The Thermal Evolution of Planetesimals During Accretion and Differentiation: Consequences for Dynamo Generation by Thermally-Driven Convection

The meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 5-40 Myr after the formation of calcium-aluminum-rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a one dimensional model of the thermal evolution of planetesimals from accretion through to the shutdown of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short-lived radiogenic isotope Al-26. During differentiation, Al-26 partitions into the silicate portion of these bodies, causing their magma oceans to heat up and introducing stable thermal stratification to the top of their cores, which inhibits dynamo generation. In instantaneously accreting bodies, this effect causes a delay on the order of >10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over >0.1 Myr can minimize the development of this stratification, allowing dynamo generation from similar to 4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales >1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies.

Automated summary

Studies have shown that differentiated planetesimals generated dynamo fields shortly after calcium-aluminum-rich inclusions (CAIs) formed, attributed to thermal convection in the liquid cores. The partitioning of the short-lived radiogenic isotope Al-26 during differentiation was found to heat up magma oceans and introduce stable thermal stratification, inhibiting dynamo generation. This study compared model predictions with meteorite paleomagnetic records to constrain the physical properties of parent bodies.