Constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling

Bryson, James F. J. and Neufeld, Jerome A. and Nimmo, Francis (2019) Constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling. Earth and Planetary Science Letters, 521. pp. 68-78. ISSN 0012-821X DOI https://doi.org/10.1016/j.epsl.2019.05.046

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Official URL: https://doi.org/10.1016/j.epsl.2019.05.046

Abstract

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of 26Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites.

Item Type: Article
Uncontrolled Keywords: 2019AREP; IA75
Subjects: 03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
08 - Green Open Access
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 521
Page Range: pp. 68-78
Identification Number: https://doi.org/10.1016/j.epsl.2019.05.046
Depositing User: Sarah Humbert
Date Deposited: 27 Jun 2019 12:06
Last Modified: 27 Jun 2019 12:06
URI: http://eprints.esc.cam.ac.uk/id/eprint/4509

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