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Photometry of asteroids gives fundamental information about their spectral and physical properties. The aim of this work is two-fold: (1) to calculate phase functions of Vesta and Lutetia in the visible spectral range; and (2) to compare photometric properties of all the asteroids visited by space missions, as inferred from disk-resolved photometry. The phase functions of Vesta and Lutetia have been retrieved by performing a statistical analysis on data provided by the VIR–Dawn and the VIRTIS–Rosetta imaging spectrometers, respectively. The approach is based on the empirical procedure defined in Longobardo et al. (Longobardo, A. et al. [2014]. Icarus 240, 20–35). The Vesta phase functions have been calculated at two wavelengths, one outside (0.75 μm) and one inside (0.95 μm) the pyroxene absorption band at 0.9 μm. The steepness of the phase function at 0.75 μm decreases from dark to bright regions, due to the increasing role of multiple scattering. Otherwise, the phase function at 0.95 μm results in uniformity across Vesta surface, since darkening agents are spectrally featureless and their influence at wavelengths inside the pyroxene absorption band is negligible. Moreover, it is, on average, steeper than the phase functions at 0.75 μm, due to the more important role of single scattering at 0.95 μm. The Lutetia phase function is instead constant across the surface due to the homogeneous spectral properties of this asteroid. The obtained photometric curves (reflectance versus phase angle) of Vesta and Lutetia have been then compared with those retrieved in previous works on asteroids visited by space missions. Differently from comparisons of disk-integrated phase functions of asteroids performed in previous works at low phase angles (lower than 25°), this work restricts to asteroid observations that are disk-resolved and occur at solar phase angles between 20° and 60°. The S-type asteroids (Gaspra, Ida, Eros and Annefrank) show similar photometric curves. The phase functions found in bright material units on Vesta are similar to those found for Steins (E-type in the Tholen taxonomy, Xe-type in the Bus one), suggesting a photometric analogy between achondritic surfaces. The latter are brighter and with a flatter phase function with respect to chondritic surfaces: we argued that this behavior is driven by optical properties of asteroid surfaces (e.g. albedo, role of multiple scattering) rather than by physical ones (e.g. grain size, roughness). Dark material units on Vesta show an intermediate behavior between achondrites and the C-type Mathilde, confirming once again that these regions are characterized by mixtures of HED and carbonaceous chondrites. While a clear anti-correlation is observed between reflectance and steepness of phase function for V, S and C asteroids, Lutetia shows an anomalous photometric behavior, presenting both a low reflectance and a flat phase curve, and hence cannot be grouped with other spectral classes here considered. This behavior is similar to some X-type asteroids ground-observed at low phase angles and is consistent with a chondritic composition of its surface. [ABSTRACT FROM AUTHOR]

This work is aimed at developing and interpreting infrared albedo, pyroxene and OH band depths, and pyroxene band center maps of Vesta’s Gegania and Lucaria quadrangles, obtained from data provided by the Visible and InfraRed (VIR) mapper spectrometer on board NASA’s Dawn spacecraft. The Gegania and Lucaria quadrangles span latitudes from 22°S to 22°N and longitudes from 0°E to 144°E. The mineralogical and spectral maps identify two large-scale units on this area of Vesta, which extend eastwards and westward of about 60°E, respectively. The two regions are not associated to large-scale geological units, which have a latitudinal distribution rather than longitudinal, but are defined by different contents of carbonaceous chondrites (CC): the eastern region, poor in CCs, is brighter and OH-depleted, whereas the western one, rich in CCs, is darker and OH-enriched. A detailed analysis of the small-scale units in these quadrangles is also performed. Almost all the units show the typical correspondence between high albedo, deep pyroxene bands, short band centers and absence of OH and vice versa. Only a few exceptions occur, such as the ejecta from the Aelia crater, where dark and bright materials are intimately mixed. The most characteristic features of these quadrangles are the equatorial troughs and the Lucaria tholus. The equatorial troughs consist of graben, i.e. a depression limited by two conjugate faults. The graben do not present their own spectral signatures, but spectral parameters similar to their surroundings, in agreement to their structural origin. This is observed also in graben outside the Gegania and Lucaria quadrangles. However, it is possible to observe other structural features, such as tectonic grooves, characterized by a changing composition and hence an albedo variation. This result is confirmed not only by mineralogical maps of Vesta, but also by analyzing the VIRTIS-Rosetta observations of Lutetia. The albedo change is instead a typical behavior of geomorphic grooves. Finally, ridges are characterized by a bluer color and, in some cases, shorter band centers than their surroundings, suggesting that they are composed of fresher materials. We also performed a comparative analysis between the three tholi of Vesta, i.e. Lucaria (which gives the name to its quadrangle), Aricia (in the Marcia quadrangle) and Brumalia (Numisia quadrangle). Whereas Brumalia tholus is a young magmatic intrusion, the absence of diogenites, the low albedo, and the orientation of Aricia and Lucaria tholi suggest that they are older features, which are covered by dark materials and therefore experienced a different geological history than Brumalia. [ABSTRACT FROM AUTHOR]

The discovery of olivine on Vesta’s surface by the VIR imaging spectrometer onboard the Dawn space mission has forced us to reconsider our views of Vestan petrogenetic models. Olivines were expected to be present in the interior of Vesta: in the mantle of a vertically layered body as invoked by the magma ocean models, or at the base (or within) the mantle–crust boundary as proposed by fractionation models. Olivines have been detected by VIR-Dawn in two wide areas near Arruntia and Bellicia, regions located in the northern hemisphere. Interestingly, these olivine-rich terrains are far from the Rheasilvia and the more ancient Veneneia basins, which are expected to have excavated the crust down to reach the mantle. In this work we present our attempts to identify other undetected olivine rich areas on Vesta by using spectral parameters sensitive to olivine such as the Band Area Ratio (BAR) and other specific parameters created for the detection of olivines on Mars (forsterite, fayalite and a generic olivine index). As a preliminary step we calibrated these parameters by means of VIS–IR spectra of different HED meteorite samples: behaviors versus sample grain size and albedo were analyzed and discussed. We selected the BAR and the Forsterite Index as the best parameters that can be used on Vesta. A cross-correlation analysis has been applied in order to detect olivine signature on the VIR hyperspectral cubes. These detections have then been confirmed by an anti-correlation analysis between the BAR and one of the olivine parameters, independent of the first method applied. In agreement with the recent discovery, Arruntia and Bellicia were found to be as the most olivine-rich areas, i.e. where the parameter values are strongest. In addition we detected 6 new regions, all but one located in the Vesta north hemisphere. This result confirms again that the old petrogenetic models cannot be straightforwardly applied to Vesta and should be reshaped in the view of these new detections. An alternative and very recent option can be represented by the model according to which surface “eruption” of material from the mantle, including olivine can reach the surface of Vesta. [ABSTRACT FROM AUTHOR]

Vesta is the asteroid with the largest albedo variation among the known rocky Solar System objects and shows a widespread occurrence of dark material (DM) and bright material (BM) units. In the first observation phases by the Dawn spacecraft, two main extensions of low albedo areas were identified on Vesta and found to be closely correlated with carbonaceous, OH-rich, material. In this work we use the hyperspectral data provided by the VIR-Dawn imaging spectrometer onboard Dawn to detect and analyze individual, well-defined, dark material units. We define DM units assuming a relative criterion, i.e. reflectance lower than the surroundings. By coupling visible and infrared images of the same area we are able to select real dark material units, discarding false detections created by shadowing effects. A detailed final catalogue of 123 dark units is presented, containing the geographical parameters and the main spectral characteristics for each unit. Independently of the geological context of the dark units, all DMs show similar spectral properties, dominated by the pyroxene absorption features, as is the average spectrum of Vesta. This finding suggests a similar composition, with the presence of darkening agents that also weaken pyroxene band depths. The majority (90%) of the DM units shows a positive correlation between low albedo and an OH band centered at 2.8 μm, confirming the hypothesis that the darkening agents are carbonaceous chondrites, probably delivered by low-velocity impacts of primitive asteroids. A comparison with laboratory spectra allows us to better constrain the size and the composition of the darkening agents. These DM areas seem to be made of eucritic material. The regolith grain size seems to be nearly constant around an average value of 25 μm, and is quite homogenous at least in the first hundreds of meters beneath the Vesta surface, suggesting similar processing mechanisms for both DM and BM. [ABSTRACT FROM AUTHOR]

NASA’s Dawn spacecraft orbited Vesta for approximately one year, collecting thousands of hyperspectral images of its surface. The mission revealed that Vesta’s surface shows the largest variations in surface albedo on asteroids visited thus far, due to the presence of dark and bright materials at the local scale (i.e. 0.1–10 km). The aim of this work is to characterize the photometric properties of bright and dark regions, and thus derive and apply an empirical photometric correction to all the hyperspectral observations of Vesta. The very large dataset (i.e. more than 20 million spectra) provided by the VIR imaging spectrometer onboard Dawn enabled accurate statistical analysis of the spectral dataset, aimed at retrieving empirical relations between several spectral parameters (i.e. visible and infrared reflectance, band depths, band centers, Band Area Ratio) and the illumination/viewing angles. The derived relations made it possible to derive photometrically corrected maps of these spectral parameters and to infer information on the regolith shadowing effect in the Vestan dark and bright regions. As an additional analysis, we also evaluated the correlation between surface temperature and band center position. A general conclusion of this analysis is that, from a photometric point of view, the distinction between bright and dark material units lies mainly in the larger contribution due to multiple scattering in the bright units. We observed reflectance and band depth variations over Vesta’s entire surface, but these variations were much larger in the dark regions than in the bright ones. Band centers have been found to shift to longer wavelengths at increasing temperatures, with a trend that is the same observed for HED meteorites (Reddy et al. [2012]. Icarus 217, 153–158). Finally, the Band Area Ratio (i.e. the ratio between areas of the main pyroxene absorption bands located at 1.9 μm and at 0.9 μm, respectively) did not show any dependence on observational geometry, again a behavior similar to laboratory results obtained on HED meteorites (Reddy et al. [2012]. Icarus 217, 153–158). [ABSTRACT FROM AUTHOR]

Abstract We report a comprehensive analysis of the global spectrophotometric properties of Ceres using the images collected by the Dawn Framing Camera through seven color filters from April to June 2015 during the RC3 (rotational characterization 3) and Survey mission phases. We derived the Hapke model parameters for all color filters. The single-scattering albedo of Ceres at 555 nm wavelength is 0.14 ± 0.04, the geometric albedo is 0.096 ± 0.006, and the bolometric Bond albedo is 0.037 ± 0.002. The asymmetry factors calculated from the best-fit two-term Henyey-Greenstein (HG) single-particle phase functions (SPPFs) show a weak wavelength dependence from −0.04 at 438 nm increasing to 0.002 at >900 nm, 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° ± 6°, with no wavelength dependence. The phase function of Ceres presents appreciably strong scattering around 90° phase angle that cannot be fitted with a single-term HG SPPF, suggesting possible stronger forward scattering component than other asteroids previously analyzed with spacecraft data. We speculate that such a scattering characteristic of Ceres might be related to its ubiquitous distribution of phyllosilicates and high abundance of carbonates on the surface. We further grouped the reflectance data into a 1° latitude-longitude grid over the surface of Ceres, and fitted each grid independently with both empirical models and the Hapke model to study the spatial variations of photometric properties. Our derived albedo maps and color maps are consistent with previous studies [Nathues, A., et al., 2016, Planet. Space Sci. 134, 122–127; Schröder, S.E., et al., 2017, Icarus 288, 201–225]. The SPPF over the surface of Ceres shows an overall correlation with albedo distribution, where lower albedo is mostly associated with stronger backscattering and vice versa, consistent with the general trend among asteroids. On the other hand, the Hapke roughness parameter does not vary much across the surface of Ceres, except for the ancient Vendimia Planitia region that is associated with a slightly higher roughness. Furthermore, the spatial distributions of the SPPF and the Hapke roughness do not depend on wavelength. Based on the wavelength dependence of the SPPF of Ceres, we hypothesize that the regolith grains on Ceres either contain a considerable fraction of μm-sized or smaller particles, or are strongly affected by internal scatterers of this size. Highlights • Hapke photometric parameters for Ceres derived using Dawn color images. • Albedo, single particle phase function, and roughness all vary over Ceres' surface. • Stronger light scattering for Ceres at intermedium phase angle than other asteroids. • Some correlations between light scattering properties and geological features seen. [ABSTRACT FROM AUTHOR]