Blue marble, stagnant lid: Could dynamic topography avert a waterworld?

TL;DR

This study models dynamic topography on stagnant lid exoplanets to assess if it can support land above water, revealing that even modest topography might prevent planets from becoming waterworlds.

Contribution

We develop new scaling relationships for dynamic topography on stagnant lid planets using 2D convection models and apply them to explore land-water distribution.

Findings

Dynamic topography can support land on Earth-size planets with small water inventories.

Basin capacity increases less steeply with planet mass than water volume.

Dynamic topography amplitudes are around 1 km, providing a lower limit estimate.

Abstract

Topography on a wet rocky exoplanet could raise land above its sea level. Although land elevation is the product of many complex processes, the large-scale topographic features on any geodynamically-active planet are the expression of the convecting mantle beneath the surface. This so-called "dynamic topography" exists regardless of a planet's tectonic regime or volcanism; its amplitude, with a few assumptions, can be estimated via numerical simulations of convection as a function of the mantle Rayleigh number. We develop new scaling relationships for dynamic topography on stagnant lid planets using 2D convection models with temperature-dependent viscosity. These scalings are applied to 1D thermal history models to explore how dynamic topography varies with exoplanetary observables over a wide parameter space. Dynamic topography amplitudes are converted to an ocean basin capacity, the minimum water volume required to flood the entire surface. Basin capacity increases less steeply with planet mass than does the amount of water itself, assuming a water inventory that is a constant planetary mass fraction. We find that dynamically-supported topography alone could be sufficient to maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately $10^{-4}$ times their mass, in the most favourable thermal states. By considering only dynamic topography, which has ~1-km amplitudes on Earth, these results represent a lower limit to the true ocean basin capacity. Our work indicates that deterministic geophysical modelling could inform the variability of land propensity on low-mass planets.