Growth and evolution of secondary volcanic atmospheres: I. Identifying the geological character of hot rocky planets

The geology of Earth and super-Earth sized planets will, in many cases, only be observable via their atmospheres. Here, we investigate secondary volcanic atmospheres as a key base case of how atmospheres may reflect planetary geochemistry. We couple volcanic outgassing with atmospheric chemistry models to simulate the growth of C-O-H-S-N atmospheres in thermochemical equilibrium, focusing on what information about a planet's mantle fO$_2$ and bulk silicate H/C ratio could be determined by atmospheric observation. 800K volcanic atmospheres develop distinct compositional groups as the mantle fO$_2$ is varied, which can be identified using sets of (often minor) indicator species: Class O, representing an oxidised mantle and containing SO$_2$ and sulfur allotropes; Class I, formed by intermediate mantle fO$_2$'s and containing CO$_2$, CH$_4$, CO and COS; and Class R, produced by reduced mantles, containing H$_2$, NH$_3$ and CH$_4$. These atmospheric classes are robust to a wide range of bulk silicate H/C ratios. However, the H/C ratio does affect the dominant atmospheric constituent, which can vary between H$_2$, H$_2$O and CO$_2$ once the chemical composition has stabilised to a point where it no longer changes substantially with time. This final atmospheric state is dependent on the mantle fO$_2$, the H/C ratio, and time since the onset of volcanism. The atmospheric classes we present are appropriate for the closed-system growth of hot exoplanets, and may be used as a simple base for future research exploring the effects of other open-system processes on secondary volcanic atmospheres.Comment: Accepted for publication in JGR:Planet

Can volcanism build hydrogen-rich early atmospheres?

Hydrogen in rocky planet atmospheres has been invoked in arguments for extending the habitable zone via N2-H2 and CO2-H2 greenhouse warming, and providing atmospheric conditions suitable for efficient production of prebiotic molecules. On Earth and Super-Earth-sized bodies, where hydrogen-rich primordial envelopes are quickly lost to space, volcanic outgassing can act as a hydrogen source, provided it balances the hydrogen loss rate from the top of the atmosphere. Here, we show that both Earth-like and Mars-like planets can sustain atmospheric H2 fractions of several percent across relevant magmatic ranges. In general this requires hydrogen escape to operate somewhat less efficiently than the diffusion limit. We use a thermodynamical model of magma degassing to determine which combinations of magma oxidation, volcanic flux and hydrogen escape efficiency can build up appreciable levels of hydrogen in a planet's secondary atmosphere. On a planet similar to the Archean Earth and with a similar magmatic , we suggest that the mixing ratio of atmospheric H2 could have been in the range 0.2-3%, from a parameter sweep over a variety of plausible surface pressures, volcanic fluxes, and H2 escape rates. A planet erupting magmas around the Iron-Wüstite (IW) buffer (i.e., ∼3 log units lower than the inferred Archean mantle ), but with otherwise similar volcanic fluxes and H2 loss rates to early Earth, could sustain an atmosphere with approximately 10-20% H2. For an early Mars-like planet with magmas around IW, but a lower range of surface pressures and volcanic fluxes compared to Earth, an atmospheric H2 mixing ratio of ∼2-8% is possible. On early Mars, this H2 mixing ratio could be sufficient to deglaciate the planet. However, the sensitivity of these results to primary magmatic water contents and volcanic fluxes show the need for improved constraints on the crustal recycling efficiency and mantle water contents of early Mars

The Atmospheric Fingerprints of Volcanism: Simulating Volcanic Outgassing and Secondary Atmospheres on Rocky Planets

The study of the atmospheric composition and evolution of rocky planet atmospheres is key to understanding both the conditions required to develop a habitable planet, and to analyse the link between the deep interior and atmosphere of rocky bodies. This thesis uses volcanism as a chemical link between the mantle of a planet and its atmosphere, with the aim of analysing how a volcanically derived or supplemented atmosphere may appear, both under the end-member case where volcanism is the only factor affecting the atmosphere, and when changing surface temperatures and atmospheric escape is considered. Chapter 2 describes a newly developed model of volcanic degassing for COHSN elements, designed with the broad range of exoplanet geochemistry in mind. It also describes a model for simulating the evolution of a volcanic atmosphere through time, based on the initial volatile content of a planetary mantle, the surface temperature and a stipulation for the escape of hydrogen. Chapter 3 demonstrates that volcanic activity can sustain a fraction of hydrogen in planetary atmospheres undergoing hydrogen escape, which may have contributed to a cold, wet early Mars, and expands the liquid water habitable zone for exoplanets. Chapter 4 shows that on planets with Venus-like atmospheric temperatures, the mantle fO2 of a planet can be inferred from the chemistry and composition of a volcanic atmosphere as three distinct classes (defined by the presence/absence of certain indicator species) are formed. Specifically, Chapter 4 presents a set of volcanic atmospheres as an important base case for future research, exploring the effects of other processes on volcanic secondary atmospheres as produced by a range of geological conditions. Chapter 5 utilises chemical kinetics models to show that volcanic atmospheres must be at temperatures of 700K and above in order to be accurately modelled as in thermochemical equilibrium, with the reactions of key species (NH3, CO and CH4) being quenched over geological time below this point. Chapter 6 returns to the effect of hydrogen escape on volcanic atmospheres, exploring how escape modifies the atmospheric classes discussed in Chapter 4 and reduces or removes all indicators of mantle fO2 from the atmosphere. This thesis presents a new volcanic degassing model and a number of use-cases, demonstrating the wide range of chemical speciations which volcanically generated atmospheres can form.Embiricos Trust Scholarship, Jesus College Cambridg

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

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

The sulfur solubility minimum and maximum in silicate melt

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The sulfur solubility minimum and maximum in silicate melt

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