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Comets evolve due to sublimation of ices embedded inside porous dust, triggering dust emission (that is, erosion) followed by mass loss, mass redistribution and surface modifications. Surface changes were revealed by the Deep Impact and Stardust NExT missions for comet 9P/Tempel 1 (ref. 1), and a full inventory of the processes modifying cometary nuclei was provided by Rosetta while it escorted comet 67P/Churyumov–Gerasimenko for approximately two years2–4. Such observations also showed puzzling water-ice-rich spots that stood out as patches optically brighter and spectrally bluer than the average cometary surface5–9. These are up to tens of metres large and indicate macroscopic compositional dishomogeneities apparently in contrast with the structural homogeneity above centimetre scales of pebble-made nuclei10. Here we show that the occurrence of blue patches determines the seasonal variability of the nucleus colour4,11,12 and gives insight into the internal structure of comets. We define a new model that links the centimetre-sized pebbles composing the nucleus10 and driving cometary activity13,14 to metre-sized water-ice-enriched blocks embedded in a drier matrix. The emergence of blue patches is due to the matrix erosion driven by CO2-ice sublimation that exposes the water-ice-enriched blocks, which in turn are eroded by water-ice sublimation when exposed to sunlight. Our model explains the observed seasonal evolution of the nucleus and reconciles the available data at micro (sub-centimetre) and macro (metre) scales.

Clay minerals detected on Mars are valuable targets to seek traces of life on the planet, where biosignatures might be preserved. Here, we report an in-depth spectral analysis of clay-rich outcrops identified in northern Xanthe Terra (300°–320° E, 10°–20° N). We focused particularly on the absorptions centered in the 1.0–2.6 μm spectral range to (1) constrain the mineralogy of the clay outcrops, (2) map their strength and distribution throughout the region, and thus (3) develop a better understanding of the geologic environment at circum–Chryse Planitia. We then compared the infrared signatures in Xanthe Terra and Oxia Planum. Like in Oxia Planum, Xanthe’s clays are consistent with either Fe-bearing saponites or vermiculites. However, the spectral signatures in Xanthe are weaker relative to Oxia Planum, perhaps due to significant dust cover in the region. Besides the spectral signatures, northern Xanthe Terra displays several morphological features similar to Oxia Planum, indicating long-lasting aqueous activity (fluvial channels and fan deltas). Clays found at the fan deltas could be detrital (fluvial transport) or authigenic (lacustrine or deltaic sedimentation), while the origin of clays seen elsewhere on the surrounding plateaus remains undetermined. Oxia Planum has been selected as the landing site for ESA’s ExoMars “Rosalind Franklin” rover, where its instruments will search for signs of life and constrain the nature and origin of the clays. This exploration will indubitably provide new clues on the clays found in the circum-Chryse basin.

We describe the AMBITION project, a mission to return the first-ever cryogenically-stored sample of a cometary nucleus, that has been proposed for the ESA Science Programme Voyage 2050. Comets are the leftover building blocks of giant planet cores and other planetary bodies, and fingerprints of Solar System’s formation processes. We summarise some of the most important questions still open in cometary science and Solar System formation after the successful Rosetta mission. We show that many of these scientific questions require sample analysis using techniques that are only possible in laboratories on Earth. We summarize measurements, instrumentation and mission scenarios that can address these questions. We emphasize the need for returning a sample collected at depth or, still more challenging, at cryogenic temperatures while preserving the stratigraphy of the comet nucleus surface layers. We provide requirements for the next generation of landers, for cryogenic sample acquisition and storage during the return to Earth. Rendezvous missions to the main belt comets and Centaurs, expanding our knowledge by exploring new classes of comets, are also discussed. The AMBITION project is discussed in the international context of comet and asteroid space exploration.

The prominent bright deposit Cerealia Facula, Ceres, coincides with the central depression (or central pit) of the recently formed 92 km-wide complex crater Occator. The central pit is 9–10 km wide and up to 1 km deep and is partially filled with a 700 m-high 2 km-wide dome. The upper surface of the central dome is densely fractured but the flanks are not, indicating that uplift of the dome surface occurred after the bright deposit was emplaced, and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion. The pit is rimless except for two prominent massifs that flank it to the east and west. This pit-dome morphology bears a strong resemblance in morphology, topography, and dimensions to central pit craters observed on Ganymede and Callisto, but is essentially absent on the similar sized ice-rich moons of Uranus, Saturn, and Pluto. The lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of Ceres suggest that temperature is a significant controlling factor in central pit formation, beyond the canonical inverse-gravity scaling of complex crater transitions. While renewed modeling will benefit from these new constraints, the mapping of central pits on Ceres, including Occator, suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on Ceres (lack of similar high-resolution imaging on the Galilean icy satellites precludes definitive evaluation of models there). Large volumes of lobate floor-fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of Occator, and small outcrops of this unit are found perched high on closed topographic depressions on the Occator terrace zone and other higher areas of the crater floor. The swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as Tycho. These observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates, salts and/or carbonates, and large quantities of fragmented debris. Melt of ice phases within Ceres crust is also supported by independent numerical models of impact at Occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright Na-carbonates on the surface of the central pit at Occator, if the deposit formed within a million years or so of impact. The central dome finds analogs in salt domes on Earth and pingos (frost-mounds) on Earth and Mars, which form by water and solute migration and freeze expansion. Later freezing of a central reservoir of volatiles beneath the central pit at Occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome. Finally, no changes were detected in the Cerealia and Vinalia Facula bright deposits during the course of the prime Dawn mapping mission, placing additional constraints on their formation time and limits on ongoing activity.

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 H2O 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 NH4, 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.

We present an investigation of the emission intensity of CO2and H2O and their distribution in the coma of 67P/ Churyumov–Gerasimenko obtained by the VIRTIS-M imaging spectrometer on the Rosetta mission. We analyze 4 data cubes from Feb. 28, and 7 data cubes from April 27, 2015. For both data sets the spacecraft was at a sufficiently large distance from the comet to allow images of the whole nucleus and the surrounding coma. We find that unlike water which has a reasonably predictable behavior and correlates well with the solar illumination, CO2outgasses mostly in local regions or spots. Furthermore for the data on April 27, the CO2evolves almost exclusively from the southern hemisphere, a region of the comet that has not received solar illumination since the comet's last perihelion passage. Because CO2and H2O have such disparate origins, deriving mixing ratios from local column density measurements cannot provide a meaningful measurement of the CO2/H2O ratio in the coma of the comet. We obtain total production rates of H2O and CO2by integrating the band intensity in an annulus surrounding the nucleus and obtain pro-forma production rate CO2/H2O mixing ratios of ∼5.0% and ∼2.5% for Feb. 28 and April 27, respectively. Because of the highly variable nature of the CO2evolution from the surface we do not believe that these numbers are diagnostic of the comet's bulk CO2/H2O composition. We believe that our investigation provides an explanation for the large observed variations reported in the literature for the CO2/H2O production rate ratios. Our mixing ratio maps indicate that, besides the difference in vapor pressure of the two gases, this ratio depends on the comet's rotational orientation combined with its complex geometric shape which can result in quite variable rates of erosion for different surface areas such as the northern and southern hemisphere. Our annulus measurement for the total water production for Feb. 28 at 2.21AU from the Sun is 2.5×1026molecules/s while for April 27 at 1.76 AU it is 4.65×1026. We find that about 83% of the H2O resides in the illuminated portion of our annulus and about 17% on the night side. We also make an attempt to obtain the fraction of the H2O production coming from the highly active neck of the comet versus the rest of the illuminated surface from the pole-on view of Feb. 28 and estimate that about 60% of the H2O derives from the neck area. A rough estimate of the water surface evaporation rate of the illuminated nucleus for April 27 yields about 5×1019molecules/s/m2. Spatial radial profiles of H2O on April 27 on the illuminated side of the comet, extending from 1.78 to 6.47km from the nucleus center, show that water follows model predictions quite well, with the gas accelerating as it expands into the coma. Our dayside radial profile allows us to make an empirical determination of the expansion velocity of water. On the night side the spatial profile of water follows 1/ρ. The CO2profiles do not exhibit any acceleration into the coma but are closely matched by a 1/ρprofile.

Focus of this work is the analysis of rock slabs by means of the Ma_Miss BreadBoard instrument. Ma_Miss (Mars Multispectral Imager for Subsurface Studies, Coradini et al., 2001; De Sanctis et al., 2017) is the miniaturized imaging spectrometer onboard the ESA Exomars 2020 mission. Here we report the results of the analysis carried out on rock slabs using the Ma_Miss breadboard (BB) (De Angelis et al., 2014, 2015) and a Spectro-Goniometer (SPG). The samples are three volcanic rocks (from the Aeolian Islands and Montiferru volcanoes, Italy) and two carbonate rocks (from Central Apennines, Italy). Visible and near infrared spectroscopic characterization has been first performed on all the samples with a Spectro-goniometer (SPG). Successively, higher spatial resolution spectra were acquired with the Ma_Miss BB setup in each of the areas analyzed with the SPG. We compared the spectra of the same areas of the slabs, acquired with SPG and Ma_Miss BB. Three different analysis approaches have been performed on the spectra: arithmetical averaging of the spectra, linear mixing of reflectances and linear mixing of Single Scattering Albedoes (using Hapke model). The comparison shows that: (i) Ma_Miss instrument has great capabilities for the investigation of rock surfaces with high detail; a large number of different mineralogical phases can be recognized thanks to Ma_Miss high resolution within each millimeter-sized analyzed area; (ii) the agreement with SPG spectra is excellent especially when linear mixing is applied for the convolution of Ma_Miss BB spectra.

Comets are usually considered to be the most primitive bodies in the Solar System. The level of truth of this paradigm, however, is a matter of debate, especially if by primitive we mean that they represent a sample of intact, unprocessed material. We now have the possibility of analysing the comet 67P/Churyumov-Gerasimenko with an unprecedented level of detail, but its interior remains largely unprobed and unknown. The questions we address in this paper concern the depth of the processed layers, and whether the comet nucleus, under these processed layers, is really representative of the original material. We applied the Rome model for the thermal evolution and differentiation of nuclei to give an estimation of the evolution and depth of the active layers and of the interplay between the erosion process and the penetration of the heat wave. In order to characterize the illumination regime and the activity on the nucleus, two locations with very different illumination histories were chosen for the simulation. For both locations, the bulk of the activity tends to be concentrated around the perihelion time, giving rise to a high erosion rate. As a consequence, the active layers tend to remain close to the surface, and the interior of the comet, below a layer of few tens of centimetres, can be considered as pristine.

H2O-rich materials are locally exposed at the surface of Ceres as discovered from infrared reflectance spectra of the Visible and InfraRed mapping spectrometer (VIR) of the Dawn mission. Nine locations on Ceres exhibit diagnostic absorption bands of the H2O molecule at 2.00, 1.65 and 1.28 μm. The detections are all consistent with H2O ice mixed with low-albedo components. All the reported H2O exposures occur at latitudes poleward of 30° in fresh craters near rim shadows, have a surface area < 7 km2, and are associated with one or more surface features such as a morphological flow or landslide, fractures, high albedo, or a pole-facing slope (one case is confirmed to be adjacent to persistent shadow). In four occurrences, these detections are associated with small (< 0.1 km2) high-albedo areas that can be recognized in high-resolution imagery (∼35 m/pixel) from the Framing Camera (FC). Since all these observations are compatible with an H2O-rich subsurface, the replenishment of surficial H2O likely comes from the ice that is present underneath. In four other occurrences, H2O is detected on walls and floors of fresh impact craters, either in the shadow or adjacent to shadows, which suggests that local thermodynamical conditions may also favor the concentration of H2O in these areas.

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.

Herein we present the geology of the Nawish quadrangle, located in the equatorial region of dwarf planet Ceres, named after one of the most prominent craters of the area. Geologic mapping was based on the image mosaics and digital terrain models derived from Dawn Framing Camera data. Interpretation of geologic units was supported by supplemental data, such as the multi spectral color images from the Framing Camera, and the spectral parameters derived from the Visible and Infrared Spectrometer (VIR) data, as well as Dawn gravity data. There is not a primary feature that dominates the geology of Nawish quadrangle, but rather several terrains overlap, and their relations explain the geology of the area. Crater size frequency distributions show that Nawish quadrangle is dominated by two distinct time domains. The central and eastern part of the quadrangle is topographically elevated, which we define as cratered highlands, and contains the older domain. The western lowlands show two younger domains related to impact craters Kerwan and Dantu, including the Kerwan smooth material and Dantu ejecta. This variation of elevation within the Nawish quadrangle is more than the half of the global topographic altitude variation on Ceres. Analysis and comparison of the topography of the Nawish quadrangle with surrounding ones shows that this quadrangle is dominated by the topography of the rim sector of a large, >800 km ancient impact basin, most likely the putative Vendimia Planitia. The Nawish quadrangle thus represents a sector of Ceres which has not undergone large-scale, post-Kerwan, intermediate age-events, but rather represents a place on Ceres where a well-preserved relict of old cerean crust can be studied, together with ejecta from more recent impact events.

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.

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.

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.

From August to November 2014 the Rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67P/CG nucleus properties and to the selection of the Philae landing site. The campaign led to the production of a global map of the illuminated portion of 67P/CG nucleus. During this prelanding phase the comet's heliocentric distance decreased from 3.62 to 2.93 AU while Rosetta was orbiting around the nucleus at distances between 100 to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer - Mapping channel (Coradini et al. 2007) onboard the orbiter, has acquired 0.25-5.1 micron hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission removal in the 3.5-5.1 micron range and for surface's photometric response. The resulting reflectance spectra have been used to compute several Cometary Spectral Indicators (CSI): single scattering albedo at 0.55 micron, 0.5-0.8 micron and 1.0-2.5 micron spectral slopes, 3.2 micron organic material and 2.0 micron water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the Sun. Indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5-0.8 and 1.0-2.5 micron spectral slopes, indicating a flattening of the reflectance. We attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line., 19 Figures, 5 Tables. Accepted for publication in Icarus journal on 29 February 2016

We present new reflectance spectra of 12 V-type asteroids obtained at the 3.6 m Telescopio Nazionale Galileo (TNG) covering the spectral range 0.7 to 2.5 μm. This spectral range, encompassing the 1 and 2 μm, pyroxene features, allows a precise mineralogical characterization of the asteroids. The spectra of these asteroids are examined and compared to spectra for the Howardite, Eucrite and Diogenite (HED) meteorites, of which Vesta is believed to be the parent body. The observed objects were selected from different dynamical populations with the aim to verify if there exist spectral parameters that can shed light on the origin of the objects. A reassessment of data previously published has also been performed using a new methodology. We derive spectral parameters from NIR spectra to infer mineralogical information of the observed asteroids. The V-type asteroids here discussed show mainly orthopyroxene mineralogy although some of them seem to have a mineralogical composition containing cations that are smaller than Mg cations. Most of the observed Vestoids show a low abundance of Ca (

A stereo camera (STC) with panchromatic and 4 visible-near infrared (NIR) filters and a visible-near infrared hyperspectral imager (VIHI) with 400–2000 nm spectral range, integrated into the spectrometers and imagers for BepiColombo integrated observatory-system (SIMBIO-SYS) selected for BepiColombo, are dedicated to the geologic exploration of Mercury's surface. In this study the responses of the two sensors were simulated and degraded by the addition of random noise, with the purpose of evaluating their suitability to resolve the spectroscopic features diagnostic of rock forming minerals and discriminate among different lithotypes. Two cumulates (norite and anorthosite) from a Late Proterozoic layered intrusion and two basaltic lavas from Mount Etna complex were selected as possible terrestrial analogues of Mercury. Interband spectral reflectance ratios of STC noise-free data provided a first order discrimination among the different rock types. VIHI noise-free data compare well with laboratory reflectance data for the identification of the spectral features of the most important rock forming minerals. STC noisy spectra simulated for S / N =200 showed that, because of the low reflectance of the surface rocks, the band ratios of the different rocks can partly overlap, with some uncertainty in rock discrimination. VIHI noisy spectra simulated for S / N =100 indicated that for well defined spectral features with band depth 0.1 reflectance unit, the shift in band position corresponds to a variation of less than 0.03 atoms/formula unit of the spectrally active phases. Weaker bands have a high probability of being misidentified.

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.

We wish to thank the Italian Space Agency (ASI) for funding and fully supporting the Ma_Miss experiment (ASI/INAF grant I/ 060/10/0). We also thank G.F. De Astis (INGV) for providing the Aeolian samples, and A.Frigeri for providing the red micritic limestone. We thank an anonymous reviewer for useful comments and suggestions. The Ma_Miss instrument (Mars Multispectral Imager for Subsurface Studies, Coradini et al. (2001)) is a Visible and Near Infrared miniaturized spectrometer that will observe the Martian subsurface in the 0.4-2.2 μm spectral range. The instrument will be entirely hosted within the Drill of the ExoMars-2018 Pasteur Rover: it will allow analyzing the borehole wall excavated by the Drill, at different depths, down to 2 m. The aim will be to investigate and characterize the mineralogy and stratigraphy of the shallow Martian subsurface. A series of spectroscopic measurements have been performed in order to characterize the spectral performances of the laboratory model of the instrument (breadboard). A set of six samples have been analyzed. Each sample (four volcanic rocks, a micritic limestone and a calcite) has been reduced in particulate form, ground, sieved and divided into nine different grain sizes in the range d

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.

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 (npFe0), 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 npFe0 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 npFe0. 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.

The relation among V-type asteroids in the Main Belt, the large basaltic asteroid (4) Vesta and the HED meteorites is a long standing issue. To better constraint this relation we analysed the spectral parameter distribution of a sample of V-type asteroids, located in the vicinity of (4) Vesta, and of basaltic achondrites meteorites. Our results indicate that among the V-type asteroids there exist slightly distinct mineralogies which could be explained either as coming from different depths of the asteroid (4) Vesta or from different bodies.

We present reflectance spectra of 19 V-type asteroids obtained at the 3.6 m Telescopio Nazionale Galileo covering 0.8 to 2.5 μm. For 8 of these asteroids we obtained also visible spectra in the same observational run. The range from 0.8 to 2.5 μm, encompassing the 1 and 2 μm pyroxene features, allows a precise mineralogical characterization of these asteroids. The obtained data suggests the possible coexistence of distinct mineralogical groups among the V-type asteroids, either probing different layers of (4) Vesta or coming from different bodies. No clear correlation was found between mineralogies and the objects being, or not, member of the Vesta dynamical family.

The nuclear spin temperature, which is derived from the ortho-to-para abundance ratio of molecules measured in cometary comae, is a clue to the formation conditions of cometary materials, especially the physical temperature at which the molecules were formed. In this paper we present new results for the nuclear spin temperatures of ammonia in comets Hale-Bopp (C/1995 O1) and 153P/Ikeya-Zhang based on observations of NH2 at 26 and 32 K, respectively. These results are similar to previous measurements in two other comets, and the nuclear spin temperatures of ammonia in the four comets are concentrated at about 30 K. We emphasize that the nuclear spin temperatures of water measured thus far have also been about 30 K. In particular, the spin temperatures of ammonia and water are equal to each other within ±1 σ error bars in the case of comet Hale-Bopp. These nuclear spin temperatures of ammonia and water were measured under quite different conditions (heliocentric distances and gas production rates). There is no clear trend between the nuclear spin temperatures and the heliocentric distances, the gas production rates, or the orbital periods of the comets. The possibilities of the ortho-to-para conversion in the coma and in the nucleus are discussed. The present data set implies that the ortho-to-para ratios were not altered after the molecules were incorporated into the cometary nuclei. It appears that cometary ammonia and water molecules formed on cold grains at about 30 K.

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (approximately 63 kilometers diameter) and Calpurnia (approximately 53 kilometers diameter) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between approximately 40 and 60 million years (depending upon choice of chronology system), and Marcia's ejecta blanket ranges in age between approximately 120 and 390 million years (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major Vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative Vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of approximately 280-990 million years based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta's ancient crust preserved on Vestalia Terra.

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 Fe2+, 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.

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).

We studied high-resolution images of asteroid Vesta's surface (~70 and 20–25 m/pixel) obtained during the High- and Low-Altitude Mapping Orbits (HAMO, LAMO) of NASA's Dawn mission to assess the formation mechanisms responsible for a variety of lobate, flow-like features observed across the surface. We searched for evidence of volcanic flows, based on prior mathematical modeling and the well-known basaltic nature of Vesta's crust, but no unequivocal morphologic evidence of ancient volcanic activity has thus far been identified. Rather, we find that all lobate, flow-like features on Vesta appear to be related either to impact or erosional processes. Morphologically distinct lobate features occur in and around impact craters, and most of these are interpreted as impact ejecta flows, or possibly flows of impact melt. Estimates of melt production from numerical models and scaling laws suggests that large craters like Marcia (~60 km diameter) could have potentially produced impact melt volumes ranging from tens of millions of cubic meters to a few tens of cubic kilometers, which are relatively small volumes compared to similar-sized lunar craters, but which are consistent with putative impact melt features observed in Dawn images. There are also examples of lobate flows that trend downhill both inside and outside of crater rims and basin scarps, which are interpreted as the result of gravity-driven mass movements (slumps and landslides).

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.

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.