Abstract
AbstractThe 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 26Al. During differentiation, 26Al 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 ∼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.
Highlights
Our model considers the thermal evolution of a planetesimal, from accretion and differentiation through to magnetic field generation and the cessation of magma ocean convection
Accretion starts at 0.8 Myr after calcium-aluminium-rich inclusions (CAIs) formation with the accretion duration varying between the three cases
We have shown an effective core-mantle boundary (CMB) heat flux and magnetic Reynolds number in Figures 7c and d, which is the average of the respective quantity for each episode of core formation and thermal evolution between episodes in order to give a clearer picture of the evolution of both these values
Summary
Advances in rock magnetism and paleomagnetic techniques over the past two decades have revealed that many meteorites carry primary magnetic remanences imparted by magnetic fields generated in the first few 100 Myr after the formation of the solar system.This primary remanence has been found in both achondrites (e.g. Fu et al (2012), Bryson et al (2015), Wang et al (2017)), which sample the mantles of differentiated planetesimals, as well as chondritic meteorites (e.g. Carporzen et al (2011), Cournede et al (2015), Gattacceca et al (2016), Shah et al (2017), Bryson et al (2019a), Cournede et al (2020), Maurel et al (2020)), which are usually considered to be samples of unmelted, undifferentiated planetesimals. Advances in rock magnetism and paleomagnetic techniques over the past two decades have revealed that many meteorites carry primary magnetic remanences imparted by magnetic fields generated in the first few 100 Myr after the formation of the solar system. This primary remanence has been found in both achondrites (e.g. Fu et al (2012), Bryson et al (2015), Wang et al (2017)), which sample the mantles of differentiated planetesimals, as well as chondritic meteorites Long-lived dynamo activity driven by mechanical stirring from impacts (Le Bars et al, 2011) or perturbation of orbital parameters such as precession (Reddy et al, 2018) is unlikely due to the short < 10 kyr spin-down timescales of asteroid-sized bodies (Burns et al, 1973).
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