  From August to November 2014 the Rosetta orbiter has performed an extensive
observation campaign aimed at the characterization of 67P/CG nucleus properties
and to the selection of the Philae landing site. The campaign led to the
production of a global map of the illuminated portion of 67P/CG nucleus. During
this prelanding phase the comet's heliocentric distance decreased from 3.62 to
2.93 AU while Rosetta was orbiting around the nucleus at distances between 100
to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer -
Mapping channel (Coradini et al. 2007) onboard the orbiter, has acquired
0.25-5.1 micron hyperspectral data of the entire illuminated surface, e.g. the
north hemisphere and the equatorial regions, with spatial resolution between
2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission
removal in the 3.5-5.1 micron range and for surface's photometric response. The
resulting reflectance spectra have been used to compute several Cometary
Spectral Indicators (CSI): single scattering albedo at 0.55 micron, 0.5-0.8
micron and 1.0-2.5 micron spectral slopes, 3.2 micron organic material and 2.0
micron water ice band parameters (center, depth) with the aim to map their
spatial distribution on the surface and to study their temporal variability as
the nucleus moved towards the Sun. Indeed, throughout the investigated period,
the nucleus surface shows a significant increase of the single scattering
albedo along with a decrease of the 0.5-0.8 and 1.0-2.5 micron spectral slopes,
indicating a flattening of the reflectance. We attribute the origin of this
effect to the partial removal of the dust layer caused by the increased
contribution of water sublimation to the gaseous activity as comet crossed the
frost-line.
