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

TL;DR

This work presents a framework to quantify how interior composition (notably FeO-driven mantle oxidation) and surface topography set the water inventory and flooding potential of rocky planets. Using ExoPlex-based interior modeling and defect-chemistry water storage in the transition zone, it links Mg/Si and FeO to transition-zone capacity, while a hypsometric approach evaluates surface flooding and seafloor pressures. The study applies a homogeneous set of host-star abundances from APOGEE and GALAH to 689 rocky planets, deriving a scaling relation for water storage with planet radius and showing that mantle chemistry and topography jointly govern climate-relevant water distribution. Key findings include a strong FeO effect on TZ water storage, Earth-like topographies achieving partial to full flooding for moderate FeO, and Mercury-like topographies enabling widespread surface oceans at relatively low water inventories, with implications for habitability and climate stability on rocky exoplanets.

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.

Figures (8)

• Figure 1: The normalized hypsometry of 1 M $_\oplus$ planet calculated using topographic data from Earth (teal)amante2009, Mars (purple) fergason2018(MARSData), Venus (pink) VenusData, and Mercury (gray) MercuryData. Mercury and Earth have the most shallow topographies. Given the steep topographies of Mars and Venus, they are most useful when considering the surfaces of Earth-like or smaller ($<$ 1 R$_\oplus$)
• Figure 2: Planet sample as a function of radius and period. Our total sample consists of 689 rocky planets. We use a period-dependent radius gap VanEylen2018Ho2023 to classify super-Earth and Earth-like planets.
• Figure 3: Transition zone (TZ) water storage capacity as a function of Mg/Si and FeO for a 1 M$_\oplus$ planet with a mantle potential temperature of 1600 K. We use cubic interpolation between data points for smoothing. The colorbar indicates the water storage capacity in Earth oceans. Earth's composition is indicated by $\oplus$. The TZ water storage capacity increases with FeO. The drastic increase in water storage capacity at FeO $\sim$11 wt% corresponds with a pressure change in the transition of olivine to wadsleyite.
• Figure 4: TZ water storage capacity as a function of composition and mantle potential temperature assuming a 1 M$_\oplus$ planet. The colorbar indicates the water storage capacity in Earth oceans. We use cubic interpolation between data points for smoothing.Right: We show the variation of water storage capacity with temperature as a function of FeO. Decreasing temperature corresponds with a higher water storage capacity. Left: We show the variation of water storage capacity with temperature as a function of Mg/Si. Generally, (Mg,Fe)$_2$SiO$_4$ minerals (i.e., wadsleyite and ringwoodite) show a greater dependence on temperature than other minerals.
• Figure 5: Water storage capacity as a function of planet radius and composition, assuming a Mg/Si= 1.2. We show the impact of FeO on the water storage capacities as a function of radius and their corresponding fits given by equation (\ref{['eq:WSC']}).