The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko

• Published in: Nature (London) 2015-09, Vol.525 (7570), p.500-503
• Main Authors: De Sanctis, M. C., Capaccioni, F., Ciarniello, M., Filacchione, G., Formisano, M., Mottola, S., Raponi, A., Tosi, F., Bockelée-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.
• Format: Article
• Language: English
• Subjects: 639/33; 639/33/445/848; Astrophysics; Comets; Diurnal variations; Extraterrestrial Environment - chemistry; Humanities and Social Sciences; Hydrologic cycle; Ice; Ice - analysis; Ice environments; letter; Meteoroids; multidisciplinary; Natural history; Observations; Physics; Relative abundance; Science; Space exploration; Surface water; Temperature; Time Factors; Volatilization; Water cycle; Water vapor
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14869

Observations of water ice on the surface of comet 67P/Churyumov–Gerasimenko show the ice appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity and leading to cycling modification of the ice abundance on the surface. A cometary hydrologic cycle Maria Cristina De Sanctis et al . report observations from the VIRTIS imaging spectrometer onboard the Rosetta mission that show a diurnal water ice on the surface of comet 67P/Churyumov–-Gerasimenko. Surface water ice appears and disappears in a cyclic pattern that follows local illumination conditions, providing a source of localized activity. The authors suggest that the cyclic sublimation–condensation of ice triggered by varying illumination conditions may be a general process acting on cometary nuclei. Observations of cometary nuclei have revealed a very limited amount of surface water ice 1 , 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 regions 8 . The observations 6 , 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 illumination 1 , 2 , 3 , 5 , but that has not hitherto been observed. Periodic condensations of water vapour very close to, or on, the surface were suggested 3 , 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.