Thermal evolution of early solar system planetesimals and the possibility of sustained dynamos

We present a study investigating the possible presence and longevity of a stable dynamo powered by thermal convection in early solar system planetesimals. We model the thermal evolution of planetesimals that start from an initially cool state. After melting, core formation, and onset of mantle convection have occurred, we use a numerical model that relies on thermal boundary layer theory for stagnant lid convection to determine a cooling rate and thermal boundary layer thickness that are dynamically self consistent. We assess the presence, strength and duration of a dynamo for a range of planetesimal sizes and other parameters. The duration of a dynamo depends foremost on the planetesimal's radius. Given a particular magnetic field strength and dynamo duration we are able to place a constraint on the minimum radius of planetesimals, for example: bodies smaller than similar to 500 km will be unable to generate a dynamo with magnetic field strength of the order of 20 mu T for a duration of 10 Myr or longer. We find that dynamo duration also depends, to a lesser extent, on the effective temperature dependence of the mantle viscosity and on the rotation rate of the body. These dependencies are made explicit by our derivation of an analytical approximation for the cooling rate of a planetesimal and its dynamo duration. (C) 2012 Elsevier B.V. All rights reserved.