MI2023 - 05 Kathryn Dodds: Dynamics of inwards solidification in asteroids
Channel: MagNetZ
Date: 2023-01-16

Magnetic Interactions 2023 conference at the University of Cambridge
Session 1: Solar System and Asteroid Magnetism

Title: Dynamics of inwards core solidification in asteroids: insights from analogue experiments
Author: Kathryn Dodds from the University of Cambridge, UK

Given their small sizes and low interior pressures, the cores of meteorite parent bodies likely crystallized inwardly from their core-mantle boundary (CMB) instead of nucleating an inner core as is the case for the Earth. However, the iron snow model of dynamo generation may not be applicable to these cores as the small adiabatic temperatures between the CMB and interior of these cores may prevent remelting of the iron snow, removing the source of buoyancy required to drive a dynamo field. Motivated by this, we developed a set of analogue experiments to investigate the possible mechanisms of core crystallization in asteroid cores. Our experiments found that the mean convective speeds of the fluid increase when the fluid crystallizes. This increase in speed is driven by an increase in the bulk density of the fluid in the snow zone due to the presence of a small, entrained crystal fraction.

In this study, we apply the results of our experiments to a model of asteroid thermal evolution. In the absence of remelting of the iron snow below the CMB, core convection during solidification is driven by the addition of dense sub-mm sized iron crystals to the core fluid at the CMB. This crystal-rich layer grows until it becomes convectively unstable, producing dense, crystal-rich plumes which mix in with the interior. We find that this convection can drive a dynamo field in cores of  20 – 100 km in mantled asteroids. Our model will also allow us to constrain the parent body properties, such as size, of the meteorites that experienced a compositionally-driven dynamo field during core solidification in the first 65 – 200 Myr of the solar system.

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00:00 - Intro
00:15 - Presentation
16:50 - Questions