# Dissecting the puzzle of tectonic lid regimes in terrestrial planets

## TL;DR

This study uses models to classify tectonic regimes on rocky planets, including a new episodic-squishy lid regime, to better understand how Earth and Venus evolved as they cooled.

## Contribution

The discovery of the episodic-squishy lid regime and mapping of tectonic transitions during planetary cooling.

## Key findings

- Six tectonic regimes were identified, including the newly discovered episodic-squishy lid regime.
- Planetary cooling conditions influence transitions between tectonic regimes.
- The episodic-squishy lid regime may have played a role in early Earth and present-day Venus.

## Abstract

The surface tectonic style of rocky planets controls interior evolution, geologic activity, dynamo action, atmospheric composition, and thus, habitability. In the solar system, Earth is unique in exhibiting plate tectonics, but it also displays a protracted history of diverse tectonic regimes. To understand the dynamics of the coupled plate-mantle system, here we explore 2D hemispheric-scale thermochemical mantle convection models with self-consistent magmatism that reach the statistical steady state. By statistical analysis of model predictions, we quantitatively distinguish between six tectonic regimes, including the mobile, stagnant, sluggish, plutonic-squishy and episodic lids. In addition, we discover the episodic-squishy lid regime, characterized by alternating episodes of plutonic-squishy lid and mobile-lid behavior. By mapping out these regimes over a wide parameter space, we constrain the conditions for potential regime transitions during planetary cooling. Based on geological evidence of Earth’s tectonic history, our results point to decreasing effective lithospheric strength during planetary evolution, consistent with previously-proposed physical weakening mechanisms. We also suggest an important role of the episodic-squishy lid for early Earth and present-day Venus. Thus, our study helps to understand the tectonic history of terrestrial planets as they cool over time.

This study uses numerical models to classify six tectonic regimes on rocky planets, including an episodic-squishy lid regime. We map regime transitions during planetary cooling, thus contributing to the understanding of Earth and Venus evolution.