Photometric Properties of Comet 67P/CG as Seen by VIRTIS-M Onboard Rosetta: Light Curves and Disk-integrated Phase Curves.

Abstract:

VIRTIS-M is the Visible InfraRed Thermal Imaging Spectrometer onboard the Rosetta Mission orbiter (Coradini et al., 2007) devoted to investigate the spectrophotometric properties of the comet 67P/Churyumov-Gerasimenko in the 0.25-5.1 µm spectral range. Here we present data acquired during the first observations of the comet, starting from mid-July 2014, when the spacecraft-target distance was approximately 13000 km and the comet nucleus as seen by the instrument was pixel-size, up to the more recent acquisitions in which the surface was resolved. These far-approach data have allowed us to produce both light curves of the nucleus at different solar phase angles and disk-integrated phase curves over the entire instrument spectral range. The light curve is characterized by two asymmetric peaks due to the elongated, e.g. double-lobed, shape of the nucleus. The disk-integrated phase curves exhibit a back-scattering behavior. Comparisons with light curves and phase curves of other comets will be discussed, as well as the dependence of color ratios on rotational phase and phase angle in order to exploit surface compositional variations. We also present a retrieval of the main photometric parameters (single scattering albedo, surface roughness, single particle phase function and opposition effect width and amplitude) by means of Hapke model.

Authors acknowledge the funding from Italian and French Space Agencies.