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

• Published in: Nature (London) 2016-01, Vol.529 (7586), p.368-372
• Main Authors: Filacchione, G., De Sanctis, M. C., Capaccioni, F., Raponi, A., Tosi, F., Ciarniello, M., Cerroni, P., Piccioni, G., Capria, M. T., Palomba, E., Bellucci, G., Erard, S., Bockelee-Morvan, D., Leyrat, C., Arnold, G., Barucci, M. A., Fulchignoni, M., Schmitt, B., Quirico, E., Jaumann, R., Stephan, K., Longobardo, A., Mennella, V., Migliorini, A., Ammannito, E., Benkhoff, J., Bibring, J. P., Blanco, A., Blecka, M. I., Carlson, R., Carsenty, U., Colangeli, L., Combes, M., Combi, M., Crovisier, J., Drossart, P., Encrenaz, T., Federico, C., Fink, U., Fonti, S., Ip, W. H., Irwin, P., Kuehrt, E., Langevin, Y., Magni, G., McCord, T., Moroz, L., Mottola, S., Orofino, V., Schade, U., Taylor, F., Tiphene, D., Tozzi, G. P., Beck, P., Biver, N., Bonal, L., Combe, J-Ph, Despan, D., Flamini, E., Formisano, M., Fornasier, S., Frigeri, A., Grassi, D., Gudipati, M. S., Kappel, D., Mancarella, F., Markus, K., Merlin, F., Orosei, R., Rinaldi, G., Cartacci, M., Cicchetti, A., Giuppi, S., Hello, Y., Henry, F., Jacquinod, S., Reess, J. M., Noschese, R., Politi, R., Peter, G.
• Format: Article
• Language: English
• Subjects: 639/33/445/3928; 639/33/445/848; Astrophysics; Comets; Diffusion; Extraterrestrial Environment - chemistry; Gases - analysis; Gases - chemistry; Humanities and Social Sciences; Ice; Ice - analysis; Ice environments; letter; Meteoroids; multidisciplinary; Natural history; Observations; Phase transitions; Physics; Science; Space exploration; Spectrum Analysis; Stratigraphy; Water; Water vapor; Wavelengths
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature16190

Using infrared wavelengths, micrometre-sized water-ice grains have been identified on the nucleus (which is mostly coated in a dark material) of comet 67P/Churyumov–Gerasimenko. Water ice on the surface of comet 67P/Churyumov–Gerasimenko Until now there has been little evidence for the presence of large regions of exposed water ice on the surfaces of comets, despite the fact that water is the major constituent of cometary nuclei. Here Gianrico Filacchione et al . report the identification at infrared wavelengths of water ice in the form of millimetre-sized grains on two debris falls in the Imhotep region of the nucleus of comet 67P/Churyumov–Gerasimenko, based on data from the VIRTIS imaging spectrometer onboard ESA's Rosetta probe. The ice is exposed on the walls of elevated structures and at the base of the walls, and 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 layering in which the outer dehydrated crust is superimposed on water ice enriched layers. Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko 1 , 2 and water is the major constituent of cometary nuclei 3 , 4 , limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far 5 , 6 . The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far 7 , 8 , 9 . The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material 10 . 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 6 , 7 , 8 , 9 . Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers 6 , 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 cr