From Underground Oceans to Continents: A Glimpse into the Water Inventory on Rocky Planets using Host Star Abundances

TL;DR

This study investigates how interior composition, topography, and planet size influence the water inventory of rocky planets, revealing the significant roles of mantle oxidation state and surface features in determining surface water presence and distribution.

Contribution

It is the first to analyze the combined effects of interior composition, topography, and radius on planetary water inventory using a large sample of spectroscopically characterized rocky planets.

Findings

Mantle oxidation state (FeO content) affects water storage capacity.

Flat topographies are more prone to flooding and surface water presence.

Mars-like topographies may lead to high-pressure ice formation, reducing weathering.

Abstract

The amount of surface water is thought to be critical for a planet's climate stability and thus habitability. However, the probability that a rocky planet may exhibit surface water at any point its evolution is dependent on multiple factors, such as the initial water mass, geochemical evolution, and interior composition. To date, studies have examined the influence of interior composition on the water inventory of the planet or how surface oceans may be impacted by planet topography individually. Here, we provide the first exploration on the impact of interior composition, topography, and planet radius on the water inventory of rocky planets using a sample of 689 rocky planets with spectroscopically derived stellar abundances from APOGEE and GALAH. We find that the oxidation state of the mantle (FeO content) significantly impacts the mantle water storage capacity and potential for surface flooding. For an FeO ~11 wt%, the water storage capacity of a 1 M$_\oplus$ is 2 times that of Earth, indicating that the oxidation state may reduce the amount of surface water. We quantify the impact of topography on seafloor pressures, showing that flat topographies are more likely to be flooded for all planet compositions and radii. We also find that Mars-like topographies are more likely to have seafloor pressures that may form high-pressure ice, reducing seafloor weathering. Thus, for the first time, we show that the composition and topography of the mantle influence the water inventory of rocky planets.