Introduction:
Of the multiple theories of formation of the Moon, it is the Giant Impact Theory, proposed in the 1970s that stands out above the all and is today, in general, the most favoured view of lunar formation by Scientists. it was quickly favoured as a mechanism for the Moon's formation because it provided answers to some of the physical and chemical differences seen between the Moon and the Earth [10]
Following the Big Bang the Universe was in an accretionary stage, with new matter and elements coming together to form early Planets and Suns. During this period the Universe was relatively small and rapidly expanding. Over the first few 10,000,000s of years of the universe, this period of passive accretion transformed into a time of large and numerous collisions, at higher velocities, as large planets were forming and material caught within the Roche limit of large bodies was drawn in. [11] It is not hard to believe therefore, that collisions during this time could disperse huge amounts of matter that could coalesce to form an orbiting body such as the Moon. The name given to the body that collided with the Earth is Theia. The problem in accounting for the presence of the Earth's Moon is that it has a large mass in relation to the primary body - the Earth and the fact that it is depleted in volatiles and metals - in particular, Fe. [12]
Of the multiple theories of formation of the Moon, it is the Giant Impact Theory, proposed in the 1970s that stands out above the all and is today, in general, the most favoured view of lunar formation by Scientists. it was quickly favoured as a mechanism for the Moon's formation because it provided answers to some of the physical and chemical differences seen between the Moon and the Earth [10]
Following the Big Bang the Universe was in an accretionary stage, with new matter and elements coming together to form early Planets and Suns. During this period the Universe was relatively small and rapidly expanding. Over the first few 10,000,000s of years of the universe, this period of passive accretion transformed into a time of large and numerous collisions, at higher velocities, as large planets were forming and material caught within the Roche limit of large bodies was drawn in. [11] It is not hard to believe therefore, that collisions during this time could disperse huge amounts of matter that could coalesce to form an orbiting body such as the Moon. The name given to the body that collided with the Earth is Theia. The problem in accounting for the presence of the Earth's Moon is that it has a large mass in relation to the primary body - the Earth and the fact that it is depleted in volatiles and metals - in particular, Fe. [12]
The Giant Impact Hypothesis (GIH):
The reason that the GIH was later in coming than the other theories of Moon formation is because before the 1970s, it was thought that there were no impactors that had a mass larger than 0.1% of that of the Earth hitting the Earth [12]. However, subsequent research has shown that collision of a body 10% of the mass of the Earth during late Earth accretion is not to be unexpected [10].
Computer models have shown that in order for the GIH to be feasible the impactor must have been 10% the mass of the Earth, it must have hit the Earth at an oblique angle so as to blast material off the planet into orbit rather than just sending it up into the atmosphere just for it to fall back to Earth (See figure 2). The impactor must have hit the Earth at a velocity of at least 2km/s greater than the escape velocity in order to send material outside the Earth's Roche limit where the material could accrete to form an orbiting body [12].
The Giant Impact Hypothesis gained significant recognition as a mechanism for the formation of the Moon after geochemical analysis is the Earth's mantle and the Moon's mantle revealed that they have very similar oxygen and chromium isotope ratios [13]. This discovery was an important milestone because it provided evidence for the Moon forming in our Solar System and thus invalidated the Capture Theory.
After the Apollo missions to the Moon, studies of lunar material showed a rough resemblance between lunar material and Earth-mantle material [14] (Earth's Mantle is 8% FeO compared to 13% for the Moon [13]. However as a weight percentage iron is 7% of Earth's Mantle [15]and 7-9% of the Moon [16]). These similarities led scientists to conclude that the Moon formed, at least in part, from Earth material. Due to the deficiency of iron in the Moon the collision of Earth with a Mars-size body must have occurred after the differentiation of the Earth's core and mantle during which most of the iron sunk to the center of the Earth along with other heavy elements. This puts additional time constraints on the time at which the collision occurred and this is now thought to have happened 30-50 million years into Earth's history [13].
Perhaps the most compelling evidence for the GIH is observations of the angular momentum of the Earth-Moon system that cannot easily be explained by any of the other hypotheses. Compared to other planetary systems the Earth and Moon have an unusually high angular momentum (the amount of rotation an object has based on is size, mass and shape [17]) [18]. This can only be explained by a high velocity, large impactor hitting the Earth at an oblique angle and thus rules out Fission Theory.
Oxygen isotopic composition of the Moon and the Earth's mantle have also been pivotal in the development of the GIH. In 1973 a paper was published that proposed that oxygen isotope ratios change in relation to how close to the Sun a body formed [19]. this was backed up in 1981 when analysis of oxygen isotopes of different types of meteorites was undertaken [20]. Of all the meteorites C3 carbonaceous chondrites are the most different in term of oxygen isotope ratios to Earth and thus they must have formed far away from the Earth. SNC meteorites which come from Mars have much similar ratios to Earth but are still discriminable. Lunar rock samples however have isotope ratios characteristic of Earth leading us to assume that the Moon formed from material in the same 'planetary zone' as the Earth [14].
The final major piece of evidence for the impact theory is that of lunar bulk density. The mean density of the Moon is 3.344 g/cm^3 and the mean density of the Earth's Upper Mantle is 3.3-3.4 g/cm^3 [21]. This matching up of densities is easily explained by the impact hypothesis but is a strange coincidence when looking at other theories.
In conclusion, many of the first-order observations we make about the Earth and the moon: iron deficiency, mantle composition similarities, angular momentum, ratio of oxygen isotopes, and density come together to produce a strong case for the Moon forming by a giant impact.