The Visible-Infrared Mapping Spectrometer (VIR) on board the Dawn spacecraft revealed that aqueous secondary minerals -- Mg-phyllosilicates, NH4-bearing phases, and Mg/Ca carbonates -- are ubiquitous on Ceres. Ceres' low reflectance requires dark phases, which were assumed to be amorphous carbon and/or magnetite (~80 wt.%). In contrast, the Gamma Ray and Neutron Detector (GRaND) constrained the abundances of C (8-14 wt.%) and Fe (15-17 wt.%). Here, we reconcile the VIR-derived mineral composition with the GRaND-derived elemental composition. First, we model mineral abundances from VIR data, including either meteorite-derived insoluble organic matter, amorphous carbon, magnetite, or combination as the darkening agent and provide statistically rigorous error bars from a Bayesian algorithm combined with a radiative-transfer model. Elemental abundances of C and Fe are much higher than is suggested by the GRaND observations for all models satisfying VIR data. We then show that radiative transfer modeling predicts higher reflectance from a carbonaceous chondrite of known composition than its measured reflectance. Consequently, our second models use multiple carbonaceous chondrite endmembers, allowing for the possibility that their specific textures or minerals other than carbon or magnetite act as darkening agents, including sulfides and tochilinite. Unmixing models with carbonaceous chondrites eliminate the discrepancy in elemental abundances of C and Fe. Ceres' average reflectance spectrum and elemental abundances are best reproduced by carbonaceous-chondrite-like materials (40-70 wt.%), IOM or amorphous carbon (10 wt.%), magnetite (3-8 wt.%), serpentine (10-25 wt.%), carbonates (4-12 wt.%), and NH4-bearing phyllosilicates (1-11 wt.%).
Phase angle-induced spectral effects are important to characterize since they affect spectral band parameters such as band depth and band center, and therefore skew mineralogical interpretations of planetary bodies via reflectance spectroscopy. Dwarf planet (1) Ceres is the next target of NASA's Dawn mission, which is expected to arrive in March 2015. The visible and near-infrared mapping spectrometer (VIR) onboard Dawn has the spatial and spectral range to characterize the surface between 0.25-5.0 microns. Ceres has an absorption feature at 3.0 microns due to hydroxyl- and/or water-bearing mi…
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 de…
The NASA Dawn spacecraft took off from Cape Canaveral in September 2007 atop a Delta II rocket starting an ambitious journey to Vesta and Ceres, the two most massive worlds in the largest reservoir of asteroids in the Solar System, the Main Belt. Prior to the Dawn launch, Earth-bound observations of Vesta and Ceres revealed intriguing features--from Vesta's rugged shape to Ceres' tenuous water exosphere--, but these objects remained fuzzy speckles of light even through the lenses of the most powerful telescopes. With Dawn's exploration of Vesta (2011-2012) and Ceres (2015-2018) these two world…
Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres' visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin & Volquardsen (2010, Icarus 206, 327) who reported rotati…
The NASA/Dawn mission has acquired an unprecedented amount of data from the surface of the dwarf planet Ceres, providing a thorough characterization of its surface composition. The current favorite compositional model includes a mixture of ultra-carbonaceous material, phyllosilicates, carbonates, organic, Fe-oxides, and volatiles, as determined by the Dawn/VIR-IR imaging spectrometer, with the constraints on the elemental composition provided by the Dawn/GRAND instrument. The recent calibration of the VIS channel of the VIR spectrometer provided further constraints on the overall compositional…
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…
We present a global spectrophotometric characterization of the Ceres surface using Dawn Framing Camera (FC) images. We identify the photometric model that yields the best results for photometrically correcting images. Corrected FC images acquired on approach to Ceres were assembled into global maps of albedo and color. Generally, albedo and color variations on Ceres are muted. The albedo map is dominated by a large, circular feature in Vendimia Planitia, known from HST images (Li et al., 2006), and dotted by smaller bright features mostly associated with fresh-looking craters. The dominant col…
We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet. We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software. Ceres surf…
We mapped all boulders larger than 105 m on the surface of dwarf planet Ceres using images of the Dawn framing camera acquired in the Low Altitude Mapping Orbit (LAMO). We find that boulders on Ceres are more numerous towards high latitudes and have a maximum lifetime of $150 \pm 50$ Ma, based on crater counts. These characteristics are distinctly different from those of boulders on asteroid (4) Vesta, an earlier target of Dawn, which implies that Ceres boulders are mechanically weaker. Clues to their properties can be found in the composition of Ceres' complex crust, which is rich in phyllosi…
We study the spectrophotometric properties of dwarf planet Ceres in the VIS-IR spectral range by means of hyper-spectral images acquired by the VIR imaging spectrometer on board the NASA Dawn mission. Disk-resolved observations with a phase angle within the $7^{\circ}<α<132^{\circ}$ interval were used to characterize Ceres' phase curve in the 0.465-4.05 $μ$m spectral range. Hapke's model was applied to perform the photometric correction of the dataset, allowing us to produce albedo and color maps of the surface. The $V$-band magnitude phase function of Ceres was fitted with both the classical …
A diurnal varying haze layer at the bright spots of Occator on dwarf planet Ceres has been reported from images of the Dawn Framing Camera. This finding is supported by ground-based observations revealing diurnal albedo changes at Occator's longitude. In the present work, we further investigate the previously reported haze phenomenon in more detail using additional Framing Camera images. We demonstrate that the light scattering behavior at the central floor of Occator is different compared to a typical cerean surface and is likely inconsistent with a pure solid surface scatterer. The identifie…
Context. Ceres is the most massive body of the asteroid belt and contains about 25 wt.% (weight percent) of water. Understanding its thermal evolution and assessing its current state are major goals of the Dawn Mission. Constraints on internal structure can be inferred from various observations. Especially, detailed knowledge of the rotational motion can help constrain the mass distribution inside the body, which in turn can lead to information on its geophysical history. Aims. We investigate the signature of the interior on the rotational motion of Ceres and discuss possible future measuremen…
Variations and spatial distributions of bright and dark material on dwarf planet Ceres play a key role in understanding the processes that have led to its present surface composition. We define limits for bright and dark material in order to distinguish them consistently, based on the reflectance of the average surface using Dawn Framing Camera data. A systematic classification of four types of bright material is presented based on their spectral properties, composition, spatial distribution, and association with specific geomorphological features. We found obvious correlations of reflectance …
Context: Despite the observed signs of large impacts on the surface of Ceres, there is no confirmed collisional family associated with this dwarf planet. After a dynamical and photometric study, a sample of 156 asteroids was proposed as candidate members of a Ceres collisional family. Aims: Our main objective is to study the connection between Ceres and a total of 14 observed asteroids among the candidate's sample to explore their genetic relationships with Ceres. Methods: We obtained visible spectra of these 14 asteroids using the OSIRIS spectrograph at the 10.4 m Gran Telescopio Canarias(GTC…
The Visible InfraRed (VIR) mapping spectrometer onboard Dawn mission has obtained the spatial distribution of the spectral reflectance of Vesta in the wavelength ranges between 0.25-1.07 micrometers and 0.95-5.1 micrometers. A photometric correction allows to characterize the intrinsic variability of the surface albedo by removing the dependance of the reflectance from the observing geometry. In this work, we present the photometric correction obtained for observations from the Survey, HAMO and HAMO 2 mission phases at Vesta in the whole spectral range investigated by VIR, as well as the surfa…
Previous observations suggested that Ceres has active but possibly sporadic water outgassing, and possibly varying spectral characteristics in a time scale of months. We used all available data of Ceres collected in the past three decades from the ground and the Hubble Space Telescope, and the newly acquired images by Dawn Framing Camera to search for spectral and albedo variability on Ceres, in both a global scale and local regions, particularly the bright spots inside Occator crater, over time scales of a few months to decades. Our analysis has placed an upper limit on the possible temporal …
Earth and orbital-based radar observations of asteroids provide a unique opportunity to characterize surface roughness and the dielectric properties of their surfaces, as well as potentially explore some of their shallow subsurface physical properties. If the dielectric and topographic properties of asteroid's surfaces are defined, one can constrain their surface textural characteristics as well as potential subsurface volatile enrichment using the observed radar backscatter. To achieve this objective, we establish the first dielectric model of asteroid Vesta for the case of a dry, volatile-po…
We report a comprehensive analysis of the global spectrophotometric properties of Ceres using Dawn Framing Camera images collected from April to June 2015 during the RC3 and Survey mission phases. The single-scattering albedo of Ceres at 555 nm is 0.14$\pm$0.04, the geometric albedo is 0.096$\pm$0.006, and the Bond albedo is 0.037$\pm$0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase function (SPPF) show a wavelength dependence, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multi…
Context: The dwarf planet (1) Ceres - next target of the NASA Dawn mission - is the largest body in the asteroid main belt; although several observations of this body have been performed so far, the presence of surface water ice is still questioned. Aims: Our goal is to better understand the surface composition of Ceres, and to constrain the presence of exposed water ice. Methods: We acquired new visible and near-infrared spectra at the Telescopio Nazionale Galileo (TNG, La Palma, Spain), and reanalyzed literature spectra in the 3-$μ$m region. Results: We obtained the first rotationally-resolv…
We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology. We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite (HED) meteorites data as a reference, we …
Spectral Evolution of Ceres' Surface and Implications for Space Weathering
Sun Yat-sen University（中山大学）；Xinjiang Astronomical Observatory, Chinese Academy of Sciences（中国科学院新疆天文台）；National Space Science Center, Chinese Academy of Sciences（中国科学院国家空间科学中心）；University of Chinese Academy of Science（中国科学院大学）
This work describes the correction method applied to the dataset acquired at the asteroid (4) Vesta by the visible channel of the visible and infrared mapping spectrometer. The rising detector temperature during data acquisitions in the visible wavelengths leads to a spectral slope increase over the whole spectral range. This limits the accuracy of the studies of the Vesta surface in this wavelength range. Here, we detail an empirical method to correct for the visible detector temperature dependency while taking into account the specificity of the Vesta dataset.
Data acquired at Ceres by the visible channel of the Visible and InfraRed mapping spectrometer (VIR) on board the NASA Dawn spacecraft are affected by the temperatures of both the visible (VIS) and the infrared (IR) sensors, which are respectively a CCD and a HgCdTe array. The variations of the visible channel temperatures measured during the sessions of acquisitions are correlated with variations in the spectral slope and shape for all the mission phases. The infrared channel (IR) temperature is more stable during the acquisitions, nonetheless it is characterized by a bi-modal distribution wh…
Anhydrous pyroxene-rich interplanetary dust particles (IDPs) have been proposed as surface analogs for about two-thirds of all C-complex asteroids. However, this suggestion appears to be inconsistent with the presence of hydrated silicates on the surfaces of some of these asteroids including Ceres. Here we report the presence of enstatite (pyroxene) on the surface of two C-type asteroids (Ceres and Eugenia) based on their spectral properties in the mid-infrared range. The presence of this component is particularly unexpected in the case of Ceres because most thermal evolution models predict a …
We present an analysis of the olivine-rich exposures at Bellicia and Arruntia craters using Dawn Framing Camera (FC) color data. Our results confirm the existence of olivine-rich materials at these localities as described by Ammannito et al. (2013a) using Visual Infrared Spectrometer (VIR) data. Analyzing laboratory spectra of various Howardite-Eucrite-Diogenite meteorites, high-Ca pyroxenes, olivines and olivine-orthopyroxene mixtures, we derive three FC spectral band parameters that are indicators of olivine-rich materials. Combining the three band parameters allows us, for the first time, t…
