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Internal Dynamics and Magnetic field generation in rocky bodies: planets and large moons in the solar system

Periodic Reporting for period 1 - InDyMag (Internal Dynamics and Magnetic field generation in rocky bodies: planets and large moons in the solar system)

Reporting period: 2019-03-13 to 2021-03-12

The IndyMag project goal is to better understand the generation of magnetic fields in the rocky bodies of the solar system and beyond, combining models of the long term evolution of planetary interiors and planetary cores (how they evolve, how they crystallize), and observations of evidence of magnetic fields on such bodies.

The innermost planets in the Solar System are rocky. To first order their composition is similar to the Earth’s one, with an iron core, a silicate mantle and a surrounding crust. Their internal dynamics, in the core and mantle, present a large diversity of features, and this work is interested in the main product of the core dynamics: the magnetic field. Amongst the solar system planets, only Venus shows no sign of a past or present magnetic field. Other rocky bodies present evidence of present (the Earth, Mercury) or past (Mars, the Moon) magnetic fields, with various time and spatial variability. This diversity is poorly understood, and often planets are studied in separate studies.

The goal of the IndyMag project is to (i) model quantitative thermal and compositional evolution of rocky bodies’ cores, including variations in size and compositions and to (ii) generate synthetic observables of the time evolution and structure of the magnetic field of telluric bodies, to be compared to existing observations and models.

We showed that the thermal and chemical evolution of a planetary core is controlled primarily by its crystallization. We modeled the crystallization through two complementary approach: study of the effect of the crystallization on the solid fraction (Lasbleis et al. 2020, Geoph. Res. Lett.) and model of the thermal and compositional evolution of the outer core for various planetary properties (Bonati et al, 2021)
The crystallization is a major driving force for the generation of the magnetic field. The crystallization of iron and other elements leaves the liquid devoid of heavy elements, making it lighter than the surrounding. Its buoyancy generates flows into the core, powering the geodynamo. The IndyMag project focused on the impact of the crystallization of solids on the magnetic field generation, with three aspects in mind: (1) the development of a parameterized model for the thermal and chemical evolution, applicable to all rocky planets, (2) the effect of crystallization on the solid fraction dynamics and in particular the existence of trapped liquids, (3) the chemical stratification in the liquid core induced by a "snow" effect, when crystals are formed in the bulk of the liquid and settle through the liquid.

During the duration of the project, results have been published in four articles in international peer-review journals, and presented at six international conferences (two talks, five posters, one invitation for a review talk), some of them online due to sanitary restrictions during the duration of the project. Two open source Python packages have been published online, for the scientific community.
The IndyMag showed that the thermal and chemical evolution of a planetary core is controlled primarily by its crystallization. The thermal and chemical evolution of planetary cores are often based on the Earth's core, for which we have more information from ground and space observations. This project explored how good this approximation is, providing tools and methods to consider when modeling planetary cores. This will be especially useful when modeling exoplanets and their likeliness to host a magnetic field, one of the component that would make an exoplanet more "Earth-like"
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