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

Abstract

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.

The Main Belt Comets and ice in the Solar System

“…We discard the possibility of in situ investigation for now; although it would certainly be effective, it is a very different prospect to astronomical observation in terms of cost. We also leave detection of absorption features out of this comparison, as this appears to be only possible for larger bodies: even for much more active comets, surface ice features are not detected remotely, and even in situ exploration shows only relatively small and variable ice patches on surfaces (De Sanctis et al 2015).…”

“…However, direct detection of water ice in comets is surprisingly scarce. To date, solid ice features have only been observed in a handful of comets via ground-based facilities (Davies et al 1997;Kawakita et al 2004;Yang et al 2009Yang et al , 2014 and in situ observations (Sunshine et al 2006;A'Hearn et al 2012;Protopapa et al 2014;De Sanctis et al 2015). Knowledge of ices in comets mainly comes from spectroscopic observations of emission lines and bands from the gas coma, which are discussed in more detail in Sect.…”

On the Mass and Origin of Chariklo’s Rings

“…The symbol sizes indicate body sizes. 11 The recent detection of diurnal cycle of water ice sublimation on Comet 67P (De Sanctis et al 2015) is an exact analogy of this time-dependent process, but happens on the diurnal timescale. 12 Also, once an optically thick ring forms, it should accrete grains efficiently.…”

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

“…When in shadow or on the night-side a temperature inversion is present between the now colder surface and a subsurface that remains warmer for a longer duration. These observed diurnal variations probably trigger the sublimation-condensation cycle on the nucleus of 67P (De Sanctis et al 2015, see also below).…”

“…De Sanctis et al (2015) discussed in detail a temperature gradient that exists between the top layer of the surface, which is reported to have a temperature between about 180 K and 230 K during daytime whereas the temperature of the subsurface, at a depth of 4 cm is, even during daytime, only about 130 K. Therefore, we infer that Philae's excavation would have suddenly exposed the colder subsurface to the higher temperatures of the solar insolated surface and that this process would have rapidly caused the sublimation of volatile species from dust grains present in the subsurface. Further, these volatilized species entered the COSAC and Ptolemy mass spectrometers.…”