The tectonic mode of rocky planets: Part 1 – Driving factors, models & parameters

Abstract

A recent debate as to whether plate tectonics should occur on super-Earths raises two questions: (1) how can this disagreement between previous models be disentangled, and (2) what controls the propensity of plate tectonics on Earth and other planets?To tackle these questions, we use a 1D thermal evolution model to study the ratio of driving to resistive forces of plate tectonics for a variety of initial conditions, two intrinsically different plate tectonics models, and for a large range of model parameters. This wide approach allows us to crystallize some fundamental factors driving plate tectonics.We find that the way plate tectonics reacts to changes of interior temperature is key for understanding how plate tectonics depends on a planet’s mass (and composition) and derive a new approach to better constrain appropriate scaling parameters for 1D models (i.e., for heat flux (β), convective velocity (γ), and aspect ratio (ε)). This allows us to track back the discrepancy between various groups to different 1D scaling parameters (β, γ, ε), or to different yield stress scalings, interior temperatures, or initial conditions in 2D models. Our results also show that planet structure, composition, and initial conditions significantly affect plate tectonics.By re-analyzing previous 2D plate tectonics models and setting them in relation to our results, we suggest that: (1) increasing interior temperatures and planet mass make plate tectonics less likely; (2) plate tectonics is more likely with increasing mantle viscosity (if vigorously convecting) and not generally with increasing Rayleigh number Ra.Moreover, our results demonstrate that trying to understand distant worlds teaches us how some present assumptions used to describe the dynamics of the Earth (e.g., β=1/3, γ=2/3, ε=0, or plate tectonics more likely with increasing Ra) might not be appropriate – implying that we have to partially revise our current understanding of the Earth’s evolution and rock cycle.