Change of outgassing pattern of 67P/Churyumov-Gerasimenko during the March 2016 equinox as seen by ROSINA

Abstract

As the spin axis of comet 67P/Churyumov-Gerasimenko (67P) is not normal to the orbital plane, 67P has strong seasonal changes in the illumination conditions on the nucleus' surface, with a short and intense summer in the Southern hemisphere. We have been monitoring these seasonal variations in the gas coma with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument suite aboard the ESA's Rosetta spacecraft. Rosetta followed 67P from its rendezvous in 2014 August, from a distance of almost 3.5 au to Sun, through perihelion at 1.24 au, and away from Sun again. In this study, we present the change of outgassing pattern during the 2016 March equinox based on measurements acquired with the ROSINA instruments: while H2O, O2 and NH3 abundances rapidly decreased during this period, CO2, CO, H2S, CH4 and HCN abundances decreased much less and showed a strong south-north heterogeneity for the whole period, thus not following Sun. Sublimation temperatures of the pure ices are found to be uncorrelated with the slope of the decrease for the minor species. This can be interpreted as a consequence of two different ice phases, water ice and CO2 ice, in which the minor species are embedded in different relative abundances.