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

Abstract

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• < α < 132• interval were used to characterize Ceres' phase curve in the 0.465-4.05 µm 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 µm are w = 0.14 ± 0.02 and ξ = −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 isθ = 29• ± 6• . 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% kÅ • gives H = 3.14 ± 0.04 and G = 0.10 ± 0.04, while the classical linear model provides V(1, 1, 0 • ) = 3.48 ± 0.03 and β = 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.

Resolved spectrophotometric properties of the Ceres surface from Dawn Framing Camera images

“…5.1). Color maps from VIR data are presented by Ciarniello et al (2016). The VIR map in the visual wavelength range is broadly consistent with the FC map in Nathues et al (2015).…”

“…It seems that analyzing full-disk images at low phase angles does not suffice to reliably derive the roughness parameter; high phase angle images are required in accordance with Helfenstein and Veverka (1989). Ciarniello et al (2016) find a photometric roughness of θ = 29…”

“…Our aim is to relate these photometric parameters to the physical properties of the regolith and thereby uncover the physical processes shaping the surface of Ceres. Other investigations of Ceres' spectrophotometric properties using Dawn data are presented by Ciarniello et al (2016), Li et al (2016a), and Longobardo et al (2016).…”

The mineralogy of Ceres’ Nawish quadrangle

“…The data have been calibrated following the procedure described by Filacchione and Ammannito (2014). Afterwards, (i) we removed the instrumental artifacts from the calibrated data (Carrozzo et al, 2016); (ii) we removed the thermal emission from the spectra (Raponi et al 2017, this issue); (iii) then we applied a photometric correction based on the Hapke model (Ciarniello et al, 2017); (iv) finally, we computed and produced proper spectral parameters maps, as described by Frigeri et al, 2018a (this issue). To complement our analysis, we included also some relevant maps derived by Framing Camera (FC; Sierks et al, 2011) data.…”

“…Carrozzo et al, 2018) showed that occurrence of carbonate content increases the reflectance while the opaque causes darkening. Albedo was calculated by applying a Hapke photometric correction to the entire dataset as described by Ciarniello et al, 2017. Hapke photometric correction to standard observation geometry (i = 30°, e = 0°, α = 30°).…”