Observed changes in the physical environment and chemistry in the inner coma of 67P/Churyumov-Gerasimenko

The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) on board Rosetta consists of two bore-sighted cameras (Keller et al. 2007). Its Wide Angle Camera includes narrow-band filters to image gaseous emission by CS, OH, NH, CN, NH2, Na, and [O I].OSIRIS monitored gas about once every two weeks for heliocentric distances greater than 2 AU and weekly afterward. Images were calibrated using the OSIRIS pipeline (Tubiana et al. 2015), and further processed to remove the continuum flux and the emission from other molecules with lines that fall within the narrowband filters’ passbands.Many collimated jets are visible in the continuum images (Lara et al. 2015; Lin et al. 2015). Those jets are not present in the gas images. The morphology of the gas images is either focused in a projected cone (OI, CN) or diffuse and roughly isotropic (OH, NH, CS, NH2). We interpret the first as the signature of prompt emission (where fragmentation of a molecule released from the nucleus directly produces a radical in an excited state), and the second as that of excitation of fragments species after they were formed.Surprisingly, up to the spring of 2015 all measured surface brightnesses are more than 10x higher than can be accounted for by formation and excitation processes as understood from remote sensing of comets. This changed in the summer of 2015 and current measurements are consistent with photodissociation and fluorescence rates. This change reflects the transition of 67P from a distant comet with extremely low gas production rates to conditions under which comets are normally observed.