Effect of surface-mantle water exchange parameterizations on exoplanet ocean depths

TL;DR

This study investigates how different models of water exchange between the surface and interior of exoplanets influence their long-term water distribution, revealing conditions under which planets become waterworlds.

Contribution

It introduces and compares various volatile cycling models, highlighting how different dependencies on temperature and pressure affect planetary water distribution.

Findings

Steady-state water cycling occurs after about 2 Gyr.

High water abundance (>0.3%) is needed for waterworlds if cycling depends solely on temperature or pressure.

Hybrid models suggest super-Earths can become waterworlds with Earth-like water fractions.

Abstract

Terrestrial exoplanets in the canonical habitable zone may have a variety of initial water fractions due to random volatile delivery by planetesimals. If the total planetary water complement is high, the entire surface may be covered in water, forming a "waterworld." On a planet with active tectonics, competing mechanisms act to regulate the abundance of water on the surface by determining the partitioning of water between interior and surface. Here we explore how the incorporation of different mechanisms for the degassing and regassing of water changes the volatile evolution of a planet. For all of the models considered, volatile cycling reaches an approximate steady-state after $\sim 2 \ \mathrm{Gyr}$. Using these steady-states, we find that if volatile cycling is either solely dependent on temperature or seafloor pressure, exoplanets require a high abundance ($\gtrsim 0.3\%$ of total mass) of water to have fully inundated surfaces. However, if degassing is more dependent on seafloor pressure and regassing mainly dependent on mantle temperature, the degassing rate is relatively large at late times and a steady-state between degassing and regassing is reached with a substantial surface water fraction. If this hybrid model is physical, super-Earths with a total water fraction similar to that of the Earth can become waterworlds. As a result, further understanding of the processes that drive volatile cycling on terrestrial planets is needed to determine the water fraction at which they are likely to become waterworlds.