ALBERT

Description: 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉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 -- England -- Nature. 2016 Jan 21;529(7586):368-72. doi: 10.1038/nature16190. Epub 2016 Jan 13.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy. ; LESIA, Observatoire de Paris/CNRS/UPMC/Universite Paris-Diderot, Meudon, France. ; Institute for Planetary Research, DLR, Berlin, Germany. ; Universite Grenoble Alpes, CNRS, IPAG, Grenoble, France. ; INAF-Osservatorio di Capodimonte, Napoli, Italy. ; UCLA, Los Angeles, California, USA. ; European Space Agency-ESTEC, Noordwijk, The Netherlands. ; Institut d'Astrophysique Spatial CNRS, Orsay, France. ; Dipartimento di Matematica e Fisica "Ennio De Giorgi", Universita del Salento, Lecce, Italy. ; Space Research Centre, Polish Academy of Sciences, Warsaw, Poland. ; NASA JPL, Pasadena, California, USA. ; Space Physics Research Laboratory, The University of Michigan, Michigan, Ann Arbor, USA. ; Universita di Perugia, Perugia, Italy. ; Lunar Planetary Laboratory, University of Arizona, Tucson, Arizona, USA. ; National Central University, Taipei, Taiwan. ; Department of Physics, Oxford University, Oxford, UK. ; Bear Fight Institute, Winthrop, Washington, USA. ; Helmholtz-Zentrum Berlin fur Materialien und Energie, Berlin, Germany. ; INAF-Osservatorio Astrofisico di Arcetri, Firenze, Italy. ; Agenzia Spaziale Italiana, Rome, Italy. ; Istituto di Radioastronomia-INAF, Bologna, Italy. ; Institute of Optical Sensor Systems, DLR, Berlin, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26760209" target="_blank"〉PubMed〈/a〉

Description: Observations of cometary nuclei have revealed a very limited amount of surface water ice, which is insufficient to explain the observed water outgassing. This was clearly demonstrated on comet 9P/Tempel 1, where the dust jets (driven by volatiles) were only partially correlated with the exposed ice regions. The observations of 67P/Churyumov-Gerasimenko have revealed that activity has a diurnal variation in intensity arising from changing insolation conditions. It was previously concluded that water vapour was generated in ice-rich subsurface layers with a transport mechanism linked to solar illumination, but that has not hitherto been observed. Periodic condensations of water vapour very close to, or on, the surface were suggested to explain short-lived outbursts seen near sunrise on comet 9P/Tempel 1. Here we report observations of water ice on the surface of comet 67P/Churyumov-Gerasimenko, appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity. This water cycle appears to be an important process in the evolution of the comet, leading to cyclical modification of the relative abundance of water ice on its surface.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉De Sanctis, M C -- Capaccioni, F -- Ciarniello, M -- Filacchione, G -- Formisano, M -- Mottola, S -- Raponi, A -- Tosi, F -- Bockelee-Morvan, D -- Erard, S -- Leyrat, C -- Schmitt, B -- Ammannito, E -- Arnold, G -- Barucci, M A -- Combi, M -- Capria, M T -- Cerroni, P -- Ip, W-H -- Kuehrt, E -- McCord, T B -- Palomba, E -- Beck, P -- Quirico, E -- VIRTIS Team -- England -- Nature. 2015 Sep 24;525(7570):500-3. doi: 10.1038/nature14869.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Istituto di Astrofisica e Planetologia Spaziali - INAF, via del fosso del cavaliere 100, 00133 Rome, Italy. ; Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany. ; LESIA-Observatoire de Paris, CNRS, Universite Pierre et Marie Curie, Universite Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France. ; Universite Grenoble Alpes - CNRS Institut de Planetologie et Astrophysique de Grenoble, Batiment D de Physique, BP 53, 38041 Grenoble Cedex 9, France. ; University of California, Los Angeles, California 90095, USA. ; Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, Michigan 48109, USA. ; National Central University, No. 300, Jhongda Road, Jhongli District, Taoyuan City, 32001 Taipei, Taiwan. ; Bear Fight Institute, 22 Fiddler's Road, Box 667, Winthrop, Washington 98862, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26399830" target="_blank"〉PubMed〈/a〉