Formation of Early Water Oceans on Rocky Planets

TL;DR

This paper explores how rocky planets can form surface water oceans early in their history through volatile degassing and atmospheric collapse, with implications for exoplanet habitability.

Contribution

It identifies mechanisms for early water ocean formation on rocky planets, including accretion with low water content and atmospheric collapse during solidification.

Findings

Planets with 1-3% water can extrude oceans after magma solidification.

Low water content (as low as 0.01%) can lead to atmospheric collapse into oceans.

Exoplanets may develop water oceans within hundreds of millions of years after formation.

Abstract

Terrestrial planets, with silicate mantles and metallic cores, are likely to obtain water and carbon compounds during accretion. Here I examine the conditions that allow early formation of a surface water ocean (simultaneous with cooling to clement surface conditions), and the timeline of degassing the planetary interior into the atmosphere. The greatest fraction of a planet's initial volatile budget is degassed into the atmosphere during the end of magma ocean solidification, leaving only a small fraction of the original volatiles to be released into the atmosphere through later volcanism. Rocky planets that accrete with water in their bulk mantle have two mechanisms for producing an early water ocean: First, if they accrete with at least 1 to 3 mass% of water in their bulk composition, liquid water may be extruded onto the planetary surface at the end of magma ocean solidification. Second, at initial water contents as low as 0.01 mass% or lower, during solidification a massive supercritical fluid and steam atmosphere is produced that collapses into a water ocean upon cooling. The low water contents required for this process indicate that rocky super-Earth exoplanets may be expected to commonly produce water oceans within tens to hundreds of millions of years of their last major accretionary impact, through collapse of their atmosphere.