Last year I wrote a review on Lunar Origin. If you want the answer in one sentence, this is it:
The formation of our Moon is unknown.
Is it just me or is that quite embarrassing? Here is the breakdown:
The Giant Impact Hypothesis – a leading model on Lunar Origin that is not empirically correct.
Taylor’s Axiom – The best models for lunar origin are the testable ones.
Taylor’s Corollary – The testable models for lunar origin are wrong.
-S.Ross Taylor, Paraphrased by Sean Solomon, The conference on the Origin of the Moon, Kona, 1984.
Since the promising Apollo programme ended, an empirically correct conclusion to the formation of our Moon has not been agreed upon. This is owed to the attempt of scientists to reconcile both dynamical and geochemical evidence (Canup, 2013) by creating complex and implausible models (Canup 2012; and Ćuk Stewart, 2012; Reufer et al, 2012). This has led to a wavering popularity of the original impactor hypothesis. The isotope crisis (Canup, 2013; Taylor, 1986; Battisti et al 2015; Canup, 2012; Clery, 2013; Drake, 1986; Turcotte and Kellogg, 1986.) and the fortuitous coincidence for impactors with Earth-like compositions, awards speculation to whether the Giant Impact Hypothesis should be regarded as the leading modern theory on lunar origin.
Validation of the Giant Impact Hypothesis lies in the geochemical analysis of the Mars-Phobos system and in a mission to Venus (Canup 2013) – not the continual attempt to model events that require special circumstances.
The current leading theory on the origin of the Moon was conceived by William Hartmann and Donald Davis and presented in 1974. The original Giant Impact Hypothesis states that late in the Earth’s formation, a smaller planetary body collided with the Earth in such a direction relative to Earth’s rotation, that abundant amounts of rocky debris were blown into orbit, which eventually coalesced (by accretionary processes) to form the Moon. This model complies with a number of dynamical, geochemical and geophysical constraints and has adopted the title of the leading modern theory.
The 1984 conference about lunar origin held in Kona, is often referred to as ‘a paradigm-shifting, pivotal meeting in the history of planetary science.’ This meeting successfully eliminated out-dated and impossible theories surrounding the lunar origin; nonetheless, it initiated an undesired complexity in the attempt to achieve an empirically correct model of the Moon’s formation.
It is paramount to define how accurately the original Giant Impact Hypothesis accounts for the constraints generated by meticulous science, in order to contemplate why the theory is titled as leading and modern.
In many cases, the hypothesis does not apply to new constraints and requires a remodelling of the sequence of steps that created our Moon. Conversely, more complex models equate to an increased likelihood of implausibility. The original hypothesis under scrutiny proves it is not an empirically correct answer to lunar origin, and more research is needed to solve the genesis of our Moon.
Evidence and Interpretations
The challenge is to provide a single solution for both the Earth and Moons dynamics and their chemical similarities and discrepancies.
Nonetheless, the myriad of facts surrounding these similarities and differences cannot be avoided, which calls for new, simple and common models altogether (Canup, 2013; Canup, 2012; Ćuk and Stewart, 2012; Reufer et al 2012).
An earlier account of an impactor by Reginald A. Daly (1950) was crowded by three, now seemingly preposterous, theories.
(i) Co-accretion of a sister body orbiting Earth (which seemed ruled out by the Moon’s low bulk density and lack of an iron core); (ii) capture (which might explain the lack of iron if the moon formed far away, but was difficult to achieve, and eventually rejected due to the Moon’s isotopic near-equality with Earth); and (iii) George Darwin’s idea of fission (which was generally dismissed as ruled out by angular momentum considerations.) (Hartmann, 2014)
The Kona conference (1984) first saw the Giant Impact Hypothesis as a potential solution to lunar origin. Research provides constraints to eliminate impossible or implausible explanations for lunar origin.
- Dynamical Constraints (Original Impactor Theory)
A.P. Boss and S.J. Pearle (1986) divide theories into six categories and conclude the only mechanism that is not deemed impossible by dynamical constraints, is the Giant Impact Hypothesis; consequently the Giant Impact Hypothesis emerges as the most plausible model for lunar origin simply due to its lack of flaws. The authors suggest that “[the] model is relatively new and has not been extensively developed nor thoroughly criticized,” which is interesting as the current model lacks development despite its thorough criticism; yet lends itself to be called the ‘modern leading hypothesis,’ suggesting accuracy.
- Geochemical Constrains (Original Impactor Theory)
A valid hypothesis must comply with a vast majority of constraints offered by geochemical analysis (figure 1); the Giant Impact Hypothesis does this, yet only to a certain extent.
Figure 1: Geochemical data allows Fission to be completely eliminated from the probable theories of lunar origin. Intact and disintegrative capture are highly unlikely. Binary accretion is viable yet Collisional Ejection is the only hypothesis that fits the criteria for dynamical and geochemical constraints, despite not satisfying all constraints and being an ambiguous hypothesis. (Modified from Drake, 1986 in Origin of the Moon.)
The work (figure 1) concludes that despite the development of the Giant Impact Hypothesis, an unambiguous, empirical solution is yet to be attained. In 1984 it was right to be titled the leading modern theory, despite its uncertainty.
- The Theory under Scrutiny
The manned Apollo mission returned samples rich in chemical and isotopic information to potentially validate the Giant Impactor theory. However, recent findings make headline news (Howard, 2015; Clery, 2014; Clery, 2013) outlining uncertainty about the accuracy of the Hypothesis.
The findings posed queries as to whether all the necessary conditions occurred simultaneously to comply with the discovery of compositional constraints. (Canup, 2013.) The issue began concerning the bulk proportions of the Moon (Drake, 1986; Canup, 2012; Ćuk and Stewart 2012), whether it is composed mostly of projectile (Cameron, 1986), or target (Ringwood, 1979; Wanke and Dreibus, 1984) or an equal mix of both (Melosh and Sonett, 1986). Originally, the Hypothesis was clearly consistent with the concept that Earths mantle made up most of the Moon’s geology, which accounted for the Moon’s chemical similarity to Earth (Taylor, 1986). Recently however, complex numerical simulations show that the vast majority of the Moon derived from the impactor, and that to achieve a Moon with mostly Earth’s mantle requires another impact after the giant impactor event, to remove the evidence of a first event (Ćuk and Stewart, 2012). This depends on special circumstances and initiated the dawn of uncertainty surrounding the theory.
The ‘fortuitous coincidence’ for the impactor being chemically similar to earth (Taylor, 1986) has not been accepted as solution to the isotope crisis. It remains to be demonstrated that the same conditions (target/projectile ratios or thermal processing) can satisfy all geochemical constraints.
- Isotopic Geochemical Complexity
Isotopes, unlike elements, are not significantly fractionated from each other via magmatic processes; hence erupted melts should record the composition of deeper sources (T. Elliott 2012). From the Kona conference (1984) to 2013, L. Elkins-Tanton (2013) states how recent work could have undone previous study due to the fact that emerging constraints are proving impossible for the original large impactor hypothesis to meet. This is down to the ever-increasing fact that the Moon and the Earth are more similar than different (Canup, 2013). These recent constraints are summarised by A. Halliday (2012),
First, the Moon formed relatively late given its size. Tungsten isotopic data require that the Moon formed more than 30 million years after the start of the solar system, whereas most objects this size are predicted to have formed in the first few hundred thousand years. Second, the oldest rocks appear to have formed from a magma ocean, implying an intensely energetic fiery start at a time when heat-producing, short-lived nuclides (26Al and 60Fe) were extinct. Third, the oxygen isotopic composition of the Moon is identical to that of the Earth to within 5 parts per million, whereas that of nearly all asteroidal and planetary objects are different.
An isotopic dissimilarity is expected, yet not found between the Earth and Moon.
A theory posed by Pahlevan and Stevenson (2007) suggests the debris disk mixed to a homogeneous composition prior to coalescing to form the Moon. This would account for the Earth-Moon similarities, and would not disrupt geophysical and dynamical constraints.
Logically, volatile elements will remain in the vapour/liquid phase longer as a consequence of their lower condensation temperatures. They, therefore, have a greater chance of reaching homogeneity via mixing, whereas refractory elements will not remain in the vapour/liquid phase as long due to their higher condensation temperatures and thus not reach a great a level of homogeneity (L. Elkins-Tanton, 2013). But geochemical data provides results that do not abide with this logic. Kato et al (2015) acknowledges the contrasting fates of volatiles and refractory elements between the Earth and Moon.
The data presented for the following elements (Oxygen, Zinc, Titanium and Silica) have been corrected for change associated with cosmogenic effects, radioactive decay, unusual chemical processes and the inheritance of nucleosynthetic anomalies. Data that has not been corrected is not included.
Oxygen is the third most abundant element in the solar system and documents fractionation processes in the solar nebula. This makes them viable indicators for understanding the evolution of solid and gaseous phases in the early solar system (H. Yurimoto et al 2004). Compositions of nearly all-terrestrial samples fall on the terrestrial fractionation line (TFL), whilst the majority of extra-terrestrial samples deviate from this (figure 2)
Figure 2: A diagram to represent the comparison of oxygen isotopic signatures from samples off of Mars, Vesta, the Earth and the Moon; the Earth and Moon correlate, which is an unusual phenomenon. (Modified from Greenwood 2013).
Interestingly so, lunar samples also fall on this line reflecting “mass-dependant fractionation from a single homogeneous source during chemical and physical processes that results from differences in masses of oxygen isotopes” (figure 3; H. Yurimoto et al 2004).
Figure 3: This figure indicates how all the values from lunar sampling fall on the terrestrial fractionation line (TFL), depicting terrestrial and lunar samples similarities. (Modified from Kevin Righter, NASA Johnson Space centre.)
Approximately 80% of the debris thrown into orbit originated from the impactor, (Canup and Asphaug, 2001) yet the concept that the impactor formed at equal heliocentric distances (Weichert et al 2001) is not consistent. The similarities of volatile oxygen isotopes between the Earth and the Moon are not a recent discovery. Michael Drake (1986) wrote a paper detailing this phenomenon yet Taylor (1986) created premature provisos about the impactor, stating “The impactor must form in the general vicinity of the Earth, to account for the similarity in oxygen isotopic signatures between the Earth and Moon.” Although this would account for the oxygen isotope similarities, this requires special pleading, and the likelihood is implausible. If the target and impactor did form at the same place in the inner solar system, they should have collided sooner (figure 4). The first provisos made more recently by A. Halliday (2012) states “…the Moon formed relatively late given its size. Tungsten isotopic data require that the Moon formed more than 30 million years after the start of the solar system, whereas most objects this size are predicted to have formed in the first few hundred thousand years.” This eliminates the assumptions generated by Taylor (1986).
Figure 4: This gives a perspective of relative timings after the origin of the Solar System. The Earth and Moon significantly differ in timing episodes; eliminating the possibility they formed in the same area of the inner solar system, at the same time. Their isotopic similarities remain ambiguous. (Taken from Mikhail, 2015).
More so, this theory would also be inconsistent with the formation of terrestrial planets by “stochastic collisions of embryos over much of the inner solar system” (Chambers, 2001). As a result, it leaves only Pahlevan and Stevenson’s (2005) theory of equilibration or a high angular momentum scenario as a plausible explanation of oxygen isotope similarities between the Earth and Moon.
Zinc is termed a volatile element due to its chemical properties of 50% condensation at 726K under solar nebula conditions. It is also a good indicator of the Moons evolution and growth as its isotopic fractionation during magmatic processes has negligible effects (Kato et al 2015).
Oxygen isotopes seem to fit to the currently unclear hypothesis of equilibration. Contrastingly, zinc (also a volatile) does not correlate with this, being highly refracted with uneven abundances in the Moon and Earth. Kato et al (2015) expresses the general truth that the depletion of volatiles on the Moon is not well understood and recently Chlorine and Zinc are shown to be highly depleted compared to those found on the Earth. Both Paniello et al (2012) and Kato et al (2015) confirm that although there is a general depletion of volatiles, the heavy isotopes of volatile elements like zinc are abundant on the Moon. This coincides with the satellite experiencing a specific event of volatile depletion. 66Zn and/64Zn are approximately 1.5‰ (Figure 5).
Figure 5: d66Zn Values for lunar rocks, the orange square showing the terrestrial mantle composition. The grey dashed box (a) represents the area of magnification shown in (b). Lunar mare basalts, alkali and magnesium suite samples are enriched in the heavy Zn isotopes where as the Ferroan anorthosites show isotopic variability. Overall, a heavy isotopic abundance in lunar samples (Taken from Kato et al 2015).
(Elliott 2012) also speculates that the mechanisms for this contrast in geochemistry between terrestrial and lunar samples remain unclear. For example, water is highly volatile compared to zinc yet is retained in some samples (McCubbin et al 2010). Nakajima & Stevenson (2015) discover that hydrodynamic escape cannot be used to explain the loss of water from the Earth-Moon system and suggest the volatile depletion must come from the precursor material (Kato et al 2015). The most reliable and representative rock type is that of the mare basalts, yet this model does not include their deprivation of volatile elements. Kato et al (2015) concludes that new potential model prospects must account for this, yet states that “continuous volatile depletion in lunar magma oceans is required to explain the differences in d66Zn between mare basalts and pristine lunar crustal rocks.” The complexity of the theory questions its right to be renowned as a leading modern theory.
Other work (Wieczorek, 2013; R. Canup 2013; Pahlevan and Stevenson 2007), suggests for evidence for a mixed homogenous composition. These conclusions were reached by observations that were partly made of the Moon from NASA’s gravity recovery and the Interior laboratory spacecraft coupled with the topography data from NASA’s lunar reconnaissance Orbiter (figure 6; Canup, 2013). The results reduced the predictions for the thickness of the Moons crust and its alluvium abundance, which suggests that the refractory elements – previously assumed abundant in the Moon – were similarly abundant in both the Earth and the Moon.
Figure 6: NASA’s gravity recovery and the Interior laboratory spacecraft coupled with the topography data from NASA’s lunar reconnaissance Orbiter.
Meier (2012) accordingly titles a paper “Earths Titanium Twin” and details that the Earth and Moon are more similar than models permit due to new, high precision analysis. Terrestrial rocks have 50Ti/47Ti ratios to within 0.0001% compared to meteorites with deviations of 0.05%. Considering that the majority of material that made up the Moon derived from the impactor, this similarity is surprising. More so, refractory elements such as titanium are predicted to not reach high levels of homogeneity in the Earth-Moon system due to their high condensation temperatures. The simplest solutions again refer back to special circumstances not empirically determined situations.
Many more isotopes represent similar, complex, findings to those presented above. Lunar formation models must include the later stages of the Giant Impact hypothesis to explain similarities and differences in volatile and refractive elements respectively. In conclusion, constraints must address whether the Giant Impact hypothesis can be revised or if new hypotheses are needed to account for the current, juxtaposing nature of volatile and refractory observations (Zhang et al 2012).
The original Hypothesis readily explains both the total angular momentum of the system and the depletion of a lunar iron core and density variation to Earth. It does not account for the isotopic homogeneity with silicate Earth. Pahlevan (2014) outlines two reasons for this,
First, potential Moon-forming impacts that leave the system with approximately its present level of angular momentum derive a major fraction of the proto-lunar disk (40-90%) from the less massive proto-planet. Second, the range of tracers that display isotopic homogeneity in the silicate Earth and Moon is such as to leave little doubt that the atoms composing these objects represent a single planetary reservoir.
A simple solution to this would be that the impactor contained the same chemistry as the Earth’s upper mantle, as if it had contrasting isotopic signatures, this would be apparent in its composition post-impact; this again requires special circumstances (Canup, 2013).
This implies additional mechanisms must have involved in the Giant Impactor Hypothesis to account for this isotopic discrepancy. Firstly, that convective mixing of the target and impactor created homogeneity and a single reservoir prior to the accretion phase of lunar formation. Secondly, that the Moon forming impact was of a high angular momentum event, which threw predominantly Earth-mantle material into orbit and the loss of angular momentum, was achieved by the later gravitational resonance with the sun (Pahlevan, 2014).
Due to the advance in analytical techniques, an array of observations provides evidence that the lunar and terrestrial mantle came from a single planetary reservoir, not a single nebula. The two situations applicable of creating homogeneity in the isotopic Earth-Moon system must be analysed in order to determine which, if any, of these scenarios accounts for the bulk composition of the Earth and Moon (Pahlevan, 2014).
Equilibration “requires convective mixing across its radial extent such that the proto-lunar disk can inherit the isotopic composition of the innermost disk (and post-impact Earth)” (figure 7; Pahlevan, 2014). The determination of the chemical transport processes will clarify the feasibility of this scenario.
Figure 7: This schematic details the scenario of convective mixing post-impact event. It is complex and not entirely feasible due to individual element characteristics and the abundances of volatile and refractory elements presented in scientific study. (Taken from Pahlevan and Stevenson, 2007)
Despite this, the example of the lunar abundances of volatile oxygen and refractory titanium and their homogeneity with the silicate Earth are complex. If the equilibration scenario was accepted, the mixing must also involve liquid-liquid mixing alongside the convecting vapour atmosphere, in order to abide by the geochemical evidence of volatile and refractory elements (Pahlevan, 2014).
The high angular momentum scenario, where there is a projection of Earths mantle directly into orbit, does not reply on complex mixing and can be argued to be more plausible if it can be proven the angular momentum was slowed due to resonance with the Sun.
Ćuk and Stewart (2012) break the mould of assuming a Mars-sized impactor. They suggest that impacts with a higher angular momentum become viable (including two simulations, which involve the debris disk composing of the vast majority of earth’s mantle) when acknowledging the resonant state between the Moon and Sun, “occurring when the lunar precession period matches one-year of earth’s orbit.”
If this persisted long enough, it could half the earth spin rate. Canup (2013) describes this scenario below:
The ‘fast-spinning Earth’ scenario, proposed by Ćuk and Stewart, invokes the collision of an object slightly smaller than Mars with an Earth that is already rotating with a 2–2.5-hour day owing to a previous large impact. Because Earth is spinning close to the critical rate at which it becomes unstable, the Moon-forming impact ejects part of Earth’s mantle into orbit, leading to a disk.
Canup (2012) defined another impactor scenario, presented in figure 8.
Figure 8: Canup (2013) models a collision between two planets of half-Earths size produce a final planet and a disk created of half the impactors composition and half the planets composition. This would account for the similarities between the Earth and Moon.
Canup is critical of her own theory, suggesting that though it is simpler than Ćuk and Stewart (2012) as it does not require a prior impact event, it still demands a large impactor so may even be less probable than the original canonical impact theory.
Although both models seem to meet the chemical constrains, they both rely heavily on the fact that the impactors iron core remains intact as it descends though the Earth’s mantle thus avoiding metal-silicate interactions.
In both scenarios, the conditions for making the Moon rely on an ‘improbable narrow range of conditions,’ therefore scientists are either missing some vital piece of knowledge, or the Moon forming event is a rare exception. Pahlevan (2014) states that samples from Venus or Mercury can place the composition of lunar samples into a constructive context and will aid the search for an empirically correct model of lunar origin.
Canup (2013) states, “No impact model stands out as more compelling than the rest” whilst L. Elkins-Tanton (2013) suggests “either scientists are modelling the wrong process, or that they are modelling the processes wrong.” Either way, this initiates the call for better models to satisfy all the constraints whilst excluding the plea for special circumstances. Only then will we be provided with an empirically correct model of lunar origin that justifies the title of a leading theory.
Progress must take the form of addressing the uncertainty within current models. These include understanding the scenarios of equilibration and a high angular momentum impact to answer the isotopic similarities of the silicate Earth and Moon. This is paramount to development as these processes potentially govern the chemical attributes and aid the evolution of understanding lunar origin.
The remodelling and dependence on fortuitous circumstances is not a systematic way to find the solution, Canup (2013) impresses that “current impact models are more complex and seem less probable than the original Giant Impact Hypothesis.” It is disconcerting that many models rely on a process post-impact to remove the evidence of the original event! It is paramount to strive for the simplest outcomes even though complex sequences of events do happen in nature (Canup, 2013).
To this day we assume that the impactor must have had a contrasting composition to Earth, based on our knowledge on Mars. Venus is the planet most similar to Earth (mass and distance from the Sun), however if the composition of Venus is similar to Earth, then the outlier would be Mars, and the impactors composition being similar to Earths, would not be unreasonable. As a result the Giant Impact Hypothesis would rightly be considered as the leading theory on lunar origin. As Pahlevan (2014) rightly suggests, the isotopic composition of extra-terrestrial bodies aids allows lunar samples to be set in a more constructive context.
The leading modern theory on lunar origin is not entirely empirically correct. The sequence of events that led to lunar origin must be assessed in order to achieve a stochastic model at the very least. Ad hoc assumptions have no place in this generation and scientists like Robin Canup are correct in calling for better models that do not rely on special circumstances and fortuitous coincidences. The reconciliation of dynamical and geochemical evidence is a long way from completion, yet attaining the knowledge to complete the story of the Moon’s formation is achievable. The future of the hypothesis does indeed lie within missions to Venus and understanding the Mars-Phobos system, as it seems we have exhausted material provided by the lunar missions, meteors and complex simulations.
Considering that we owe so much to our Moon – that its very formation determined the future evolution of planet Earth – it is embarrassing that we cannot understand its origin. ‘The Origin of the Moon’ (1986), begins with an axiom and corollary by S.R. Taylor that comically remain to be true in 2015. “The best models for lunar origin are the testable ones. The testable models for lunar origin are wrong.” In the same way, this review begins with these statements, emphasising the lack of development accomplished since 1984.
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