Thermal Evolution and Crust Formation of Tidally-Locked Rocky Exoplanets

Many of the observed rocky exoplanets orbiting close to their host stars are likely tidally locked, with one hemisphere in permanent daylight and the other in complete darkness. Phase curve observations of GI-486b, TRAPPIST-1b and c, and LHS 3844b suggest that such planets may exhibit extreme temperature contrasts between their day and night sides, with differences potentially exceeding hundreds of kelvins. While these surface temperature gradients are widely recognized for their impact on atmospheric dynamics, their influence on interior thermal evolution and crust formation remains less explored. This study addresses how asymmetric surface heating affects mantle processes and crust generation on tidally locked rocky exoplanets operating under stagnant-lid convection.

Using the 1D parametrized thermal evolution model TEMPURA (Baumeister et al. 2023) and accounting for hemispherical surface temperature differences (adapted from Bonnet Gibet et al., 2022), we simulate the long-term interior evolution of tidally-locked planets for a number of day/night temperature differences. The model assumes a stagnant-lid regime, where the lithosphere acts as a rigid, immobile shell through which heat is primarily lost by conduction. By assigning different surface temperatures to the dayside and nightside boundaries, the model can capture the first-order effects of tidal locking on internal dynamics without the need for more complex 2D or 3D models.

Our simulations show that large day/night temperature asymmetries can greatly enhance volcanic activity and crust production on the dayside. A cold nightside leads to the growth of a thick lithosphere, which thermally insulates the mantle efficiently. Partial melting is minimal or entirely absent, producing thinner or no crust. In contrast, on the hot dayside, the lid is thin, thus enhancing melting and crust production. This effect is enhanced the greater the difference between the day and night temperatures. 

Importantly, the findings suggest that tidally locked rocky planets with significant day–night temperature contrasts may evolve into geologically asymmetric bodies, where crustal thickness, heat retention, and possibly surface volcanism vary strongly between day and nightside. Specifically, we find that the ages of crust can vary significantly between day and nightside, with typically a very young surface on the dayside, and old and potentially even primordial crust on the nightside.

This study provides a foundation for interpreting geophysical evolution on tidally locked rocky exoplanets, especially those in close-in orbits around M-dwarf stars. As observational techniques improve, coupling such models with surface and atmospheric simulations may provide critical insight into the nature of exoplanetary surfaces, volcanic histories, and long-term habitability.