Seasonal evolution of comet 67P activity from Rosetta/VIRTIS-H observations

Spectroscopic infrared observations of the coma of 67P/Churyumov-Gerasimenko were carried out with the VIRTIS-H instrument onboard Rosetta. Vibrational bands of H2O, CO2, 13CO2, OCS, and CH4 were detected, from which column densities and abundance ratios were measured. The data set show that seasons play an important role in the activity of the comet. We report here on data obtained from July 2015 to the end of the Rosetta mission. Raster maps obtained from July to November 2015 show prominent CO2 and H2O outgassing from the southern regions. The abundance ratios relative to water strongly increased 6 days after perihelion, reaching 32%, 0.5%, and 0.2% for CO2, CH4, and OCS, respectively. This is interpreted as resulting from the ablation of devolatilized surface layers in the southern hemisphere, and the subsequent exposure of non-differentiated volatile-rich material to solar heat. Comparison with data obtained pre-equinox suggests that the low CO2/H2O values measured above the illuminated northern hemisphere during pre-equinox are characteristics of outgassing from differentiated, dust-covered regions. Unexpectedly, the CO2 outgassing remained high long after perihelion. A strong pre/post perihelion asymmetry in CO2 activity is observed on both the southern and northern hemispheres. The large CO2/H2O ratio measured around perihelion indicates that 67P is a CO2-rich comet.