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.

During accretion, terrestrial bodies attain a wide range of thermal and rotational states, which are accompanied by significant changes in physical structure (size, shape, pressure and temperature profile, etc.). However, variations in structure have been neglected in most studies of rocky planet formation and evolution. Here we present a new code, the Highly Eccentric Rotating Concentric U (potential) Layers Equilibrium Structure (HERCULES) code, that solves for the equilibrium structure of planets as a series of overlapping constant‐density spheroids. Using HERCULES and a smoothed particle hydrodynamics code, we show that Earth‐like bodies display a dramatic range of morphologies. For any rotating planetary body, there is a thermal limit beyond which the rotational velocity at the equator intersects the Keplerian orbital velocity. Beyond this corotation limit (CoRoL), a hot planetary body forms a structure, which we name a synestia, with a corotating inner region connected to a disk‐like outer region. By analyzing calculations of giant impacts and models of planet formation, we show that typical rocky planets are substantially vaporized multiple times during accretion. For the expected angular momentum of growing planets, a large fraction of post‐impact bodies will exceed the CoRoL and form synestias. The common occurrence of hot, rotating states during accretion has major implications for planet formation and the properties of the final planets. In particular, the structure of post‐impact bodies influences the physical processes that control accretion, core formation, and internal evolution. Synestias also lead to new mechanisms for satellite formation. Finally, the wide variety of possible structures for terrestrial bodies also expands the mass‐radius range for rocky exoplanets.

In the giant impact hypothesis for lunar origin, the Moon accreted from an equatorial circumterrestrial disk; however the current lunar orbital inclination of 5 • requires a subsequent dynamical process that is still debated [1][2][3] . In addition, the giant impact theory has been challenged by the Moon's unexpectedly Earth-like isotopic composition 4, 5 . Here, we show that tidal dissipation due to lunar obliquity was an important effect during the Moon's tidal evolution, and the past lunar inclination must have been very large, defying theoretical explanations. We present a new tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modeling, we show that the solar perturbations on the Moon's orbit naturally induce a large lunar inclination and remove angular momentum from the 1 arXiv:1802.03356v1 [astro-ph.EP] 9 Feb 2018Earth-Moon system. Our tidal evolution model supports recent high-angular momentum giant impact scenarios to explain the Moon's isotopic composition [6][7][8] and provides a new pathway to reach Earth's climatically favorable low obliquity.The leading theory for lunar origin is the giant impact 9, 10 , which explains the Moon's large relative size and small iron core. Here we refer to the giant impact theory in which the Earth-Moon post-impact angular momentum (AM) was the same as it is now (in agreement with classic lunar tidal evolution studies 11, 12 ) as "canonical". In the canonical giant impact model 13 , a Mars-mass body obliquely impacts the proto-Earth near the escape velocity to generate a circum-terrestrial debris disk. The angular momentum of the system is set by the impact, and the Moon accretes from the disk, which is predominantly (> 60 wt%) composed of impactor material. However, Earth and the Moon share nearly identical isotope ratios for a wide range of elements, and this isotopic signature is distinct from all other extraterrestrial materials 4, 5 . Because isotopic variations arise from multiple processes 4 , the Moon must have formed from, or equilibrated with, Earth's mantle 5,14 . Earth-Moon isotopic equilibration in the canonical model has been proposed by Pahlevan and Stevenson 15 , but has been questioned by other researchers 16 , who suggest that the large amount of mass exchange required to homogenize isotopes could lead to the collapse of the proto-lunar disk.Cuk and Stewart 6 proposed a new variant of the giant impact that is based on an initially high AM Earth-Moon system. In this model, a late erosive impact onto a fast-spinning proto-Earth produced a disk that was massive enough to form the Moon, and was composed primarily of material from Earth, potentially satisfying the isotopic observations. Canup 7 presented a variation of a high-2 AM origin in which a slow collision between two similar-mass bodies produces a fast-spinning Earth and disk with Earth-like composition. Subsequently, Lock et al. 8 have argued that a range of hig...

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Tides and Earth-Moon system evolution are coupled over geological time. Tidal energy dissipation on Earth slows  Earth Es rotation rate, increases obliquity, lunar orbit semi-major axis and eccentricity, and decreases lunar inclination. Tidal and core-mantle boundary dissipation within the Moon decrease inclination, eccentricity and semi-major axis. Here we integrate the Earth-Moon system backwards for 4.5 Ga with orbital dynamics and explicit ocean tide models that are "high-level" (i.e., not idealized). To account for uncertain plate tectonic histories, we employ Monte Carlo simulations, with tidal energy dissipation rates (normalized relative to astronomical forcing parameters) randomly selected from ocean tide simulations with modern ocean basin geometry and with 55, 116, and 252 Ma reconstructed basin paleogeometries. The normalized dissipation rates depend upon basin geometry and  Earth E s rotation rate. Faster Earth rotation generally yields lower normalized dissipation rates. The Monte Carlo results provide a spread of possible early values for the Earth-Moon system parameters. Of consequence for ocean circulation and climate, absolute (un-normalized) ocean tidal energy dissipation rates on the early Earth may have exceeded  today E s rate due to a closer Moon. Prior to  E 3 Ga, evolution of inclination and eccentricity is dominated by tidal and core-mantle boundary dissipation within the Moon, which yield high lunar orbit inclinations in the early Earth-Moon system. A drawback for our results is that the semi-major axis does not collapse to near-zero values at 4.5 Ga, as indicated by most lunar formation models. Additional processes, missing from our current efforts, are discussed as topics for future investigation. Plain Language Summary Tidal dissipation ins oceans and solid body cause the distance to the Moon and the length of day to increase over time. Tides also change the eccentricity and tilt of the lunar orbit, and  Earth E s obliquity (the tilt between the equator plane and the ecliptic plane of our orbit around the Sun). This paper attempts to calculate the evolution of the Earth-Moon system over the whole of  Earth E s history using sophisticated ocean tide and orbit models. Over long time scales, the rate at which tidal energy is being dissipated is affected by the geometrical configuration of the continents, the length of day, and mean sea level, which is affected by plate tectonic forces and the presence or absence of large ice caps. The faster rotating Earth of the past was less efficient at dissipating energy and the present placement of the continents enhances some tides due to resonances. In addition, tidal dissipation in the Moon slows the orbit evolution by absorbing energy from the orbit and there was a time in the distant past when the  Moon s E tidal dissipation was large. The evolution of the Earth-Moon system is complex and uncertain, but it can be addressed with advanced models.

Giant impacts dominate the final stages of terrestrial planet formation and set the configuration and compositions of the final system of planets. A giant impact is believed to be responsible for the formation of Earth's Moon, but the specific impact parameters are under debate. Because the canonical Moon-forming impact is the most intensely studied scenario, it is often considered the archetypal giant impact. However, a wide range of impacts with different outcomes are possible. Here we examine the total energy budgets of giant impacts that form Earth-mass bodies and find that they differ substantially across the wide range of possible Moon-forming events. We show that gravitational potential energy exchange is important, and we determine the regime in which potential energy has a significant effect on the collision outcome. Energy is deposited heterogeneously within the colliding planets, increasing their internal energies, and portions of each body attain sufficient entropy for vaporization. After gravitational re-equilibration, post-impact bodies are strongly thermally stratified, with varying amounts of vaporized and supercritical mantle. The canonical Moon-forming impact is a relatively low-energy event and should not be considered the archetype of accretionary giant impacts that form Earth-mass planets. After a giant impact, bodies are significantly inflated in size compared to condensed planets of the same mass, and there are substantial differences in the magnitudes of their potential, kinetic, and internal energy components. As a result, the conditions for metal-silicate equilibration and the subsequent evolution of the planet may vary widely between different impact scenarios. Plain Language Summary Collisions between large planetary bodies, known as giant impacts,dominate the final stages of the formation of rocky planets like the Earth and set the configuration and compositions of the final planets. A giant impact is believed to have formed Earth's Moon, but the specific configuration of this impact is under debate. Understanding giant impacts is crucial for understanding the formation and evolution of the Earth and the Moon as well as rocky planets around other stars. The traditional Moon-forming impact model is often considered the archetype of a giant impact; however, a wide range of impacts with substantially different outcomes are possible. In this work, we examine the total energies involved in giant impacts that form Earth-like planets and find that there are large differences across the wide range of possible impacts. The internal energy increases cause large portions of each body to vaporize as the result of impacts. Giant impacts produce planetary bodies that are significantly inflated in size compared to condensed planets of the same mass, and there are substantial differences in their potential, kinetic, and internal energies. As a result, how planets and their cores evolve after different impact scenarios may vary widely.

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Quantifying the energy budget of Earth in the first few million years following the Moon-forming giant impact is vital to understanding Earth's initial thermal state and the dynamics of lunar tidal evolution. After the impact, the body was substantially vaporized and rotating rapidly, very different from the planet we know today. The subsequent evolution of Earth's energy budget, as the body cooled and angular momentum was transferred during lunar tidal recession, has not been accurately calculated with all relevant energy components included. Here, we use giant impact simulations and planetary structure models to calculate the energy budget at stages in Earth's evolution. We show that the figure and internal structure of Earth changed substantially during its post-impact evolution and that changes in kinetic, potential, and internal energy were all significant. These changes have important implications for the dynamics of tidal recession and the thermal structure of early Earth.