Search

Thanks to the VIR spectrometer onboard NASA’s Dawn spacecraft, which orbited Vesta in 2011–2012, thousands of hyperspectral images of its surface have been collected. The mission confirmed the HED (Howardite–Eucrite–Diogenite) meteorite composition of Vesta. Moreover, the VIR spectrometer detected the 2.8 µm absorption band, due to the presence of the OH molecule. In this work, we took advantage of the newly calibrated data of the VIR spectrometer by characterizing new spectral features thanks to the improved signal-to-noise (S/N) ratio for these spectra. The main goals of this work are as follows: (1) to characterize Vesta’s surface in the visible range and (2) to confirm, reinforce and characterize the OH distribution on Vesta by studying the 2.8 µm band and looking for OH combination bands around 2.2–2.4 µm. A possible relation between the 1.9 µm absorption band due to the presence of pyroxenes and the one at 0.5 µm was analyzed. Finally, the analysis of hydroxyl absorption bands evidenced an anti-correlation between the abundance of hydroxyl-bearing molecules and the surface reflectance. This confirms that the hydroxyl presence is linked to the dark units on Vesta.

Introduction: This study is focused on the investigation of the reflected solar light on planetary atmosphere-less bodies, as a function of the viewing geometry (direction of the solar light and direction of the ob-server with respect to the normal to the surface, identifying angles named respectively incidence and emission, and the relative angle between them, named phase). In particular, we focus on the photometric properties of large-scale roughness (hereafter referred to as roughness), which, being not spatially resolved, can be only derived by photometric models. The investigation of the photometric response as a function of roughness is mandatory to disentangle properties of the surface regolith such as albedo, porosity, and thermal inertia (the latter when thermal emission data are avail-able). Moreover, the information on the roughness can be important by itself for interpretation of the geological and physical processes in place, or for engineering use in a case of a landing site selection. Photometric Models: Among the interpretative physically-based models describing the reflection of the light from surfaces, Hapke [1] theory is among the most used in literature, thanks to its completeness in the description of the surface, and the practicality of use, being an analytical model. According to Hapke, a rough surface can be described as a collection of facets, each with a certain slope (θ), in a way that the distribution of slopes is completely identified by a unique parameter (the Mean Slope θ). Random Gaussian and fractal terrains are well-represented by the slope distribution described by Hapke’s theory. This is the case with several minor bodies, targets of recent space missions (e.g., [2]). Hapke developed an analytic formulation to include the effect of the roughness into a general relation describing the reflectance of planetary surfaces, which is a function of several photometric parameters. However, some authors, including Hapke himself, have high-lighted some limitations of this formulation: (i) the parameter that describes roughness is derived in theory only for small angles of the slope of the facets relative to the average surface. This prevents reliable derivation of roughness for more general conditions [1]; (ii) unresolved shadows and self-illumination can affect the overall reflected signal, and if not accurately modeled, can be confused with albedo variation (e.g., [3]). In Hapke’s theory, the dependence of these contributions on observation geometry is treated by means of simpli-fied approximations. An improved approach to the study of rough surfaces has been proposed to overcome the above issues [4]. It consists of a statistical approach in which N unresolved facets, each one having its own viewing geometry, are generated. The integrated resulting signal corresponding to these facets is calculated starting from the response of all facets visible to the observer. This statistical-multi-facets algorithm (SMFA) also takes into account the self-illumination and shadows projected along the observer's line of sight, so that the photometric response is not calculated through analytical ap-proximations, but with the sum of each of the N facets that describe the simulated terrain (see Fig. 1). Roughness retrieval: We investigate the properties of spatially resolved roughness of the dwarf planet Ceres and comet 67P from their shape models, derived by Dawn/NASA and Rosetta/ESA data. From this, we infer the possibility to safely apply the above-mentioned models for retrieval of non-resolved roughness. The latter is performed by the comparison of signals of the surface detected in different viewing geometries. We discuss results coming from both Hapke model and the statistical multi-facets algorithm. Figure 1. An example of the different output between SMFA and Hapke’s models: S = shadowing function, µ0eff = effective incidence angle cosine, µeff = effective emission angle cosine. All these parameters are used by the Hapke formulation, and they are functions of the Mean Slope (θ) parameter. In this example, the incidence, emission, and phase angles are fixed respectively to 30°, 0°, 30° [4]. References: [1] Hapke, Theory of Reflectance and Emittance Spectroscopy, 1993 [2] Davidsson et al., Icarus, 252, 2015 [3] Cuzzi et al., Icarus, 289, 2017 [4] Raponi et al., 14th EPSC 2020, id. EPSC2020-761

Introduction: V-type asteroids are those whose spectra is similar to Vesta [1].Vesta underwent two huge impact events [2], that could have formed objects of this taxonomic class [3,4] and have eventually supplied the Vesta meteorites (HEDs) escaped from the Main Belt.In this work, we look for statistically significant differences among the modal mineralogy of V-type asteroids belonging to different dynamical subclasses through a combination of spectroscopic analysis and Hapke Radiative Transfer Model (RTM).Data and Methods: 76 Vis-NIR V-type asteroid spectra were downloaded from the Planetary Data System (PDS). In this work, we considered four dynamical families of V-type as follows:(i) Vestoid: V-type members of the Vesta dynamical family, as defined by [4] using the hierarchical clustering method (HCM).(ii) Fugitive: following the definition of [4], they are V-type asteroids with a < 2.3 au and comparable e and i with the Vesta family.(iii) Low-I: according to [4], they are V-type asteroids having i < 6◦ and 2.3 < a < 2.5 au.(iv) IOs (Inner Others): the remaining V-type asteroids in the inner main belt.VNIR spectra of Vesta’s, V-type asteroids surface and HED show absorption bands centred at approximately 0.9 and 1.9μm, confirming the pyroxenes presence [5]. Through scatterplot analysis of spectral parameters (in particular, band depth and half width half maximum) of V-type asteroids and of HED, these two bands give evidence of the grain size range of each asteroid. The obtained information is used as constrain in the Hapke’s RTM (to prevent possible degeneration in the creation of synthetic spectra) to retrieve modal composition for each V-type asteroid. To simulate asteroids spectra in the Hapke RTM, we used the main endmembers of HED and we also added a darkening agent (Murchison spectrum) to find possible correlations between V-type asteroids and dark regions on Vesta [6].Spectral fits of the asteroid’s dataset are robust with a global R mean square of 0.98, while the retrieved particle sizes are small with a mean value of 20 µm.Results and Discussion: Comparing the modal mineralogy obtained from Hapke RTM and the [7] studies, it is possible to assign a lithology to each dynamical subclass. In particular, we can distinguish diogenites (approximately 100 % of pyroxene), noritic diogenites (2-42 % plagioclase, 52-97 pyroxene), cumulate eucrite (30-64 % plagioclase and 36-70 % pyroxene) and basaltic eucrite (39-46% plagioclase, 46-60 % pyroxene).Then, in Figure 1, we report the mean and the standard deviation of sample of the four subclasses (Fugitive, Vestoids, Inner Others and Low-i). It seems that Vestoids and Fugitive are diogenitic (diogenitic and noritic diogenites respectively), while Low-I and Inner Other (IOs) are eucritic (Figure 2).Figure 1: Results of mean mineralogy for each subclass from Hapke retrivalFigure 2: Probability density function of Pyroxene (up) and of Plagioclase (bottom) for each subclass The most ‘ancient’ family, that is Low-i asteroids (they may date back to the LHB epoch or even to the 4.48-Ga impact on Vesta), seems to have an eucritic composition. In the most recent models of the structure of Vesta, it has a massive eucritic crust. Moreover, [2] found that the density of craters increases from the Rheasilvia rim to northern latitudes. This dichotomy in terms of crater density indicates that heavily cratered terrains (centered at about 348°E, 17°N and 110°E, 17°N) are much older than Rheasilvia basin, whose most probable age is estimated to be 1.0 ± 0.2 Ga [2]. Thus, it is compatible with the mineralogy of the Low-I asteroids that ‘swept out’, in the past, the outermost layer of Vesta.Fugitive better match with a diogenitic composition. For this subclass, the probable age of formation is at least 1 Ga so that they could migrate away from Vesta [4], then it is possible that they were ejected in an older (2 Ga) cratering event that produced the Veneneia basin that underlies Rheasilvia [8].Furthermore, [9] found from Dawn data that Rheasilvia basin (probable age is estimated to be 1.0 ± 0.2 Ga [9]) is consistent with a larger abundance of diogenite on the surface, while spectra from the equatorial regions are consistent with a larger eucrite amount. Thus, there is a possible chronologic link between impact events on Rheasilvia crater and the Vestoids, which, from our work, have a mineralogy associated to Diogenites.In any family, there is no signature of dark material as carbonaceous chondrites (here we used Murchison spectra to simulate it). This could be explained by the fact that it has an exogenous origin [6,10], and it could have impacted on Vesta after the V-type family formation.From this work, a possible chronologic link between impact events on Vesta and the V-type family is possible. We plan to augment the database of V-type asteroids to improve the statistical analysis. References: [1] De Meo,F. E. et al. (2009), Icarus, n. 202,p.160–180; [2] Marchi, S. et al. (2012), SCIENCE, Vol 336, Issue 6082, pp. 690-694; [3] Carruba, V. et al. (2007), A&A 465, 315-330;[4] Nesvorny, D. et al. (2008), Icarus, vol 183, 85–95; [5] De Sanctis,M.C. et al. (2013), Meteorit. Planet. Sci, vol. 48, n. 11, p. 2166–2184; [6] Palomba,E et al. (2014), Ica-rus, n. 240, pp. 58-72; [7] Mittlefehldt,D.W. et al. (2015), Chemie der Erde, vol. 75, pp. 155-183 ; [8] McSween, H.Y.J. et al. (2013), Meteorit. Planet. Sci, vol. 48, n. 11, p. 2090–2104; [9] De Sanctis, M.C. et al. (2012), SCIENCE ,VOL 336; [10] Turrini, D. et al. (2014), Icarus, 240, 86-102

Cometary outbursts are well-known and offer a valuable window into the composition of comet nuclei with their forceful ejection of dust and gas that reveals interior components of the comet. Understanding how different types of outbursts influence the observed dust properties and volatile abundances is necessary to better interpret what signatures can be attributed to primordial composition and what features are the result of processing. As such, it is an important task best undertaken with a multi-instrument approach. During the period between July and November 2015, the Rosetta spacecraft had monitored the inner coma of comet 67P/Churyumov-Gerasimenko (67P/CG). This period encompassed the passage at perihelion (August 2015) resulting in the most active part of its orbit. The Visible InfraRed the Thermal Imaging Spectrometer (VIRTIS) [1] and the ALICE ultraviolet spectrograph, [2] onboard Rosetta observed and detected a series of transient events associated with outburst [Fig. 1]. Previous Alice observations of outbursts have revealed a range of compositions and emission processes within these periods of increased activity. H2O, CO2, CO, and O2 were all indirectly observed within outbursts via emission from the daughter products H, C, and O, identified in the spectra as the first three members of the H I Lyman series, O I multiplets at 1152, 1304, and 1356 Å, and weak multiplets of C I at 1561 and 1657 Å [3]. VIRTIS detected and characterized the dust properties of the outburst in terms of light curve, color, and dust mass loss in the VIS and IR wavelength range. The aim of this work is to take advantage of the capabilities of two instruments to analyze the dust and gas coma trends during these transient events in the perihelion and post-perihelion period. Alice and VIRTIS observe two outbursts on November 7, 2015, displaying both gas and dust components. The gas components show different gas emission characteristics. The outburst B was approximately twice as strong, with respect to the outburst A [Fig. 2], based on the intensity of atomic emissions, which is inverse of the VIRTIS-M measured dust radiances. VIRTIS-M observations of the dust component show that outburst A had a maximum radiance larger than outburst B [Fig. 3]. Alice outburst spectra show the transient events characterized by a different CO2/H2O and O2/H2O ratio as determined from spectral modeling and different ob-served O I 1356/O I 1304 Å ratio. In cases where dissociative electron impact excitation on O2 or CO2 is dominant, we would expect the O I 1356/O I 1304 Å ratio greaten than 1, while if dissociative electron impact occurs on H2O, this value would be less than 1 [5, 10]. Outburst B contained more CO2 and O2, while outburst A was relatively richer in H2O. Elevated CO2 content could indicate a more pristine surface origin (i.e., fracture deepening) and the subsequent activity was sustained for over two hours. The outbursts are characterised by a sudden increase of the dust radiance continuum, fol-lowed by a gentle decrease lasting a few minutes to tens of minutes. The VIRTIS observations show two kind of outbursts. The first type is characterize by a strong color gradient values in the dust continuum with respect to the surrounding coma and the second type doesn’t show colour difference [4,5,7]. The outburst colour sequence in the VIS and IR show a colour gradient pattern which seems correlated to the intensity of the dust radiance within the outburst [7,8]. The first type of outburst shows a VIS colour behaviour reaching the bluer values of 6±1.4 % /100 nm and returning to the pre-outburst value of about 14 % /100 nm [4,5]. The IR continuum emission is also characterised by high colour temperatures of about 600 K and a bolometric albedo of 0.6 [5]. Colour temperatures of 600 K thus reveals the presence of very small grains (less than 100 nm) in the outburst material. The bright grains in the ejecta could be silicate grains, implying the thermal degradation of the carbonaceous material, or icy grains. The rapid increase in radiance at the start of an outburst event is not due primarily to an increase in the number of existing dust particles, but rather to the release of small and bright silicate or icy particles with a high geometric albedo and a filling factor between 1.3 and 5.0 % [4,5]. For the second type of outburst, we found no clear evidence of different reddening values in the dust continuum with respect to the surrounding coma. The reason is probably that this is a faint outburst and the signal from the background coma dominates, so the colour of the outburst is not measurable. The VIS dust color is around 13.1% /100 nm [4,5,7]. Nearly 30 new outbursts observed by VIRTIS have been identified and have corresponding Alice UV spectra. Following the successful characterization of the 7 November outbursts we will apply the same methodology to the full pre/post-perihelion sample. By increasing the sample size by a factor of nearly 15 we will be able to more rigorously understand the link between gas intensity and outburst composition and explore the correlation between dust colour and outburst strength. References: [1] Coradini, A. et al. (2007), SSR 128, 1-4, 529-555; [2] Stern, S. A., Slater, D., Scherrer, J., et al. 2007, SSRv, 128, 507; [3] Feldman, P. D., A’Hearn, M. F., Feaga, L. M., et al. 2016, ApJL, 825, L8.[4] Bockelee-Morvan D., et al., 2017, MNRAS, 469, S443; [5] Rinaldi, G., Bockelée-Morvan, D., Ciarniello, M., et al. 2018, MNRAS, 481, 1235; [6] Fornasier, S., Hoang, V. H., Hasselmann, P. H., et al. 2019b, A&A, 630, A7; [7] Noonan, J. G., Rinaldi, S. A., Feldman, P. D., et al. 2021, AJ, 162, 4

In this work, we apply a method previously discussed in [1] to assess the presence of water ice in Mercury’s Polar Shadowed Regions (PSR) mixed to S-rich volatile species like SO2, H2S, and volatile organics, intending to verify their detectability from Bepi Colombo’s orbit and to optimize SIMBIO-SYS/VIHI observations. PSR icy deposits located within the floor of the Kandinsky crater are simulated in terms of I/F spectra corresponding to mixtures of H2O-H2S and H2O-SO2 with grain sizes of 10, 100, and 1000 µm as resulting from indirect illumination by the scattered solar light coming from the crater’s rim. The spectral simulations, performed following the method described in [2], and including the ice-regolith mixing (areal or intimate) as modeled in [3], allow for exploring different volatile species abundances and grain size distribution.The resulting ice detection threshold are evaluated by means of the computation of VIHI’s instrumental signal-to-noise ratio as given by the instrumental radiometric model [4]. In this way, a synergistic use of illumination models, spectral simulations and resulting SNR calculations will help us to optimize the VIHI’s observation strategy across Mercury’s polar regions with the aim to detect, identify and map volatile species. This task is one of the primary scientific goals of the 0.4-2.0 µm Visible and Infrared Hyperspectral Imager (VIHI) [5], one of the three optical channels of the SIMBIO-SYS experiment [6] on ESA’s BepiColombo mission.Due to orbital characteristics and proximity to the Sun, Mercury’s polar regions undergo large variations in illumination conditions during the hermean year [1]. At poles, Permanent Shadowed Regions (PSRs) occur on deep craters and rough morphology terrains that are not directly illuminated by the Sun during the hermean day. Nevertheless, some of these areas could experience partial illumination caused by multiple scattered light coming from nearby illuminated areas. Despite the orbital vicinity to the Sun, Mercury’s PSRs can maintain cryogenic temperatures across geological timescales resulting in the condensation and accumulation of volatile species [7]. While water ice is the more certain species in Mercury’s PSR, it is not precluded the occurrence of other secondary species rich in sulfur or even organic matter.In fact, the total surface area of PSRs between latitudes 80−90° south is not negligible, being estimated at about 25.000 km2 [8], about two times larger than the same geographic area on the North Pole [9]. References: [1] Filacchione G. et al., MNRAS, 498, 1308-1318, 2020. [2] Raponi A. et al., Sci. Adv., 4, eaao3757, 2018. [3] Ciarniello. M. et al., Icarus, 214, 541, 2011. [4] Filacchione G. et al., Rev. Sci. Instrum., 88, 094502, 2017. [5] Capaccioni F. et al., IEEE Trans. Geosci. Remote Sens., 48, 3932, 2010. [6] Cremonese G. et al., Space Sci. Rev., 216, 75, 2020. [7] Paige D. A. et al., Science, 339, 300, 2013. [8] Chabot N. L. et al., J. Geophys. Res., 123, 666, 2018. [9] Deutsch A. N. et al., Icarus, 280, 158, 2016. Acknowledgments: We gratefully acknowledge funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2017- 47-H.0.

Introduction: Various minerals have been detected on Ceres and their abundance and spatial distribution has been mapped in the near infrared spectral domain [i.e. 1, 2]. In this work we study the 1 µm absorption to map Ceres mineralogy at a global scale. It is based on the determination of the whole 1 µm absorption from VIR spectra. VIR is the imaging spectrometer on board the NASA mission Dawn in orbit around Ceres and still operating. VIR is composed of two channels: the VIS channel working in the visible wavelengths between 0.26-1.07 μm and the NIR channel operating in the near infrared between 1.02-5.1 µm. The 1 µm absorption is, indeed, in between the instrument channels. Method: To study the 1 micron spectral range, an automatic method to co-register the VIR and NIR channels, where they overlap, has been implemented, thus allowing the study of this band. After the spatial co-registration, a residual difference in the I/F at 1 µm between the two channels can still remain due to the difference in the PSF’s. To avoid this problem, each VIS spectrum is rescaled to the value of the I/F at ~1 µm of the corresponding NIR spectrum. The exact wavelength at which the two VIR channels are matched, inside the overlapping range between 0.92–1.8 µm, is selected for each spectrum on the basis of the best match. The difference between the VIS and NIR reflectance at ~1 µm (ΔR) is usually 1-2% and only about 1% of data show a ΔR>10%. The spectra with the largest mismatch between the two channels are located along the walls of the craters and valleys where the influence of PSF differences is expected to be larger. In this study the data with ΔR>10% are discarded. The maps have global longitudinal coverage, latitudinal coverage from 66°S to 66°N, and a spatial resolution of ~1.86 km/pixel at the equator. Each spectrum of the surface is the result of areal or intimate mixing of different minerals, and the resulting spectral reflectance properties are a complex combination of the spectra of each mineral end-member. Several studies have shown that it is possible to explore Ceres mineralogical diversity utilizing specific spectral parameters [1,2]. In order to extract compositional information from the spectra, we have introduced some spectral indices and studied the correlations between them all over the VIR data set. Global maps of the 1 µm signature on Ceres have been derived from various spectral indices, such as band centers, depths, integrated areas, etc. Results: A better understanding of the spatial distribution and the content of the different mineral phases can thus be obtained by investigating the spectral characteristics of the 1 µm band. In general, the VIR spectrum of Ceres is dominated by a broad absorption at about 1.2 µm. It is mostly uniform across the surface, but some differences can be seen in the map of the band parametres, like the band depth. Variation in the band depth could be due to various factors, such as relative abundance and grain size of the present minerals. However, the spectral properties of the 1 µm band have been related to other spectral parameters in the NIR domain where mineralogy is known. For example, the attribution of the 3.1 µm band to ammonia-bearing species [1] show that this mineral phase dominant where we see for low values of 1 micron band. Infact, the spatial variability in the 1.0 µm band intensity is anticorrelated respect to the 3.1 µm band. Some terrains show a different behavior of the 1 micron band respect to 3.1 band. This suggests the presence of a distinctive mineralogy. This work, together with the results of other authors [1,2], completes the global mapping of the Ceres mineralogy. Acknowledgments: We thank the Italian Space Agency (ASI). The Visible and Infrared Mapping Spectrometer (VIR) was funded and coordinated by the Italian Space Agency, with the scientific leadership of the Institute for Space Astrophysics and Planetology, Italian National Institute for Astrophysics, Italy. References: [1] E. Ammannito et al. (2016) Science, 353, issue 6303. [2] Carrozzo et al. (2018), Science Advances, vol. 4, 3, e1701645.

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.

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.

Introduction: The 4 Vesta asteroid is a differentiated asteroid, i.e. it has experienced melting and its structure is distinct layered: crust, mantle, nucleus. Hence, study of materials from different depths of Vesta would be helpful to understand its evolutionary history. Moreover, Vesta preserves information on the composition of primordial smaller asteroids that impacted on its surface. Vesta bright surface shows indeed dark units [1], that are supposed to be linked to exogenous material, and are therefore useful to understand the initial stages of the Solar System. The VIR spectrometer on board the NASA’s Dawn spacecraft [2], that orbited Vesta for approximately one year, collected thousands of hyperspectral images of its surface. Vesta spectra show the 2.7-2.8 μm absorption band, due to the presence of the OH molecule, whose distribution is inversely correlated with the reflectance of its surface [3]. In fact, it is included in carbonaceous chondrites, that are the darkening agents of the Vesta’s dark units [4]. Scientific Objectives: In this work we took advantage of the newly calibrated data of the VIR spectrometer, which are characterized by a better signal to noise (S/N) ratio, giving us the opportunity to search for spectral features that were never seen before, due to noise. Basing on this, the goals of this work were to: 1) confirm, reinforce and characterize the OH distribution on Vesta by studying the 2.7-2.8 µm band and by looking for OH combination bands that occur around 2.2-2.4 µm; 2) search for and characterize possible carbonates, that are supposed to be included in carbonaceous chondrites, i.e., the darkening agents of Vesta; 3) characterize the Vesta surface in the visible range. OH absorption band: The analysis on absorption bands related to the hydroxyl evidenced an anticorrelation between the abundance of hydroxyl and the reflectance of the surface (Figure 1). This confirms that the OH molecule is linked to the presence of the dark units on Vesta [3]. This is also a definitive proof that dark hydrated impactors (likely carbonaceous chondrites) are the darkening agents on Vesta, ruling out other interpretations suggested in literature (regolith grain size, impact shock and presence of opaque-rich eucrites and metals) [1]. Figure 1: Anti-correlation between the 2.8 µm band depth and the reflectance of the surface. We searched for the presence of possible combination bands centered around 2.2-2.4 µm. Weak features at 2.4 µm were found but no correlation with the 2.8 µm absorption band has been proved, so we can say that this feature is not related to the OH molecule and is likely a spectral artifact (Figure 2). Figure 2: This scatterplot shows that the 2.4 µm feature is not correlated to the 2.8 µm absorption band. Search for carbonates: We were not able to identify the 3.9 µm and the 3.4 µm absorption bands related to the presence of carbonates. This does not necessarily mean that carbonates are absent, but their abundance is not sufficient to observe an absorption band emerging from spectral noise. We compared VIR spectra with synthetic spectra of Vesta analogous mixtures (developed by applying a radiative transfer model) to calculate the upper limit of the abundance of carbonates on Vesta. The results show that carbonaceous chondrites that impacted on Vesta, contain no more than 0.5-2% of carbonates, in agreement with petrochemical analyses of typical Carbonaceous chondrites [5]. Caracterization in the visible range: The visible spectra showed an absorption band around 0.5 µm in addition to the strongest well known pyroxenes absorption band centered at 0.9 µm. We made a scatterplot between the 0.5 µm band depth and the 1.9 µm band depth related to the pyroxenes (Figure 3). In the scatterplot a linear dependance between the two features is evident. This proves that the 0.5 µm absorption band is due to the presence of the pyroxenes. Other features found in the visible range were instead associated to spectral artifacts. This emphasizes the results found by [6]. Figure 3: Correlation between the absorption band at 0.5 µm and the absorption band of the pyroxenes at 1.9 µm. References: [1] Palomba E. et al. (2014) Icarus, 240, 58–72. [2] De Sanctis M. C. et al. (2011) SSR, 163, 329. [3] De Sanctis M. C. et al. (2012) The astrophysical Journal Letters., 758.2, p. L36. [4] McCord T. B. et al. (2012) Nature, 491.7422 , 83-86. [5] de Leuw S., Rubin A. E., Wasson, J. T. (2010) Meteoritics & Planetary Science, 45.4, 513-530. [6] Rousseau B. e al. (2021) Astronomy and Astrophysics, 653.

Remote sensing data of comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko (67P hereafter) indicate the occurrence of water-ice-rich spots on the surface of cometary nuclei [1-5]. These spots are up to tens of metres in size and appear brighter and bluer than the average surface at visible wavelengths. In addition, the extensive observation campaign performed by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS, [6]) and the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS, [7]) during the Rosetta escort phase at 67P revealed a seasonal cycle of the nucleus colour. This is characterised by blueing of the surface while approaching perihelion followed by progressive reddening and restoral of the original colour along the outbound orbit. The temporal evolution of the colour has been interpreted in previous studies as the result of increasing exposure of water ice at smaller heliocentric distances [8, 9], however, an explanation of such seasonal cycle in the context of a quantitative cometary activity model was not yet been provided. Recently, in [10] we showed that the seasonal colour cycle observed on comet 67P is determined by the occurrence of the above-mentioned water-ice-rich spots (referred to as Blue Patches – BPs –, given their colour). This can be explained in the context of activity models [11, 12] of pebble-made cometary nuclei [13], i.e. in terms of nucleus surface erosion induced by H2O and CO2 ices sublimation, driving the cometary activity. According to the scenario proposed in [10] (Fig. 1), the presence of the BPs is due to the exposure of subsurface sub-metre-sized Water-ice-Enriched Blocks (WEBs) thanks to surface erosion triggered by CO2 sublimation ejecting decimetre-sized chunks [12]. The WEBs are composed of ice-rich pebbles (dust-to-ice mass ratio δ=2, [14]), embedded in a matrix of drier pebbles (δ>>5) forming most of the nucleus. Once exposed to illumination as BPs, the WEBs are eroded by water-ice sublimation ejecting sub-cm dust [11]. By means of dedicated spectral and thermophysical modelling, we match the nucleus colour temporal evolution measured by the VIRTIS Mapping channel in the 0.55-0.8 µm spectral range. In doing this, we take into account the competing effects of CO2- and H2O-driven erosion that expose and remove the BPs, respectively, and are seasonally modulated by the insolation conditions, primarily depending on the heliocentric distance. The new nucleus model proposed in [10], implying an uneven distribution of water ice in cometary nuclei, reconciles the compositional dishomogeneities observed on comets (the BPs) at macroscopic (up to tens of metres) scale, with a structurally homogeneous pebble-made nucleus at small (centimetre) scale, and with the processes determining the cometary activity at microscopic (sub-pebble) scales. Figure 1. 67P surface gets bluer approaching perihelion as a consequence of the progressive exposure to sunlight of subsurface WEBs (from Figure 4 in Ciarniello et al., 2022, Nature Astronomy, https://doi.org/10.1038/s41550-022-01625-y). The comet nucleus is made of two types of pebbles, both including refractories and CO2 ice, with different water ice content: pebbles with high content of H2O ice form the WEBs, while H2O-ice-poor pebbles represent the rest of the nucleus. CO2 ice is stable beneath the CO2 sublimation front at depths >0.1 m [12]. Approaching perihelion, the CO2 ice sublimation rate increases, eroding the surface by chunk ejection and exposing the WEBs. Once exposed, WEBs lose CO2 and are observable as BPs. Water-ice sublimation erodes the BPs ejecting sub-cm dust from their surface and preventing the formation of a dry crust [11]. The BPs survive until their water-ice fraction is sublimated, producing the observed surface blueing. Please refer to ref. [10] for complete details. References [1] Sunshine, J. M. et al. (2006) Science 311, 1453–1455. [2] Filacchione, G. et al. (2016) Nature 529, 368–372. [3] Raponi, A. et al. (2016) Mon. Not. R. Astron. Soc. 462, S476-S490. [4] Barucci, M. A. et al. (2016) Astron. Astrophys. 595, A102. [5] Oklay, N. et al. (2017) Mon. Not. R. Astron. Soc. 469, S582–S597. [6] Coradini, A. et al. (2007) Space Sci. Rev. 128, 529–559. [7] Keller, H. U. et al. (2007) Space Sci. Rev. 128, 433–506. [8] Fornasier, S. et al. (2016) Science 354, 1566–1570. [9] Filacchione, G. et al. (2020) Nature 578, 49-52. [10] Ciarniello, M. et al. (2022) Nat. Astron. doi:10.1038/s41550-022-01625-y. [11] Fulle, M. et al. (2020) Mon. Not. R. Astron. Soc. 493, 4039–4044. [12] Gundlach, B. et al (2020). Mon. Not. R. Astron. Soc. 493, 3690–3715. [13] Blum, J. et al. (2017) Mon. Not. R. Astron. Soc. 469, S755–S77. [14] O’Rourke, L. et al. (2020) Nature 586, 697–701. Acknowledgements We thank the Italian Space Agency (ASI, Italy; ASI-INAF agreements I/032/05/0 and I/024/12/0), Centre National d’Etudes Spatiales (CNES, France), and Deutsches Zentrum für Luft-und Raumfahrt (DLR, Germany) for supporting this work. VIRTIS was built by a consortium from Italy, France and Germany, under the scientific responsibility of IAPS, Istituto di Astrofisica e Planetologia Spaziali of INAF, Rome, which also led the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI (Italy), with contributions from Observatoire de Meudon (France) financed by CNES and from DLR (Germany). The VIRTIS instrument industrial prime contractor was former Officine Galileo, now Leonardo Company, in Campi Bisenzio, Florence, Italy. Part of this research was supported by the ESA Express Procurement (EXPRO) RFP for IPL-PSS/JD/190.2016. D.K. acknowledges DFG-grant KA 3757/2-1. This work was supported by the International Space Science Institute (ISSI) through the ISSI International Team "Characterization of cometary activity of 67P/Churyumov-Gerasimenko comet". This research has made use of NASA’s Astrophysics Data System.

Visual-to-infrared (VIS-IR) remote sensing observations of different classes of outer solar system objects indicate the presence of water ice and organics. Here, we present laboratory reflectance spectra in the 0.5–4.2 μm spectral range of binary particulate mixtures of water ice, organics analogue (kerite), and an opaque iron sulphide phase (pyrrhotite) to investigate the spectral effects of varying mixing ratios, endmember grain size, and mixing modality. The laboratory spectra are also compared to different implementations of the Hapke reflectance model (Hapke, 2012). We find that minor amounts (≲1 wt%) of kerite (investigated grain sizes of 45–63 μm and &lt, 25 μm) can remain undetected when mixed in coarse-grained (67 ± 31 μm) water ice, suggesting that organics similar to meteoritic insoluble organic matter (IOM) might be characterized by larger detectability thresholds. Additionally, our measurements indicate that the VIS absolute reflectance of water ice-containing mixtures is not necessarily monotonically linked to water ice abundance. The latter is better constrained by spectral indicators such as the band depths of water ice VIS-IR diagnostic absorptions and spectral slopes. Simulation of laboratory spectra of intimate mixtures with a semi-empirical formulation of the Hapke model suggests that simplistic assumptions on the endmember grain size distribution and shape may lead to estimated mixing ratios considerably offset from the nominal values. Finally, laboratory spectra of water ice grains with fine-grained pyrrhotite inclusions (intraparticle mixture) have been positively compared with a modified version of the Hapke model from Lucey and Riner (2011).

Laboratory measurements were performed to study the spectral signature of H2O ice between 0.4 and 4.2 µm depending on varying temperatures between 70 and 220 K. Spectral parameters of samples with particle sizes up to ~1360 µm, particle size mixtures, and different particle shapes were analyzed. The band depth (BD) of the major H2O-ice absorptions at 1.04, 1.25, 1.5, and 2 µm offers an excellent indicator for varying particle sizes in pure H2O ice. The spectral changes due to temperature rather, but not exclusively, affect the H2O-ice absorptions located at 1.31, 1.57, and 1.65 µm and the Fresnel reflection peaks at 3.1 and 3.2 µm, which strongly weaken with increasing temperature. As the BDs of the H2O-ice absorptions at 1.31, 1.57, and 1.65 µm increase, the band centers (BCs) of the H2O-ice absorptions at 1.25 and 1.5 µm slightly shift to shorter wavelengths. However, the BCs of the strong H2O-ice absorptions can also be affected by saturation in the case of large particles. The collected spectra provide a useful spectral library for future investigations of icy satellites such as Ganymede and Callisto, the major targets of ESA’s JUICE mission.

We analyzed the surface of Vesta at visible wavelengths, using the data of the Visible and InfraRed mapping spectrometer (VIR) on board the Dawn spacecraft. We mapped the variations of various spectral parameters on the entire surface of the asteroid, and also derived a map of the lithology. We took advantage of the recent corrected VIR visible data to map the radiance factor at 550 nm, three color composites, two spectral slopes, and a band area parameter relative to the 930 nm crystal field signature in pyroxene. Using the howardite-eucrite-diogenite (HED) meteorites data as a reference, we derived the lithology of Vesta using the variations of the 930 nm and 506 nm (spin-forbidden) band centers observed in the VIR dataset. Our spectral parameters highlight a significant spectral diversity at the surface of Vesta. This diversity is mainly evidenced by impact craters and illustrates the heterogeneous subsurface and upper crust of Vesta. Impact craters also participate directly in this spectral diversity by bringing dark exogenous material to an almost entire hemisphere. Our derived lithology agrees with previous results obtained using a combination of infrared and visible data. We therefore demonstrate that it is possible to obtain crucial mineralogical information from visible wavelengths alone. In addition to the 506 nm band, we identified the 550 nm spin-forbidden one. As reported by a laboratory study for synthetic pyroxenes, we also do not observe any shift of the band center of this feature across the surface of Vesta, and thus across different mineralogies, preventing use of the 550 nm spin-forbidden band for the lithology derivation. Finally, the largest previously identified olivine rich-spot shows a peculiar behavior in two color composites but not in the other spectral parameters., Comment: 23 pages, 11+7 figures

Introduction: The Dawn NASA mission investigated Ceres with its payload, including the Visual (0.25–1 μm) and InfraRed (1–5 μm) mapping spectrometer (VIR) [1]. The first findings from the VIR-IR channel confirmed some previous results from ground-based observations [2]: composition of surface includes a dark component (carbon and/or magnetite) and Mg-carbonates. The detection of a strong absorption at 2.7 μm was assigned to Mg-phyllosilicates; while the 3.06-μm absorption was attributed to NH4-phyllosilicates [3]. The elemental data from the GRaND instrument [4] provided additional constraints on the abundance of C, H, K, and Fe, [5,6], establishing that the dark material that makes up most of Ceres’ surface should be like carbonaceous chondrite [7,8]. The recent improvement in the VIR-VIS channel calibration [9,10] allows taking advantage of the full VIS spectral range. Following [9] and [10], IR channel calibration was also refined [9,10,11,12]. Data were photometrically corrected to standard viewing geometry using Hapke modeling [13] according to [14]. Thermal emission was removed [11]. The resulting spectrum is shown in Fig. 1. The VIR data have shown a clear absorption at 1 μm confirming previous ground observations [2]. The interpretation of such feature, in the context of the average spectrum of Ceres, provides new constraints on Ceres mineralogy. The 1μm feature: The 1-micron band that characterizes the average spectrum of ceres is also common in the spectra of several types of carbonaceous chondrites, (e.g., Ivuna CI, MAC87300 C2-ung, and DAV92300 CK4) (Fig. 2). Previous work [15,16,17] has suggested that these meteorites are characterized by the presence of mafic silicates and magnetite, which are often the main constituents of their fine-grained matrix in addition to low-crystallinity components. In the VIS-NIR range, the main bands of mafic silicates are between 0.9 and 1.1 μm. These absorption features are induced by Fe2+, whose coordination in the crystal structure determines the position of the bands. Ferrous oxides, such as magnetite, show very shallow absorption in the VIS-NIR range. Iron-containing carbonates also have a broad band near 1 μm, but they also show several other vibrational bands between 1.7 and 2.6 μm, allowing them to be distinguished from silicates and oxides. The near-1 μm band associated with the presence of iron is also evident in the spectra of low-crystallinity Fe-containing materials, such as tektites and volcanic glasses. In this case, the depth of the 1μm band varies depending on the amount of iron in octahedral coordination [18]. Mineralogical implications: The possible presence of magnetite on the surface of Ceres as one of the agents responsible for the low albedo of the dwarf planet may be supported by the mineral assemblage as detected by the VIR. Thus, the simultaneous presence of Mg-serpentine could suggest that most of the magnetite could have formed during the serpentinization process because of a serpentinization reaction [21]. The contemporaneous presence of magnetite and serpentine could disguise the spectral contribution of olivine, which may be present as a partially serpentinized primary crust. The occurrence of olivine has also been identified in laboratory heated serpentine and in heated carbonaceous chondrites [22,23]. These laboratory data show that the incipient crystallization of metamorphic olivine changes spectral properties by introducing a bump near 0.8 μm and a broad band near 1 μm, which resembles that observed in the average Ceres spectrum. This does not prove that Ceres experienced a high-temperature metamorphic process because the metamorphic olivine nucleation involves high temperatures that are inconsistent with the stability of ammonium hosted in clay minerals [24,25]. In this case, the presence of olivine could be an indication of a partial serpentinization process that affected the surface of Ceres. During the serpentinization process, the dissolution of olivine is also related to the formation of poorly crystalline phases [26] that represent metastable precursors of crystalline serpentine. Amorphous and nanocrystalline material has also been found in the matrix of several carbonaceous chondrites [27]. Spectral data obtained on mixtures of crystalline olivine and ferrous glass indicate that by increasing the content of the amorphous component, the band at 1 μm becomes shallower [28]. An additional complication in the interpretation of the 1 μm band in the average Ceres spectrum can be introduced by the presence of a fine-grained coating of iron-containing material that can introduce a spectral slope [29] consistent with the VIR data. Conclusions: Remnants of primary olivine, magnetite, and/or low-crystallinity iron phases may be responsible for the 1-μm band in Ceres' average spectrum. Further spectral modeling and laboratory activities will allow a better comprehension of this absorption feature giving important hints on the geological evolution of the dwarf planet. Acknowledgments: VIR data are available through the Planetary Data System (PDS) online data archive at:https://sbn.psi.edu/pds/resource/dawn/dwncvirL1.html. The VIR is funded by ASI and was developed under the leadership of INAF-IAPS. Selex-Galileo, Italy, built the instrument. The authors acknowledge the support of the Dawn Science, Instrument, and Operations Teams. This work was supported by ASIINAF n. I/004/12/0 and NASA. References: [1]De Sanctis MC et al SpaceSciRev (2011) [2]Rivkin AS et al SpaceSciRev (2011) [3]De Sanctis MC et al Nature (2015) [4]Prettyman TH et al SpaceSciRev (2011) [5]Prettyman TH Science (2017) [6]Prettyman TH et al 49th LPSC (2018) [7]Marchi S et al Nat Astron (2019) [8]Kurokawa H et al J Geophys Res Planets (2020) [9]Rousseau B et al Rev Sci Instrum (2019) [10]Rousseau B et al Astron Astrophys (2020) [11]Raponi et al Life (2021) [12]Carrozzo FG et al Rev Sci Instrum (2016) [13]Hapke B Theory of Reflectance and Emittance Spectroscopy Cambridge UK (1993) [14]Ciarniello M et al Astron Astrophys (2017) [15]King AJ et al Earth Planets and Space (2015) [16]Kallemeyn G W Meteoritics (1992) [17]Marlow R et al Antarct Met Newslett (1993) [18]Bell PM et al Proc 7th Lunar Sci Conf (1976) [19]Castillo‐Rogez J et al Meteorit Planet Sci (2018) [20]McSween HY et al Meteorit Planet Sci (2018) [21]Evans BW Geology (2010) [22]Hiroi T et al Antarct Met Res (1999) [23]Cloutis EA et al Icarus 221 (2012) [24]De Angelis et al JGR Planets (2021) [25]Bishop et al Planet Space Sci (2002) [26]King HE et al Environ Sci Tech (2010) [27]Abreu NM et al Microsc Microanal Suppl2 (2019) [28]Horgan BHN et al Icarus (2014) [29]Fisher EM Pieters CM Icarus (1993)

Introduction The Visible channel of the Rosetta VIRTIS spectrometer is known to be affected by significant straylight, which makes it difficult to take advantage of its whole 230-1040 nm spectral range [1]. Straylight is by definition an extra signal due to photons reaching the detector through anomalous optical paths. The main evidence of contamination is the presence of residual solar (Fraunhofer) lines in calibrated reflectance spectra and anomalously high reflectance levels in the short-wavelength edge of the spectral range. The origin of such straylight in VIRTIS is twofold: a diffracted component, related to the IR diffraction grating, and a scattered one, related to internal reflections. Diffracted straylight The VIRTIS circular convex diffraction grating is formed by an outer corona optimized to disperse IR wavelength photons towards the IR detector through its diffraction order mIR = +1 and an inner circular part optimized to disperse VIS-NIR wavelength photons towards the VIS detector, through its diffraction order mV = -1 [2]. Both IR and VIS detectors are only sensitive to photons in their respective spectral range, but visible photons can reach the VIS detector also through diffraction on the IR grating portion. E.g., being VIS grating’s grooves 5 times denser (268 grooves/mm) than IR one (54 grooves/mm), the VIS detector is also illuminated by the order mIR = -5, which owns the same dispersion law. These photons simply add to those from order mV = -1, both forming the nominal VIS-NIR signal, calibrated by standard radiometric pipeline. However, also other diffraction orders of the IR grating illuminate the VIS detector (namely orders mIR = -2,-3,-4,-6,-7,-8), spreading VIS wavelength photons each with its own dispersion law. As a result, the photons of a given wavelength, forming the signal at a given VIS spectel b once diffracted through the VIS grating, are actually contributing to the signal in other spectels, through IR grating higher diffraction orders [3]. The amount of spectral mixing introduced this way in the final spectrum depends on the overall shape of the radiance spectrum incident on the grating. This unfavourable diffraction effect has been characterized, during ground calibration, by measuring in the laboratory the spectral efficiencies of IR grating higher orders. These lab data allow to model the diffracted straylight, enabling an effective technique for removing it from the VIRTIS-V space observations. The correction amount for any given spectrum is evaluated from the whole VIS spectrum itself, using an iterative method to gradually approximate the solution. The non-linearity of the problem forces evaluating the correction independently for each spectrum, hence no simple corrective factors can be found effective for the whole dataset. Scattered straylight Contrary to the diffracted one, scattered straylight is an out-of-field contamination, since the scattered photons impinging on a given detector pixel can also come from locations on the entrance slit different from that pertaining to that pixel. VIRTIS-V images across body limbs make clear the presence of such straylight, readily identifiable as an off-limb signal against the dark sky. Its spatial distribution, gradually dimming in going farther from the limb, demonstrates the interpixel correlation introduced by this effect. The correction developed for this kind of contamination is based on two main points: a) the stability of the spectral shape of the off-limb straylight against variation of the target body and its location in the field of view, and b) the capability of VIRTIS-V of resolving some Fraunhofer lines and measuring their depth. However, the latter is only possible when the SNR in the blue side of the spectrum is high enough, and unfortunately this condition is not often met. A systematic application of this technique to the whole VIRTIS-V dataset is therefore not possible so far. Discussion and conclusions We describe separately two techniques for the removal of the two straylight components identified in VIRTIS-V spectra. Of course, the two phenomena take place at the same time and a complex interplay can exist between them. In good SNR condition, the estimation of the total straylight signal proceeds by alternatively iterating the two corrections, removing the straylight modeled at each iteration step from the original measured spectrum until convergence, which is usually reached in a dozen iterations. However, the scattered straylight model shows a high sensitivity to the noise, amplifying and propagating the fluctuations in the Fraunhofer lines region to the whole spectral range. As a consequence, a full straylight correction can only be achieved in selected spectra subsets, e.g. pertaining to bright surface regions of target bodies. Since the straylight correction here presented has been included in a full recalibration process of the entire VIRTIS dataset, to be provided to the ESA PSA archive, only the correction for diffracted straylight has been implemented. Even if this approach keeps significant straylight levels in the data, on the other hand it represents a minimal, deterministic, correction, which can make the basis for future developments. Finally, it is worth noting the special case of stellar targets: while in the full pixel case the amount of scattered and diffracted straylight is comparable, in the case of point sources the former is expected much lower, if any. The diffracted straylight correction is in this case very effective in recovering the unperturbed spectrum in the full V range. Since these observations are used for in-flight calibration updates, the discussed correction may have significant consequences on the radiometric calibration of the whole dataset. References: [1] Ammannito, A. et al., RSI, 2006, doi: 10.1063/1.2349308 ; [2] Filacchione, G. PhD Thesis 2006, doi: 10.6092/UNINA/FEDOA/1462; [3] Filacchione, G. et al., RSI 2006, doi: 10.1063/1.2360786.

With the introduction of visible and infrared Imaging Spectrometers about 20 years ago, we have assisted to a dramatic enhancement of the scientific return achieved by space missions launched for the exploration of the Solar System. This innovative class of instruments are in fact able to conjugate together the imaging capabilities of the cameras with the spectral resolution accomplished by spectrographs. By exploiting such capabilities, planetary scientists can retrieve and map the physical and chemical properties of a target body and correlate them with surface morphological features or with dynamical structures of the atmospheres. It is not by chance that such instruments have become an essential payload on many planetary exploration missions since they have demonstrated their capabilities to allow a better understanding of the composition, processes and evolution of Solar System bodies. In the meanwhile, ongoing photonic research has made available new devices whose technology is mature enough to allow to investigate new optical concepts for color cameras and imaging spectrometers architectures to be implemented in a single integrated instrument (⨏ISPEx), which we present here. The ⨏ISPEx camera design uses a Liquid Crystal Tunable Filter (LCTF) to ensure high flexibility in the selection of the bandpass (between 0.42-0.73 µm) and bandwidth (35 nm at 0.55 µm) of the color filters, which can then be adapted to very different scientific targets. The camera has a broad 3.3°x1.6° FOV and an IFOV of 14 µm allowing to reach a spatial resolution of 0.14 m/px from 10 km distance. Conversely, the ⨏ISPEx VIS (0.4-1.05 µm spectral range, 3.2 nm/band sampling) and IR (0.95-5.0 µm, 8 nm/band) integral field imaging spectrometers use Coded Mask Optical Reformatters (CMOR) based on optical fibers bundles to collect the full hyperspectral cube through a single acquisition resulting in a great reduction of the acquisition time with respect to traditional whiskbroom and pushbroom instruments. The FOV/IFOV are 0.4° (circular)/100 µrad and 0.28°x0.28° (square)/225 µrad for the VIS and IR channel spectrometers, respectively. These values correspond to a 70 m-wide circular hyperspectral image made of 4500 pixels at 1 m/px resolution for the VIS channel and to a 50x50 m image made of 484 pixels at 2.25 m/px for the IR channel for acquisitions made from a distance of 10 Km. These requirements are optimized for remote sensing of asteroids and comets from close distances. Camera’s and spectrometers’ FOV are co-located on the same boresight thanks to the use of a common Three Mirror Afocal Telescope operating with a 2.4X beam reduction factor (entrance pupil diameter 84 mm). By means of a beamsplitter and a dichroic filter the collimated beam generated by the telescope is split to the camera and spectrometers’ pupils. Thanks to the solutions we are developing, the camera and imaging spectrometers will operate at the same time on the same boresight, resulting in a great simplification of their operations. In this respect, the ⨏ISPEx will offer the advantage to allow a better exploitation of the data collected by the three channels resulting in a tremendous advantage for many scientific investigations. With this synergic approach it will be possible to analyze high resolution images to constrain morphology interpretation of a target, while hyperspectral data collected at the same time allow the retrieval of composition and physical properties. The availability of camera images makes possible to apply sharpening algorithms to spectrometer’s ones. Apart this, an integral-field spectrometer will keep the capabilities of more traditional whiskbroom and pushbroom spectrometers but it will overcome them when the target scene is rapidly evolving and changing during the acquisition: traditional instruments are limited by the fact that the cube acquisition process may take a time longer than the temporal scale of the investigated event. This is the case of observations enquiring into the dynamical evolution of planetary atmospheres, lightning events, outbursts, hypervelocity impact or fast-moving targets. By operating with fast readout detectors, an Integral Field spectrometer can adequately resolve the four dimensions of data (2D spatial, spectral and temporal) opening the possibility to perform time-resolved hyperspectral movies. Another substantial advantage of Integral Field spectrometers is their better operability during fast flyby phases, where the distance from the target and illumination geometry are rapidly changing, resulting in limited time periods suitable to observe the target with optimal conditions. By collecting the entire hyperspectral cube in a fraction of the time necessary to complete the scan for a traditional scanning spectrometer, an Integral Field spectrometer can reach a level of imaging flexibility similar to the one achieved by a camera. Within this study we are defining the configuration of the ⨏ISPEx space model operating in the 0.4-5.0 µm spectral range including optical performance analyses, and thermomechanical and electronic architecture. Moreover, we are realizing a development breadboard limited to the 0.4-1.0 µm spectral range to conduct performance tests at system level on LCTF and CMOR devices. We gratefully acknowledge financial contribution from the Agreement ASI-INAF n.2018-16-HH.0.

We performed spectral analyses on four serendipitous observations of Europa acquired by the JIRAM instrument (Adriani et al., 2017) onboard the Juno mission. Due to the fixed orientation of the spacecraft spin axis and orbital plane placed along terminator, Europa is observed at approximately 90° phase angle from distances >3.35E5 km resulting in a best spatial resolution of 80 km/px. The one thousand spectra dataset returned to the ground offers a limited coverage in longitude but an almost complete excursion in latitude from poles to equator. With a 2 to 5 μm spectral range and a sampling of about 10 nm/band, JIRAM reflectance spectra allow extraction of different spectral indicators suitable to characterize surface composition, grain size distribution and temperature: the 2 μm water ice band center appears on average centered at 2.038 μm as a result of the presence of both amorphous and crystalline ice phases and non-ice materials. The reflectance level on the 2.4 μm inflection shoulder is comparable to the maximum absorption on the 2 μm band pointing to the presence of non-ice materials, like magnesium chlorinated or hydrated magnesium salts, mixed with water ice. Assuming water ice as the principal endmember, weestimate the surface regolith grain size by comparing the 2 μm band depth and the ratio between I/F at 3.63 and 2.27 μm (Filacchione et al., 2012). On average, we measure values of 0.6 for the first and 0.1 for the latter: such values are compatible with grain sizes in the range of tens to hundreds of microns. Despite the low signal-to-noise beyond 2.7 μm, due to the intrinsic low-reflectance of Europa's surface and very high phase angle values occurring during the observations, JIRAM is able to measure the wavelength of the reflectance peak around 3.6 μm, which is a proxy of water ice temperature (Filacchione et al., 2016a). In the dataset JIRAM has obtained the peak's maximum wavelength occurs at 3.64 μm corresponding to a maximum diurnal temperature of 132 K.

The distinctive bright regions within Occator crater are one of the most remarkable discoveries of the Dawn mission's exploration of Ceres. The central region is named Cerealia Facula and the additional regions in the eastern crater floor are named Vinalia Faculae. Here we summarize and synthesize the results of this special issue, which aimed to identify the driving forces behind the formation of Occator and the faculae, and thus lead us to a new understanding of the processes and conditions that occurred in Ceres’ past, and potentially in its present. The investigations presented here used Dawn data, theoretical modeling and laboratory experiments to deduce the sequence of events that led to the formation of Occator, Cerealia Facula and Vinalia Faculae, which are broken into stages 1–3. Stage 1: Occator's ejecta blanket, terraces and hummocky crater floor material formed during and shortly after crater formation. These features are located in many Cerean complex craters. However, Occator also contains the lobate materials and faculae, which are unique to Occator. We interpret the lobate materials as a slurry of water, soluble salts and boulders of unmelted silicates/salts, which flowed around the crater interior before solidifying. At least portions of the lobate materials were solidified prior to the formation of the central pit, which is suggested to form via the ‘melted uplift model’ and provides insights into central pit formation across the Solar System. We propose the outer edge of Cerealia Facula formed shortly after the Occator-forming impact, via impact-induced hydrothermal brine deposition or via salt-rich water fountaining (perhaps sourced in a pre-existing reservoir). Stage 2: the majority of Cerealia Facula is located within the central pit and is interpreted to have formed later, at least ∼18 Myr after the Occator-forming impact, via multiple depositional events. The Cerealia-Facula-forming brines may have flowed out of fractures in the walls of the central pit and/or been driven to the surface by freezing of a subsurface reservoir and/or deposited via salt-rich water fountains. Further investigations are required to identify whether the formation of the majority of Cerealia Facula is driven by processes triggered by an impact, i.e. an exogenic event, or by a combination of impact-driven and endogenic processes. Cryomagmatic intrusions are suggested to uplift the crater floor, resulting in concentric floor fractures and an asymmetric dome. Injection of a similar material is proposed to inflate part of the lobate materials, giving them a hummocky texture. The resulting stresses formed fractures in the hummocky lobate material, which allowed the Vinalia-Faculae-forming brines to travel to the surface, where they ballistically erupted. The central dome within the central pit was one of the last features to form, by laccolithic intrusion, or by volume expansion from freezing of volatiles, or by extrusion of brines. Stage 3: mixing with Ceres’ average materials and/or space weathering darken the faculae over time. Cerealia Facula and Vinalia Faculae are the brightest and freshest of the bright regions identified on Ceres’ surface. Bright regions darken over time until their eventual erasure. Thus, it is likely that faculae formation has occurred throughout Ceres’ history, but that Occator's faculae are visible today because they are geologically young. Through synthesis of the studies presented in this special issue, we find that entirely exogenic driving forces, triggered by the impact, or a combination of endogenic and impact-derived forces could explain the formation of Occator and its faculae. Whether activity is impact-triggered and/or endogenic in nature is a key question for all investigations of Ceres, and future studies may favor one possibility over the other. The investigations presented in our special issue indicate Ceres is an active world where brines have been mobile in the geologically recent past. As such, Ceres is an intriguing world that we have only begun to explore.

Occator Crater on dwarf planet Ceres hosts the so-called faculae, several areas with material 5 to 10 times the albedo of the average Ceres surface: Cerealia Facula, the brightest and largest, and several smaller faculae, Vinalia Faculae, located on the crater floor. The mineralogy of the whole crater is analyzed in this work. Spectral analysis is performed from data of the VIR instrument on board the Dawn spacecraft. We analyse spectral parameters of all main absorption bands, photometry, and continuum slope. Because most of the absorption features are located in a spectral range affected by thermal emission, we developed a procedure for thermal removal. Moreover, quantitative modeling of the measured spectra is performed with a radiative transfer model in order to retrieve abundance and grain size of the identified minerals. Unlike the average Ceres surface that contains a dark component, Mg-Ca-carbonate, Mg-phyllosilicates, and NH4-phyllosilicates, the faculae contain mainly Na-carbonate, Al-phyllosilicates, and NH4-chloride. The present work establishes unambiguously the presence of NH4-chloride thanks to the high-spatial resolution data. Vinalia and Cerealia Faculae show significant differences in the concentrations of these minerals, which have been analyzed. Moreover, heterogeneities are also found within Cerealia Facula that might reflect different deposition events of bright material. An interesting contrast in grain size is found between the center (10-60 μm) and the crater floor/peripheral part of the faculae (100-130 μm), pointing to different cooling time of the grains, respectively faster and slower, and thus to different times of emplacement. This implies the faculae formation is more recent than the crater impact event, consistent with other observations reported in this special issue. For some ejecta, we derived larger concentrations of minerals producing the absorption bands, and smaller grains with respect to the surrounding terrain. This may be related to heterogeneities in the material pre-existent to the impact event.

At the beginning of the Ceres investigation, the Dawn-NASA mission discovered a large bright spot (BS) in the Occator crater floor. Several other smaller bright spots were discovered during the following phases of the mission. In this paper, a complete survey for the detection of BS on the Ceres surface have been made by using the hyperspectral data acquired by Visible and Infrared Mapping Spectrometer (VIR). The hyperspectral images span the spectral range from 0.2 to 5 μm, by using two channel, the VIS channel with a spectral sampling of 1.8 nm and a IR channel with a spectral sampling of 9.8 nm. Finally a catalogue of 92 BS has been compiled and their compositional properties have been examined. In particular, five spectral parameters have been applied to perform the analysis: the photometrically corrected reflectance and four band depths, related to spectral absorptions at 2.7 μm (OH fundamental indicative of phyllosilicates), at 3.05 μm (due to ammoniated clays), at 3.4 and 4.0 μm (carbonate overtones). The 90% of BS are impact-related features (ejecta, crater rim, crater floor, crater wall). The two brightest BS, Cerealia and Vinalia Faculae, are located on the Occator crater floor. Most of BSs show features similar to the average Ceres surface, which has low reflectance and is composed of Mg-phyllosilicates and ammoniated clays, with a reduced abundance of Mgsbnd Ca carbonates. Cerealia and Vinalia Faculae are a peculiar BS family, with a high abundance of Na-carbonates and Al-rich phyllosilicates. Oxo and a companion bright spot represents a third category, depleted in phyllosilicates and with a high to moderate albedo. Carbonate composition ranges from Mg/Ca to Na components. Haulani, Ernutet, Kupalo, and other two BS's represent another group, with intermediate properties between the typical BS and the Oxo family: they are moderately rich in carbonates and slightly depleted in Mg- and ammoniated phyllosilicates. The four families probably explain a single evolutionary path followed by the BS from the formation to their maturity: initially the very fresh bright spots would possess characteristics similar to Cerealia and Vinalia Faculae; with time, salts and OH volatilize and a light mixing with surrounding material would produce Oxo-like BS's; additional strong mixing would form Haulani-like BS, which finally become a typical bright spots.

We present an analysis of the areal distribution of spectral parameters derived from the VIR imaging spectrometer on board NASA/Dawn spacecraft. Specifically we studied the Occator quadrangle of Ceres, which is bounded by latitudes 22°S to 22°N and longitudes 214°E to 288°E, as part of the overall study of Ceres’ surface composition reported in this special publication. The spectral parameters used are the photometrically corrected reflectance at 1.2 µm, the infrared spectral slope (1.1–1.9 µm), and depths of the absorption bands at 2.7 µm and 3.1 µm that are ascribed to hydrated and ammoniated materials, respectively. We find an overall correlation between 2.7 µm and 3.1 µm band depths, in agreement with Ceres global behavior, and band depths are shallower and the spectral slope is flatter for younger craters, probably due to physical properties of regolith such as grain size. Spectral variations correlated with the tali geological unit also suggest differences in physical properties. The deepest band, indicating enrichment of ammoniated phyllosilicates, are associated with ejecta generated by impacts that occurred in southern quadrangles. The most peculiar region of this quadrangle is the Occator crater (20°N 240°E). The internal crater area contains two faculae, which are the brightest areas on Ceres due to exposure of sodium carbonates, and by two types of ejecta, dark and bright, with different spectral properties, probably due to different formation, evolution or age.

This article presents the spectral parameter maps used in this Surface Composition of Ceres Special Issue. The definition and use of spectral parameters has always played a fundamental role in understanding the properties and composition of a planetary surface. Mapping proper spectral parameters shows the global mineralogical diversity across Ceres. In this work, we discuss the production process of Ceres spectral parameter maps derived by the data of the Visible and Infrared mapping spectrometer (VIR) onboard NASA’s Dawn mission. We describe the data processing of the VIR spectra and the procedure to retrieve the geometries (latitude, longitude and illumination angles) of the acquired data. Spectra and geometries are used to project and mosaic this data to produce Geographic Information System-compatible spectral parameters maps of Ceres. An overview of the variability of the data across the quadrangles is given, addressing the specific analysis to each quadrangle mapping paper included in this special issue.

The dwarf planet Ceres is a heavily cratered rocky body, and complex craters with a central peak are widely observed on its surface. These types of craters form when a large body impacts the surface, generating extreme temperatures and pressures. During the impact event a large volume of rock is raised from the subsurface and a central uplift is formed. The material composing the central uplift is called crater central peak material (ccp) and the spectral analysis of such geologic areas can provide information about the composition of Ceres’ subsurface. Reflectance spectra of 32 ccps, acquired by the VIR spectrometer on board the NASA/Dawn spacecraft, were analyzed and shows absorption bands located at about 2.7, 3.1, 3.4 and 4.0 µm which are also common on the Cerean surface. These absorptions are related, respectively, to Mg-phyllosilicates, NH4-phyllosilicates and Mg/Ca-carbonates. The spectral parameters considered in this work are: spectral slopes estimated between 1.2 µm and 1.9 µm, band depths at 2.7-, 3.1-, 3.4- and 4.0-µm, and band centers near 4.0-µm. The ccps spectral parameters were analyzed in conjunction with other Cerean parameters, such as the estimated depth of excavation of the material composing the central peak, in order to search for correlations and information about Ceres’ subsurface. Central peak material located polewards show stronger 2.7- and 3.1-µm band depths with respect to those at the equatorial region, suggesting that subsurface deposits closer to poles are probably richer in Mg- and NH4-phyllosilicates. The 3.4-µm spectral feature is also deeper in ccps located at poleward latitudes, similar to the phyllosilicates. Conversely, the 4.0-µm band does not show this trend with latitude. An increase in both 3.1- and 3.4-µm band depths with the estimated depth of excavation indicates that the spectral feature at 3.4-µm is the result of different contributions from carbonates and NH4-phyllosilicates, as expected. However, depending on their relative influence, the shape of the 3.4-µm spectral feature can vary. Phyllosilicates and carbonates are the resulting products of aqueous alteration of chondritic material and, given the increasing abundance of such minerals (in particular ammoniated phyllosilicates) with depth of excavation, it is likely that our investigation involved subsurface layers nearby the boundary between the volatile-rich crust and the silicate-rich mantle. Na-carbonate is found in the crater central peak material of Ernutet, Haulani and Ikapati, characterized by an estimated depth of excavation of about 6–9 km, where deposits of sodium carbonates could be locally present.

We studied the distribution in the Urvara–Yalode region of Ceres (latitudes 21–66°S, longitudes 180–360°E) of main spectral parameters derived from the VIR imaging spectrometer onboard the NASA/Dawn spacecraft, as an overall study of Ceres mineralogy reported in this special issue. In particular, we analyzed the distribution of reflectance at 1.2 µm, band depth at 2.7 and 3.1 µm, ascribed to magnesium and ammoniated phyllosilicates, respectively. Whereas the average band depths of this region are lower than eastern longitudes, reflecting the E-W dichotomy of abundance of phyllosilicates on Ceres, spectral variations inside this region are observed in the following units: (a) the central peak of the Urvara crater (45.9°S, 249.2°E, 170 km in diameter), showing a deep 3.1 µm band depth, indicating an ammonium enrichment; (b) the cratered terrain westwards of the Yalode basin (42.3°S, 293.6°E, 260 km in diameter), where absorption bands are deeper, probably due to absence of phyllosilicates depletion following the Yalode impact; (c) the hummocky cratered floor of Yalode and Besua (42.4°S, 300.2°E) craters, characterized by lower albedo and band depths, probably due to different roughness; (d) Consus (21°S 200°E) and Tawals (39.1°S, 238°E) craters, whose albedo and band depths decreasing could be associated to different grain size or abundance of dark materials. Twenty-two small scale (i.e., lower than 400 m) bright spots are observed: because their composition is similar to the Ceres average, a strong mixing may have occurred since their formation.

Ceres has been explored by NASA/Dawn spacecraft, which allowed for the discovery of the main mineralogical and compositional characteristics of Ceres' surface. Here, we use mainly data from the Visible and InfraRed imaging spectrometer (VIR) in order to investigate the main spectral characteristics of the quadrangle Ac-H-2 Coniraya, one of the 15 quads in which Ceres' surface has been divided. Coniraya quadrangle is characterized by the presence of mostly highly degraded impact craters of diameters between 50 and 200 km and clusters of small to midsize impact craters. Although the composition over the quadrangle appears to be quite uniform, significant differences have been detected between different craters by spectral parameters analysis and spectral modeling. Ernutet crater presents two regions with very peculiar band at 3.4 μm, typical of organics aliphatic material. One region result to be correlated with larger amount of carbonates, the other region does not present such correlation. Ikapati crater shows strong absorption bands at 4.0 μm, indicating the presence of Na-carbonates in the floor and ejecta. Ikapati, Gaue and other craters present smaller spectral features of NH4 and/or OH stretching, suggesting a volatile depletion process induced by the heating of the impact event.

This paper focuses on the identification and distribution of compositional units and their stratigraphic relationships in the Fejokoo quadrangle of Ceres (Ac-5) located between 21–66°N and 270–360°E and named after one of its prominent and well-preserved impact craters, Fejokoo (centered at 26°N and 312°E). In this quadrangle, we observed that hydroxylated- (OH-rich) and ammoniated- (NH4-rich) phyllosilicates are present everywhere, in various abundances; low abundance is observed on bright terrains and higher abundances are observed on lobate materials associated with craters. Carbonates are mostly correlated with the high-albedo areas surrounding Oxo (359.7°E, 42.2°N) and other major craters, and are mixed with Ceres’ most common surface composition type (i.e. low albedo, phyllosilicate-rich material). There are a few locations where carbonates and phyllosilicates co-exist, indicating a range of geological or chemical processes produced them in co-existence. No correlation between the surface composition and the age of the craters was found. Instead, the composition observed on impact craters depends on the size of the impact and the composition of the different stratigraphic layers excavated. The compositional interpretation inferred from Dawn's visible-infrared spectrometer data of the Fejokoo quadrangle is consistent with the presence of an internal ocean that produced carbonates and phyllosilicates via aqueous alteration of minerals.

Mineralogical maps of the Ac-H-11 Sintana and the Ac-H-12 Toharu quadrangles of the dwarf planet Ceres were produced in order to access the composition of this planetary body. We used data from NASA's Dawn spacecraft, in particular the spectra returned by VIR, the imaging spectrometer on board. Different spectral parameters in the infrared range have been computed to study the composition of this portion of Ceres’ surface and its large and small scale variability. We studied the variation and distribution of the phyllosilicate bands at 2.73 µm and 3.07 µm, the reflectance at 1.2 µm, and the overall spectrum in specific locations. We did not observe variations of the center of the bands at 2.73 µm and 3.07 µm, with the exception of a few pixels in the Kupalo crater. We found that this southern region, extending from 0° to 180° and from 21°S to 66°S, show an overall increase of phyllosilicate band intensity from the equatorial areas to the southern areas. Superimposed to the large-scale trend, we observe many smaller localized variations of band intensity. The observed variations can indicate large and small-scale heterogeneities in the abundance of the different species in the Ceres subsurface. However, the small-scale variation that is mostly associated with young craters, can be also due to processes related with impacts, such as de-hydration or delivery of exogenous material that, mixed with the original surface, could change the band intensity. Several craters, such as Kupalo and Juling, show a different composition with respect to the background, displaying water ice and sodium carbonates.

Quadrangle Ac-H-08 Nawish is located in the equatorial region of Ceres (Lat 22°S-22°N, Lon 144°E- 216°E), and it has variable mineralogy and geology. Here, we report on the mineralogy using spectra from the Visible and InfraRed (VIR) mapping spectrometer onboard the NASA Dawn mission. This quadrangle has two generally different regions: the cratered highlands of the central and eastern sector, and the eastern lowlands. We find this dichotomy is also associated with differences in the NH4-phyllosilicates distribution. The highlands, in the eastern part of the quadrangle, appear depleted in NH4-phyllosilicates, conversely to the lowlands, in the north-western side. The Mg-phyllosilicates distribution is quite homogeneous across Nawish quadrangle, except for few areas. The 2.7 µm band depth is lower in the south-eastern part, e.g. in the Azacca ejecta and Consus crater ejecta, and the band depth is greatest for the Nawish crater ejecta, and indicates the highest content of Mg-phyllosilicates of the entire quadrangle. Our analysis finds an interesting relationship between geology, mineralogy, topography, and the age in this quadrangle. The cratered terrains in the highlands, poor in NH4 phyllosilicates, are older (2 Ga). Conversely, the smooth terrain, such as with Vindimia Planitia, is richer in ammonia-bearing phyllosilicates and is younger (1 Ga). At the local scale, Ac-H-8 Nawish, displays several interesting mineralogical features, such as at Nawish crater, Consus crater, Dantu and Azzacca ejecta, which exhibit localized Na-carbonates deposits. This material is superimposed on the cratered terrains and smooth terrains and shows the typical depletion of phyllosilicates, already observed on Ceres in the presence of Na-carbonates.

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.

The surface mineralogy of dwarf planet Ceres appears to be dominated by products of rock–fluid interactions, such as phyllosilicates—some of which are NH4-bearing—and carbonates1–3. Elemental concentrations derived from the inferred mineral mixing fractions, however, do not match measurements of H, C, K and Fe on Ceres4. A complicating factor in assessing Ceres’s unique surface composition is the secular accretion of asteroids typical of chondritic compositions. Here we show that Ceres’s mineral and elemental data can be explained by the presence of carbonaceous chondritic-like materials (~50–60 vol%), possibly due to infalling asteroids, admixed with aqueously altered endogenic materials that contain higher-than-chondritic concentrations of carbon. We find that Ceres’s surface may contain up to 20 wt% of carbon, which is more than five times higher than in carbonaceous chondrites. The coexistence of phyllosilicates, magnetite, carbonates and a high carbon content implies rock–water alteration played an important role in promoting widespread carbon chemistry. These findings unveil pathways for the synthesis of organic matter, with implications for their transport across the Solar System. Infrared and neutron spectroscopic observations by Dawn give contrasting results on the elemental composition of Ceres’s surface, which can be reconciled by assuming that Ceres’s surface contains ~20 wt% of carbon, coming from impacts by carbonaceous asteroids and/or generated by extensive aqueous alteration.

International audience; Organic matter directly observed at the surface of an inner planetary body is quite infrequent due to the usual low abundance of such matter and the limitation of the infrared technique. Fortuitously, the Dawn mission has revealed, thanks to the Visible and InfraRed mapping spectrometer (VIR), large areas rich in organic matter at the surface of Ceres, near Ernutet crater. The origin of the organic matter and its abundance in association with minerals, as indicated by the low altitude VIR data, remains unclear, but multiple lines of evidence support an endogenous origin. Here, we report an experimental investigation to determine the abundance of the aliphatic carbon signature observed on Ceres. We produced relevant analogues containing ammoniated-phyllosilicates, carbonates, aliphatic carbons (coals), and magnetite or amorphous carbon as darkening agents, and measured their reflectance by infrared spectroscopy. Measurements of these organic-rich analogues were directly compared to the VIR spectra taken from different locations around Ernutet crater. We found that the absolute reflectance of our analogues is at least two orders of magnitude higher than Ceres, but the depths of absorption bands match nicely the ones of the organic-rich Ceres spectra. The choices of the different components are discussed in comparison with VIR data. Relative abundances of the components are extrapolated from the spectra and mixture composition, considering that the differences in reflectance level is mainly due to optical effects. Absorption bands of Ceres’ organic-rich spectra are best reproduced by around 20 wt.% of carbon (a third being aliphatic carbons), in association with around 20 wt.% of carbonates, 15 wt.% of ammoniated-phyllosilicate, 20 wt.% of Mg-phyllosilicates, and 25 wt.% of darkening agent. Results also highlight the pertinence to use laboratory analogues in addition to models for planetary surface characterization. Such large quantities of organic materials near Ernutet crater, in addition to the amorphous carbon suspected on a global scale, requires a concentration mechanism whose nature is still unknown but that could potentially be relevant to other large volatile-rich bodies.

1. Introduction (4) Vesta is the second most massive body in the main asteroid belt and exhibits absorption features indicative of basaltic minerals [1]. The discovery of new basaltic objects taxonomically classified as V-type asteroids [2] is probably linked to the fragments produced during a catastrophic event where Vesta is believed to have lost about one percent of its mass less than a billion years ago. VIR spectra of Vesta’s and V-type asteroids surface show absorption bands centred at approximately 0.9 and 1.9µm, confirming the pyroxenes presence [1] and they have a reflectance spectrum which is very similar to HED. We aim to infer the modal mineralogical compositions and regolith grain size of some V-type asteroids using the Hapke radiative transfer model, basing on the results of scatterplot analysis of spectral parameters such as Band Center I (hereafter BCI) and Band Center II (hereafter BCII), Band depth (hereafter BDI) and Band depth II (hereafter BDII) and Half-width-half-maximum (hereafter HWHM). It is also foreseen to test the genetic linkages between V-type asteroids and HED meteorites and Vesta. 2. Data and method We analysed 76 V-type asteroids normalized spectra in the infrared from 0.7-0.8 µm to 2.5 µm, divided in two datasets. The first group was from [3] and the second was from [4, 5, 6]. All the spectra were downloaded from the Planetary Data System (PDS). To study the spectral properties and to understand the mineral composition and possibly the physical characteristics of the regolith (i.e., effective grain size), we introduced spectral parameters such as the band BC, BD and HWHM as defined in [7] and compared the results obtained from V-type asteroid with HED meteorites used in [7]. We also applied Hapke’s bidirectional reflectance model to retrieve compositional and grain size information for the V-type asteroids and HED analysed in this work. The whole procedure can be schematized as follows: Retrieve the mineralogy of each V-type asteroid dataset, by comparing them with HED in BCI versus BCII scatterplot (Figure 1) Retrieve the grain size regolith range of each V-type asteroid dataset, by comparing them with HED in BDI versus BDII and HWHM I dx versus BDII scatterplot (Figure 2, Figure 3) Application of the Hapke’s bidirectional reflectance model to the spectral deconvolution of the selected asteroids and meteorites. The used end-members in the Hapke retrieval are clinopyroxene, orthopyroxene and plagioclase [8]. We assume that the scattering is isotropic and no backscattering is considered. In order to simulate HED mineralogy, abundance limits on fractional mass of pyroxene (40% to 100%) and of plagioclase (0% to 60 %) were set in the Hapke retrieval. In addition, the grain size for each mixture was varied (in step of 5 µm) from 5 µm up to a limit of 80 µm, in agreement with scatterplot analysis (Figure 2, Figure 3). Figure 1: Band I Center versus Band II Center scatterplot of HED and V-type group 1 and group 2. For the entire legend see [9] . Figure 2: BDI versus BDII scatterplot on Eucrite, Howardite and V-type group 1 and group 2. Figure 3: HWHM I DX vs BDII scatterplot on Eucrite, Howardite and V-type group 1 and group 2. 3. Results and conclusions Through the scatterplot analysis (Figure 1) we observed that the asteroids have the same mineralogy of HED (same trend line) and an upper limit of grain size of 80 µm. Basing on these results, we applied Hapke retrieval on asteroid sample using the main endmembers of HED (plagioclase, clinopyroxene and orthopyroxene) and a fixed grain size range (Figure 2, Figure 3) to simulate spectra. In order to find the best fit, a R2 statistical test was performed. Hapke procedure tested on HED spectra with known composition from [8] gave a global R2 with a mean value of 0.92. The 86 % of grain size, resulting from Hapke model on HED (with known and unknown composition [8]), lied in the declared range . The Hapke procedure on asteroids sample had a R2 mean value of 0.97, in addition a mean grain size of ≈20 µm is given. This is in agreement with previous analyses. So, our Hapke implementation, basing on the results of scatterplot analysis, is able to reproduce the spectral shapes and absorption features for all the V-type asteroids examined here, providing a globally satisfactory fit, confirming a link between V-type asteroids, HED meteorites and Vesta. References [1] M. C. De Sanctis, Meteoritics & Planetary Science 48, n. Nr 11, p. 2166–2184, 2013. [2] F. E. DeMeo, Icarus 202, p. 160–180, 2009. [3] N. A. Moskovitz, Icarus 208, p. 773–788, 2010. [4] P. S. Hardersen, «,» The Astronomical Journal, 156:11, p. 16, 2018. [5] P. S. Hardersen, The Astrophysical Journal Supplement Series, 221:19, p. 12, 2015. [6] P. S. Hardersen, Icarus 242, p. 269–282, 2014. [7] E. Palomba, Icarus 240, p. 58–72, 2014. [8] D. W. Mittlefehldt, Chemie der Erde 75, pp. 155-183, 2015. [9] M. Angrisani, «Master degree thesis,» Università Federico II, 2021.

Abstract The Dawn spacecraft orbited the dwarf planet (1) Ceres from March 2015 to October 2018. This allowed the Visible InfraRed spectrometer (VIR) [1] to achieve a nearly global hyperspectral mapping of the surface. Here we present, through several spectral indices, the characteristics of the Ceres’ surface as observed by VIR in the visible wavelengths.Introduction The spectral signatures observed in the infrared range by VIR reveal the presence of (Mg-, NH4)-phyllosilicates and (Mg, Ca)-carbonates all over the surface of Ceres [2, 3, 4, 5]. Localized spots of Na-carbonates have been detected [5] and an organic rich material patch has been identified [6]. Buried ice is present in the Ceres subsurface [7] with several exposures of water ice visible on the surface [8]. The mineralogical composition of Ceres shows that the surface experienced a pervasive and lasting aqueous alteration. The surface of Ceres is very dark – reflectance of 0.034 at 550nm – due to the presence of a yet unidentified dark material. However, high albedo features, so-called “bright spots”, are widespread over the surface and mostly associated with crater peaks, rims or ejecta [9]. Furthermore, some other features of endogenous origin, like Ahuna Mons or the Occator faculae [10, 11], also show a high albedo.While the surface of Ceres has been thoroughly studied in the infrared range, data coming from the VIR visible channel suffered from an instrumental artefact which prevented a detailed analysis in the 400-1000 nm spectral range. This issue has been recently solved [12] and here we analyse the Ceres’ surface with diverse spectral parameters.Data and methods We analyse the VIR visible data from 400 nm to 950 nm (Fig. 1), taking advantage of all of the available corrections: denoising [13], spectral shape correction [14], photometric correction [15], and finally a correction that involved the detector’s temperature [12]. Then, by merging the data from four different VIR mission phases, we were able to build an 8 million pixels dataset, covering the surface of Ceres from 75°N to 60°S. We build maps of calibrated radiance factor at 550nm, of two RGB composites and three spectral slopes. This allows us to study the variation of albedos, colours and spectral behaviour across the surface of Ceres. In addition, we make those maps available for the community through the free software Aladin.Figure 1 - Median spectra of Ceres in the visible range with the spectral slope parameters used in this study.Map of the spectral parameters We observed that the RGB ratio composite, which highlights colour variations, and the spectral slope between 480 nm and 800 nm (Fig. 2), provide similar results, with impact craters and associated ejecta which appear spectrally flat to blue over a globally red surface. Some peculiar places are also well visible with those spectral maps, e.g. Ahuna Mons, Occator faculae or the organics-rich material at Ernutet crater.Figure 2 - Ceres global map of the spectral slope between 480nm and 800nm.We will also present the spectral slope between 405 nm and 480 nm which characterizes the steep slope observed in that range and present interesting regional variations. The location and the shape of the slope 405-480 nm make us suggesting that this could be a proxy to follow an absorption due to the O2- -> Fe3+ charge transfer in a mineral which remain nevertheless unidentified with our data.Conclusion The investigation of the Ceres surface at visual wavelengths reveals different spectral behaviours. While no absorption bands are revealed in the visible range, the spectral slope 405-480nm could be a proxy to follow a O2- -> Fe3+ charge transfer feature. On the other hand, the colours and the slope between 480nm and 800nm highlight impact craters and their ejecta as their composition or the physical properties could have been modified through different processes [16,17].References [1] De Sanctis, M. C., Coradini, A., et al. 2011, Space Science Reviews, 163, 329[2] De Sanctis, M. C., Ammannito, E., Raponi, A., et al. 2015, Nature, 528, 241[3] De Sanctis, M. C., Raponi, A., Ammannito, E., et al. 2016, Nature, 536, 54[4] Ammannito, E., DeSanctis, M. C., Ciarniello, M., et al. 2016, Science, 353, aaf4279[5] Carrozzo, F. G., De Sanctis, M. C., Raponi, A., et al. 2018, Science Advances, 4, e1701645[6] De Sanctis, M. C., Ammannito, E., McSween, H. Y., et al. 2017, Science, 355, 719[7] Prettyman, T. H., Yamashita, N., Toplis, M. J., et al. 2016, Science, 355, 55[8] Combe, J.-P., McCord, T. B., Tosi, F., et al. 2016, Science, 353, aaf3010[9] Stein, N., Ehlmann, B., Palomba, E., et al. 2019, Icarus, 320, 188[10] Zambon, F., Raponi, A., Tosi, F., et al. 2017, Geophysical Research Letters, 44, 97[11] Scully, J. E., Bowling, T., Bu, C., et al. 2019, Icarus, 320, 213[12] Rousseau, B., Raponi, A., Ciarniello, M., et al. 2019, Review of Scientific Instruments, 90[13] Carrozzo, F. G., Raponi, A., De Sanctis, M. C., et al. 2016, Review of Scientific Instruments, 87, 124501[14] Rousseau, B et al., submitted[15] Ciarniello, M., De Sanctis, M., Ammannito, E., et al. 2017, Astronomy & Astrophysics [arXiv:1608.04643][16] Stephan, K., Jaumann, R., Krohn, K., et al. 2017, Geophysical Research Letters[17] Schröder, S., Mottola, S., Carsenty, U., et al. 2017, Icarus, 288, 201

Abstract From July 2011 to September 2012, the Dawn spacecraft orbited the large asteroid (4) Vesta. A nearly global coverage of the surface was achieved by the Visible and InfraRed mapping spectrometer (VIR) [1]. The change of CCD temperature during the data acquisition affects the instrument spectral response. As a consequence, the acquired spectra experienced a change in spectral slope. Here we present a method to correct this issue; this method is similar to the one we applied for the VIR VIS data of Ceres [2]. CCD detector temperature dependencies The acquisition of VIR data are organized in sequences during which several hyperspectral cubes are acquired. Within a given sequence, which may last several hours, the CCD temperature increases over time, while it goes back down in the time period between two sequences. The increase of CCD temperature induces a reddening of the spectra at visible wavelengths, as displayed in Fig. 1. To avoid misinterpretation and to retrieve a coherent shape of the Vesta spectra, a correction is therefore required. Figure 1 – Median spectral variations observed for each CCD temperature recorded during the VH2 mission phase (normalized at 550nm). Spectral correction As in [2], we defined a correction factor for the different mission phases at Vesta, considering their particularities if necessary. This correction factor requires the definition of a reference temperature, at which the spectral response is considered reliable, and a corresponding reference spectrum. A previous analysis of the same effect on Ceres observations [2], revealed a reference CCD temperature of 177K. The correction has been applied on the mission phases during which VIR acquired VIS data. It allows to obtain a globally coherent dataset. Mapping and reliable spectral studies are then possible (see Fig. 2). Figure 2 - Map of the spectral slope for the VH2 mission phase before (top) and after (bottom) the correction. The gradient observed, in each sequences, in the top panel vanishes in the bottom panel, after the application of the correction. Conclusion The empirical method developed in [2] has now been applied to the Vesta data acquired by the VIS channel of the VIR spectrometer. This correction is mandatory to carry out reliable analysis of the Vesta surface at global and local scale. References [1] De Sanctis, M. C., Coradini, A., et al. 2011, Space Science Reviews, 163, 329, https://doi.org/10.1007/s11214-010-9668-5 [2] Rousseau, B., Raponi, A., Ciarniello, M., et al. 2019, Review of Scientific Instruments, 90, arXiv:1912.09136

We developed a fast algorithm to model a rough planetary surface by means of a statistical multi-facets approach. Consistently with Hapke theory (2012) we model the surface with a distribution of non-spatially resolved facets, being the distribution of their slopes completely described by roughness parameter

Like the Moon and Mercury, Ceres’s rotation axis is inclined by just a few degrees. The obliquity of 4 degrees between the normal to the orbital plane and the rotational axis of Ceres implies that Ceres' poles terrains are always slightly angled toward the sun. The combination of low angle illumination over a cratered surface creates areas never reached by the sun. In the presence of ice, these permanently shadowed regions (PSRs) create “cold traps” protecting ice from sublimation. In 2016 Platz et al. [1] explored the topography and imagery [2,3] of the northern polar regions of Ceres, looking into 634 craters in permanent shadows, finding bright deposits in 10 of them. Out of this, 1 crater has part of the deposits exposed, so that it has been possible to detect the signature of water thanks to the data returned by Dawn’s mapping spectrometer VIR [4]. The last phase of the Dawn mission, which ended in 2017 collected enough imagery to reconstruct the topography of the southern pole of Ceres by stereophotogrammetry [Preusker]. Since then, no evidence of icy terrain from the south polar terrain of Ceres has been reported. Our study will explore the PSRs in this region for a better understanding of the ice distribution on the dwarf planet. The first step of our study is to locate and characterize the southern PSRs. From the topography, we computed the PSRs using the Integrated Software for Imagers and Spectrometers (ISIS3) version 4.8.3, a suite of applications for the rigorous spatial treatment of planetary data acquired by various missions/instrument, including NASA Dawn at Vesta and Ceres [5]. Figure 1 shows the map of PSRs of the southern terrain of Ceres in stereographic projection, latitude grids are spaced by 10 degrees, central is at the top. The PSRs are reported in blue shades corresponding to their area (see legend), over the shaded relief extracted from the topography. The hatched area at the south pole locates the place where stereo reconstruction of topography was not possible and was excluded from PSRs computation. We have located about 950 PSRs with areas ranging from 1 to over 500 square kilometers. The largest PSRs locates in an unnamed 75 km wide crater at 223.6°E 74.9°S. Figure 2 shows the distribution of the sizes of PSRs. Most of PSRs are in areas smaller than 10 square km. The map of PSRs represents the base for observations into the dark areas with Framing Camera and possibly the VIR spectrometer, looking for sings on icy deposits in the south polar regions of Ceres. Figure 1: The PRSs maps of the south polar terrain of Ceres (stereographic polar projection over a spheroid with a radius of 470km, prime meridian is at top and latitude grid are spaced by 10 degrees). The blue patches are the area permanently in shadow and color indicates the area in square kilometers. The etched pattern indicates the area without topographic data. Figure 2: The frequency distribution of the areal extents of PSRs, which ranges from less than a 1 to more than 500 square kilometers. References [1] Platz, T. (2017) Nature Astronomy, 1, 7 [2] Sierks, H. et al. The Dawn Framing Camera. Space Sci. Rev. 163, 263–327 (2011). [3] Preusker, F., (2016) LPSC 48 Abstract #1954 [4] De Sanctis, (2011) Space Science Reviews, 163, 1-4,329-369 [5] Becker, K.J., (2013) LPSC 44 Abstract #2829

We study the surface of Ceres at visible wavelengths, as observed by the Visible and InfraRed mapping spectrometer (VIR) onboard the Dawn spacecraft, and analyze the variations of various spectral parameters across the whole surface. We also focus on several noteworthy areas of the surface of this dwarf planet. We made use of the newly corrected VIR visible data to build global maps of a calibrated radiance factor at 550 nm, with two color composites and three spectral slopes between 400 and 950 nm. We have made these maps available for the community via the Aladin Desktop software. Ceres surface shows diverse spectral behaviors in the visible range. The color composite and the spectral slope between 480 and 800 nm highlight fresh impact craters and young geologic formations of endogenous origin, which appear bluer than the rest of the surface. The steep slope before 465 nm displays very distinct variations and may be a proxy for the absorptions caused by the $O_2^{-}$ -> $Fe^{3+}$ or the $2Fe^{3+}$ -> $Fe^{2+} + Fe^{4+}$ charge transfer, if the latter are found to be responsible for the drop in this spectral range. We notice several similarities between the spectral slopes and the abundance of phyllosilicates detected in the infrared by the VIR, whereas no correlation can be clearly established with carbonate species. The region of the Dantu impact crater presents a peculiar spectral behavior (especially through the color and the spectral slope before 465 nm) suggesting a change in composition or in the surface physical properties that is not observed elsewhere on Ceres., 22 pages, 14 + 5 figures. Published online on Astronomy and Astrophysics on 07-10-2020