Earth and Planetary Sciences
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
One of the most prominent features of Mars is the hemispherical dichotomy. The Martian surface consists of a heavily cratered elevated southern hemisphere and a resurfaced depressed northern hemisphere. The dichotomy seems to have formed very early in the history of the planet. Another interesting feature is a remnant magnetization of the crust, which suggests that early Mars had a magnetic field. Investigation of the origin of these features provides insights into the early history of Mars as well as other terrestrial planets including Earth. We develop a hypothesis that the dichotomy is caused by an early transient superplume produced by a hot Martian core. At first glance, the superplume hypothesis seems unlikely because the number of plumes in typical fluids heated from below is very large and the plumes are relatively small. However, solid rocks are rather unusual fluids whose viscosity varies with temperature by many orders of magnitude. Plume formation in such fluids is a complex and poorly understood phenomena. Thus, we begin with a systematic two-dimensional numerical and theoretical investigation of plume formation in strongly temperature-dependent viscosity fluids. Then we extend both the numerical calculations and the theory to fully three- dimensional geometry. We find the conditions under which a single transient superplume forms. One of the most important conditions is the requirement that the core was at least several hundred degrees Kelvin hotter than the mantle. Geophysical data and theoretical models of core formation suggest that this is likely to be the case. We find that the superplume can easily satisfy the timing constraints on the formation of the dichotomy. In the last part we consider the coupled core-mantle thermal evolution and investigate the cooling of the initially superheated core and the generation of the magnetic field on early Mars. We show that the core cooling is sufficiently rapid to induce convection inside the core and allow the operation of the magnetic dynamo. In our models, the magnetic field exists for millions to hundreds of millions of years after planetary formation, which is consistent with observations.
Ke, Yun, "Plume formation in strongly temperature-dependent viscosity fluids: Application to early Mars" (2009). All Theses and Dissertations (ETDs). 178.