Heterogeneous Reactivity in the Atmosphere of Rocky Planets

The current context of space exploration missions is extremely favourable, especially regarding rocky planets. Several orbiters and rovers currently investigate the surface and atmosphere of Mars; more are planned in the future. Three Medium-class missions have been approved by NASA and ESA to study Venus in the coming years. Space telescopes, recently launched or scheduled, will be able to determine the composition of exoplanet atmospheres. Rocky (exo)planets are preferred targets, because they hold similarities with our own planet. Past and present signs of habitability are eagerly scrutinized, in the first place as atmospheric biomarkers.

Descriptions of planetary atmospheres based on gas-phase (homogeneous) chemistry are globally robust. Nonetheless, recent key detections remain hotly debated, in part because they remain unexplained by gas-phase chemistry alone. The most illustrative example of this is the controversial detection of methane on Mars. Recent work has shown that this apparent contradiction may be lifted by heterogeneous photo-induced reactions¹.

Different types of aerosols exist in planetary atmospheres: dust, clouds, hazes,… They exist as liquids and solids, depending on the physical and chemical parameters (T, P, chemical composition) characterizing the atmosphere. Models of planetary atmospheres usually describe gas-phase reactions and photochemistry, but overlook heterogeneous reactions. Meanwhile, these interactions are key to account for crucial atmospheric processes; the most famous example of this is the “hole” in the stratospheric ozone layer of our planet.

In this talk, I will focus on heterogeneous processes on solid surfaces, and on their influence on the atmosphere of rocky bodies. Mineral dust and ice surfaces represent chemical factories for reactions to take place. Important heterogeneous reactions are known in the atmosphere of Earth, but heterogeneous chemistry is an (almost) virgin field when it goes to other planets. This talk will be an opportunity to discuss the current knowledge on atmospheric heterogeneous processes, which will be illustrated with recent examples. The need for dedicated experimental setups to study heterogeneous reactions under conditions of relevance for planetary atmospheres will be stressed, and perspectives for the study of exoplanets will be drawn².

 

¹ Zhang, X. et al., Icarus 376, Article number 114832 (2022)

² Lasne, J. ACS Earth Space Chem. 5, 149 (2021)