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

Abstract

Water is a major constituent of comets and is one of the major volatile species controlling cometary activity. Thus, it is crucial to understand water’s behavior on and in a comet. All close up observations of cometary nuclei have revealed no, or a very limited amount, of surface water ice, insufficient to explain the observed water outgassing. From this, it was concluded that water vapor was generated in ice-rich subsurface layers with a mechanism of transport linked to solar illumination. In addition, short lived outbursts were seen near sunrise on Tempel1. Periodical condensations of water vapor very close, or on the surface, were suggested to explain these outburst. Up to now, though, no clear observational evidence was brought forward to prove or disprove this proposed link. In this paper we report observations by VIRTIS -Rosetta of water ice on the surface of comet 67P/Churyumov- Gerasimenko, appearing and disappearing in a cyclic pattern, which follows local illumination conditions, thus providing a source of localized activity. The total amount of water vapor produced by this mechanism can be roughly estimated by considering the relative extent of water-rich areas in our observations; a value of 3% of the total water flux is derived, but this is only a lower limit. We can extrapolate that a much larger area is affected by the same mechanism, although not directly observed, thus contributing by a larger amount to the total water flux. This water cycle was never actually observed until Rosetta and it appears to be a fundamental process in the evolution of the comet, contributing to local cometary water activity.