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Infrared observations of the coma of 67P/Churyumov-Gerasimenko were carriedout from July to September 2015, i.e., around perihelion (13 August 2015), withthe high-resolution channel of the VIRTIS instrument onboard Rosetta. Wepresent the analysis of fluorescence emission lines of H$_2$O, CO$_2$,$^{13}$CO$_2$, OCS, and CH$_4$ detected in limb sounding with the field of viewat 2.7-5 km from the comet centre. Measurements are sampling outgassing fromthe illuminated southern hemisphere, as revealed by H$_2$O and CO$_2$ rastermaps, which show anisotropic distributions, aligned along the projectedrotation axis. An abrupt increase of water production is observed six daysafter perihelion. In the mean time, CO$_2$, CH$_4$, and OCS abundances relativeto water increased by a factor of 2 to reach mean values of 32%, 0.47%, and0.18%, respectively, averaging post-perihelion data. We interpret these changesas resulting from the erosion of volatile-poor surface layers. Sustained dustablation due to the sublimation of water ice maintained volatile-rich layersnear the surface until at least the end of the considered period, as expectedfor low thermal inertia surface layers. The large abundance measured for CO$_2$should be representative of the 67P nucleus original composition, and indicatesthat 67P is a CO$_2$-rich comet. Comparison with abundance ratios measured inthe northern hemisphere shows that seasons play an important role in cometoutgassing. The low CO$_2$/H$_2$O values measured above the illuminatednorthern hemisphere are not original, but the result of the devolatilization ofthe uppermost layers.

Evolution of CO$_2$, CH$_4$, and OCS abundances relative to H$_2$O in the coma of comet 67P around perihelion from Rosetta/VIRTIS-H observations