The Moon is the most extensively studied planetary body outside of Earth. Despite this, its formation mechanism is still a mystery.
Hypotheses have bounced around the scientific community for decades, evolving under the influence of popular opinion. Early theories included: George Darwin’s 1879 fission model, in which a rapidly spinning early-Earth expelled material, possibly from the region now occupied by the Pacific Ocean, which later coalesced into the Moon; and the capture model, initially investigated by Gerstenkorn, H., 1955, claiming the Moon formed spatially distinct from Earth and was subsequently captured by Earth’s gravitational field. Without evidence to the contrary, Darwin’s theory prevailed and was still taught in schools during the Apollo era in the late 60s early 70s.
However, the acquisition and subsequent analysis of Apollo samples uncovered a crucial compositional similarity between lunar and Earth rocks leading to the capture model being abandoned. Moreover, advanced models were used to disprove the fission model as it appeared physically impossible. Therefore, a lack of reasonable hypothesis initiated a re-evaluation of the Moon’s formation. The demise of the past theories stresses the need for new ideas.
Ted Ringwood’s 1969 binary formation model (or “precipitation” hypothesis), was the first model to take into account the compositional similarity between the Earth and the Moon discovered from the Apollo samples. It stated that the Moon formed simultaneously as a sister planet to Earth. However, this model neglected to account for the current angular momentum of the Earth-Moon system and the presence of a very small iron core in the Moon.
In summary each past theory had its strengths; however, the exposure of endless plot holes and continuity errors have resulted in their catastrophic rejection.
Yet again, another hypothesis for the Moon’s formation was craved by the scientific community. However, this time it must satisfy the identified core constraints defined by current observations:
- Conversation of angular momentum - requiring the angular momentum for the system before the Moon’s formation to be equal to the angular momentum of the Earth-Moon system today.
- Structure of the Moon – the Moon has a much lower average density than the Earth, due to it having a very small iron core compared to its mantle, therefore the mechanism must account for a lack of iron present at the time of formation.
- Surface composition of the Moon – presence of an anorthosite (low density rock made of plagioclase feldspar and mafic minerals) crust on the surface of the Moon, likely to have derived from the crystallisation and subsequent floating of anorthositic rocks in a magma ocean to form an anorthosite crust on the surface. Therefore, a new hypothesis must result in the formation of a magma ocean.
- Matching oxygen isotopic signature – Differences in oxygen isotopic ratio trends are indicative of different planetary bodies, however it is observed that the Moon and Earth’s oxygen isotopic ratios plot on the same line compared to other planetary bodies (see Fig. 1). Therefore, a new hypothesis must ensure the formed Earth and Moon have similar isotopic ratios, which can be achieved through deriving from the same material and undergoing similar fractionation.
Through time hypotheses have increased in resolution and complexity attempting to satisfy the core constraints as well as finer more detailed constraints that have arisen to question each hypothesis as it passes. Since the Apollo missions only one hypothesis has stood the test of time, making it the current prevailing theory.
Giant impact Hypothesis
A single giant impact is the only theory to survive the Apollo era In 1975 Dr. William K. Hartmann and Dr. Donald R. Davis first presented the giant impact theory for the formation of the Moon. In a nutshell, they suggested that a Mars-sized body named Theia impacted the early-Earth generating a large volume of ejecta, which surrounded Earth as a disk to later coalesce and form the Moon we observe today (see Fig. 2). It is thought that this phenomenon occurred in the much busier early history of the solar system (perhaps within the first 60-100 million years), when impacts between planets clearing out their orbits were frequent in comparison to today.
- The Earth-Moon system’s high angular momentum - satisfied by the combination of the early-Earth and impactor’s mass and rotational velocity, as well as, the impact velocity and off-centre collision.
- The Moon’s depletion in iron - caused by the ejection of mostly mantle material from the early-Earth.
- The accretion of very hot material from the impact would generate a body of molten material, satisfying the constraint that the initial stages of the Moon had to be a magma ocean in order to derive the predominantly anorthosite crust we observe today.
- The Earth and Moon’s oxygen isotopic signature similarity, suggests they formed from similar material, which is satisfied by the mixing that would occur during a giant impact.
- Earth’s current obliquity (axial tilt) - caused by the large-scale off-centre impact knocking the Earth over slightly.
- The Moon’s depletion in volatiles (elements that easily evaporate into gases) - due to volatile escape under the high energies and temperatures of the giant collision.
- The Moon’s enrichment in refractory elements – due to these elements preferentially condensing into dust under the high temperatures of the collision, to later accrete to form the Moon.
What is tidal force? Mutual gravitational attraction of the moonlets and the early-Earth as well as their difference in rotational velocity cause an energy transfer from the early-Earth to the Moon. This results in very minor slowing of the early-Earth’s rotation and an increase in the moonlet’s orbital energy, which causes the moonlet to move further away from the early-Earth to conserve the energy balance. This tidal force reduces as the distance between the bodies increases, therefore the moonlets initially accelerate away from the early-Earth, then slow down and settle at a distance called the Hill radius.
Multiple impact Hypothesis