Oxygen isotope fractionation during spin-forbidden photolysis of CO2: Relevance to the atmosphere of Mars

Abstract:

The oxygen isotope composition of the Martian atmosphere is of interest for comparison with recent MSL SAM results, and to understand the origin of oxygen isotope anomalies (i.e., mass-independent fractionation or MIF) in secondary minerals in SNC meteorites. Our focus here is on spin-forbidden photolysis of CO₂, CO₂ + hv (>167 nm) → CO(X¹S) + O(³P). The spin-forbidden photolysis of CO₂ is unusual in the Martian atmosphere because of its high reaction rate from the upper atmosphere (80 km) all the way to the ground. This range of altitudes spans 4 orders of magnitude in atmospheric pressure, and occurs because of the gradual decrease in the CO₂ cross sections from 167 to ~200 nm. Previous laboratory photolysis experiments on CO₂ in the spin-allowed and spin-forbidden regions have yielded a remarkably large MIF signature (¹⁷O excess ~ 100 permil) in O₂ product for photolysis at 185 nm. Recent theoretical cross sections for CO₂ isotopologues argue for a much smaller MIF signature from spin-forbidden photolysis. Here, we report the results of photolysis experiments on CO₂ at the Soleil synchrotron DESIRS beamline. High purity, natural isotope abundance CO₂ was placed in a 20 cm photocell with MgF₂ windows. Experiments were performed at 3 wavelengths (7% FWHM): 160 nm (spin-allowed), and at 175 nm and 185 nm (spin-forbidden). After VUV exposure, aliquots of the photolyzed CO₂ were sent to the Department of Isotope Geology at the University of Goettingen for O isotope analysis. The isotope results show that the spin-allowed photolysis yields normal, mass-dependent fractionation in agreement with earlier work. Photolysis at 175 nm, which is mostly spin-forbidden, yields a small positive (or zero) MIF signature. Photolysis at 185 nm, which is entirely spin-forbidden, yields O₂ with a negative MIF signature (D¹⁷O ~ -8 to -10 permil). The results at 185 nm disagree in magnitude and sign with the very large positive MIF signature previously reported, and provides support for the theoretical cross sections. Photochemical simulations of the experiments are in progress to test the possibility that the negative MIF in O₂ results from O₃ formation. Incorporation of these results into Mars atmosphere photochemical models is in progress. We anticipate isotopic O₂ predictions that will be testable by MSL SAM.