Ceres observed at low phase angles by VIR-Dawn

Abstract

Context. Particulate surfaces exhibit a surge of reflectance at low phase angles, a phenomenon referred to as the opposition effect (OE). Two mechanisms are recognized as responsible for the OE: shadow hiding (SH) and coherent backscattering. The latter is typically characterized by a small angular width of a few degrees at most and according to the theoretical prediction should exhibit wavelength and albedo dependence. Aims. We characterize the OE on the surface of Ceres using Dawn Visible InfraRed mapping spectrometer hyperspectral images at low phase angles. Furthermore, this dataset, coupled with previous observations, allows us to perform a complete spectrophotometric modeling at visual-to-infrared (VIS-IR) wavelengths (0.465–4.05 μm) in the broad phase angle range ≈0°−132°. Methods. We applied Hapke’s theory to the average phase curve for Ceres. Disk-resolved properties of the OE were investigated through an empirical model. Results. Across the investigated phase angle interval, Ceres’ average phase curve exhibits a smaller back-scattering contribution for increasing wavelengths. This determines a progressive spectral reddening at larger phase angles that we hypothesize as being related to the effect of submicron roughness on the grain surface. In the OE region, the shape of the phase curves is fairly constant across the VIS range and no sharp opposition surge at very small phase angles (α < 2°) can be recognized. This would suggest a major contribution from SH to Ceres’ OE. Assuming SH as the dominant mechanism, from the OE angular width we infer a high surface porosity (≈0.9), which appears in good qualitative agreement with Ceres’ low thermal inertia. Thanks to the OE observations we derive Ceres’ VIS-IR geometric albedo with a reference value at 0.55 μm of 0.098 ± 0.007. Mapping of the VIS normal albedo and OE angular width across a portion of the surface of Ceres does not reveal a spatial correlation between these quantities, consistent with SH dominating in the α = 0°−7° interval. The comparison of Ceres’ V -band magnitude curve with that of other asteroids indicates that Ceres’ OE is typical of a low-albedo object and compatible with the C-class type.

Small lobe of comet 67P: Characterization of the Wosret region with ROSETTA-OSIRIS

“…The phase reddening coefficient determined for the Wosret region is comparable to the one found for other dark and presumed organic rich bodied such as D-type asteroids (0.05 ± 0.03 × 10 −4 nm −1 deg −1 in the 0.45-2.45 µm range, Lantz et al 2017), and the dwarf planet Ceres (γ = 0.046 × 10 −4 nm −1 deg −1 , Ciarniello et al 2017Ciarniello et al , 2020. It is however two to four times higher than the value reported for low albedo and carbonaceous rich bodies such as the near-Earth asteroids: (101955) Bennu and ( 162173) Ryugu (Table 2), that were recently visited by the OSIRIS-REx and Hayabusa 2 missions, respectively.…”

A Probabilistic Approach to Determination of Ceres' Average Surface Composition From Dawn Visible‐Infrared Mapping Spectrometer and Gamma Ray and Neutron Detector Data

“…Thus, we assumed p = 1 in the absence of further constraints. We note that analysis of VIR data obtained for different phase angles showed that the SPPF of Ceres' averaged surface is fairly symmetric in IR (Ciarniello et al., 2017, 2020).…”

“…Assumption of isotropic scattering in our model may underestimate the darkening effect of magnetite due to its forward scattering (Mustard & Pieters, 1989). However, analysis of Ceres' photometric properties (Ciarniello et al., 2017, 2020) showed back scattering or symmetrical scattering, depending on different assumptions, which suggests that strong forward scattering is unlikely to be a major cause of Ceres' low albedo.…”

Phase reddening on asteroid Bennu from visible and near-infrared spectroscopy

“…For the dwarf planet Ceres, Ciarniello et al (2017Ciarniello et al ( , 2020) reported a monotonic spectral phase reddening throughout visible to NIR wavelengths, with γ values of 0.0046 µm −1 deg −1 in the 0.55-0.8 µm range and 0.0015 µm −1 deg −1 in the 1.2-2 µm range. Thus phase reddening is progressively less sensitive to wavelength in the IR, that is, it gets smaller at longer wavelengths.…”