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. Hapke’s 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 $w=0.14\pm 0.02$ and $\xi =-0.11\pm 0.08$, respectively (two-lobe Henyey-Greenstein phase function); the modeled geometric albedo is $0.094\pm 0.007$; the roughness parameter is $\bar{\theta }={29}^{\circ }\pm 6^{\circ }$. 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 $1.1\mathrm{\%}k{\mathring{A}}^{-1}$ and $0.85\mathrm{\%}k{\mathring{A}}^{-1}$ at VIS and IR wavelengths, respectively. Broadband color indices are $V-R=0.38\pm 0.01$ and $R-I=0.33\pm 0.02$. H-G modeling of the $V$-band magnitude phase curve for $\alpha <{30}^{\circ }$ gives $H=3.14\pm 0.04$ and $G=0.10\pm 0.04$, while the classical linear model provides $V(1,1,0^{\circ })=3.48\pm 0.03$ and $\beta =0.036\pm 0.002$. 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.