Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission

Aims. We present a study of the spectrophotometric properties of dwarf planet Ceres in the visual-to-infrared (VIS-IR) spectral range by means of hyper-spectral images acquired by the VIR imaging spectrometer on board the NASA Dawn mission. Methods. Disk-resolved observations with a phase angle within the 7 degrees < alpha < 132 degrees interval were used to characterize Ceres' phase curve in the 0.465-4.05 mu m spectral range. Hapke's model was applied to perform the photometric correction of the dataset to standard observation geometry at VIS-IR wavelength, allowing us to produce albedo and color maps of the surface. The V-band magnitude phase function of Ceres has been computed from disk-resolved images and fitted with both the classical linear model and H-G formalism. Results. The single-scattering albedo and the asymmetry parameter at 0.55 mu m are w = 0.14 +/- 0.02 and xi = 0.11 +/- 0.08, respectively (two-lobe Henyey-Greenstein phase function); at the same wavelength, Ceres' geometric albedo as derived from our modeling is 0.094 +/- 0.007; the roughness parameter is (theta) over bar = 29 degrees +/- 6 degrees. Albedo maps indicate small variability on a global scale with an average reflectance at standard geometry of 0.034 +/- 0.003. Nonetheless, isolated areas such as the Occator bright spots, Haulani, and Oxo show an albedo much higher than average. We measure a significant spectral phase reddening, and the average spectral slope of Ceres' surface after photometric correction is 1.1% k angstrom(-1) and 0.85% k angstrom(-1) at VIS and IR wavelengths, respectively. Broadband color indices are V-R = 0.38 +/- 0.01 and R I = 0.33 +/- 0.02. Color maps show that the brightest features typically exhibit smaller slopes. The H-G modeling of the V-band magnitude phase curve for alpha < 30 degrees gives H = 3.14 +/- 0.04 and G = 0.10 +/- 0.04, while the classical linear model provides V(1; 1; 0 degrees) = 3.48 +/- 0.03 and beta = 0.036 +/- 0.002. The comparison of our results with spectrophotometric properties of other minor bodies indicates that Ceres has a less back-scattering phase function and a slightly higher albedo than comets and C-type objects. However, the latter represents the closest match in the usual asteroid taxonomy.