Sublimation of volatiles from H₂O:CO₂ bulk ices in the context of comet 67P/Churyumov-Gerasimenko

Context. Comets are considered to be remnants from the formation of the Solar System. ESA’s Rosetta mission targeted comet 67P/Churyumov-Gerasimenko and was able to record high-quality data on its chemical composition and outgassing behaviour, including low abundances of N 2 that are observed to be correlated with H 2 O and CO 2 in approximately a 63:37 ratio. Aims. In this work, the thermal desorption behaviour of N 2 in H 2 O:CO 2 ices was studied in the laboratory to investigate the co-desorption behaviour of N 2 within the two most abundant cometary ices in 67P and to derive desorbing fractions in different temperature regimes. Methods. H 2 O:CO 2 :N 2 ices of various ratios were prepared in a gas mixing system and co-deposited at 15 K onto a copper sample holder. Sublimation of the ice was measured using temperature programmed desorption mass spectrometry. Quantitative values were derived for the fraction of N 2 co-desorbing with CO 2 and H 2 O respectively. To validate the results, H 2 O:CO 2 : 13 CO ices were prepared as well. Results. The experiments show that the co-desorption of N 2 with CO 2 in H 2 O:CO 2 :N 2 ices depends on the bulk amount of CO 2 present in the ice. The fraction of N 2 trapped in H 2 O reduces as more N 2 and CO 2 are added to the mixture. CO behaves qualitatively similar to N 2 , but more CO is found to co-desorb with CO 2 . To reproduce the ratio of N 2 desorbing with H 2 O over that of CO 2 (N 2 (H 2 O)/N 2 (CO 2 )), our ice analogues need to contain ≥15% CO 2 , while 67P contains ≤7.5% CO 2 . Large fractions of N 2 can be removed from the ice due to heating up to 70 K, but for ice that most closely resembles that of 67P, the loss fraction of pure phase N 2 is expected to be ≤20%. Therefore, N 2 is suggested to be a minor carrier of nitrogen in the comet.