title: Interior dynamics of tidally locked super-Earths
Creators: name: Meier, Tobias Gabriel
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 520 Astronomy
subject: 530 Physics

description: Within the last few years, a large number of rocky exoplanets with masses larger than Earth have been detected. These so-called super-Earths do not exist in our Solar System and their tectonic regimes could be substantially different than Earth’s. Current and near-future astronomical observations are capable of measuring the masses, radii and sometimes even the surface temperature of these planets with unprecedented precision, and we can now use these measurements to probe their interior dynamics. To achieve this, we run geodynamic simulations of the interior mantle flow using the mantle convection code StagYY in the two-dimensional spherical annulus geometry. We constrain the longitudinal surface temperature dependence using the results from thermal phase curve observations and general circulation models. Many super-Earths are on very short orbits and are therefore most likely tidally locked. This means that the same hemisphere always faces the host star, which can lead to a strong temperature contrast between the dayside and nightside.
In Chapter 1, I present our results on the interior dynamics of super-Earth LHS 3844b. The thermal phase curve of this planet suggests that this planet lacks a substantial atmosphere. The dayside temperature is around 1040K and the nightside temperature is consistent with zero K. Our numerical simulations suggest that this planet could exhibit hemispheric tectonics if it has a strong lithosphere. In such a regime, a cold and viscous downwelling is located predominantly on one hemisphere and hot upwellings are getting flushed around the core-mantle-boundary (CMB) towards the other hemisphere, where they rise towards the surface. Such a regime is currently absent in our Solar System and this work is the first observationally constrained indication that such a regime could operate on exoplanets.
In Chapter 2, we investigate the case of super-Earth GJ 486b that is very similar to LHS 3844b in terms of size. For this planet, it is not (yet) known whether it was able to retain its atmosphere. However, it is a suitable target for future follow-up observations that aim to characterise its atmosphere and we therefore investigate in this study what tectonic regimes could operate on this planet for different surface temperature contrasts that are derived from different types of atmospheres. This study provided also an opportunity to investigate further what causes hemispheric tectonics and if it could operate on planets with smaller temperature contrasts between dayside and nightside. As with LHS 3844b, we find that hemispheric tectonics is favoured for planets with strong lithospheres. However, a strong temperature contrast between dayside and nightside and overall higher (dayside) surface temperatures are needed in order for the downwelling to be pinned to one hemisphere. In Chapter 3, we study the case of lava world 55 Cancri e. This is the first rocky exoplanet for which a thermal phase has been observed. This planet’s dayside temperature is around 2700K and the nightside is roughly 1380K. Therefore, the dayside most likely harbours a magma ocean which could potentially even extend towards the nightside. In this study, we investigate the impact of such a global magma ocean on the interior mantle dynamics. We parametrise heat transport inside the magma ocean through a very high eddy diffusivity. We find that if both hemispheres are molten, the magma ocean will tend to equilibrate the temperatures at the interface between the magma ocean and the underlying solid mantle and dichotomous mantle convection is therefore less likely. If only the dayside is molten, we find that a massive plume forms on the dayside of the planet which rises from the CMB towards the magma ocean.
In conclusion, we find that rocky super-Earths that are on very short orbits around their host star could exhibit tectonic regimes that are unlike anything we currently observe in our Solar System. A planet’s tectonic regime is inherently related to its atmosphere through long-term cycling of volatiles through outgassing. Our work has therefore important consequences for current and upcoming space missions, such as CHEOPS, JWST, ARIEL or PLATO that will target super-Earths and aim to characterise their atmospheres and surface composition. A spatially-variable release of volatiles might for example lead to secondary features in thermal phase curve observations.