The Seasonal Cycle of Water Ice at the Surface of Comet 67P/Churyumov-Gerasimenko as Observed by Virtis Onboard Rosetta

Rosetta was the first mission escorting a comet, 67P/Churyumov-Gerasimenko, for two years along its orbit (August 2014 - September 2016). This allowed us to follow the evolution of the nucleus surface and activity as 67P was approaching the Sun from a heliocentric distance of 3.62 AU to 1.25 AU (perihelion), and along the outbound part of the orbit up to 3.8 AU. Here we report about the observations performed by the VIRTIS instrument (Coradini et al., 2007), which investigated the nucleus surface composition by means of its reflectance spectrum in the 0.2-5.1 micron wavelength range. Results from Filacchione et al. (2016), for the pre-landing phase (August 2014-November 2014), extended to observations until early May 2015 by Ciarniello et al. (2016), indicate a global enrichment of surface water ice content, as inferred from analysis of comet spectral indicators (such as slope in the VIS and IR ranges, and the 3.2 micron band area and band center). Recent analysis from Ciarniello et al. (in preparation), carried out by means of the sole VIS channel (0.24-1.04 micron) shows that the water ice enrichment continues up to perihelion, then followed by a monotonic reduction of water ice abundance, as indicated by a progressive increase of the VIS spectral slope. In April 2016, on the outbound leg of the orbit at 2.9 AU, a plateau was reached, followed by a possible enrichment of water ice until the end of mission in September 2016 at 3.8 AU. The described behavior suggests the presence of a seasonal cycle of water ice on the surface. These results will be compared to the temporal variability of dust production with the aim to characterize the thermo-physical processes influencing the surface water ice content.