolution of the Moon video by NASA circa 2012.[32]

2011

In 2011, it was theorized that a second moon existed 4.5 billion years ago, and later had an impact with the Moon, as a part of the accretion process in the formation of the Moon.[33]

2013

One hypothesis, presented only as a possibility, was that the Earth captured the Moon from Venus.[34]

2017

Uranium–lead dating of Apollo 14 zircon fragments shows the age of the Moon to be about 4.51 billion years.[35][36]

2020

A team of researchers of the Miniature Radio Frequency (Mini-RF) instrument on NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft concluded that the Moon's subsurface may be richer in metals, like iron and titanium, more than scientists had believed.[37]

In July 2020 scientists report that the Moon formed 4.425 ±0.025 bya, about 85 million years later than thought, and that it hosted an ocean of magma for substantially longer than previously thought (for ~200 million years).[38][39][40]

2023

On 1 November 2023, scientists reported that, according to computer simulations, remnants of a protoplanet, named Theia, could be inside the Earth, left over from a collision with the Earth in ancient times, and afterwards becoming the Moon.[41][42]

See also

  • Geology of the Moon – Structure and composition of the Moon
  • Late Heavy Bombardment – Hypothesized astronomical event
  • Lunar theory – theoretical description of motion of Earth's moon
  • Claimed moons of Earth – Claims that Earth may have other natural satellites
  • Lunar Reconnaissance Orbiter – NASA robotic spacecraft orbiting the Moon
  • MoonRise – proposed NASA mission to the Moon (Lunar lander proposal)
  • Spaceship Moon, a non-scientific hypothesis of the Moon's origin
  • Phaeton, another non-scientific hypothesis of the Moon's origin
  • Magma Ocean, general description of magma oceans and their formation

References

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  9. ^ Spudis, Paul D. (May 14, 2013). "Earth-Moon: A Watery "Double-Planet"". Air & Space/Smithsonian. Archived from the original on 2013-08-07.
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  11. ^ Erik Ian Asphaug: Impact Origin of the Moon?. In: Annual Revue of Earth and Planetary Sciences. March 2014.
  12. ^ Jones, J. H. "TESTS OF THE GIANT IMPACT HYPOTHESIS" (PDF). Origin of the Earth and Moon Conference. Retrieved 2006-11-21.
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  34. ^ Did We Steal Our Moon From Venus? Is our planet a dirty thief?, Popular Science, 09.27.2013 at 5:07 pm
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  39. ^ Lennon, Annie (13 July 2020). "Earth's Moon Had Magma Ocean for 200 Million Years | Space". LabRoots. Retrieved 16 August 2020.
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  41. ^ Chang, Kenneth (1 November 2023). "A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon". The New York Times. Archived from the original on 1 November 2023. Retrieved 2 November 2023.
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External links