Ceres' opposition effect observed by the Dawn framing camera

• Main Authors: Schröder, Stefan E, Li, Jian-Yang, Rayman, Marc D, Joy, Steven P, Polanskey, Carol A, Carsenty, Uri, Castillo-Rogez, Julie C, Ciarniello, Mauro, Jaumann, Ralf, Longobardo, Andrea, McFadden, Lucy A, Mottola, Stefano, Sykes, Mark, Raymond, Carol A, Russell, Christopher T
• Format: Journal Article
• Language: English
• Published: 13-11-2018
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.1811.05218

A&A 620, A201 (2018) The surface reflectance of planetary regoliths may increase dramatically
towards zero phase angle, a phenomenon known as the opposition effect (OE). Two
physical processes that are thought to be the dominant contributors to the
brightness surge are shadow hiding (SH) and coherent backscatter (CB). The
occurrence of shadow hiding in planetary regoliths is self-evident, but it has
proved difficult to unambiguously demonstrate CB from remote sensing
observations. One prediction of CB theory is the wavelength dependence of the
OE angular width. The Dawn spacecraft observed the OE on the surface of dwarf
planet Ceres. We characterize the OE over the resolved surface, including the
bright Cerealia Facula, and to find evidence for SH and/or CB. We analyze
images of the Dawn framing camera by means of photometric modeling of the phase
curve. We find that the OE of most of the investigated surface has very similar
characteristics, with an enhancement factor of 1.4 and a FWHM of 3{\deg} (broad
OE). A notable exception are the fresh ejecta of the Azacca crater, which
display a very narrow brightness enhancement that is restricted to phase angles
$< 0.5${\deg} (narrow OE); suggestively, this is in the range in which CB is
thought to dominate. We do not find a wavelength dependence for the width of
the broad OE, and lack the data to investigate the dependence for the narrow
OE. The prediction of a wavelength-dependent CB width is rather ambiguous. The
zero-phase observations allow us to determine Ceres' visible geometric albedo
as $p_V = 0.094 \pm 0.005$. A comparison with other asteroids suggests that
Ceres' broad OE is typical for an asteroid of its spectral type, with
characteristics that are primarily linked to surface albedo. Our analysis
suggests that CB may occur on the dark surface of Ceres in a highly localized
fashion.