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Linear features are very common on asteroid surfaces. They are generally formed after impact and provide information about asteroid evolution. This work focuses on a mineralogical and spectral analysis of the main linear features on the 1/Ceres surface, having both tectonic (Samhain Catena's pit chains) and geomorphic origins, i.e., generated by ejecta material (Occator ejecta, Dantu's secondary radial chains, secondary radial chains generated from the Urvara impact). The analysis is based on spectral parameters defined by the Dawn's VIR imaging spectrometer data, as albedo and depths of the bands centered at approximately 2.7, 3.1, 3.4 and 3.9 mm. The geomorphic linear features show spectral variations with respect to the surroundings, i.e., ammoniated phyllosilicates band depth shallowing is caused by the presence of material originating in a different region or dehydration caused by impact. The Samhain Catena does not show any mineralogical variation, due to its tectonic origin. The spectral behavior of Ceres' linear features is similar to that observed on other asteroids (Vesta, Eros) and can be diagnostic in discerning the origin of linear features. Then, we searched spectral signatures of organics in the Samhain Catena region, since they are expected to form at depth due to internal processes: the absence of such signatures indicates that either they form at a larger depth or that their subsurface distribution is uneven. [ABSTRACT FROM AUTHOR]

Highlights • Carbonate-rich surface compositions are associated with H 2 O ice-rich crust. • Dark, smooth lobate materials at the foot of Occator's northern rim have a noticeably strong positive spectral slope that may be due to small grain size or the presence of spectrally featureless organic molecules such as tholins. • The deeper absorption band depth of OH and NH 4 on the rim of several impact craters is consistent with a stratification of the phyllosilicate abundance. Abstract We studied the surface composition of Ceres within the limits of the Ezinu quadrangle in the ranges 180–270°E and 21–66°N by analyzing data from Dawn's visible and near-infrared data from the Visible and InfraRed mapping spectrometer and from multispectral images from the Framing Camera. Our analysis includes the distribution of hydroxylated minerals, ammoniated phyllosilicates, carbonates, the search for organic materials and the characterization of physical properties of the regolith. The surface of this quadrangle is largely homogenous, except for small, high-albedo carbonate-rich areas, and one zone on dark, lobate materials on the floor of Occator, which constitute the main topics of investigation. (1) Carbonate-rich surface compositions are associated with H 2 O ice rich crust. Weaker absorption bands of hydroxylated and ammoniated minerals over the carbonate-rich areas can be explained by higher abundances of carbonates at the topmost surface. (2) Dark, smooth lobate materials at the foot of Occator's northeastern wall possibly reveal fresh slumping of phyllosilicate-rich materials with fine grain size, or local enrichment in carbon-rich materials such as tholins. (3) The deeper absorption band depth of OH and NH 4 , on the rim of several impact craters, is one observation that is consistent with a stratification of the phyllosilicate abundance that has been inferred previously from global investigations. [ABSTRACT FROM AUTHOR]

Exposure to ion and micrometeoroid bombardment in the space environment causes physical and chemical changes in the surface of an airless planetary body. These changes, called space weathering, can strongly influence a surface’s optical characteristics, and hence complicate interpretation of composition from reflectance spectroscopy. Prior work using data from the Dawn spacecraft (Pieters, C.M. et al. [2012]. Nature 491, 79–82) found that accumulation of nanophase metallic iron (npFe 0 ), which is a key space-weathering product on the Moon, does not appear to be important on Vesta, and instead regolith evolution is dominated by mixing with carbonaceous chondrite (CC) material delivered by impacts. In order to gain further insight into the nature of space weathering on Vesta, we constructed model reflectance spectra using Hapke’s radiative-transfer theory and used them as an aid to understanding multispectral observations obtained by Dawn ’s Framing Cameras (FC). The model spectra, for a howardite mineral assemblage, include both the effects of npFe 0 and that of a mixed CC component. We found that a plot of the 438-nm/555-nm ratio vs. the 555-nm reflectance for the model spectra helps to separate the effects of lunar-style space weathering (LSSW) from those of CC-mixing. We then constructed ratio–reflectance pixel scatterplots using FC images for four areas of contrasting composition: a eucritic area at Vibidia crater, a diogenitic area near Antonia crater, olivine-bearing material within Bellicia crater, and a light mantle unit (referred to as an “orange patch” in some previous studies, based on steep spectral slope in the visible) northeast of Oppia crater. In these four cases the observed spectral trends are those expected from CC-mixing, with no evidence for weathering dominated by production of npFe 0 . In order to survey a wider range of surfaces, we also defined a spectral parameter that is a function of the change in 438-nm/555-nm ratio and the 555-nm reflectance between fresh and mature surfaces, permitting the spectral change to be classified as LSSW-like or CC-mixing-like. When applied to 21 fresh and mature FC spectral pairs, it was found that none have changes consistent with LSSW. We discuss Vesta’s lack of LSSW in relation to the possible agents of space weathering, the effects of physical and compositional differences among asteroid surfaces, and the possible role of magnetic shielding from the solar wind. [ABSTRACT FROM AUTHOR]

Mapping Vesta’ surface composition in the Sextilia region offers the unique possibility to investigate stratigraphic relationship of the surface compounds throughout the transition from Rheasilvia, Vesta’s prominent impact basin, to Vesta’s northern equatorial region. The VIR data point to an upper eucrite-dominated layer overlaying extended deposits of diogenite in the subsurface, which reaches the surface at Matronalia Rupes, the uplifted rim of the Rheasilvia impact structure. The dominance of diogenite in Matronalia Rupes suggests the existence of an extended diogenite-rich layer and supports the magma-ocean model as the most plausible formation model for Vesta’s interior. Local enrichment of diogenite outside of the Rheasilvia impact basin are interpreted as outcrops of diogenite-dominated breccia excavated during the Rheasilvia impact event and re-excavated by more recent impacts. On the contrary, the asymmetry in the distribution of diogenite-rich outcrops in the norther parts, i.e., the depletion of these deposits in the Veneneia region, is at odds with the equally distributed plutonic intrusions of diogenite-rich material as proposed in the serial magmatism model but could be explained by an uneven distribution of its ejecta on Vesta’s surface due to an oblique Rheasilvia impact. [ABSTRACT FROM AUTHOR]

Vesta’s surface albedo variations and hydrated material content share similar spatial distribution. This observation is consistent with carbonaceous chondrite meteorites as a likely source material for dark surface units observed by the Dawn spacecraft, as presented by numerous publications. While these deposits have been studied extensively by analysis of data from the Framing Camera (FC) and the Visible and Infrared Spectrometer (VIR), we performed a new analysis based on an improved calibration of VIR. First we identified instrument and calibration artifacts, and we therefore developed corrections of the VIR flat field and response function. Then we developed a photometric correction for Vesta based on the lunar model by Shkuratov et al. (Shkuratov, Yu.G. et al. [1999]. Icarus 141, 132–155. http://dx.doi.org/10.1006/icar.1999.6154 ), and a semi-analytical inversion of the photometric parameters. This photometric model combines minimization of the scattering effects due to the topography (a disk function) and variations of multiple-scattering with phase angle (the phase function) caused by microscopic physical properties of the regolith. The improved calibration and photometric correction enable more accurate analysis of the spectral properties of Vesta’s surface material, especially the reflectance at 1.4 μm and the 2.8 μm hydroxyl absorption band depth. We produced global and quadrangle maps that are used as a common dataset for this Icarus special issue on Vesta’s surface composition. The joint interpretation of both the 1.4 μm reflectance and the 2.8 μm absorption band depth reveals unusual spectral properties for a number of impact craters and ejecta compared to the rest of Vesta. An area including the Bellicia, Arruntia and Pomponia craters, where olivine might be present, has relatively high reflectance and a strong hydroxyl absorption band. Another area in the vicinity of Capparonia crater has a high content of hydrated materials, although with moderate reflectance and typical pyroxene-rich composition. Ejecta blankets west of Oppia crater have a spectral behavior similar to Capparonia, except for the wider and more complex shape of the hydroxyl absorption band. On the other hand, some low-hydrated areas associated to crater floors and ejecta have higher reflectance and steeper spectral slope than most low-hydrated terrains Vesta. A broad lane that extends from Rheasilvia rim at Matronalia Rupes to the northern regions hosts little to no hydrated materials and exhibits a moderate spectral slope, similar to Rheasilvia’s basin floor. These properties reinforce the hypothesis that the lane is composed of ejecta from Rheasilvia, as indicated by the distribution of pyroxene compositions by previous results from Dawn. A few small and fresh craters exhibit an association between low-reflectance, little to no hydrated materials and a strong positive spectral slope, suggesting optical effects by opaque coatings, as opposed to carbonaceous chondrite deposits, and possible coarser grains. [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 surface composition of the northern regions of Vesta, observed by the Dawn spacecraft, offers the possibility to test several hypotheses related to impact-related processes. We used mostly imaging spectrometry in the visible and near infrared to assess the distribution of mafic lithologies, hydrated components and albedo properties, and use the link with howardite, eucrite and diogenite meteorites (HEDs) to investigate the origin of those materials. We established that Rheasilvia ejecta reached part of the northern regions, and have a diogenitic-rich composition characteristic of the lower crust. Investigations of the antipodes of the two major impact basins (Rheasilvia and Veneneia) did not reveal any correlation between geographic location, geological features and the surface composition. The northern wall of Mamilia crater, which is one of the freshest craters above 22°N, contains relatively pure eucritic-rich, diogenitic-rich and dark, hydrated materials, which are representative of the rest of the northern regions (and most of Vesta), with the exception of an olivine-like component found in Bellicia crater by Ammannito et al. (Ammannito, E. et al. [2013a]. Nature 504(7478), 122–125). We determined that similar types of materials are found in various proportions over a large region, including Bellicia, Arruntia and Pomponia craters, and their origin does not seem to be related to Rheasilvia ejecta. These materials are hydrated, which could indicate an exogenous origin, and not as dark as expected for carbonaceous chondrites, which likely compose the majority of dark hydrated materials on Vesta. Spectral mixture analysis reveals that mixtures of pyroxenes (hypersthene, pigeonite and diopside) could offer an alternative interpretation to olivine in this area. [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 spectra have prominent near-infrared absorption bands characteristic of pyroxenes, indicating a direct link to the howardite, eucrite and diogenite meteorites. Many localized dark and bright materials are present on Vesta’s surface. Here we focus on the bright material (BM) units to determine their spectral properties, their origin, the presence of mineralogical phases different from pyroxenes, and whether different bright units share a common lithology. VIR, the Visible and Infrared spectrometer onboard Dawn, allows us to first do a detailed analysis of the spectral properties of a large number of bright material units on Vesta including examples of the different morphological classes. The spectral parameters used are band centers, band depths, and Band Area Ratio (BAR) for the pyroxene bands at ∼0.9 and ∼1.9 μm. The mineralogies of most bright regions are consistent with those of the howardite, eucrite and diogenite meteorites typical of Vesta’s surface. We find that bright material units exhibit the full range of HED pyroxene composition, from eucrites to diogenites. Large part of the bright materials are eucrite-rich, according with the Vesta’s mineralogy. In most cases, the bright materials have the same mineralogy of the surrounding terrain, but have larger band depth values. The band depths can be related to the abundance of the absorbing minerals, the abundance of Fe 2+ , grain size, and/or to the abundance of opaque materials. We found a positive correlation between albedo and band depth, which suggests that the grain size is not the main factor responsible for the higher albedo. The analysis of the band parameters indicates that most of the bright materials, excluding the few olivine-rich units, represent fresh uncontaminated Vestan pyroxenes from a variety of lithologies exposed from beneath the surface by impacts. [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]

The Dawn mission's Framing Camera (FC) observed Asteroid (4) Vesta in 2011 and 2012 using seven color filters and one clear filter from different orbits. In the present paper we analyze recalibrated HAMO color cubes (spatial resolution ~60m/pixel) with a focus on dark material (DM). We present a definition of highly concentrated DM based on spectral parameters, subsequently map the DM across the Vestan surface, geologically classify DM, study its spectral properties on global and local scales, and finally, compare the FC in-flight color data with laboratory spectra. We have discovered an absorption band centered at 0.72μm in localities of DM that show the lowest albedo values by using FC data as well as spectral information from Dawn's imaging spectrometer VIR. Such localities are contained within impact-exposed outcrops on inner crater walls and ejecta material. Comparisons between spectral FC in-flight data, and laboratory spectra of meteorites and mineral mixtures in the wavelength range 0.4-1.0μm, revealed that the absorption band can be attributed to the mineral serpentine, which is typically present in CM chondrites. Dark material in its purest form is rare on Vesta's surface and is distributed globally in a non-uniform manner. Our findings confirm the hypothesis of an exogenic origin of the DM by the infall of carbonaceous chondritic material, likely of CM type. It further confirms the hypothesis that most of the DM was deposited by the Veneneia impact. [ABSTRACT FROM AUTHOR]

Highlights: [•] BVRI photometry of multiple V-type NEOs spanning wide range in solar phase angle. [•] Spectroscopy of main-belt and near-Earth asteroids reveal 2 new vestoids beyond 3:1 resonance. [•] Composite solar phase curve allowed for generation of empirical photometric model of Vesta. [•] Photometric modeling reveal macroscopic surface roughness higher than Sand C-type asteroids. [•] We derive a visible phase integral of 0.44±0.02 and a corresponding Bond albedo of 0.15±0.03. [ABSTRACT FROM AUTHOR]

Abstract: We report a comprehensive review of the UV–visible spectrum and rotational lightcurve of Vesta combining new observations by Hubble Space Telescope and Swift Gamma-ray Burst Observatory with archival International Ultraviolet Explorer observations. The geometric albedos of Vesta from 220nm to 953nm are derived by carefully comparing these observations from various instruments at different times and observing geometries. Vesta has a rotationally averaged geometric albedo of 0.09 at 250nm, 0.14 at 300nm, 0.26 at 373nm, 0.38 at 673nm, and 0.30 at 950nm. The linear spectral slope as measured between 240 and 320nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude of 20°, and maximum in the eastern hemisphere. This is consistent with the longitudinal distribution of the spectral slope in the visible wavelength. The photometric uncertainty in the ultraviolet is ∼20%, and in the visible wavelengths it is better than 10%. The amplitude of Vesta’s rotational lightcurves is ∼10% throughout the range of wavelengths we observed, but is smaller at 950nm (∼6%) near the 1-μm band center. Contrary to earlier reports, we found no evidence for any difference between the phasing of the ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth longitude. Vesta’s average spectrum between 220 and 950nm can well be described by measured reflectance spectra of fine particle howardite-like materials of basaltic achondrite meteorites. Combining this with the in-phase behavior of the ultraviolet, visible, and near-infrared lightcurves, and the spectral slopes with respect to the rotational phase, we conclude that there is no global ultraviolet/visible reversal on Vesta. Consequently, this implies a lack of global space weathering on Vesta, as previously inferred from visible–near-infrared data. [Copyright &y& Elsevier]