The Ice Composition Close to the Surface of Comet 67P/Churyumov-Gerasimenko

The relation between ice composition in thenucleus of comet 67P/Churyumov-Gerasimenko on the onehand and relative abundances of volatiles in the coma on theother hand is important for the interpretation of densitymeasurements in the environment of the cometary nucleus. Forthe 2015 apparition, in situ measurements from the two ROSINA(Rosetta Orbiter Spectrometer for Ion and Neutral Analysis)sensors COPS (COmet Pressure Sensor) and DFMS (DoubleFocusing Mass Spectrometer) determined gas densities at thespacecraft position for the 14 gas species H2O, CO2, CO, H2S, O2,C2H6,CH3OH, H2CO, CH4,NH3, HCN, C2H5OH, OCS, andCS2. We derive the spatial distribution of the gas emissions on the complex shape of the nucleus separately for 50 subintervals of the2 year mission time. The most active patches of gas emission are identified on the surface. We retrieve the relation between solarirradiation and observed emissions from these patches. The emission rates are compared to a minimal thermophysical model to inferthe surface active fraction of H2O and CO2. We obtain characteristic differences in the ice composition close to the surface betweenthe two hemispheres with a reduced abundance of CO2ice on the northern hemisphere (locations with positive latitude). We do notsee significant differences for the ice composition on the two lobes of 67P/C-G.