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

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Rimmer, Paul B 


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. Mod- els simulating volcanic growth of atmospheres often make one of two assumptions: ei- ther that atmospheric speciation is set by the high-temperature equilibrium of volcan- ism; or, that volcanic gases thermochemically re-equilibrate to the new, lower, temper- ature 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 un- derlying such inferences by performing chemical kinetic calculations to estimate the re- laxation timescale of volcanically-derived atmospheres to thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry and reaction catalysis. We demon- strate that for planets with volcanic atmospheres, thermochemical equilibrium over ge- ological timescales can only be assumed if the atmospheric temperature is above ∼700 K. Slow chemical kinetics at lower temperatures inhibit the relaxation of redox-sensitive species to low-temperature thermochemical equilibrium, precluding the production of two in- dependent biosignatures through thermochemistry alone: 1. ammonia, and 2. the co- occurrence of CO2 and CH4 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 there- fore complicates linking atmospheres to the interiors of rocky exoplanets, even when their atmospheres are purely volcanic in origin.



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Journal of Geophysical Research: Planets

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American Geophysical Union (AGU)