Séminaire ISTerre

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Exoplanetary magnetic fields as a tool for future interior characterization

mardi 30 juin 2020 - 10h00

Irène Bonati - Earth and Life Science Institute, Tokyo
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Most planets located within our solar system display evidence of past and/or current magnetic activity. Magnetic fields of rocky bodies are thought to be driven by thermo-chemical convection taking place in an electrically conducting fluid in their deep interior (the liquid core for Earth), and are thus evidence of strong internal dynamics. Furthermore, magnetism is thought to play a crucial role for the development and the long-term stability of habitable surface conditions, as it shields the upper atmosphere from mass loss induced by stellar winds and extreme space weather events.

The discovery of a large number of rocky exoplanets motivates the search and the study of magnetic fields beyond the solar system. While current observations are limited to providing the planetary radius and minimum mass, the future exploration of exoplanetary atmospheres will open up new avenues for the inversion of interior properties starting from atmospheric parameters. Such a goal requires knowledge of the cores and magnetic fields of exoplanets, as well as their influence on atmospheric evolution.

In the current study we investigate the evolution of the cores of planets having different mass and iron content. Starting out from the internal temperature profile after the complete solidification of a global magma ocean, we determine the size and structure of the core, and model its subsequent thermal and magnetic evolution. We compute the thermal and compositional buoyancy fluxes, as well as the generated magnetic field strength and lifetime. We also vary the content of light alloys and determine their effect on the evolution of an inner core and the lifetime of the field. While the planetary mass does not seem to be a determining factor, we find that iron-rich planets having a high mantle iron number and high light element concentration start out and end up with inner cores that are substantially larger than iron-poor bodies, sometimes reaching up the radius of the outer core and shutting down magnetic activity. We therefore find that there is a “sweet spot” for longer-lasting magnetic fields. Field strengths can reach up to several times the one of Earth, even though such a signal might still be too weak to be detected by current instrumentation.

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Equipe organisatrice : Géodynamo

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