The origin of the Moon within a terrestrial synestia (1802.10223v1)

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 (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.

• The paper proposes a novel model where the Moon formed within a terrestrial synestia following a giant impact, offering a solution for the observed isotopic similarities between Earth and the Moon.
• Simulations show that as the synestia cools, silicate vapor condensation within its photosphere leads to the accretion of a lunar-mass moon, ensuring isotopic equilibration with Earth's bulk silicate vapor.
• The model suggests synestias could be common in early planet formation and explains volatile depletion and isotopic composition, though boundary layer dynamics and accretion efficiency need further study.

The Origin of the Moon within a Terrestrial Synestia

This scholarly article presents a novel model for the origin of the Moon, positing that it was formed within a terrestrial synestia following a high-energy, high-angular momentum giant impact. Traditionally, the giant impact hypothesis has suggested that the Moon resulted from a colossal collision, but the isotopic similarities between the Earth and the Moon have been challenging to explain. This paper introduces the concept of a synestia—a distinctive planetary configuration exceeding the corotation limit (CoRoL)—as a potential solution.

The authors explore the creation and evolution of synestias, celestial bodies with sufficient thermal energy and angular momentum to distribute material in a continuous, non-condensed, supercritical fluid state, transcending traditional mantle and disk distinctions. Simulations demonstrate that when a synestia cools, significant pressure support sustains the fluid structure, permitting mass and angular momentum redistribution via rain-like condensation of silicate vapor. This condensation within the photosphere initiates the accretion of a lunar mass moon, which equilibrates isotopically with the bulk silicate Earth vapor.

Several calculations underscore the feasibility of this model. The simulations reveal that the majority of condensates form at high altitudes before migrating radially through the synestia, generating moonlets that accrete into a nascent Moon. This process happens in tandem with the synestia's contraction as vapor pressure support reduces with cooling. Moreover, the paper suggests that a synestia could have been a common occurrence at the terminal stages of terrestrial planet formation, given the prevalent high-energy impacts in the solar system’s nascent accretionary environment.

Key characteristics of this model are the equilibration of material between the Moon and synestia at high temperatures buffered by the silicate vaporization limit and tens of bars of pressure. This evokes a scenario where the Moon, acquiring its composition within this buffered environment, exhibits the isotopic congruence observed with Earth, inclusive of moderately volatile element depletion.

However, practical and theoretical challenges remain. The boundary layer dynamics at the moon's surface and the efficiency of condensate accretion require further exploration, together with the implications of rotational dynamics on mass transport in such thick vapor envelopes.

This paper posits a significant paradigm shift in lunar origin models, emphasizing the potential for a broader array of impact scenarios to create the requisite environment for lunar formation. It suggests a reevaluation of isotopic similarity explanations between Moon and Earth, proposing mixing during high-energy impacts, and identifies terrestrial synestias as catalyst stages facilitating the lunar formation within a few tens of years post-impact. The implications for understanding volatile retention, core formation, and isotopic composition in resultant early Earth and Moon systems invite further investigation and refinement through advanced simulation and theoretical modeling.