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

Abstract

Water ice-rich patches have been detected on the surface of comet 67P/Churyumov-Gerasimenko by the VIRTIS (Visible InfraRed and Thermal Imaging Spectrometer) hyperspectral imager on board the Rosetta spacecraft, since the orbital insertion in late 2014 August. Among these, three icy patches have been selected, and VIRTIS data are used to analyse their properties and their temporal evolution while the comet was moving towards the Sun. We performed an extensive analysis of the spectral parameters, and we applied the Hapke radiative transfer model to retrieve the abundance and grain size of water ice, as well as the mixing modalities of water ice and dark terrains on the three selected water ice-rich areas. Study of the spatial distribution of the spectral parameters within the ice-rich patches has revealed that water ice follows different patterns associated with a bimodal distribution of the grains: ∼50-μm sized and ∼2000-μm sized. In all three cases, after the first detections at about 3.5-au heliocentric distance, the spatial extension and intensity of the water ice spectral features increased, it reached a maximum after 60-100 d at about 3.0 au, and was followed by an approximately equally timed decrease and disappearance at about 2.2 au, before perihelion. The behaviour of the analysed patches can be assimilated to a seasonal cycle. In addition, we found evidence of short-term variability associated with a diurnal water cycle. The similar life cycle of the three icy regions indicates that water ice is uniformly distributed in the subsurface layers, and no large water ice reservoirs are present.