Origin of the Earth and Moon

Colloquium: Astronomy Colloquia | November 29 | 4 p.m. | 2 LeConte Hall

Speaker/Performer: Sarah Stewart, Harvard

Sponsor: Department of Astronomy

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 implications for core formation and Earth’s early atmosphere.

Event Contact: rhelgens@astro.berkeley.edu, 510-642-5275