The Origin of the Moon Within a Terrestrial Synestia

Abstract

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, which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super‐corotation‐limit 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 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 bulk silicate Earth 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.

Accretion of the Earth—Missing Components?

“…The similarity of Earth and the Moon in nucleosynthetic isotope compositions can be exploited to constrain the origin of the hypothetical impactor Theia that collided with the Earth to form the Moon. Recently, a new variation of the giant impactor scenario has been proposed, in which the Moon formed from an originally completely vaporized accretion disk that fully equilibrated with the silicate Earth (Synestia; Lock et al 2018). In such a scenario, tracing of the impactor material using nucleosynthetic isotope anomalies is not possible.…”

Thermal and magnetic evolution of a crystallizing basal magma ocean in Earth's mantle

“…Exsolution mechanisms explicitly occur across a metal-silicate interface that is liquid on both sides (Badro et al, 2016(Badro et al, , 2018, thus invoking a longlived basal magma ocean (BMO) atop the core (Labrosse et al, 2007;Laneuville et al, 2018) which would initiate at mid-mantle depths and crystallize downwards to the core. A giant impact as large as one suggested to lead to the formation of the Moon may have been energetic enough that Earth's initial condition was completely molten (Canup and Asphaug, 2001;33Ć uk and Stewart, 2012;Lock et al, 2018), however the initial depth of an emergent BMO is subject to uncertainty in the equation of state of lower mantle composition, its melting curve, the adiabatic gradient as determined by its material properties, and the dynamics of phase separation (Stixrude et al, 2009;De Koker and Stixrude, 2009;Boukaré et al, 2015;Boukaré and Ricard, 2017;Wolf and Bower, 2018;Caracas et al, 2019). The scenario of whether the BMO, if electrically conductive enough, could be capable of generating a dynamo was explored as a potential mechanism for providing a magnetic field during the early Earth (Ziegler and Stegman, 2013).…”

A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system

“…Given the relatively large mass of planet c, it seems unlikely that there would have been sufficient time for such a scenario to operate, or that it may have occurred without the eccentricities of the lower-mass planets becoming large enough to destabilize the system. Giant impacts are thought to have occurred in our Solar System and have been invoked to explain the composition of Mercury 27 , the origin of the Earth-Moon system 28 and the high orbital obliquity of Uranus 29 . We have shown that they likely occurred in the exoplanetary system Kepler-107 and shaped the compositional properties of its two inner planets.…”