Tectonic Diversity in Rocky Exoplanets: The Impact of Planet Mass and Magmatism

In recent years, there has been a significant increase in the detection of exoplanets, revealing a remarkable diversity of exoplanetary systems that stand in sharp contrast to our Solar System. These systems exhibit a wide range of variations, including size, mass, orbital distance, and host star type. Among them, rocky exoplanets are particularly intriguing because of their potential to harbour life. Tectonic activity is often considered a crucial ingredient in terms of sustaining life-friendly surface conditions.  Therefore, modelling the interior processes of these terrestrial exoplanets is required to understand their tectonic regimes and identify potentially habitable worlds.

Progress made in numerical modelling has greatly enhanced our understanding of tectonically active “mobile lid” and inactive “stagnant lid” tectonic regimes. Alternative tectonic modes, e.g. the episodic lid, sluggish lid, and plutonic-squishy lid, have also been characterised, but are not fully confirmed by observations. In the context of exoplanet discoveries, the question arises whether the mobile lid regime is more or less likely on larger planets, or if alternative surface tectonic regimes become more prevalent. While this is not a completely unexplored topic, previous research yields conflicting results. Moreover, most existing studies overlook factors such as mantle melting, crustal production, and the occurrence of intrusive magmatism.

In this work, we use the mantle convection code StagYY to model generic sub- and super-Earths in 2D spherical annulus geometry, incorporating crustal formation due to extrusive and intrusive magmatism. We focus on determining the trends in tectonic regimes as a function of planet mass (from 0.5 to 2 times that of Earth), surface yield stress, and the ratio of intrusive-to-extrusive magmatism. Our models suggest that the propensity of the mobile lid regime at low surface yield stresses only depends weakly on planet mass. Additionally, the plutonic-squishy lid regime emerges in models with high intrusion efficiency and high yield stresses, whereas the stagnant lid regime occurs at high extrusion efficiency and high yield stresses. Another noteworthy finding is the identification of the episodic-squishy lid regime at intermediate yield stresses, characterised by an alternation between a mobile and a plutonic-squishy lid. Future research will explore the effects of varying surface temperatures within the model. This study holds significant implications for advancing our understanding of planetary thermal and tectonic evolution.