Retention of Water in Terrestrial Magma Oceans and Carbon-rich Early Atmospheres

Abstract

Massive steam and CO2 atmospheres have been proposed for magma ocean outgassing of Earth and terrestrial planets. Yet formation of such atmospheres depends on volatile exchange with the molten interior, governed by volatile solubilities and redox reactions. We determine the evolution of magma ocean–atmosphere systems for a range of oxygen fugacities, C/H ratios, and hydrogen budgets that include redox reactions for hydrogen (H2–H2O), carbon (CO–CO2), methane (CH4), and solubility laws for H2O and CO2. We find that small initial budgets of hydrogen, high C/H ratios, and oxidizing conditions suppress outgassing of hydrogen until the late stage of magma ocean crystallization. Hence, early atmospheres in equilibrium with magma oceans are dominantly carbon-rich, and specifically CO-rich except at the most oxidizing conditions. The high solubility of H2O limits its outgassing to melt fractions below ∼30%, the fraction at which the mantle transitions from vigorous to sluggish convection with melt percolation. Sluggish melt percolation could enable a surface lid to form, trapping water in the interior and thereby maintaining a carbon-rich atmosphere (equilibrium crystallization). Alternatively, efficient crystal settling could maintain a molten surface, promoting a transition to a water-rich atmosphere (fractional crystallization). However, additional processes, including melt trapping and H dissolution in crystallizing minerals, further conspire to limit the extent of H outgassing, even for fractional crystallization. Hence, much of the water delivered to planets during their accretion can be safely harbored in their interiors during the magma ocean stage, particularly at oxidizing conditions.

A detailed analysis of the Gl 486 planetary system

“…We allowed the mantle to vary in iron content, which is why an iron-rich mantle with no core was allowed in the posterior solution. Whenever we allowed for water to be present in dissolved form in possible magma layers, we followed the approach of Dorn & Lichtenberg (2021), while the partitioning of water between the surface reservoir and the magma was determined by solubility laws (Kessel et al 2005;Lichtenberg et al 2021;Bower et al 2022). We accounted for the fact that water increases the melt fraction by lowering the melting temperature (Katz et al 2003).…”

Deep Two-phase, Hemispherical Magma Oceans on Lava Planets

“…However, vapor can also degas from the melt at depths when it becomes supersaturated in volatile, as observed in terrestrial volcanoes (e.g., Le Guern et al 1979;Tazieff 1994). This process might be an alternative to surface degassing when the magma ocean is isolated from the atmosphere by a solid conductive lid (e.g., Bower et al 2022), and thus reinforces the efficiency of a lava planet in degassing its volatile content in regions where the surface temperature is below the solidus, i.e., near the terminator and on the nightside.…”

“…Models of magma oceans generally consider the liquid-gas equilibrium at the planet surface as a primary mechanism for atmosphere formation (e.g., Abe & Matsui 1986;Bower et al 2022). However, vapor can also degas from the melt at depths when it becomes supersaturated in volatile, as observed in terrestrial volcanoes (e.g., Le Guern et al 1979;Tazieff 1994).…”

The Long-Term Evolution of the Atmosphere of Venus: Processes and Feedback Mechanisms

“…Understanding the evolution of CO 2 in the atmosphere is not only crucial for predicting the potential for the existence of liquid water on early Venus, but also provides insights into the divergent evolution of Earth and Venus. As discussed in Salvador et al (2022), the solidification of a magma ocean may have outgassed large amounts of CO 2 into the early atmosphere, although it is difficult to constrain (e.g., Bower et al 2022;Gaillard et al 2022a). The present day atmosphere could be a combination of an early atmosphere resulting from magma ocean solidification, outside contribution from impactors (Gillmann et al 2020, both early and late) and a later contribution from subsequent long-term magmatic mantle outgassing .…”

“…For example, the fraction of CO 2 released by metamorphic crustal decarbonation that makes it to the surface is a major uncertainty in coupled atmosphere-interior models. In addition, volatile partitioning during magma ocean solidification controls the post-crystallization atmosphere (Elkins-Tanton 2008;Massol et al 2016;Hier-Majumder and Hirschmann 2017;Salvador et al 2017;Nikolaou et al 2019;Bower et al 2022;Gaillard et al 2022a,b). The composition of this initial atmosphere may in turn dictate whether the precipitation of water oceans, and therefore CO 2 drawdown and climate regulation via silicate weathering is permissible.…”