Condensing the Moon from a MAD Earth

Abstract:

The favored theory for lunar origin is the giant impact hypothesis, where a protoplanet collides with the growing Earth and creates an orbiting disk of material that forms the Moon. However, the astonishing isotopic similarity between the Earth and Moon cannot be explained by current giant impact models without appealing to highly specific circumstances. Here, we demonstrate that a condensation model for lunar origin, achieved via a previously unrecognized class of post-impact states, produces the Moon's major characteristics. The required class of post-impact states is defined by (i) a high degree of vaporization and (ii) rapid rotation. When these two criteria are met, the mantle, atmosphere and disk (MAD) form a dynamically and thermodynamically continuous structure that quickly mixes, thereby diluting initial compositional heterogeneities. Then, partial condensation from the pressure-supported mass beyond the Roche limit produces a Moon that is isotopically similar to the bulk silicate Earth and depleted in volatile and moderately volatile elements. Initially, the condensed liquid is composed of silicates. As the structure cools, metal exsolves in the accreting Moon and moonlets. We calculate ~2wt% metal is exsolved from a bulk silicate Earth composition, which is consistent with estimates of the mass of the lunar core. Thus, similar tungsten isotopes are established in the Earth and Moon as metal is exsolved in both bodies after mixing. In our model, the criterion for lunar origin shifts, away from specific impact parameters that inject terrestrial material into orbit, to any collision that transforms the Earth into a rapidly rotating and substantially vaporized MAD planet. Impacts that can transform the Earth are common during the end stages of planet formation. Therefore, the characteristics of our Moon are a natural consequence of forming the Earth.