Convective Pattern and Type of Volcanism on Mars

The remnant crustal magnetic field discovered by the MGS mission suggests that Mars had an internal magnetic field during the Noachian period. At the same time, hot-spot type volcanism occurred at least at Tharsis in the Syria Planum area. We investigate the range of temperature variations at the core-mantle boundary assuming that gravitational energy converted to heat is not necessarily equally partitioned between metal and silicate phases. Assuming a given fraction of heat is partitioned in the metal phase as it differentiates into an iron-rich core in the few tens of millions years after accretion, we use scaling laws describing heat transfer through a hot thermal boundary layer for a fluid having strongly temperature dependent viscosity to assess the cooling rate of the core. For an initial temperature difference less than 800K, the core cools down in less than one billion years, leading to a core temperature gradient smaller than adiabatic and stopping the dynamo. We are currently exploring estimates of the amount of melt produced and compare it with the volume of crust necessary to account for Tharsis. Mars'mantle is also cooled from above, and heated from within by the decay of radiogenic elements. In the absence of plate tectonics, stationary convective patterns for both isoviscous and temperature dependent viscosity can be described by strong cold downwellings and global hot upwelling. Recent studies, based on both laboratory and numerical experiments, suggest that such a stationary pattern would take some hundreds of million years to be established (Choblet and Sotin, GRL, 2001). The present study investigates the amount of warming accompanying the establishment of stationary convective pattern. We consider whether it could coincide with widespread centers of volcanism during the Early Hesperian.