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Many super-Earths are very close-in to their host star and are therefore likely to be tidally locked. Tidally locked super-Earths experience intense solar heating on their permanent dayside, whereas the nightside surface can reach extremely cold temperatures. For the case of super-Earth LHS 3844b, a bare-rock super-Earth with a radius around 1.3 Earth radii, we have shown that this strong contrast between the dayside and nightside surface temperature can lead to a so-called hemispheric tectonic regime. Such a regime is characterised by a strong downwelling on one hemisphere and upwellings that rise on the other side. We further define hemispheric tectonics as a special case of degree-1 convection (one upwelling and one downwelling), where the downwelling gets pinned to either the dayside or nightside and upwellings are preferentially on the other hemisphere. Here, we focus on super-Earth GJ 486b, which has also a radius around 1.3 Earth radii, but for which it is unknown whether it was able to retain an atmosphere. We investigate how different surface temperature contrasts affects the likelihood of hemispheric tectonics. For this, we run 2D geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is mostly composed of perovskite and post-perovskite. The lithospheric strength is modelled through a plastic yielding criterion and the models are basally heated. We use general circulation models (GCMs) of potential atmospheres to constrain the surface temperature assuming different efficiencies of atmospheric heat circulation. We find that degree-1 convection is a consequence of the strong lithosphere, while hemispheric tectonics is favoured for strong surface temperature contrasts between the dayside and nightside, and higher surface temperatures. Our results show that hemispheric tectonics or degree-1 convection could operate on super-Earth GJ 486b (or other tidally locked super-Earths), even if the surface temperature contrast between the dayside and nightside is not as strong as for LHS 3844b. Upwellings that rise preferentially on one hemisphere could lead to generation of melt and subsequent outgassing of volatiles on that side. Imprints of such outgassing on the atmospheric composition could possibly be probed by current and future observations such as JWST, ARIEL, or the ELT.

Super-Earth LHS 3844b is a rocky exoplanet with a radius around 1.3 Earth radii. Its thermal phase curve suggests that the dayside temperature is around 1040 K and the nightside temperature is around 0 - 700 K, indicating inefficient atmospheric heat circulation. Therefore, this planet most likely lacks an atmosphere. In a previous study, we have shown that such a strong surface temperature dichotomy can lead to a so-called hemispheric tectonic regime. In such a regime, a cold downwelling forms preferentially on one side and hot upwellings are getting pushed towards the other hemisphere. GJ 486b is a super-Earth that is very similar to LHS 3844b in terms of size and it is currently unknown whether this planet has an atmosphere. In this study, we are investigating under which circumstances hemispheric tectonics can operate on GJ 486b. We also investigate the stability of hemispheric tectonics. We run 2D geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is mostly composed of perovskite and post-perovskite. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). We use general circulation models (GCMs) of potential atmospheres to constrain the surface temperature assuming different efficiencies of atmospheric heat circulation. We find that a hemispheric tectonic regime is also possible for surface temperature contrasts with moderate heat redistribution. The location of the strong downwelling depends on several factors such as the surface temperature contrast and strength of the lithosphere. By reducing the temperature contrast, the location of the downwelling becomes less stable and it can start to move from one side towards the other over very long timescales (Gyrs). Our results show that hemispheric tectonics could operate on tidally-locked super-Earths, even if the surface temperature contrast between the dayside and nightside is not as strong as for LHS 3844b. Upwellings that rise preferentially on one hemisphere could lead to generation of melt and subsequent outgassing of volatiles on that side. Imprints of such outgassing on the atmospheric composition could possibly be probed by current and future observations such as JWST, ARIEL or the ELT.

The vigour 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. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. The substellar point is also substantially shifted eastwards, which requires efficient energy circulation in the atmosphere.Here, we use constraints from thermal phase curve observations to model the interior mantle flow. To constrain the surface temperature of 55 Cnc e, we use the results from general circulation models varying the atmospheric composition and optical depth.Depending on how strong the surface temperature contrast is, this can lead to hemispheric tectonic regimes. Such a regime could influence a planet's atmosphere through interior-exterior coupling mechanisms (e.g. volcanic outgassing). We run geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side (Fig. A). We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures at the interface between the magma ocean and the underlying solid mantle and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e (Fig. B). For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt.

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.