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 (2015 August 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, ¹³CO₂, 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.