Isotopic Equilibration of Earth’s Mantle and the Moon Subsequent to the Giant

Introduction: Striking, mass-independent oxygen [1], chromium [2], and tungsten [3] isotopic similarities between the Earth's mantle and the Moon, which are readily distinguished from most other Solar System materials, provide a critical but troublesome test for models of lunar formation. In the context of the giant impact theory, the leading hypothesis for the origin of the Moon [e.g., 4, 5], these isotopic similarities were once thought to document formation of both the Earth and the impactor at similar heliocentric distances in an isotopically zoned solar nebula. However, current models suggest that terrestrial planet formation culminates with a period of major impacts between growing planets and planetary embryos, thought to sample a large radial zone of the nebula extending to beyond the radius of Mars [6, 7]. The giant impactor that formed the Moon, therefore, is unlikely to have originated at one AU, or to have had isotopic characteristics indistinguishable from the proto-Earth. Suggestions that the Moon formed from material ejected from the Earth's mantle by the impactor [11], or from mass-relative proportions of Earth and impactor, are incompatible with SPH models which overwhelmingly predict that 80% or more of the protolunar material originates from the impactor [see 5 and references therein]. In view of these considerations, one must conclude either that significant aspects of current models are in need of revision, or attribute important aspects of the Earth-Moon system to a rather large coincidence. A Novel Approach: Pahlevan and Stevenson [8] explored a very different potential solution to this problem: that the Earth and protolunar disk, largely molten but isotopically dissimilar in the immediate aftermath of the giant impact, were able to achieve oxygen isotopic equilibrium via exchange of oxygen through the shared, hot, dense, silicate vapor atmosphere that prevailed for a short time between the impact and lunar accretion [5]. Subject to radiative cooling with an effective photospheric temperature of 2000°K [9], Pahlevan and Stevenson [8] argue that the cooling timescale for the disk material, which essentially defines the time available for equilibration, can be as long as 10 2 to 10 3 y (as compared to 3×10 3 y for the Earth). In this context, they construct semiquantit-ative but compelling arguments that convection within the Earth, disk and atmosphere, as well as the liquid-vapor exchange process, proceed at rates which are sufficient to permit the equilibration to occur. They conclude by noting that the limiting step is likely …