Influence of early plate tectonics on the thermal evolution and magnetic field of Mars

Recent magnetic studies of Mars suggest that (1) it possessed a periodicall y reversing magnetic field for the first similar to 500 Myr of its existenc e and (2) plate tectonics may have been operating during this time. On Eart h the geodynamo is thought to occur because of convection in the outer core . This paper estimates the amount of heat the Martian core can conduct in t he absence of convection. It uses parameterized, variable-viscosity thermal evolution models to show that the core heat; flux increases if the planet' s surface heat flux is increased above the value required to eliminate inst antaneous radiogenic heat production. Conversely, a sudden reduction in sur face heat Aux causes the mantle to heat up and the core heat flux to become negative. Thus, if plate tectonics, or some other process causing high sur face heat flux, was occurring on early Mars, it is likely to have caused co nvection in the core and hence generated a magnetic field. Conversely, a re duction in surface heat flux would probably have caused the core to stop co nvecting and shut off the magnetic field. There is thus an important link b etween surface processes and core magnet;ism, which may also be relevant to planets such as Earth and Venus.