The structure of terrestrial bodies: Impact heating, corotation limits, and synestias

Abstract

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.

The Origin of the Moon Within a Terrestrial Synestia

“…This fraction of the impactor does not have an opportunity to intimately mix with the target body, and the outer portions of the structure are enriched in impactor material. In contrast, the impact geometries of most high‐AM, high‐energy impacts (Canup, ; Ćuk & Stewart, ; Lock & Stewart, ) lead to much more contact between the silicates originating from the impactor and target, and there can be substantial shear and advective mixing during the impact event. In a large number of simulations of high‐AM, high‐energy impacts (Canup, ; Ćuk & Stewart, ), the outer regions of the structure out of which a moon would form have similar proportions of impactor and target material to the bulk (e.g., within about 10%).…”

“…Here we present a new model for lunar origin within a terrestrial synestia, an impact‐generated structure with Earth mass and composition that exceeds the corotation limit (CoRoL). Synestias are formed by a range of high‐energy, high‐AM collisions during the giant impact stage of planet formation (Lock & Stewart, , hereafter LS17). A synestia is a distinct dynamical structure compared to a planet with a condensate‐dominated circumplanetary disk, and, as a result, different processes dominate the early evolution of a synestia.…”

Modeling a Transient Secondary Paleolunar Atmosphere: 3‐D Simulations and Analysis

“…The latest research (e.g., Canup, ; Halliday, ; Herwartz et al, ; Lock & Stewart, ; Lock et al, ) tends to support the canonical hypothesis of the Moon forming ∼4.5 Ga as a result of a Mars‐sized impactor colliding with a proto‐Earth (Hartmann & Davis, ), though the hypothesis of multiple impacts has gained popularity in recent years (Rufu et al, ).…”

Silicate Melting and Vaporization During Rocky Planet Formation

“…Collisions are a key aspect of planet formation (e.g., Chambers, ). The outcomes of collisions are diverse and complex, ranging from growth of planetesimals to the formation of synestias (Leinhardt & Stewart, ; Lock & Stewart, ). Because collisions deposit energy and redistribute material, they can significantly affect the thermal and geochemical evolution of growing planets (Asphaug, ; Carter et al, , ; Stewart & Leinhardt, ).…”