Thesis Defense: Johnny Seales, Ph.D. Candidate
Defense Date: Tuesday, April 7th, 2020
Time: 2:30 p.m.
Structural Uncertainty and Hypothesis Testing in Planetary Thermal History Models
Plate tectonics convectively cools the Earth at present. Debate remains in defining plate tectonics as a dynamic system. What is the primary resistor to plate motions: plates, mantle viscosity, some mixture of the two? Here I shed some light on this debate. First, I quantify the uncertainties associated with the different hypotheses. Then, I constrain the probability that different thermal paths from each of these hypotheses match Earth’s geologic proxy data. Only a single hypothesis is true for Earth. The data, however, cannot tell us which. So, rather than using the most probable hypothesis, we can embrace this uncertainty. Each successful thermal path informs us of how other plate tectonic planetary interiors may have evolved in the past or will evolve in the future. Another question arises from inspecting Earth’s thermal history data: what is the convective mechanism for the multi-stage cooling present in some sets of thermal history data? I show that changes in the deep water cycle, namely a switch from a net dehydrating to rehydrating mantle, can act as this mechanism. Whether this hypothesis holds or not, changes in Earth’s convective efficiency may directly influence surface conditions. I show that changes in convective efficiency can alter deep carbon cycling in such a way that explains the rise of oxygen at Earth’s surface, specifically looking at the Great Oxidation Event. Interior processes, then, may play a vital role in determining the inhabitance of a planet. This leaves many searches for life on exoplanets fixated on finding Earth2.0. Depending on the question we ask, however, this may be the wrong approach. I demonstrate that if we cast a broader net in our search for inhabited planets, the increased reward likely outweighs the increased cost.