DIX Planetary Science Seminar

The early Earth experienced large-scale melting owing to giant impacts during the last phase of accretion. This indicates the possibility of substantial differentiation during the subsequent solidification of a magma ocean, and its aftermath has likely been crucial for characterizing the surface environment of the Hadean. How differentiation proceeds, however, is controversial because a number of parameters are unconstrained under high pressures, including density contrast, viscosity, grain size, and permeability. Instead of exploring the most likely scenario, I aim to bound the degree of chemical differentiation by calculating the thermochemical structure of end-member cases. A new model is developed that is consistent with both thermal and compositional aspects, and a potential differentiation mechanism is assumed to take full effect in each calculation. The results suggest that the length scale of chemical heterogeneity differs significantly depending on the mechanism that drives the differentiation. A differentiation mechanism responsible for our planet could potentially be constrained by investigating how each mantle structure affects the evolution of the surface environment. Considering that Earth already had a present-day like moderate climate by the end of the Hadean, I suggest that a wet, depleted mantle with small-scale chemical heterogeneity most naturally explains the swift transition from a fiendish to a habitable surface environment. The resemblance between the Hadean seafloor and the Lost City hydrothermal field will also be discussed, which may imply the emergence of early life.