Determining the volatile surface activity of comet 67P/CG from Rosetta remote sensing measurements

CO2 is the dominant volatile in comet 67P when the comet is farther than 2.5 AU from the Sun. With CO2 being the most abundant carbon-bearing volatile in 67P and most comets, the activity regions of CO2 on surfaces of comets may provide clues to the mechanism and the environment conditions of comet formation and allow comparisons among various comets. Abundant data obtained by various instruments from the Rosetta mission, which made both in situ and remote measurements of comet 67P/CG at a close distance over two years, offers us an unprecedented opportunity to study the cometary surface activity. Fougere et al., 2016a&b have developed a spherical harmonics inversion method applied it to ROSINA DFMS mass spectrometer measurements to obtain potential surface activity maps of H2O, CO2, CO and O2 averaged over a time period of more than one year. In this work, we will demonstrate that by applying a similar inversion method to column density maps derived from VIRTIS imaging data, we are able to derive potential surface activity maps of CO2 and H2O as well. One advantage of VIRTIS remote sensing maps over DFMS densities is that remote sensing measures the distribution of gas in coma very close to the nucleus at kilometer scales for every image, whereas the mass spectrometer measures the distribution of gas far from the nucleus (10-400 km) where detailed information of the distribution of the source region at the nucleus has been smeared out. In addition, one VIRTIS image can supply thousands of data points necessary to constrain the inversion method, and allows us to identify temporary or short-lived activity regions. Preliminary results and temporal evolution of activity regions will be discussed. This work is supported by the grant 80NSSC18K1280 from the NASA Rosetta Data Analysis Program.