Photometric Properties of Ceres from Telescopic Observations using Dawn Framing Camera Color Filters

TL;DR

This study models Ceres' phase function across wavelengths using Dawn FC data, revealing wavelength-dependent phase reddening and confirming its flat, featureless spectrum with higher albedo than previous measurements.

Contribution

It provides the first detailed phase function modeling of Ceres versus wavelength, demonstrating phase reddening and consistency with previous spectral data.

Findings

Geometric albedos are ~10% higher than previous measurements.

Phase function shows wavelength dependence, with shallower slopes at longer wavelengths.

Positive correlation between absorption band depth and albedo.

Abstract

The dwarf planet Ceres is likely differentiated similar to the terrestrial planets but with a water/ice dominated mantle and an aqueously altered crust. Detailed modeling of Ceres' phase function has never been performed to understand its surface properties. The Dawn spacecraft began orbital science operations at the dwarf planet in April 2015. We observed Ceres with flight spares of the seven Dawn Framing Camera color filters mounted on ground-based telescopes over the course of three years to model its phase function versus wavelength. Our analysis shows that the modeled geometric albedos derived from both the IAU HG model and the Hapke model are consistent with a flat and featureless spectrum of Ceres, although the values are ~10% higher than previous measurements. Our models also suggest a wavelength dependence of Ceres' phase function. The IAU G-parameter and the Hapke single-particle phase function parameter, g, are both consistent with decreasing (shallower) phase slope with increasing wavelength. Such a wavelength dependence of phase function is consistent with reddening of spectral slope with increasing phase angle, or phase-reddening. This phase reddening is consistent with previous spectra of Ceres obtained at various phase angles archived in the literature, and consistent with the fact that the modeled geometric albedo spectrum of Ceres is the bluest of all spectra because it represents the spectrum at 0 degree phase angle. Ground-based FC color filter lightcurve data are consistent with HST albedo maps confirming that Ceres' lightcurve is dominated by albedo and not shape. We detected a positive correlation between 1.1-micron absorption band depth and geometric albedo suggesting brighter areas on Ceres have absorption bands that are deeper.