Inferring interior properties of rocky exoplanets from observations of their atmospheres: evaluating the biases caused by atmospheric photochemistry and thermochemistry

With more than 5,000 exoplanet detections confirmed, most of which are small planets below the size of Neptune, a population of ‘samples’ is now at our reach to assess the diversity of interior properties permitted in planetary systems and the impact on the atmospheric composition and climate. The formation of secondary atmospheres around rocky planets via volcanic outgassing is known to be controlled by key properties of the interior, i.e., the oxygen fugacity describing the bulk redox state of the mantle, and the volatile inventory. Observations of exoplanet atmospheres can provide crucial constraints on the relative abundances of molecular species including CO2, CH4, CO, NH3 and SO2 which encode crucial information on the interior properties. A clear ambitious direction is emerging in the field aiming to use the retrieved atmospheric properties to constrain the interior properties of rocky exoplanets and thus better understand the formation of secondary atmospheres in various planetary systems including our own.

Clear correlations between the relative abundances of these observable volatiles and the interior properties (oxygen fugacity and volatile budget) are required to draw reliable conclusions. The relative abundance CO2/CO for instance is a known reliable indicator of oxygen fugacity. This approach however relies on the assumption that the volatile composition outgassed in the atmosphere and observed with telescopes is not influenced by other atmospheric mechanisms. In the present work, we address the role of atmospheric thermochemistry and photochemistry to assess the induced change on the observed relative abundances of volatiles and the bias introduced on the interior properties such as oxygen fugacity. We explore a wide range of possible conditions for rocky exoplanets in terms of volatile budget, oxygen fugacity and surface pressure. We use a self-consistent chemistry-radiative model to consider the change of gases in the atmosphere and their influence on the climate.

We find that atmospheric thermochemistry is a key process changing the relative abundances of volatiles in the atmosphere following the decrease of temperature between the interior and the surface, a process referred to as atmospheric cooling. This mechanism alone can lead to strong biases on the inferred interior oxygen fugacity as it significantly modifies the CO2/CO ratio in the atmosphere. In addition, the decrease of temperature in the atmosphere can allow significant build-up of CH4 in reducing conditions. Co-existence of CH4 and CO2 at high abundances is possible in several conditions at high temperatures. In practice, it means that one needs to have reliable constraints on the surface temperature to use CO2-CH4 as a biosignature. In reducing conditions, the formation of methane by thermochemistry triggers a climate feedback given the strong greenhouse properties of methane. This mechanism should lead to observations with clear signatures of CH4 and CO2  which do not indicate habitable conditions but rather indicate reducing conditions and high surface temperature.

For thinner atmospheres, we found that the low surface temperature make thermochemistry inefficient relative to photochemistry. Photochemistry can however also bias the CO2/CO abundance ratio as water vapor is dissociated by UV photons and oxidizes CO into CO2. This mechanism occurs over long timescales that may be compensated by the surface volcanic flux assuming that the targeted exoplanet is still volcanically active.