Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Abstract

Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov-Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov-Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet's formation.

Comparative study of water ice exposures on cometary nuclei using multispectral imaging data

“…There were more than a hundred of bright features reported in the study of Pommerol et al (2015b) and the analysis of those features from both multispectral data and spectroscopic data is an ongoing work. Water ice was found to be the most plausible explanation for the bright blue features 1 observed on comet 67P by Pommerol et al (2015b) and this is confirmed on two of the large bright blue clusters by Filacchione et al (2016c) and in eight of the bright spots by Barucci et al (2016) via IR spectroscopic observations. Besides many other investigated surface features, the multispectral properties of one of the clusters where water ice was detected (Filacchione et al 2016c, BAP2) were investigated by Oklay et al (2016) together with the isolated water-ice-rich feature (Barucci et al 2016, spot5) using OSIRIS NAC images and their multispectral similarities with the active regions observed on comet 67P were discussed.…”

“…Besides many other investigated surface features, the multispectral properties of one of the clusters where water ice was detected (Filacchione et al 2016c, BAP2) were investigated by Oklay et al (2016) together with the isolated water-ice-rich feature (Barucci et al 2016, spot5) using OSIRIS NAC images and their multispectral similarities with the active regions observed on comet 67P were discussed. Moreover, the visual detection of jets rising from BAP2 of Filacchione et al (2016c) was presented by Oklay et al (2016) and Vincent et al (2016). Via inversion techniques, the sources of some of the observed dust jets (Vincent et al 2016) and outbursts (Knollenberg et al 2016) were identified as water-ice-rich clusters and potentially water-ice-rich features.…”

“…On comet 67P, there are bright features having reflectance of up to 10 times their surrounding (Pommerol et al 2015b). The spectroscopic observations of those bright blue features in IR wavelengths unanimously identified the presence of water ice (Sunshine et al 2006(Sunshine et al , 2011Barucci et al 2016;Filacchione et al 2016c).…”

“…The water ice absorption bands at 1.5, 2.0 and 3.0 μm are common in the IR spectra of these three comets. Water ice deposits observed on comet 9P were modelled by areal mixing of 30 μm-sized water ice particles (6 per cent) with dark material (Sunshine et al 2006), while water-ice-rich clusters on comet 67P were explained by areal mixing of 2 mm-sized pure water ice (1.2 per cent) with dark material and intimate mixture of 56 μm-sized water ice particles (3.4 per cent) with dark material (Filacchione et al 2016c). The modelling of IR spectra shows that the water-ice-rich features observed on these comets contain very small amount of water ice (<6 per cent).…”

“…Physical properties of the detected water-ice-rich features were derived from the IR observations for comets 9P, 103P and 67P (Sunshine et al 2006(Sunshine et al , 2011Li et al 2013;Filacchione et al 2016c). The water ice absorption bands at 1.5, 2.0 and 3.0 μm are common in the IR spectra of these three comets.…”