The Formation, Structure and Evolution of Terrestrial Planets

Abstract

The final stage in the growth of terrestrial planets is marked by high-energy collisions between planetary bodies, called giant impacts. Giant impacts melt and vaporize the silicate mantles of the impacting bodies, and planets can acquire significant angular momentum via one or more collisions. Multiple key physical and geochemical processes occur during or in the aftermath of giant impacts, and the physical structure (e.g., shape, mass and AM distribution, internal pressures and temperatures) of post-impact bodies provides the essential framework needed to investigate planet formation and evolution. This work investigates how changes in thermal and rotational states wrought by giant impacts alter the physical structure of terrestrial bodies. I define a corotation limit beyond which bodies can not be a corotating planet and are instead a new type of planetary object, {\it synestias}. Giant impacts readily exceed the corotation limit and most rocky bodies would be synestias for periods during accretion. Synestias provide a new environment for satellite accretion, and I propose that formation of our Moon from a terrestrial synestia could reproduce the observed geochemical and physical properties of the Earth-Moon system. I demonstrate that the internal pressures in the hot, rapidly-rotating bodies produced by giant impacts can be much lower than in equivalent condensed, slowly-rotating bodies. The internal pressures of bodies would vary stochastically due to giant impacts, defining a new paradigm for pressure evolution during accretion. Lower pressures after giant impacts affect the composition and thermal evolution of the mantle. After giant impacts, the physical structure of bodies evolve by cooling and transfer of angular momentum due to the tidal recession of satellites. I show that internal pressures can increase during recovery after giant impacts, driving previously unrecognized processes. Changes in shape and physical structure also alter the planet's energy budget and influence the orbital evolution of satellites. Understanding the effects of variable thermal and rotational states during accretion on the properties of the final planets is a new field of study.