Photometry of Ceres and Occator faculae as inferred from VIR/Dawn data

Abstract

Spectral parameters of Ceres measured by the Dawn/VIR imaging spectrometer are studied as a function of illumination angles, by applying a semi-empirical method based on a statistical analysis of the VIR dataset acquired up to September 2016. The study also focuses on the photometry of the Occator faculae, i.e. the brightest spots of the Ceres surface, showing an albedo up to eight times the Ceres average. The considered semi-empirical approach takes into account the small extension (and hence small dataset) of this region and lays the groundwork to apply scattering models even on such a limited area. The behavior of Ceres visible and infrared reflectance with phase angle is similar to other asteroids belonging to its same spectral class, i.e. C-type. The depth of the bands at 2.7 μm (phyllosilicates), 3.1 μm (ammonium), 3.4 μm (magnesium carbonates) and the infrared spectral slope linearly increase with phase angle, showing analogies with other asteroids and occurrence of phase reddening. The different behavior of the 3.9 μm band depth (also due to Mg carbonates), independent of illumination angles, could indicate that other carriers contribute to the 3.4 μm band and play a more important role in photometry outside the carbonate deposits. The phase function of the Occator faculae is much steeper than expected from its high albedo. Mixture of bright and dark material and larger roughness can be at the basis of this result. The phyllosilicate bands show a steeper increase with phase angle with respect to the Ceres average, due to the lower presence of dark materials, and/or again larger roughness. The absence of trends with phase angles of the two carbonate bands and of the spectral slope suggests that carbonates do not produce phase reddening.