A Combined Astronomic-Geologic Approach to Understanding Planet Formation

Colloquium Abstract: Much of my research lies at the intersection of geology and astronomy. As the field of planet formation moves beyond just matching the orbits and masses of the planets, I have turned to geologic data to constrain the timing and characteristics of important processes in the protoplanetary disk, such as the typical mass of planetary embryos at the beginning of the epoch of giant impacts (Mars-like) and the timing of the Moon-forming impact (~100 My after the first solids in the solar system). Introducing a sophisticated astronomic-geologic model for Earth's growth and differentiation, I show that Earth’s mantle composition records evidence of multi-stage core formation as envisioned in astronomical models and as opposed to the simpler single-stage models preferred by geochemists. The step-wise increases of the pressures and temperatures of core formation during each stage change the chemistry of metal-silicate equilibration during multi-stage planetary differentiation. Elements such as palladium become less siderophile—a chemical affinity for iron—whereas other elements such as oxygen become more siderophile. In order to match the measured abundance of elements such as palladium in the mantle, we infer a Hadean matte composed of FeS in Earth’s history. The consequence of increasing light element abundances such as oxygen in core forming liquids is a stably, density stratified core. This stratification inhibits convection and prevents the creation of geodynamo. Only late and violent giant impacts are capable of sufficiently mixing the core and establishing the conditions that are observed today. Thus, during this talk I demonstrate that from modern geologic measurements, we can learn about Solar System history and from astronomical models, we can determine important events during Earth's evolution.