  The giant impact hypothesis remains the leading theory for lunar origin.
However, current models struggle to explain the Moon's composition and isotopic
similarity with Earth. Here we present a new lunar origin model. High-energy,
high-angular momentum giant impacts can create a post-impact structure that
exceeds the corotation limit (CoRoL), which defines the hottest thermal state
and angular momentum possible for a corotating body. In a typical super-CoRoL
body, traditional definitions of mantle, atmosphere and disk are not
appropriate, and the body forms a new type of planetary structure, named a
synestia. Using simulations of cooling synestias combined with dynamic,
thermodynamic and geochemical calculations, we show that satellite formation
from a synestia can produce the main features of our Moon. We find that cooling
drives mixing of the structure, and condensation generates moonlets that orbit
within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE)
vapor. The moonlets and growing moon are heated by the vapor until the first
major element (Si) begins to vaporize and buffer the temperature. Moonlets
equilibrate with BSE vapor at the temperature of silicate vaporization and the
pressure of the structure, establishing the lunar isotopic composition and
pattern of moderately volatile elements. Eventually, the cooling synestia
recedes within the lunar orbit, terminating the main stage of lunar accretion.
Our model shifts the paradigm for lunar origin from specifying a certain impact
scenario to achieving a Moon-forming synestia. Giant impacts that produce
potential Moon-forming synestias were common at the end of terrestrial planet
formation.
