In this work we compare data of two spectrometers onboard space missions directed to comets: HRI (High Resolution Imager) on board Deep Impact \citep{AHearn2005} which overflown Tempel 1 on 2005 July 4th, and VIRTIS (Visible InfraRd and Thermal Imaging Spectrometer) onboard Rosetta which nowadays is orbiting around Comet Churyumov-Gerasimenko \citep{Coradini2007}. This work is focused on the detection of water ice on the surface, which seems to be present on both comets in two distinct modalities: - small grain size (1-2 mu m), as derived in the material ejected from the surface of Tempel 1 after the impact \citep{Sunshine2007}, and on the surface of 67P/C-G as result of vapour recondensation \citep{DeSanctis2015}. - large grain size (>30 mu m), in minor amounts, detected as exposed ice on both comets \citep{Sunshine2006, Raponi2013, Filacchione2015}. These two modalities are related to different spectral features. To retrieve the physical properties of the surface we apply the Hapke scattering model to the measured spectra. The data are corrected for artifacts and thermal emission before comparing them with the model. Moreover the estimated signal to noise ratio is taken into account by a least square optimization algorithm in the fitting procedure. This comparative analysis could reveal common processes for comets, which have implication on their formation and evolution. Authors acknowledge the funding from Italian, French and German Space Agencies
