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Science, Maths & Technology

Multiple Moonlets Maketh Mystery

Updated Wednesday 5th December 2018

How did the moon form? Forget a single giant impact – relentless bombardment could explain the Moon’s formation, says Open University research student Zoe Morland.

The Moon is the most extensively studied planetary body outside of Earth. Despite this, its formation mechanism is still a mystery.

Past theories

Illustration of asteroid bodies in a police line up being judged by Earth and Moon Copyrighted image Icon Copyright: Kay Morland The search continues for the culprit or culprits responsible for the formation of the Moon. 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.
  • Copyrighted image Icon Copyright: Graph redrawn by Zoe Morland adapted from Earth Planet, Sci., doi.org/10/ddvg8g Fig. 1: Comparison of the oxygen isotope ratios between different bodies showing how they can be differentiated. The Moon and Earth plot along the same line. 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. 

Copyrighted image Icon Copyright: Images created with Universe Sandbox Fig. 2: Simple artistic step-by-step representation of the collision hypothesis model for the formation of the Moon over approximately 10s to 100s of thousands of years using Universe Sandbox. a) Mars-sized object (Theia) impacts a hot early-Earth at 45 degrees, within the first 100 million years of the solar system. b) initial impact displaying immense heating from the compression of Theia, causing vaporisation of Theia and early-Earth material, c) impact knocks the early-Earth titling its axis of rotation (changing its obliquity), excavation begins as vaporised and solid material is ejected from the impact site, d) ejected solid fragments and vaporised material spread further from the impact site, e) vaporised material dissipates and cools leaving larger ejected fragments to spread from the impact site, f) ejected material organises into a debris disk surrounding the Earth, g) accumulation of hot debris material into a hot early-Moon with a magma ocean, orbiting around a hot modified earth.
This theory could account for the core constraints as well as more specific characteristics of the current Earth-Moon system, which were unexplained by the pre-Apollo theories:
 
  • 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.
 
As time passed and observations of the Earth-Moon system swelled, the Earth-Moon system that needed to be created by models became very precise. A consequence of making the outcome of a model so specific is that it becomes very unlikely to achieve. In actual fact the probability of success reduced to ~1-2%.
 
The harder you try the harder it gets Much like, if you were boiling an egg the only detail you need to worry about was the length of time the egg was boiling. Under those circumstances you are likely to get the perfect egg. However, you then decide to make a soufflé, which requires a wider range of ingredients, advanced skill, and the correct execution of multiple cooking steps. In this situation there are many more ways in which you can get the recipe wrong, which would result in a floppy soufflé. This shows that when you make a situation more complex, it increases the number of ways you can get things wrong meaning you are less likely to reach the perfect outcome. However, when reached, this outcome could be much more delicious than the simpler version.
 
With this in mind, as scientists strove to prove the hypothesis with more accurate modelling, the hypothesis has slowly begun to disprove itself and become unrealistic. The more constrained the original conditions must have been in order to successfully create the correct Earth-Moon system by the collision hypothesis, the less likely it is that these conditions would have actually occurred.
 
This caused scientists to rethink their strategy for generating a mechanism for the Moon’s formation. They had to retrace back to consider how the debris cloud that surrounded the early-Earth after a giant impact would have realistically acted, with further acknowledgement that a single event may not be the complete solution.

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

In fact, it was Ringwood, the founder of the later disproved binary formation model, who exposed the flaws in the giant impact hypothesis and attempted to reimagine the theory. He stated in his 1989 paper “Flaws in the giant impact hypothesis of lunar origin” that in order to satisfy the conservation of high angular momentum “a giant impact occurr[ed] after the Earth had accreted to ~70% of its present size… but [this] did not itself produce the Moon.” He later goes on to say instead that “[c]ollisions by much smaller high velocity [bodies] at a very late stage of accretion were probably responsible for ejecting… material from the Earth’s mantle” which later coalesced into the Moon. This therefore, lead to the basis of the multiple-impact hypothesis for the formation of the Moon. However, several decades passed with the idea not being explored hence, the single giant impact hypothesis continued as if almost uncontested.
 
Only recently has more contemplation of this idea taken place to bring this hypothesis to the attention of the public eye. Last year Rufu, Aharonson and Perets presented their results for simulations they ran into the feasibility of this hypothesis and generated an eloquent visual representation of the process.
 
The hypothesis states that a sequence of bodies, between the Moon and Mars in size, impacted the early-Earth in several million-year intervals, generating debris disks that coalesced into moonlets. These moonlets migrated away from the early-Earth due to tidal forces.
 
Multiple impacts may be a more plausible idea Once settled in this orbit, the moonlets are joined by subsequent moonlets generated from successive impacts into the early-Earth. The close proximity of the formed moonlets causes them impact one another resulting in material either being expelled from the system or coalescing into larger moonlets. It is thought that this interaction with enough moonlets was sufficient to create the Moon we observe today.
 
This model has improved on the previously mentioned issue where models have become too complex, which makes their desired outcome almost impossible to achieve. When attempting to generate an Earth-Moon system with the perfect balance of mass, composition and angular momentum, the contribution of additional impacts into the scenario makes accomplishing the correct result much more likely. This is because the final outcome of the model is now a combination of multiple impacts of different masses, velocities and directions. This means numerous combinations of different impacts can reach the same perfect conclusion using the multiple-impact model, in comparison to a limited set of scenarios that yield the perfect conclusion when using the single giant impact model.
 
Recent modelling suggests the multiple impact hypothesis has “tens of per cent” probability of creating the Moon, which is a massive improvement on the ~1-2% success rate of the single giant impact hypothesis.
 
This model has also been favoured due to it explaining numerous recent findings, such as, entropy calculations, which regulate the number of volatiles and silicates, and also variations in 182W/184W ratios and noble gas in current Earth samples, which suggest regions Earth’s mantle are unmixed. These controls agree well with the multiple impact hypothesis and cannot be accounted for by the single giant impact hypothesis.
 
It is clear from the recent work being carried out on this topic, that the multiple-impact hypothesis solves many issues that arose after the giant impact hypothesis and it presents a more reliable mechanism for the formation of the Moon.
 
However, we must not be naive enough to believe that just because this theory is currently the least problematic, there is still a possibility that future findings could completely disprove it. This highlights an important cycle within science: a question is asked, hypotheses are designed, they are disproven by new findings creating the need once again for new hypotheses. This topic is a clear representation of how many times this cycle is repeated within the scientific community and how truly unsure we are about the vital events in history.
 
Already, variations on the hypothesis are being proposed. For example, the suggestion that an initial very large impact could have instead generated a synestia, rather than a debris disk. This is “a huge spinning, donut-shaped mass of hot… mostly vaporised rock, with no solid or liquid surface.” (see video below).
 

Clearly this hypothesis is a work in progress, however according to how well it accounts for the observations we currently have of the Earth-Moon system, it is likely that this hypothesis has the best chance of replacing the single giant impact hypothesis! Watch this space…
 
 

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