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Water ice-rich patches have been detected on the surface of comet67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board theRosetta spacecraft, since the orbital insertion in late August 2014. Amongthose, three icy patches have been selected, and VIRTIS data are used toanalyse their properties and their temporal evolution while the comet wasmoving towards the Sun. We performed an extensive analysis of the spectralparameters, and we applied the Hapke radiative transfer model to retrieve theabundance and grain size of water ice, as well as the mixing modalities ofwater ice and dark terrains on the three selected water ice rich areas. Studyof the spatial distribution of the spectral parameters within the ice-richpatches has revealed that water ice follows different patterns associated to abimodal distribution of the grains: ~50 {\mu}m sized and ~2000 {\mu}m sized. Inall three cases, after the first detections at about 3.5 AU heliocentricdistance, the spatial extension and intensity of the water ice spectralfeatures increased, it reached a maximum after 60-100 days at about 3.0 AU, andwas followed by an approximately equally timed decrease and disappearanceatabout ~2.2 AU, before perihelion. The behaviour of the analysed patches can beassimilated to a seasonal cycle. In addition we found evidence of short-termvariability associated to a diurnal water cycle. The similar lifecycle of thethree icy regions indicates that water ice is uniformly distributed in thesubsurface layers, and no large water ice reservoirs are present.

The temporal evolution of exposed water ice-rich areas on the surface of 67P/Churyumov-Gerasimenko: spectral analysis