Evolution of CO2, CH4, and OCS abundances relative to H2O in the coma of comet 67P around perihelion from Rosetta/VIRTIS-H observations

Infrared observations of the coma of 67P/Churyumov–Gerasimenko were carried out from 2015 July to September, i.e. around perihelion (2015August 13), with the high-resolution channel of the Visible and Infrared Thermal Imaging Spectrometer instrument onboard Rosetta. We present the analysis of fluorescence emission lines of H2O, CO2, CO2, OCS, and CH4 detected in limb sounding with the field of view at 2.7–5 km from the comet centre. Measurements are sampling outgassing from the illuminated Southern hemisphere, as revealed by H2O and CO2 raster maps, which show anisotropic distributions, aligned along the projected rotation axis. An abrupt increase of water production is observed 6 d after perihelion. In the meantime, CO2, CH4, and OCS abundances relative to water increased by a factor of 2 to reach mean values of 32, 0.47, and 0.18 per cent, respectively, averaging post-perihelion data. We interpret these changes as resulting from the erosion of volatile-poor surface layers. Sustained dust ablation due to the sublimation of water ice maintained volatile-rich layers near the surface until at least the end of the considered period, as expected for low thermal inertia surface layers. The large abundance measured for CO2 should be representative of the 67P nucleus original composition, and indicates that 67P is a CO2-rich comet. Comparison with abundance ratios measured in the Northern hemisphere shows that seasons play an important role in comet outgassing. The low CO2/H2O values measured above the illuminated Northern hemisphere are not original, but the result of the devolatilization of the uppermost layers.