Interior dynamics of tidally-locked super-Earths: the case of LHS 3844b

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime. Earth's surface temperature is spatially uniform at 300 K, which is sufficiently cold to drive strong downwellings into the interior (i.e. subduction). In contrast, a tidally-locked super-Earth can have a large temperature contrast between the dayside and nightside, which we infer could lead to a dichotomy of the interior dynamics. We therefore use constraints from astrophysical observations to infer the possible pattern of flow in the interior of a tidally-locked super-Earth, using super-Earth LHS 3844b as a case study. 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 (the total mantle depth is 3757 km). An upper and lower bound for the viscosity of post-perovskite is provided by previous theoretical calculations. We include plastic yielding to model the brittle nature of the lithosphere; plastic yielding occurs when the local stress state exceeds a prescribed yielding criteria and is commonly applied in studies of Earth to produce surface behaviour similar to plate tectonics.

For a low yield stress criteria (promoting a weak lithosphere), we find that plumes are generally evenly distributed between the dayside and nightside, albeit strong downwellings form on the nightside. Plumes on the nightside have less lateral mobility than on the dayside because they are confined by downwellings either side. In contrast, for a high yield stress criteria, the interior dynamics are mostly driven by a prominent downwelling on the dayside which flushes hot material from the lower thermal boundary layer around the CMB towards the nightside where plumes preferentially arise. This, in turn, leads to a return flow of colder material from the near surface of the nightside towards the dayside. This seemingly counterintuitive pattern of flow is a consequence of weak lithosphere (due to temperature) on the dayside that is able to deform and thereby subduct, whereas lithosphere on the nightside is too stiff to subduct.

Our models therefore show that the vigour of convection and the distribution of upwellings and downwellings of tidally locked super-Earths are sensitive to the strength of the lithosphere: plumes can either be equally distributed around the planet or preferentially occur on the nightside. In the first case, the cold downwellings are also equally distributed but more prominent on the nightside, whereas in the second case they are preferentially on the dayside. Somewhat unexpected, we do not observe a preference for hot plumes to congregate on the dayside. 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.