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

We study the spectrophotometric properties of dwarf planet Ceres in the VIS-IR spectral range by means of hyper-spectral images acquired by the VIR imaging spectrometer on board the NASA Dawn mission. Disk-resolved observations with a phase angle within the $7^{circ}<alpha<132^{circ}$ interval were used to characterize Ceres phase curve in the 0.465-4.05 $mu$m spectral range. Hapkes model was applied to perform the photometric correction of the dataset, allowing us to produce albedo and color maps of the surface. The $V$-band magnitude phase function of Ceres was fitted with both the classical linear model and H-G formalism. The single-scattering albedo and the asymmetry parameter at 0.55$mu$m are $w=0.14pm0.02$ and $xi=-0.11pm0.08$, respectively (two-lobe Henyey-Greenstein phase function); the modeled geometric albedo is $0.094pm0.007$; the roughness parameter is $bar{theta}=29^{circ}pm6^{circ}$. Albedo maps indicate small variability on a global scale with an average reflectance of $0.034 pm 0.003$. 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 $1.1%kAA^{-1}$ and $0.85%kAA^{-1}$ at VIS and IR wavelengths, respectively. Broadband color indices are $V-R=0.38pm0.01$ and $R-I=0.33pm0.02$. H-G modeling of the $V$-band magnitude phase curve for $alpha<30^{circ}$ gives $H=3.14pm0.04$ and $G=0.10pm0.04$, while the classical linear model provides $V(1,1,0^{circ})=3.48pm0.03$ and $beta=0.036pm0.002$. The comparison 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.