The Origin of the Moon

According to Greek mythology- Selene, the Moon goddess, is the daughter of Theia and Hyperion. The name of the hypothesized planet ‘Theia’ of the Giant Impact Hypothesis, which is the most acceptable explanation of the origin of the moon; till now, was taken. According to this hypothesis, there was a Mars-sized planet between the Earth and Venus, and it struck the Earth at an oblique angle, when the Earth was nearly fully formed, some 30-40 million years after the formation of the Solar System. Theia's iron core sunk into the young Earth's core, and most of Theia's mantle accreted onto the Earth’s mantle. However, significant portions of the mantle material from both the Theia and the Earth have been ejected into orbit around the Earth to form the moon, at a distance of about one-tenth of its current orbit. The moon, ever since is drifting away from the Earth at the rate of approximately 38±0.004 mm a year.

There are also other hypotheses on the origin of the moon. But since these are all hypotheses, we need to know some basic facts about the moon to evaluate these hypotheses.

Structure of the Moon

Moon, the lonely satellite of our planet, orbits the Earth in an elliptical path with a mean distance of about 385,000 km from the center of the Earth. With a Mean orbital velocity of 1.023 km/s, the Moon moves relative to the stars each hour by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. But most of the satellites of the solar system orbit their planets on equatorial planes. The lunar orbit plane is inclined to the ecliptic by about 5.1°, whereas the Moon's spin axis is inclined by only 1.5°.

It is also the largest natural satellite in the Solar System relative to the size of the planet it orbits around, having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 Earth’s mass. The Moon has a geo-chemically distinct crust, mantle, and core. The Moon has a low solid iron-rich inner core with a radius of only 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers. 

The radius of the core is about 20% of the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. The relatively small size of the moon’s core is suggested due to the fact that the Moon has probably formed out of material splashed into orbit by the impact of a large body into the early Earth. Differentiation on Earth had probably already separated many lighter materials toward the surface already, so that the impact removed a disproportionate amount of silicate material from Earth and the iron core of the impacting body sank into the young Earth's core, as most of its mantle and a significant portion of the Earth's mantle and crust were ejected into orbit around the Earth. This material quickly coalesced into the Moon. Estimates based on computer simulations of such an event suggest that some two percent of the original mass of the impacting body ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. 

On the Moon, a distinctive basaltic material has been found that is high in "incompatible elements" such as potassium (K), rare earth elements (REE), and phosphorus (P), (KREEP). It is also high in uranium and thorium. These elements are excluded from the major minerals of the lunar crust which crystallized out from its primeval magma ocean, and the KREEP basalt may have been trapped as a chemical differentiation between the crust and the mantle, with occasional eruptions to the surface.

This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation. 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 crystallized, lower-density plagioclase minerals could form and float into a crust on top. The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. Consistent with this, geochemical mapping from orbit shows the crust is mostly anorthosite, and moon rock samples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.

The Giant Impact Hypothesis (GIH)

The giant impact hypothesis is currently the most accepted scientific hypothesis for the formation of the Moon.

According to modern theories of planet formation of the solar system, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago. It orbited the Sun in about the same orbit as the Earth, and about 60° ahead or behind (at the L4 or L5 Lagrangian points relative to Earth), similar to a Trojan asteroid. 

However, the stability of Theia's orbit was affected when its growing mass exceeded a threshold of approximately 10% of the Earth's mass some 20-30 million years later.

In this scenario, gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent gravitational interaction with the proto-Earth locked it into an ultimate collision course.

In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the Earth 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 would have sunk into the young Earth's core, and most of Theia's mantle accreted onto the Earth's mantle, however, a significant portion of the mantle material from both Theia and the Earth would have been ejected 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 simulations of such an event suggest that some twenty percent of the original mass of Theia would have 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, and mass from such a collision. Regardless of the speed and tilt of the Earth's rotation before the impact, it experienced a day some five hours longer after the impact, and the Earth's equator shifted closer to the plane of the Moon's orbit in the aftermath of the giant impact. 

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 the Moon, stuck in Lagrangian points. Such objects may have stayed within the Earth–Moon system for as long as 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 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; instead, the material from the smaller body would have spread out across the Moon (in what would become its far side), adding a thick layer of highlands crust. The resulting mass irregularities would subsequently produce a gravity gradient that resulted in the tidal locking of the Moon so that today, only the near side remains visible from Earth.

During its formation, the Earth is thought to have experienced dozens of collisions with such planet-sized bodies. The Moon-forming collision would have been only one such "giant impact" and, perhaps, the last.

The supporting evidence of the GIH

GIH is supported by the identical direction of the Earth's spin and the Moon's orbit, the once molten surface of the Moon indicated by Moon rock samples, relatively small iron core, and lower density of the Moon, evidence of similar collisions in other stars systems (that result in debris disks), consistency with the leading theories of the formation of the star system, and identical stable isotope ratios of lunar and earth rocks, implying a common origin.

Moon is the largest natural satellite in the solar system, relative to the size of the planet it orbits. But the core is about 20% of the radius, in contrast to about 50% for most of the other terrestrial bodies, and is low in iron content. This is caused by the fact that the outer silicates of the colliding bodies (Earth and Theia) were vaporized by the heat created by the impact, but the metallic core was not. Hence, most of the impacting materials sent into orbit consisted of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision probably escaped the solar system, whereas silicates coalesced and, volatile-poor dust that could coalesce formed the Moon. This explains the unique geological and geochemical properties of the Moon.

Other evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios in moon rocks are identical to those of Earth, which can be explained if the Earth–Moon system experienced turbulent mixing in the aftermath of the giant impact. The comparison of the zinc isotopic composition of lunar samples, with that of Earth and Mars rocks has also provided further evidence for the impact hypothesis.

Oxygen has three naturally occurring isotopes: 16O, 17O, and 18O. With the most abundant 16O and a negligible percentage of 17O, only the ratio of 18O to 16O is considered for Oxygen isotope analysis. Meteorites show that other inner Solar System bodies such as Mars and Venus have very different oxygen isotopic compositions to Earth, as they were formed in distinctive different places of the solar system. But the Earth and the Moon have nearly identical isotopic compositions. Because a post-impact mixing of the vaporized materials between the forming Earth and Moon could have equalized their isotopic compositions.

Zinc is strongly fractionated when volatilized in planetary rocks, but not during normal igneous processes -as such the zinc abundance and isotopic composition can distinguish the two geological processes. Moon rocks contain more heavy isotopes of zinc and overall less zinc than corresponding igneous Earth or Mars rocks. This is consistent with zinc being depleted from Moon through evaporation, as expected for the giant impact origin.

In addition, the highly anorthositic composition of the lunar crust, as well as, the existence of KREEP-rich samples, gave rise to the idea that a large portion of the Moon once was molten, and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean. 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 impacts between rocky bodies, have been detected around the nearby (29-parsec c distant) young (~12 mln year 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 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).

Lastly, according to GIH, the moon started orbiting the Earth at nearly about one-tenth of its current orbital distance and drifting away from the Earth ever since. The distance of the Moon presently can be measured with an accuracy of a few centimeters by laser pulses that are bounced off mirrors on the surface of the moon, emplaced during the Apollo missions of 1969 to 1972, and by Lunokhod 2 in 1973. Measuring the return time of the pulse yields a very accurate measure of the distance. These measurements, from the period 1970–2007, are fitted to the equations of Newtonian mechanics of motion, and result shows the Moon recedes from the Earth at the rate of approximately 38±0.004 mm a year, because of their tidal interaction.

This also implies over millions of years, these tiny modifications caused the moon to move away from the Earth and lengthened the Earth's day by about 23 µ second a year which adds up to significant changes. During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.