Exploring the dynamics of inward core solidification using analogue tank experiments.
Abstract
Given their small sizes and low central pressures, the cores of asteroids are expected to have started crystallizing at the core mantle boundary (CMB) instead of at their centre like the Earth. This behaviour is also predicted for Ganymede and has prompted the development of the iron snow model to explain its observed dynamo field, in which iron crystals that grow at the CMB in a snow zone and subsequently sink into the hotter interior and melt, releasing dense fluid that drives convection. However, whether this process could have occurred in asteroidal cores is uncertain due to the small adiabatic temperature difference between the CMB and the centre of their cores. Instead, the power for this compositional dynamo may have come from an increase in convective velocities caused by the addition of dense crystals at the CMB or turbulence caused by the settling of the crystals themselves. In this study we use analogue tank experiments to explore the possible dynamo driving mechanisms during inward asteroid core crystallisation. Ammonium chloride solution is cooled from above with a layer of buoyant propanol separating the solution from the cold plate to prevent the growth of crystals on this boundary. For a given temperature difference across this layer, we compare the convective velocities with and without crystallization to quantify the effect of the additional buoyancy flux on the fluid flow and to develop a scaling law that we implement in our thermal models of asteroid evolution. Previous models have suggested that the CMB heat flux could be sub- or super-adiabatic at the start of core crystallization, depending on the core size and its sulfur content, which we account for by varying the driving temperature difference in the tank. A complete upcoming dataset will allow us to quantify the mechanisms by which inward core solidification could generate a dynamo field. We also record the temperature and composition as a function of depth in the tank, enabling us to determine whether thermal equilibrium is maintained. This allows us to assess whether thermal equilibrium can be assumed when modelling snow zones in cores, a problem that is also relevant to larger planetary bodies cores (e.g. Ganymede, Mars, the Moon, the early Earth). Initial results suggest that thermal equilibrium is maintained, validating this key assumption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMDI34B..05D