Detection of exposed H₂O ice on the nucleus of comet 67P/Churyumov-Gerasimenko as observed by Rosetta OSIRIS and VIRTIS instruments

Abstract : Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P/C-G) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet’s nucleus. The aim of this work is to search for the presence of H₂O on the nucleus which, in general, appears very dark and rich in dehydrated organic material. After selecting images of the bright spots which could be good candidates to search for H₂O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. Methods. The selected OSIRIS images were processed with the OSIRIS standard pipeline and corrected for the illumination condi- tions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotomet- rically and compared with the surrounding area. We selected 13 spots as good targets to be analysed by VIRTIS to search for the 2 μm absorption band of water ice in the VIRTIS spectral cubes. Results. Out of the 13 selected bright spots, eight of them present positive H₂O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H₂O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H₂O ice and dark terrain, using Hapke’s radiative transfer modeling. We also present a detailed analysis of the detected spots.