Home / Node / Sarah Stewart Colloquium Abstract (Sept 16, 2015)

Sarah Stewart Colloquium Abstract (Sept 16, 2015)

The Origin of the Moon from a MAD Earth

In the giant impact hypothesis for lunar origin, the last giant impact onto the proto-Earth generated a circumterrestrial disk that accreted to form the Moon. In the canonical version of the model, the impact also set the present-day angular momentum (AM) of the Earth-Moon system. However, the canonical model has failed to explain the near isotopic identicality between the Earth’s mantle and the Moon. A greater degree of mixing between the impactor and proto-Earth is possible during high-AM, high-energy giant impact events, but efficient mixing during the event requires highly specific impact conditions. In the high-AM cases, the present-day AM was established by a subsequent lunar orbital resonance. At present, current giant impact models do not easily explain the astonishing isotopic similarity between the Earth and Moon.

I will present a new model for lunar origin, where the Moon is a partial condensate of the bulk silicate Earth. The Moon’s major characteristics can be explained by condensation from a previously unrecognized class of post-impact states. The post-impact planet possesses a continuous pressure- and rotationally-supported fluid-to-vapor structure that spans the mantle, atmosphere and disk (MAD). The transient MAD state is unlike the current physical states of any planets in our solar system and fundamentally different than the canonical post-impact state. The continuous MAD structure quickly mixes, 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. 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 were common during the end stages of planet formation. Therefore, the characteristics of our Moon are a natural consequence of forming the Earth.