Mars Is Not Dead: Mantle Convection Controls the Observed Lateral Variations in Lithospheric Thickness on Present-day Mars

Introduction Because the transition from elastic to viscous behavior in silicate materials is controlled by temperature [1], measurements of the thickness of a planet's elastic lithosphere (T E) provide important constraints on the thermal structure and evolution of the lithosphere. For Mars, surface geophysical measurements have not yet been made, so estimates of T E have been made using orbital observations of gravity anomalies, topography, and tectonic structure [2-4]. These results show that the lithosphere has thickened with time, as expected for a cooling planet [5]. T E values for the Amazonian, the most recent geologic epoch on Mars, are typically around 100 km. An important limitation is that most of these measurements, particularly for geologic units that formed in the second half of martian history, have been made near large volcanos or extensional faulting. In either case, these measurements are likely to be in regions of upwelling mantle convection and higher than average heat flow. Phillips et al. [6] recently reported observations of the internal stratigraphy in the north polar layered deposits made with the SHARAD sounding radar on Mars Reconnaissance Orbiter. Based on the limited (< 100 meters) deflection of the ice-rock interface at the base of the layered deposits, they inferred that T E in this region is at least 300 km for deposits that are less than 5 million years old. This T E value is dramatically larger than any prior estimate of elastic lithosphere thickness on Mars and places important constraints on the planet's thermal evolution. They suggested three possible explanations for their results: (1) the bulk silicate composition of Mars contains less radioactive elements than previously believed, corresponding to just 70-80% of chondritic abundances (" chondritic abundances " in Phillips et al. [6] means the Wänke and Dreibus [7] abundance model, hereafter WD94). (2) The measured deflection is not an equilibrium elastic configuration but rather a transient visco-elastic response to recent loading of the polar cap. Successful models of this type also require a sub-chondritic abundance of radioactive elements in the martian crust and mantle. (3) There are significant spatial variations in elastic thickness and heat flow on present-day Mars. In this work, we show that it is unlikely that Mars has significantly sub-chondritic abundances of U, Th, and K. On the other hand, strong spatial variations in heat flow occur naturally in convecting systems, and we show that our existing models of mantle …