DEGREE-1 MANTLE CONVECTION AND THE ORIGIN OF THE MARTIAN HEMISPHERIC DICHOTOMY. James

The hemispheric dichotomy on Mars is largely an expression of varying crustal thickness [1]. Although there is some disagreement as to the timing of its formation, the dichotomy is very old, forming during or before the early Noachian [2]. Several formation mechanisms, including both exogenic (giant impacts) [3] and endogenic processes including mantle convection [4], plate tectonics [5], and overturn of magma ocean residue [6] have been proposed. We have investigated the possibility for mantle convection to produce the hemispheric dichotomy. To be feasible, the pattern of convection must reflect the shape of the dichotomy, primarily spherical harmonic degree-1. This pattern must also emerge on a timescale consistent with the early age of the dichotomy (within the first couple hundred Ma). We considered viscosity layering in the mantle as a method of promoting long-wavelength convection [4] addressing several key issues not previously considered. We generalized our study to three dimensions, used a more realistic temperatureand pressuredependent viscosity formulation [7] in addition to the layering, examined the extent to which internal heating controls the wavelength [8], examined the magnitude of the viscosity jump and total viscosity variation across the mantle, and noted the timescale required for the formation of degree-1. We also considered the possibility of generating degree-1 convection by means of an endothermic phase transition near the CMB [9]. We used finite-element convection codes to solve the continuity, momentum, and energy equations in 2-D axisymmetric and in 3-D (CitcomS) spherical geometry.