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Home , KREEP, Orbit of the Moon

Sam Bell The Darby Dyar Schmitt (1991)

Schmitt (1991) presents in a very comprehensive and clear way the history of lunar formation as known purely from the Apollo and Luna results. While he definitely succeeds at this, there are two places where he disagrees with modern theories of the Moon without presenting the main evidence for those theories (which does not come from Apollo results). But it is definitely a very good paper overall. The bulk of the paper consists of Schmitt breaking lunar history into seven sections: In the first section, he claims that the Moon formed by coaccretion with the (in contrast to the now-dominant impact hypothesis). The next section is the phase, where an ocean of silicate rock of ultramafic composition covered the Moon. Lighter elements floating to the top of the magma ocean formed a crust of ferroan anorthosite (pure CaAl2Si2O8 with small amounts of iron), and heavier crystals sinking to the bottom of the ocean formed an ultramafic . The last layer to crystallize is known by the acronym KREEP because it became enriched in , rare earth elements, and . The third stage that Schmitt (1991) picks out is a period from roughly 4.4Ga to roughly 4.2Ga, when the anorthosite crust was saturated with early craters. The next stage lasts until roughly 3.9Ga and consists of the formation of the oldest large impact basins and the crystallizing of the KREEP fluid layer. Some of the KREEP is thought to have intruded into the anorthosite crust. After this, Schmitt (1991) has the formation of the younger large basins from 3.9Ga to 3.8Ga. Controversially, he argues against the then dominant and now all but proven hypothesis that the Moon underwent a “lunar cataclysm,” a ferocious spike in the cratering rate around 3.8 or 3.9 Ga—an idea that is now almost fully accepted under the less exciting name of . The sixth period that Schmitt (1991) gives us is eruption of the basaltic maria. This stage included primarily flood and shield volcano activity, although there were a few pyroclastic (more explosive) volcanoes—as evidenced by the orange glass beads that the author found in his fieldwork on the Moon. The final stage, which occupied the last three eons (billion ), consisted of minor modification of the lunar crust through scattered and sporadic impacts, continued formation of the regolith, and the appearance of mysterious bright swirls. Although his discussion of lunar history is certainly very comprehensive and detailed, he does dismiss two theories that have turned out to be right and were widely accepted in 1991. The first of these is the giant impact hypothesis of lunar formation. Schmitt (1991) objects to this theory on the grounds that the source material for the orange glass beads that he found during the Apollo 17 mission was too rich in volatiles and parentless Pb to be explained by the giant impact hypothesis. This argument is somewhat absurd. Schmitt is using inferences about the parent material of one very rare sample to object to a theory that Hartmann (1986) built on overall volatile depletion, overall iron depletion, conservation, oxygen isotope similarity between the Earth and the Moon, and several other lines of evidence. While it is true that Schmitt et al. (1991) does state that it is only using the Apollo evidence, it is somewhat tacky to object to a popular theory without engaging the main thrust of the argument. The second currently accepted theory that Schmitt (1991) disagrees with is the idea of a lunar cataclysm, or as it is now called, the Late Heavy Bombardment (LHB). Early observations of the Moon coupled with dating of the Apollo samples reached the conclusion that there was a

1 massive spike in the impact flux around 3.9-3.8Ga. This was later confirmed on and Mercury. However, Schmitt (1991) lists five objections to this idea: First, he argues that a half-billion spike in cratering would be odd. Although this is probably true, it is certainly not enough to override the empirical evidence. As it turns out, the post-1991 theory of planetary migration provides a perfectly reasonable explanation for this. According to Gomes et al. (2005), Jupiter and Saturn originally formed much closer together and gradually migrated apart into their current . Part of their migration was through a 2:1 mean-motion (this basically means that Saturn makes two orbits for every one Jupiter makes). While these two were in resonance, their obits rapidly became very eccentric because both planets made their close encounter at the same part of the orbit each time (every other time for Jupiter). At each close encounter, the of the other pulled each planet into a slightly different orbit. Normally, these variations occur relatively randomly and more or less cancel each other out, but in a 2:1 resonance, each encounter builds on the previous one, distorting the orbit further and further. The new very eccentric orbits of Jupiter and Saturn destabilized the entire . This violently churned up the protoplanetary disk, hurling the planetessimals into far-flung orbits, casting them into the , and generally mixing them around. This, of course, dramatically increased the number of impactors sweeping across the and other planetary bodies, causing the LHB. The next two arguments stem from a misreading of a then outdated formulation of the argument by Tera et al. (1974): Schmitt (1991) claims that the argument tries to compress all the impact basins into this cataclysm, but the only one that Tera et al. (1974) actually associate with this event is Imbrium, although they do claim that others may be related. Schmitt (1991) then gives two reasons why this shouldn’t have happened. The final two arguments are based purely off the Apollo and Luna evidence. Schmitt (1991) points out that some pre-cataclysm impact melts exist—something the LHB theory certainly doesn’t deny. His final argument is that the Apollo and Luna missions mostly landed in the area most affected by cratering around the age of the cataclysm. It is unclear what he means by this—the Apollo and Luna landing sites cover a good deal of the lunar near side. And the modern near side should actually be less affected by the cataclysm than the modern far side because mare eruptions covered up many of the older rocks. Furthermore, the conclusive evidence for the LHB does not come from Apollo samples per se, although this is the central piece of evidence presented in Tera et al. (1974). The evidence comes from remote observations. These remote observations enable astronomers to date craters by counting the number of craters superimposed on the original crater (yes, this method is calibrated by Apollo and Luna dating of features, but Schmitt (1991) uses these dates in his paper and clearly accepts them). Although numerous papers have observed the LHB in the crater record of planets with unmodified crust old enough to show it, Wetherill (1974), which looked at the Moon, Mercury, and Mars, appears to be the first. While technically Schmitt et al. (1991) only focuses on Apollo and Luna data, it is a little absurd to criticize a theory based mainly off post-Apollo data without discussing those data. That being said, the paper does lay out lunar history in a very clear and detailed fashion, and given the author’s service as an Apollo astronaut, he should be permitted a little preference for Apollo results. Schmitt et al. (1991)’s objections to theories later proven right do not change the fact that it was a very good paper overall.

2 Gomes et al. “Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets.” Nature, vol. 435, pg. 466, 2005.

Hartmann. “Moon Origin: The Impact-Trigger Hypothesis.” In Hartmann et al. . Houston: LPI, 1986. From the 1984 Conference on the Origin of the Moon, Kona, Hawaii, 1984.

Schmitt. “Evolution of the Moon: Apollo model.” American Mineralogist, vol. 76, pg 773, 1991.

Tera et al. “Isotopic Evidence for a Terminal Lunar Cataclysm.” Earth and Planetary Science Letters, vol. 22, pg. 1, 1974.

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