Is the Composition of the Moon Consistent with the Giant Impact Hypothesis?

Details Is the Composition of the Moon Consistent with the Giant Impact Hypothesis? Is the Composition of the Moon Consistent with the Giant Impact Hypothesis?

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957q..45t&link_type=abstract

Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N

Computer Science

Impactors, Lunar Composition, Moon, Planetary Evolution, Planetary Nebulae, Solar System, Collisions, Chromium Isotopes, Magnesium Isotopes, Potassium Isotopes, Stochastic Processes

Scientific paper

Current versions of the giant impact hypothesis for the origin of the Moon call for the impact to occur when the Earth is about two thirds of its present size. The material in the Moon is derived mostly from the mantle of the impactor, although <10% of the impactor finishes up in the Moon. The metallic core of the impactor accretes to the Earth. Accretion of the last third of the Earth occurs subsequently to lunar formation. The Moon suffers net erosion rather than accumulation during this stage, consistent with the observation that there is no sign on the Moon of a late veneer that is postulated to have contributed water, volatile elements, and the excess siderophile signature to the terrestrial mantle. Relative to the Earth or the primitive solar nebula, the Moon has a strong depletion of volatile elements, water, and a probable enrichment of refractory elements (e.g., Ca, Al, Ti, REE, U, and Th), the latter observation based on heat-flow data, seismic velocity profiles, high near-surface concentrations, and the presence of a thick aluminous crust. The bulk lunar REE patterns show no Eu, Yb, or Ce anomalies based on volatility in contrast to those in CAI. The volatile elements are depleted in order of their condensation temperatures, but the refractory elements are present in the bulk Moon in solar nebular ratios, indicating that the material now in the Moon was not subjected to temperatures in excess of -1500 K. The lunar composition shows little terrestrial mantle. For the major elements, there is no correlation based on volatility between the composition of the terrestrial mantle and that of the Moon (both volatile FeO and refractory CaO are enriched). The absence of K isotopes rules out evaporative loss of K by Rayleigh type distillation processes. Magnesium isotopes likewise are normal. When did the loss of volatile elements occur? hat involve Rayleigh-type condensation from vapor are ruled out by the lack of fractionation in the REE and the absence of K-isotopic anomalies. Although volatiles were presumably lost during the giant collision, the lunar initial Sr-87/Sr-86 ratio (LUNI) is not distinguishable from BABI, indicating that Rb was also depleted close to TO. Current theories for planetary formation deplete the inner solar system in gases, and ices, including water and volatile elements close to TO due to early intense solar activity. As the impactor was clearly derived from the inner solar system as shown by the O and Cr isotopes, the possibility exists that the impactor was a primitive volatile depleted, dry body from the inner nebula before it collided with the Earth. Thus some of the lunar chemistry may reflect that of the primordial inner dry solar nebula. It would be ironic if the Moon retained some imprint of the primitive inner solar system. Possible compositions for the impactor include an body with an Fe-rich mantle, intermediate between that of Earth and Mars, but with a low Rb-Sr ratio and a low volatile content inherited from the inner nebula. However, the material now in the Moon is derived in a stochastic event from only a very small (< 10%) of the original impactor mantle and so is not necessarily representative of the bulk composition of the impactor mantle, The one-off nature of the lunar-forming event that produced a unique object makes it difficult to discern in detail the previous history of the lunar material.

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