Mantle mineralogy limits to rocky planet water inventories

TL;DR

This study predicts how the mineral composition and size of rocky planets influence their ability to hold water in their mantles, revealing that larger planets tend to have drier lower mantles and more surface water or atmosphere.

Contribution

It introduces a thermodynamic model to estimate mantle water capacities considering mineralogy, temperature, and planet size, highlighting the impact of planetary mass on water distribution.

Findings

Large planets have dry lower mantles with high-capacity transition zones.

Mantle water capacity decreases with increasing planet mass.

Massive planets likely have surface oceans or atmospheric water vapor from initial accretion.

Abstract

Nominally anhydrous minerals in rocky planet mantles can sequester oceans of water as a whole, giving a constraint on bulk water inventories. Here we predict mantle water capacities from the thermodynamically-limited solubility of water in their constituent minerals. We report the variability of mantle water capacity due to (i) host star refractory element abundances that set mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass. We find that planets large enough to stabilise perovskite almost unfailingly have a dry lower mantle, topped by a high-water-capacity transition zone which may act as a bottleneck for water transport within the planet's interior. Because the pressure of the ringwoodite-perovskite phase boundary defining the lower mantle is roughly insensitive to planet mass, the relative contribution of the upper mantle reservoir will diminish with increasing planet mass. Large rocky planets therefore have disproportionately small mantle water capacities. In practice, our results would represent initial water concentration profiles in planetary mantles where their primordial magma oceans are water-saturated. We suggest that a considerable proportion of massive rocky planets' accreted water budgets would form surface oceans or atmospheric water vapour immediately after magma ocean solidification, possibly diminishing the likelihood of these planets hosting land. This work is a step towards understanding planetary deep water cycling, thermal evolution as mediated by rheology and melting, and the frequency of waterworlds.