Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction

TL;DR

This study develops a climate model to explore how planetary weathering and habitability are affected by land-ocean distribution, revealing insensitivity of weathering behavior to land fraction and proposing a self-arrest mechanism for waterworlds.

Contribution

It introduces a low-order climate-weathering model showing weathering behavior's robustness to land fraction and proposes the waterworld self-arrest mechanism to prevent complete water loss.

Findings

Weathering behavior is weakly dependent on land fraction for partially ocean-covered planets.

Waterworlds can undergo a self-arrest process to avoid total water loss during moist greenhouse stages.

Habitable zone predictions are robust to variations in surface land coverage.

Abstract

It is likely that unambiguous habitable zone terrestrial planets of unknown water content will soon be discovered. Water content helps determine surface land fraction, which influences planetary weathering behavior. This is important because the silicate weathering feedback determines the width of the habitable zone in space and time. Here a low-order model of weathering and climate, useful for gaining qualitative understanding, is developed to examine climate evolution for planets of various land-ocean fractions. It is pointed out that, if seafloor weathering does not depend directly on surface temperature, there can be no weathering-climate feedback on a waterworld. This would dramatically narrow the habitable zone of a waterworld. Results from our model indicate that weathering behavior does not depend strongly on land fraction for partially ocean-covered planets. This is powerful because it suggests that previous habitable zone theory is robust to changes in land fraction, as long as there is some land. Finally, a mechanism is proposed for a waterworld to prevent complete water loss during a moist greenhouse through rapid weathering of exposed continents. This process is named a "waterworld self-arrest," and it implies that waterworlds can go through a moist greenhouse stage and end up as planets like Earth with partial ocean coverage. This work stresses the importance of surface and geologic effects, in addition to the usual incident stellar flux, for habitability.