Growth and Evolution of Secondary Volcanic Atmospheres: II. The Importance of Kinetics

Abstract

Volcanism is a major and long-term source of volatile elements such as C and H to Earth's atmosphere, likely has been to Venus's atmosphere, and may be for exoplanets. Models simulating volcanic growth of atmospheres often make one of two assumptions: either that atmospheric speciation is set by the high-temperature equilibrium of volcanism; or, that volcanic gases thermochemically re-equilibrate to the new, lower, temperature of the surface environment. In the latter case it has been suggested that volcanic atmospheres may create biosignature false positives. Here, we test the assumptions underlying such inferences by performing chemical kinetic calculations to estimate the relaxation timescale of volcanically-derived atmospheres to thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry and reaction catalysis. We demonstrate that for planets with volcanic atmospheres, thermochemical equilibrium over geological timescales can only be assumed if the atmospheric temperature is above ~700K. Slow chemical kinetics at lower temperatures inhibit the relaxation of redox-sensitive species to low-temperature thermochemical equilibrium, precluding the production of two independent biosignatures through thermochemistry alone: 1. ammonia, and 2. the co-occurrence of CO${}_2$ and CH${}_4$ in an atmosphere in the absence of CO. This supports the use of both biosignatures for detecting life. Quenched at the high temperature of their degassing, volcanic gases also have speciations characteristic of those produced from a more oxidized mantle, if interpreted as being at thermochemical equilibrium. This therefore complicates linking atmospheres to the interiors of rocky exoplanets, even when their atmospheres are purely volcanic in origin.

Accepted by JGR: Planets

References in corpus (17)

• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets
• Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
• VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
• Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts
• Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life
• A Comparative Study of Atmospheric Chemistry with VULCAN
• The habitability of a stagnant-lid Earth
• Vertically resolved magma ocean-protoatmosphere evolution: H${}_2$, H${}_2$O, CO${}_2$, CH${}_4$, CO, O${}_2$, and N${}_2$ as primary absorbers
• How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres
• Abundant atmospheric methane from volcanism on terrestrial planets is unlikely and strengthens the case for methane as a biosignature
• Sulfur Chemistry in the Atmospheres of Warm and Hot Jupiters
• Growth and evolution of secondary volcanic atmospheres: I. Identifying the geological character of hot rocky planets
• Can Volcanism Build Hydrogen-Rich Early Atmospheres?
• Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres
• Evolution of the Earth's Atmosphere during Late Veneer Accretion
• Coexistence of CH4, CO2 and H2O in exoplanet atmospheres

Cited by in corpus (9)

• A dry Venusian interior constrained by atmospheric chemistry
• Constraining exoplanet interiors using observations of their atmospheres
• Low 4.5 μm Dayside Emission Disfavors a Dark Bare-Rock scenario for the Hot Super-Earth TOI-431 b
• Tracing the Inner Edge of the Habitable Zone with Sulfur Chemistry
• Can tidal evolution lead to close-in planetary bodies around white dwarfs II: volcanism and transits
• Constraining the survival of HCN during cometary impacts
• The atmospheres of rocky exoplanets III. Using atmospheric spectra to constrain surface rock composition
• On the importance of laboratory experiments for interpreting exoplanet observations
• Influence of CO versus CH${}_4$ on organic haze formation in atmospheres of diverse terrestrial exoplanets