Detection of exposed H<SUB>2</SUB>O ice on the nucleus of comet 67P/Churyumov-Gerasimenko. as observed by Rosetta OSIRIS and VIRTIS instruments

Abstract

Context. Since the orbital insertion of the Rosetta spacecraft, comet 67P/Churyumov-Gerasimenko (67P) has been mapped by OSIRIS camera and VIRTIS spectro-imager, producing a huge quantity of images and spectra of the comet's nucleus. Aims: The aim of this work is to search for the presence of H<SUB>2</SUB>O 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 H<SUB>2</SUB>O 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 conditions for each pixel using the Lommel-Seeliger disk law. The spots with higher I/F were selected and then analysed spectrophotometrically 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 H<SUB>2</SUB>O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H<SUB>2</SUB>O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H<SUB>2</SUB>O ice and dark terrain, using Hapke's radiative transfer modeling. We also present a detailed analysis of the detected spots.