Two years with comet 67P/Churyumov-Gerasimenko: H2O, CO2, and CO as seen by the ROSINA/RTOF instrument ofRosetta

Context. The ESA Rosetta mission investigated the environment of comet 67P/Churyumov-Gerasimenko (hereafter 67P) from August 2014 to September 2016. One of the experiments on board the spacecraft, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) included a COmet Pressure Sensor (COPS) and two mass spectrometers to analyze the composition of neutrals and ions, the Reflectron-type Time-Of-Flight mass spectrometer (RTOF), and the Double Focusing Mass Spectrometer (DFMS). Aims. RTOF species detections cover the whole mission. This allows us to study the seasonal evolution of the main volatiles (H₂O, CO₂ , and CO) and their spatial distributions. Methods.We studied the RTOF dataset during the two-year long comet escort phase focusing on the study of H₂O, CO₂, and CO. We also present the detection by RTOF of O₂, the fourth main volatile recorded in the coma of 67P/C-G. This work includes the calibration of spectra and the analysis of the signature of the four volatiles. We present the analysis of the dynamics of the main volatiles and visualize the distribution by projecting our results onto the surface of the nucleus. The temporal and spatial heterogeneities of H₂O, CO₂, and CO are studied over the two years of mission, but the O₂ is only studied over a two-month period. Results. The global outgassing evolution follows the expected asymmetry with respect to perihelion. The CO/CO₂ ratio is not constant through the mission, even though both species appear to originate from the same regions of the nucleus. The outgassing of CO₂ and CO was more pronounced in the southern than in the northern hemisphere, except for the time from August to October 2014. We provide a new and independent estimate of the relative abundance of O₂. Conclusions.We show evidence of a change in molecular ratios throughout the mission.We observe a clear north-south dichotomy in the coma composition, suggesting a composition dichotomy between the outgassing layers of the two hemispheres. Our work indicates that CO₂ and CO are located on the surface of the southern hemisphere as a result of the strong erosion during the previous perihelion. We also report a cyclic occurrence of CO and CO₂ detections in the northern hemisphere. We discuss two scenarios: devolatilization of transported wet dust grains from south to north, and different stratigraphy for the upper layers of the cometary nucleus between the two hemispheres.