Sarah Stewart (Dept. of Earth & Planetary Sciences)
In the standard model for planet formation, Earth accreted via a series of giant impacts and the terminal giant impact produced the Moon and fully melted the Earth. The Moon and Earth are identical in multiple isotope systems that show significant variations between most meteorite groups and planetary bodies. Thus, the simplest explanation for the isotopic similarity is that the Moon and Earth’s mantle have a common origin. In contrast, canonical giant impact simulations find that the lunar disk is predominantly (>60 wt%) composed of material originating from the impactor, which should have had a different isotopic signature. Complete post-impact isotopic equilibration by mixing between the molten Earth and the boiling proto-lunar disk has been proposed as a means to remove an initial compositional difference. However, recent measurements of multiple isotope systems in samples from the deep mantle demonstrate that the early Earth was not completely mixed and preserves chemical heterogeneities established during Earth’s accretion.
Previous Moon-formation studies assumed that the angular momentum after the impact was similar to present day, but N-body simulations of the growth of Earth-mass planets typically find higher spin rates at the end of accretion. I will present a new model for the origin of the Earth–Moon system. A late giant impact onto a fast-spinning proto-Earth can produce a disk that is massive enough to form the Moon and composed primarily of material from Earth, but the system would have had more angular momentum than today. Subsequently, the excess angular momentum can be lost during tidal evolution of the Moon via a resonance between Earth’s orbital period and the period of precession of the Moon’s perigee. The impact energy is primarily deposited in the impacted hemisphere, and the mantle of the post-impact Earth is stably stratified, which would inhibit immediate deep convective mixing. Hence, the Moon-forming impact need not destroy pre-existing chemical heterogeneities in the deep mantle of the proto-Earth. Finally, I will discuss ways to test the new Moon formation model.