Effects of Temperature-Dependent Lithospheric Yield Stress on Ultra-Short Period super-Earth LHS 3844b

The discovery of exoplanets has uncovered a vast spectrum of planetary types, from enormous gas giants to smaller, rocky worlds akin to Earth. Among these, super-Earths are prevalent and are believed to exhibit a range of tectonic regimes. A portion of these have ultra-short periods and orbit their stars in mere hours to days, resulting in synchronous rotation with their host star. This establishes a surface temperature dichotomy like that seen on LHS 3844b, a bare-rock super-Earth with a radius approximately 1.3 times that of Earth, where temperatures reach 1040 K at the point receiving the most intense sunlight on the dayside and drop close to 0 K on the nightside.

We use StagYY to model mantle convection on LHS 3844b in a 2D spherical-annulus geometry. Our models incorporate a temperature-dependent yield stress that captures both near-surface and deep lithospheric rheological variations, rather than assuming a fixed effective yield stress as in previous studies. We represent the effects of various temperature-dependent microphysical processes by varying the temperature dependence of the yield stress slope. The yield stress components in our models are systematically varied to examine their impact on tectonic style and mantle dynamics.

Parameterisation of the brittle component is based on the proposition that temperature-dependent frictional weakening plays a factor in the tectonic regimes of Earth and Venus. On Earth, where low surface temperatures create a geothermal gradient that keeps much of the crust below 400°C, frictional heating can reduce the friction coefficient at high slip velocities. In contrast, Venus’ elevated surface temperatures maintain a higher friction coefficient, which helps suppress plate tectonics. In deeper lithospheric regions, elevated temperatures favour ductile deformation, which would normally weaken the lithosphere. However, these higher temperatures can also promote grain growth, counteracting dynamic strain localisation and thereby strengthening the rock.

We find that hemispheric temperature differences strongly influence lithospheric strength and deformation on LHS 3844b: the colder nightside allows brittle failure to persist over greater depths, whilst the hotter dayside promotes ductile flow at shallower depths due to a much thinner lithosphere. Importantly, we find that an increased temperature dependence of the ductile yield stress amplifies the hemispheric contrast in the planet's tectonic behaviour.