Synopsis

The main result from the Kepler mission was that every Sun-like star is likely to host a number of exoplanets. The exoplanet zoo now included definitions like: hot Jupiters, super-Earths, hot Neptunes, water worlds and exo-Earths. These definitions are mainly defined by the size of the planets and the amount of irradiation they receive from their host stars and they are thus likely to say very little about the conditions on the surface and interior of these planets. As an example Venus, Earth and Mars are all defined as exo-Earths, but the geological conditions are, as we know, very different. Geodynamics is a field of the study that focuses on dynamic processes  within the interior of the Earth and other terrestrial planets and planetoids with. Such processes include mantle convection, plate tectonics, volcanoes, mountain building and the generation of magnetic fields.

The aim of the workshop

The idea with this workshop is to take the first steps towards a Geodynamic of Exoplanets program at Aarhus University. It is clear that we have, not just, a strong growing exoplanet group at the Stellar Astrophysics Centre (SAC), but also a flourishing Geochemistry and Petrology group with the Niels Bohr Professorship of Charles Lesher. It is the goal with this workshop that we will be able to identify a number of specific research projects that we can collaborate on. We likely all have a lot to learn here, therefore teaching and supervision are also obvious places for collaboration.

Plate Tectonics

Though Venus and Earth both fall under the definition of exo-Earths, the surface of Venus suffers, with temperatures in excess of 700 K, heavily from a runaway greenhouse effect. It is likely that this effect is related to the lack of plate tectonics and thus sequestration of carbon. We do not know exactly why we do not see plate tectonics on Venus or Mars, but it could be due to the lack of surface water or magnetic dynamos. These things therefore all become important phenomena to model and understand in order to understand the geodynamic processes within not just Venus, Earth or Mars, but also the geodynamics of the known exo-planets. Could frozen water worlds have plate tectonics and dynamos? Could hot Neptunes?

Modeling the Earth

The pivot of any model of either a Sun-like star or a planet is an equation-of-state and a description of how heat is transported by advection, conduction and radiation, simultaneously. This is important to remember when astrophysicists are to understand how geodynamic modeling works, but where astrophysicists are used to work with extended opacity tables to describe the interaction between radiation and matter in the deep interior of Sun-like stars, geophysicists are more concerned withthe extended number of phase transitions that takes place for most of the different minerals in the deep interior of the Earth. An additional complexity associated with geodynamic modeling of terrestrial bodies is the large rheological variation of solid materials at different temperatures and pressure where not only viscous deformation, but also brittle and elastic deformation must be accounted for.

Geodynamic models of the Earth are formulated by assuming conservation of momentum, mass and energy and employ constitutive relationships between stress and strain rate that are consistent with the rather different physical and dynamical circumstances we have here on the Earth, including strong variations of viscosity, brittle strength and elastic moduli along with varying temperature, pressure and composition. Similarly, thermal properties such as conductivity, heat capacity and thermal expansivity are strongly temperature and composition dependent.  Hence, the thermo-chemical structure imposes a first-order control on the geodynamic evolution of the Earth. In this context, it of interest to assess the scaling of relevant geodynamic processes by comparing with other Earth-like planets. For example, what would happen if we doubled the size of the Earth?

Spectra

Using satellites like the Hubble Space Telescope and the Swift satellite infrared transmission spectra of exoplanet atmospheres have already been obtained for a number of hot Jupiters and future facilities like the James Webb Space Telescope and the European Extremely Large Telescope will be able to provide us with spectra of atmospheres around exo-Earths. So far we have seen detection of common molecules like H20, CO2, CO and CH4. A geodynamic analysis could not just be used to analyze the abundances of these molecules, but it could also be use to guide us on, which molecules would be particular interesting to observe in the future.

A particular appealing aspect here is the use of laboratory measurements of spectra of different molecules and minerals under high pressure and / or temperature using i.e. Raman, infrared or NMR spectroscopy.