Abstract
Observations of cometary nuclei have revealed a very limited amount of surface water ice1,2,3,4,5,6,7, 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 regions8. The observations6,7 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 illumination1,2,3,5, but that has not hitherto been observed. Periodic condensations of water vapour very close to, or on, the surface were suggested3,9 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.
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Acknowledgements
We thank the following institutions and agencies, which supported this work: the Italian Space Agency (ASI, Italy), Centre National d’Etudes Spatiales (CNES, France), Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany), and the 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 of the INAF (Italy), which also guides the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI, with contributions from Observatoire de Meudon financed by CNES (France), and from DLR (Germany). We also thank the Rosetta Science Ground Segment and the Rosetta Mission Operations. The VIRTIS calibrated data will be available through the ESA’s Planetary Science Archive (PSA) website.
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M.C.D.S. and F.C. contributed to data analysis and the writing of the manuscript. G.F. and F.C. provided calibrated VIRTIS data. A.R. and M.C. provided the spectral fit. M.C.D.S., M.T.C., M.F. and S.T. provided the thermal modelling. F.T. retrieved the temperatures. All authors contributed to the discussion of the results and to the writing of the paper.
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Extended data figures and tables
Extended Data Figure 1 Spectral fit.
a, b, Two mixing modalities, intimate (a) and areal (b), are used to model the same spectrum; the spectrum is identified by its position in the acquired image (‘sample’ and ‘line’) and its spacecraft event time (scet). The three missing parts of the spectra are related to the wavelength ranges covered by the junctions of the filters which produce significant artefacts. They are thus removed during the fitting procedure. The spectra are normalized with respect to λ0 = 1.8 μm. For the areal mixture case, the modelled absorption band at 2 µm is relevant, even with the small abundance (fH2O) and grain diameter (dH2O) we retrieved. This implies a worse fit, as indicated by the larger χ2 variable. The intimate mixture is thus a better model of the spectra.
Extended Data Figure 2 Spectral fits of comet nucleus spectra with different ice content.
From a to c, the depth of the absorption band at 3.2 μm decreases and the band centre moves slightly towards longer wavelengths. In all cases the spectra are well modelled with a decreasing amount of water ice (fH2O) and a constant grain diameter (dH2O) of that water ice. Missing parts of the spectra and normalization are as for Extended Data Fig. 1.
Extended Data Figure 3 Diurnal cycle of water.
In the case reported here, the sublimation of water vapour takes place in a deeper layer (see cartoon for a graphic explanation). The water vapour filters up to the surface layers (which are essentially dehydrated, as during the day we do not see any spectral signature of ice) where, finding lower temperature conditions (as in the night or in the shadow), it condenses and is trapped as ice. The subsequent illumination of the surface leads to absolute loss of the condensed water vapour. This is an effective mechanism of transport of H2O from deeper layers to the surface.
Extended Data Figure 4 Temperature and water vapour profiles.
a, Temperature profiles, and b, water vapour pressure profiles, from the nucleus surface to the interior at different times: the blue curve is the profile when the area is illuminated; the red curve is the profile obtained 6 min after passing into shadow; and the purple curve is about 40 min after passing into shadow.
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De Sanctis, M., Capaccioni, F., Ciarniello, M. et al. The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko. Nature 525, 500–503 (2015). https://doi.org/10.1038/nature14869
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DOI: https://doi.org/10.1038/nature14869
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