The thermal evolution of planetesimals during accretion and
differentiation: consequences for dynamo generation by thermally-driven
The meteorite paleomagnetic record indicates that differentiated (and
potentially, partially differentiated) planetesimals generated dynamo
fields in the first 6-20 Myr after the formation of
calcium-aluminium-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 1D model of the thermal evolution of
planetesimals from accretion through to the shutoff 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, 26Al.
During differentiation, 26Al partitions into the silicate portion of
these bodies, causing their magmas ocean to heat up and introducing
stable thermal stratifications to the tops 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 minimise the development of this
stratification, allowing dynamo generation from ~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.