The 67P nucleus: seasonal and diurnal color variations from inbound orbits to the perihelion passage

The Rosetta mission has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, observing it for about 2 years and providing the unique opportunity to continuously investigate the 67P nucleus composition and its evolution from ~4 AU to perihelion, and beyond.Here we report on seasonal and diurnal color variations of the surface of 67P's nucleus, observed in the 250-1000 nm wavelength range with the Narrow Angle Camera of the OSIRIS imaging system onboard RosettaThe analysis of colors and of the spectral slope values, evaluated in the 535-882 nm wavelength range, clearly indicates spectacular changes. The nucleus has become spectrally less red, i.e. the spectral slope has decreased, as it approached perihelion, indicating that increasing activity had progressively shed the surface dust, partially showing the underlying ice-rich layer. In addition to the change of color, the amount of phase reddening (the increase of spectral slope with phase angle) decreased when the comet approached its perihelion, by a factor of two in the 2015 observations compared to the August 2014 ones, indicating a change in the physical properties of the outermost layer of the nucleus. We also identified large ice-rich patches (1500 m² per patch) appearing and then vanishing in about 10 days, indicating small scale heterogeneities in the nucleus. While approaching perihelion, the nucleus has shown also considerable diurnal color variations on extended areas and the indisputable occurrence of water frost close to the morning shadows, moving with them and sublimating in few minutes when fully illuminated by the Sun.With an unprecedented spatial resolution and time coverage the OSIRIS images indicate that important recondensation processes of volatiles are ongoing on cometary nuclei and that solid frost/ices are widespread present on the surface but with extremely short lifetime.