The O2/H2O ratio at comet 67P/C-G
ESA's Rosetta spacecraft has made the first in situ detection of oxygen molecules (O2) outgassing from a comet.
More than 3000 samples, collected around the comet between September 2014 and March 2015, were analysed to identify the O2.
The O2/H2O ratio does not vary significantly over the study period. Short-lived strong variations are attributed to the decrease of the O2 ratio for occasionally higher H2O abundances linked to the daily water-ice cycle. The overall consistent level implies that O2 is not produced today by solar wind or UV interaction with surface ices, otherwise it would rapidly decrease due to the comet's increased activity. Instead, the O2 must have been incorporated into the comet's ices during its formation in the early Solar System, and is being released with the water vapour today.