Novel tectonic regimes on super-Earth LHS 3844b from phase curve observations

Abstract:

The vigor and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle. The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. We infer that such a large surface temperature contrast could lead to a dichotomy of the interior dynamics. Here, we use the thermal phase curve observation to constrain the interior dynamics and tectonic regimes of LHS 3844b using a numerical method of interior flow.

We run mantle convection models using the code StagYY with two-dimensional spherical annulus geometry and parameters from the literature that are appropriate for LHS 3844b. The majority of the mantle is either perovskite or post-perovskite with the phase transition occurring around 1700 km depth (total mantle depth is 3757 km). We include plastic yielding to model the brittle nature of the lithosphere.

We discover three viable interior convective regimes for LHS 3844b by varying the strength of the lithosphere and the heating mode (basal heating or basal and internal heating): (1) spatially uniform distribution of upwellings and downwellings, (2) prominent downwelling on the dayside and upwellings on the nightside, and (3) prominent downwelling on the nightside and upwellings on the dayside. Hemispheric tectonics is observed for regimes 2 and 3 and is a direct outcome of the day-night temperature contrast. This tectonic regime is absent in the present-day Solar System and has not previously been inferred from astrophysical observations of exoplanets.

Our results have implications for space missions such as TESS, CHEOPS, JWST, PLATO and ARIEL that will discover and characterise super-Earths, thereby potentially probing for signals of volatile outgassing and volcanism such that the different tectonic regimes can be distinguished.