Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko1,2 and water is the major constituent of cometary nuclei3,4, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far5,6. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far7,8,9. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material10. 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 observations6,7,8,9. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers6, 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 crust10 is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko11,12 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.
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We thank the following institutions and agencies, which supported this work: Italian Space Agency (ASI, Italy), Centre National d’Etudes Spatiales (CNES, France), Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany), National Aeronautic and Space Administration (NASA, USA). VIRTIS was built by a consortium from Italy, France and Germany, under the scientific responsibility of the Istituto di Astrofisica e Planetologia Spaziali (IAPS) of INAF, Rome (Italy), which also led the scientific operations. The VIRTIS instrument development for the ESA has been funded and managed by ASI, with contributions from Observatoire de Meudon financed by CNES and from the DLR. The VIRTIS instrument industrial prime contractor was former Officine Galileo, now Selex ES (Finmeccanica Group) in Campi Bisenzio, Florence, Italy. We also thank the Rosetta Liaison Scientists, the Rosetta Science Ground Segment and the Rosetta Mission Operations Centre for their support in planning the VIRTIS observations. This research has made use of NASA’s Astrophysics Data System. This work is dedicated to Angioletta Coradini, conceiver of the VIRTIS instrument.
The authors declare no competing financial interests.
The VIRTIS calibrated data will be available through the ESA’s Planetary Science Archive (PSA) website by early 2016 (http://www.rssd.esa.int).
Extended data figures and tables
Grains are present in two monodispersed distributions with maxima at 56 μm and at 2 mm, corresponding to the intimate and areal mixing classes, respectively. The histogram is computed by selecting only pixels showing a water-ice abundance greater than 2% for the intimate mixing class and greater than 0.3% for areal mixing.
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Filacchione, G., De Sanctis, M., Capaccioni, F. et al. Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko. Nature 529, 368–372 (2016). https://doi.org/10.1038/nature16190
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