Mars' thermal evolution constrained by the crustal magnetization and volcanic resurfacing: Present-day melting and the disappearance of the global magnetic field explained
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Convection modeling so far failed to explain the thermal state of Mars in view of the observed long-lived volcanism, absence of plate tectonics and the early generation and subsequent cessation of a core magnetic dynamo. Therefore I have applied both parameterized and full convection models of mantle convection coupled to the evolution of the core in a search for a suitable combination of model parameters that might explain the available observations. The 2D radially symmetric parameterized models allow for a fast exploration of the parameter space spanned by the thermal conductivity structure, radioactive element concentration and partitioning between mantle and crust as well as the initial temperature profile and superheating of the core. A combination of the above parameters that is both physically plausible and meets the constraints derived from the cessation of the Martian global magnetic field, growth rate of the crust, surface heat flow and the formation temperature and depth of partial melt forms the basis for the full convection models. Also, it is found that the different radioactive element concentrations used in the literature on Mars' thermal state produce such a wide range of results that these studies should be compared with care. With axisymmetric statistical equilibrium full convection models the effect of the viscosity structure -another major control on the thermal state- on the present-day Mars is investigated. However, comparison with equivalent transient full convection models shows that the equilibrium models do not accurately represent the current state of the Martian mantle. The parameterized thermal histories do agree reasonably well with equivalent time-dependent full convection models. Subsequently, using transient axisymmetric full convection models I studied the effect of radioactive element partitioning between mantle and crust, initial superheating of the core, viscosity type and structure, thermal conductivity structure, internal heating rate and compositional buoyancy on the temperature and melt productivity distribution and the convective planform of the mantle. It becomes clear that the generation of mantle plumes is highly dependable on the ratio of internal to bottom internal heating. The observational constraints on core heat flow, CMB temperature, volcanic productivity, surface heat flow and the distribution of partial melting are met by a model with Arrhenius viscosity that includes a 600 km thick top layer representing mantle material depleted through partial melting. The core is initially superheated about 200 K with respect to the mantle and only 35% of the relatively low total amount of radioactive elements is partitioned into the crust. The early generation of the core dynamo through thermal convection is explained by the large temperature difference of the core with respect to the mantle, its demise by the equilibration of mantle and core temperatures due to the isolating effect of the buoyant stagnant lid before the core has cooled enough for core crystallization to set in. Partial melting in the hot mantle shifts to greater depths upon thickening of the lithosphere during secular cooling, but the chaotic nature of thermal convection still enables periods of shallower melting that sustain an episodic volcanism through time.