Isotopic composition of CO₂ in the coma of 67P/Churyumov-Gerasimenko measured with ROSINA/DFMS

Measurements of isotopic abundances in cometary ices are key to understanding and reconstructing the history and origin of material in the solar system. Comets are considered the most pristine material in the solar system. Isotopic fractionation (enrichment of an isotope in a molecule compared to the initial abundance) is sensitive to environmental conditions at the time of comet formation. Therefore, measurements of cometary isotope ratios can provide information on the composition, density, temperature, and radiation during formation of the molecules, during the chemical evolution from the presolar cloud to the protosolar nebula, and the protoplanetary disk before accretion in solid bodies. Most isotopic abundances of ¹²C/¹³C and ¹⁶O/¹⁸O in comets to date are in agreement with terrestrial abundances. Prior to the Rosetta mission, measurements of ¹²C/¹³C in comets were only available for HCN, CN, and C₂ and for ¹⁶O/¹⁸O in H₂O. Measurements of ¹²C/¹³C in comets were only available from ground based observations and remote sensing, while ¹⁶O/¹⁸O in H₂O had also been measured in-situ. To date, no measurements of the CO2 isotopologues in comets were available. Aims. This paper presents the first measurements of the CO₂ isotopologues in the coma of 67P/Churyumov-Gerasimenko (67P). Methods. We analyzed measurements taken by the Double Focusing Mass Spectrometer (DFMS) of the ROSINA experiment on board the ESA spacecraft Rosetta in the coma of 67P. Results. The CO₂ isotopologues results for 67P are: ¹²C/¹³C = 84 ± 4, ¹⁶O/¹⁸O = 494 ± 8, and ¹³C¹⁶O2/¹²C¹⁸O¹⁶O = 5:87 ± 0:07. The oxygen isotopic ratio is within error bars compatible with terrestrial abundances but not with solar wind measurements. Conclusions. The carbon isotopic ratio and the combined carbon and oxygen isotopic ratio are slightly (14%) enriched in ¹³C, within 1δ uncertainty, compared to solar wind abundances and solar abundances. The small fractionation of ¹²C/¹³C in CO₂ is probably compatible with an origin of the material in comets from the native cloud.