CO Oxidation in the Venusian Mesosphere and Associated Isotopic Fingerprints Enabling Detection and Chemical Mechanism Insights

Venus has a thick atmosphere with many of its chemical and photochemical processes yet to be fully understood. While it mainly consists of relatively inert CO₂ and N₂, the trace gases, which include chlorine and sulfur compounds, are responsible for Venus's rich and diverse chemistry and photochemistry. Given that chlorine has two stable isotopes and sulfur has four, there are plenty of opportunities for naturally occurring isotopic effects in Venusian atmospheric chemistry. Oxygen, carbon, and hydrogen also typically participate in Venusian chlorine and sulfur chemistry, providing further diversity in isotopes to study.

Using high-level computational chemistry methods in synergy with experimental kinetics, we model temperature-dependent rate coefficients. Furthermore, the computational chemistry approaches are used to model mass-dependent rate coefficients to study how isotopic fingerprints in the reactants and products from the chemistry may arise. I will present our recent work involving CO oxidation in the Venusian atmosphere and how the measurement of isotopic signals can inform us which chemical mechanism dominates CO oxidation on Venus. The discrepancy between models and observations of O₂ abundance in the Venusian mesosphere remains an issue. The Cl radical oxidation of CO may explain the missing O₂ in the Venusian mesosphere, as the cycle consumes O₂ efficiently: Cl + CO ↔ ClCO + O₂ → ClC(O)OO → ClO + CO₂. The initial equilibrium between Cl and CO is identified as the bottleneck for propagating the radical oxidation cycle.

We show how the isotopic fingerprint in CO from the Cl radical oxidation cycle is ripe for detection on Venus through microwave spectroscopy and how important getting the isotopic ratio correct is for retrievals of other properties. Alternative CO oxidation mechanisms are also explored, and other isotopic fingerprints may be investigated, for example, for the ClO radical, which has been observed in the Venusian atmosphere. Our work provides quantitative numbers for isotopic fractionation, which are needed to interpret which chemical processes dominate on Venus. Fractionation of naturally occurring isotopes on Venus provides detectable signatures for future missions through either spectroscopy (IR or microwave) or mass spectrometry. Notably, NASA’s DAVINCI mission will include a mass spectrometer in its payload.

Our work is a collaboration between theory and experiments, providing a detailed mechanistic and quantitative understanding of CO chemistry in the Venusian mesosphere. It is additionally relevant to understanding the isotopic fractionation of the atmosphere of Archean Earth (pre-oxygen era) and Noachian Mars, where large mass-independent signatures of CO related compounds can be found, e.g., in Martian soil carbon.