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Register a new userSpectral investigation of Ceres analogue mixtures: in-depth analysis of crater central peak material (ccp) on Ceres
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In order to investigate the causes of different spectral slope in ccps, different grain-sizes of Ceres analogue mixtures were produced, heated to remove absorption of atmospheric water, and spectrally analyzed. First, the end-members which compose the Ceres surface (using the antigorite as Mg-phyllosilicate, the NH4-montmorillonite as NH4-phyllosilicate, the dolomite as carbonate and the graphite as dark component), were mixed, obtaining mixtures with different relative abundance, and identifying the mixture with the reflectance spectrum most similar to the average Ceres spectrum. The mixtures were obtained with grain size of 0-25 {mu}m, 25-50 mic and 50-100 mic, were heated and spectrally analysed at T= 300 K and T=200 K (typical for surface Ceres temperature during VIR observations). The most similar Ceres analogue mixture is composed of dolomite (18%), graphite (27%), antigorite (32%) and NH4-montmorillonite (29%) and the results of this work suggest that this mixture is more similar to the Ceres youngest region than to the Ceres average, in particular for the negative slope of spectrum. Small variation in the composition and grain size of end-members need to be considered, in addition to the occurrence of a dark component dispersed in fine size. Furthermore, the positive spectral slope that characterizes the mean Ceres spectrum can be obtained by the application of some processes simulating the space weathering on Ceres (as micro-meteoritic impacts and solar wind irradiation), i.e. laser and ion irradiation. As conclusion, youngest ccps on Ceresare probably composed by fresher and weakly processed mixture with fine dark material intimately dispersed: as a result, the reflectance spectra of youngest material show a negative slope in the 1.2-1.9 mic range. The redder slope observed in the older ccps is probably the consequence of the space weathering effects on fresher material
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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 Occators 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 identified deviation is best explained by an additional component to the scattered light of the surface, i.e., a haze layer. Our results support the water vapor detection by Herschel observations though the existence of a tenuous cerean exosphere is not yet confirmed.
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 with spectral shape (slopes) and age; however, this is not unique throughout the bright spots. Although impact features show generally more extreme reflectance variations, several areas can only be understood in terms of inhomogeneous distribution of composition as inferred from Dawn Visible and Infrared Spectrometer data. Additional material with anomalous composition and spectral properties are rare. The identification of the composition and origin of the dark, particularly the darkest material, remains to be explored. The spectral properties and the morphology of the dark sites suggest an endogenic origin, but it is not clear whether they are more or less primitive surficial exposures or excavated subsurface but localized material. The reflectance, spectral properties, inferred composition, and geologic context collectively suggest that the bright and dark material tends to gradually change toward the average surface over time. This could be because of multiple processes, i.e., impact gardening/space weathering, and lateral mixing, including thermal and aqueous alteration, accompanied by changes in composition and physical properties such as grain size, surface temperature, and porosity (compaction).
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 albedo variation on Ceres. Sporadic water vapor venting, or any possibly ongoing activity on Ceres, is not significant enough to change the albedo or the area of the bright features in Occator crater by >15%, or the global albedo by >3% over various time scales that we searched. Recently reported spectral slope variations can be explained by changing Sun-Ceres-Earth geometry. The active area on Ceres is less than 1 km$^2$, too small to cause global albedo and spectral variations detectable in our data. Impact ejecta due to impacting projectiles of tens of meters in size like those known to cause observable changes to the surface albedo on Asteroid Scheila cannot cause detectable albedo change on Ceres due to its relatively large size and strong gravity. The water vapor activity on Ceres is independent of Ceres heliocentric distance, rulling out the possibility of comet-like sublimation process as a possible mechanism driving the activity.
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$0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase function (SPPF) show a wavelength dependence, suggesting that the phase reddening of Ceres is dominated by single-particle scattering rather than multiple scattering or small-scale surface roughness. The Hapke roughness parameter of Ceres is derived to be 20$^circpm$6$^circ$ with no wavelength dependence. The phase function of Ceres shows appreciably strong scattering around 90$^circ$ phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its unique surface composition. We grouped the reflectance data into a 1$^circ$ latitude-longitude grid and fitted each grid independently to study the spatial variations of photometric properties. The albedo and color maps are consistent with previous studies. The SPPF over the surface of Ceres shows stronger backscattering associated with lower albedo and vice versa, consistent with the general trend among asteroids. The Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that has a slightly higher roughness. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that its regolith grains either contain a considerable fraction of $lessapproxmu$m-sized particles, or are strongly affected by internal scatterers of this size.
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 phyllosilicates and salts. As water ice is though to be present only meters below the surface, we suggest that boulders also harbor ice. Furthermore, the boulder size-frequency distribution is best fit by a Weibull distribution rather than the customary power law, just like for Vesta boulders. This finding is robust in light of possible types of size measurement error.
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