Geodynamics of super-Earth GJ 486b

Abstract: Many super-Earths are on very short orbits around their host star and, therefore, more likely to be tidally locked. Because this locking can lead to a strong contrast between the dayside and nightside surface temperatures, these super-Earths could exhibit mantle convection patterns and tectonics that could differ significantly from those observed in the present-day solar system. The presence of an atmosphere, however, would allow transport of heat from the dayside towards the nightside and thereby reduce the surface temperature contrast between the two hemispheres. On rocky planets, atmospheric and geodynamic regimes are closely linked, which directly connects the question of atmospheric thickness to the potential interior dynamics of the planet. Here, we study the interior dynamics of super-Earth GJ 486b ($R=1.34$ $R_{\oplus}$, $M=3.0$ $M_{\oplus}$, T$_\mathrm{eq}\approx700$ K), which is one of the most suitable M-dwarf super-Earth candidates for retaining an atmosphere produced by degassing from the mantle and magma ocean. We investigate how the geodynamic regime of GJ 486b is influenced by different surface temperature contrasts by varying possible atmospheric circulation regimes. We also investigate how the strength of the lithosphere affects the convection pattern. We find that hemispheric tectonics, the surface expression of degree-1 convection with downwellings forming on one hemisphere and upwelling material rising on the opposite hemisphere, is a consequence of the strong lithosphere rather than surface temperature contrast. Anchored hemispheric tectonics, where downwellings und upwellings have a preferred (day/night) hemisphere, is favoured for strong temperature contrasts between the dayside and nightside and higher surface temperatures.

• The paper shows that strong lithospheric yield strength, not just thermal forcing, triggers the transition to hemispheric (degree-1) mantle convection.
• The study employs a two-stage methodology, integrating Exo-FMS general circulation models with 2D spherical-annulus mantle convection simulations to explore tectonic regimes.
• The paper quantitatively pinpoints a narrow yield stress window (125–200 MPa) for the onset of anchored hemispheric tectonics, influencing surface evolution on super-Earths.

Geodynamics of Super-Earth GJ 486b: Analysis and Implications

Introduction

"Geodynamics of super-Earth GJ 486b" (2408.10851) investigates the interior mantle convection and tectonic behaviors of the exoplanet GJ 486b, a $1.3-1.34\, R_\oplus$, $3.0 M_\oplus$ rocky planet orbiting an M-dwarf. The work systematically probes how lithospheric yield strength and the planet's anticipated day-night surface temperature contrast, controlled by atmospheric heat redistribution efficiency, drive tectonic regime transitions. Through the integration of general circulation models (GCMs) and parameterized 2D spherical-annulus mantle convection simulations, the study quantifies the parameter space resulting in hemispheric (degree-1) convective patterns, elucidating the internal-external coupling relevant to hot, close-in rocky exoplanets.

Modeling Approach

The authors leverage a two-stage modeling workflow. First, they use Exo-FMS GCMs to estimate hemispheric surface temperature contrasts for a suite of plausible atmospheric scenarios, parameterized by surface pressure, infrared optical depth, and mean molecular weight. These contrast values then provide boundary conditions for the subsequent mantle convection simulations, executed with StagYY. The internal structure is reconstructed assuming Earth-like composition, with mantle rheology modeled via a stratified, pressure- and temperature-dependent Arrhenius-type viscosity law combined with a plastic yielding formulation to capture lithospheric strength. The model systematically explores a wide range of ductile yield stresses, spanning the spectrum from plate-like to stagnant lid behaviors.

Key Results

The central findings can be summarized as follows:

• Role of Lithospheric Strength: The occurrence of degree-1 (hemispheric) convective patterns is primarily a consequence of the lithospheric yield strength, not the surface temperature contrast per se. When the lithosphere is strong ($\sigma_{\text{duct}} \gtrsim 125$--$250$ MPa for the parameter space considered), convection organizes into a hemipheric mode with a dominant single downwelling.
• Impact of Day-Night Contrast: Surface temperature contrast serves to anchor the degree-1 pattern, assigning a preferred hemisphere for downwellings (typically the dayside for strong yield plus high contrast, i.e., "anchored hemispheric tectonics") or yielding a mobile/unstable arrangement ("mobile hemispheric tectonics") for moderate/low contrasts.
• Plasticity Thresholds: For low-to-moderate lithospheric strengths ($\sigma_{\text{duct}} \leq 100$ MPa), the convection remains degree-$n$ with upwellings and downwellings distributed uniformly regardless of imposed temperature contrast.
• Numerical Regime Transitions: The transition between uniform convection and degree-1 patterns is sharp over a narrow yield stress interval; the stability of the convective configuration (anchored vs. mobile) is determined robustly via rotated initial-state experiments.

Robust Numerical Findings and Claims

• Degree-1 convection is not directly caused by the imposed thermal forcing but by the nonlinear rheology of the strong lithosphere. This is a significant correction to common interpretations of hemispheric convection in tidally locked exoplanets.
• Thresholds for regime transitions are quantitative: For their adopted compositional and rheological profiles, the transition to degree-1 is at $\sigma_{\text{duct}} \approx 125$--$200$ MPa.
• Anchoring of convection patterns requires both strong lithosphere and a substantial surface temperature contrast (empirically, $\gtrsim 200$ K) and/or high dayside temperature.
• The modeling results are stable over multi-Gyr integration times, ensuring insensitivity to initial condition choice.

Implications for Exoplanet Tectonics and Observability

The strong dependence of tectonic regime on lithospheric strength implies that the tectonic diversity of close-in super-Earths is likely to be dominated as much by their interior structure/hydrostatic history as by their varying surface conditions. High yield strength promotes low-degree mantle flow, favoring hemispheric crustal dichotomies analogous to those inferred for Mars or the Moon in particular epochs, rather than Earth-like distributed tectonics.

Atmospheric retention and composition, through their control of heat transport and thus surface temperature contrast, exert a secondary but crucial role: atmosphere-free or thin-atmosphere worlds are most susceptible to anchored hemispheric tectonics, favoring side-dependent surface evolution (outgassing, volcanism, volatile cycling). Conversely, efficient atmospheric redistribution suppresses fixed hemispheric patterns, leading to more symmetric tectonic and volcanic activity.

These results are directly relevant for interpreting high-precision emission and transmission spectra of rocky exoplanets, as signatures of hemispheric differences (e.g., in outgassing-driven atmospheric chemistry or surface composition) may emerge only with strong lithosphere and significant thermal contrast. However, current observational sensitivities are inadequate to directly probe these interior-atmosphere couplings: indirect inference via dayside/nightside emission asymmetries or terminator composition gradients remains speculative.

Future Directions

• Further work extending the models to 3D geometries is necessary to capture the full spectrum of possible mantle planforms and their rotation/translation symmetries.
• Exploration of non-Earth-like mineralogies and alternative core-to-mantle ratios could reveal further diversity in tectonic outcomes, as the degree-1 regime is sensitive to internal layering and stratification.
• Quantitative coupling to outgassing rates and their possible remote signatures (thermal emission, atmospheric line profiles) remains a critical but open problem.

Conclusion

This study demonstrates that the tectonic regime of super-Earths such as GJ 486b is dictated principally by lithospheric rheology, with surface temperature contrast acting as a modulator that can "anchor" degree-1 convective structures to the planetary hemispheres if sufficient. These results directly inform the interpretation of existing and future observations of close-in rocky exoplanets, emphasizing the necessity of combining interior modeling with atmospheric and spectroscopic constraints. The work substantially clarifies the expected tectonic diversity in exoplanet systems and provides a robust framework for the theoretical-experimental feedback loop essential to exoplanet geoscience (2408.10851).