Detection of exposed H$_2$O ice on the nucleus of comet 67P/Churyumov-Gerasimenko

TL;DR

This study combines high-resolution imaging and spectral data from Rosetta to detect and analyze water ice on comet 67P's nucleus, revealing localized H2O ice presence on the dark surface.

Contribution

It introduces a method to identify water ice on the comet's surface by integrating OSIRIS imaging with VIRTIS spectral analysis, confirming H2O ice in specific bright spots.

Findings

8 out of 13 bright spots contain water ice.

Water ice detected via the 2 micron absorption band.

Spectral modeling confirms H2O ice presence.

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 H$_2$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$_2$O ice, taken at high resolution by OSIRIS, we check for spectral cubes of the selected coordinates to identify these spots observed by VIRTIS. 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 micron absorption band of water ice in the VIRTIS spectral cubes. Out of the 13 selected bright spots, eight of them present positive H$_2$O ice detection on the VIRTIS data. A spectral analysis was performed and the approximate temperature of each spot was computed. The H$_2$O ice content was confirmed by modeling the spectra with mixing (areal and intimate) of H$_2$O ice and dark terrain, using Hapke's radiative transfer modeling. We also present a detailed analysis of the detected spots.