Rosetta has made the first detection of molecular oxygen at a comet. The results presented in this graphic are based on data collected by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis Double-Focusing Mass Spectrometer (ROSINA-DFMS) between September 2014 and March 2015 when Rosetta was still on the approach to the Sun along its orbit.
Top middle: high-resolution measurements allowed molecular oxygen (O2) to be distinguished from other species like sulphur (S) and methanol (CH3OH). The detection of the coma gases is stronger closer to the comet nucleus, as expected. The contribution to the detection from contamination from the spacecraft thruster firings during manoeuvres is very low.
Top right: the strong correlation of molecular oxygen abundance with water vapour indicates a shared origin and release mechanism from the nucleus.
Bottom middle: 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.