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: 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.

The Nadir and Occultation for MArs Discovery (NOMAD, Neefs et al., 2015) instrument suite on board the Exomars Trace Gas Orbiter (TGO) spacecraft is capable to observe the Martian atmosphere at high spectral resolution with different observing modes. The data acquired in nadir observing geometry by the infrared Limb, Nadir and Occultation (LNO) channel of NOMAD are characterized by a signal to noise ratio (SNR) that is mostly limited by the instrument’s operative temperature, in turn impacting integration times. In this study we apply to LNO nadir data the Minimum Noise Fraction (MNF) technique (Green et al., 1988; Lee et al., 1990; Boardman and Kruse, 1994), usually adopted to enhance the SNR of remotely sensed hyperspectral imaging datasets (e.g. Lee et al., 1990; Amato et al, 2009; Bjorgan and Randeberg, 2015; Luo et al., 2016). In practice, the MNF projects the original data in a space in which the noise component is minimized. Such a projection is achieved by means of two consecutive Principal Component (PC) transforms (see Jolliffe and Cadima, 2016, for a comprehensive review) providing eigenvalues for the data reconstruction that are ordered with increasing noise. As first step of the analysis, we perform tests on ensembles of synthetic spectra in order to evaluate the theoretical performances of the technique in different frameworks of analysis. For example, we investigate the application of the MNF on spectral features characterized by different depth, width, correlation with other bands, and spatial dependencies, verifying that all these factors impact its effectiveness. Then, we evaluate the MNF performances on specific LNO spectral orders, in order to assess the SNR improvement for studies related to the Martian surface and aerosols/clouds (e.g. Oliva et al., 2022) and to trace gases. A limiting factor in the SNR enhancement is the presence of systematic noise linked to spectral artifacts introduced by the MNF itself. In order to remove these artifacts, the number of transform eigenvalues, and hence the noise, needs to be increased in the reconstruction of the denoised observations. Nevertheless, as result from this preliminary analysis, we report an average SNR improvement of about 20% and reaching a maximum of 50%. Acknowledgements ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (The Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-BI00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. References Amato, U., et al., 2009. IEEE Geosci. Remote Sens. Lett. 2009, 47, 153–160. Bjorgan, A. and Randeberg, L.L., 2015. Sensors 2015, 15, 3362-3378. Boardman, J.W. and Kruse, F.A., 1994. ERIM, Ed., Proc. 10th Thematic Conference on Geological Remote Sensing, San Antonio, 407-418. Green, A.A., et al., 1988. IEEE Transactions on Geoscience and Remote Sensing, Vol. 26, No. 1, pp. 65–74. Jolliffe, I.T., and Cadima, J., 2016. Phil. Trans. R. Soc. A, 374:20150202.20150202 Lee, J.B., Woodyatt, A.S. and Berman, M., 1990. IEEE Transactions on Geoscience and Remote Sensing, Vol. 28, No. 3, pp. 295–304. Luo, G., et al., 2016. Canadian Journal of Remote Sensing, 42:2, 106-116. Neefs, E., et al., 2015. Appl. Opt. 54, 8494–8520. Oliva, F., et al., 2022. Journal of Geophysical Research: Planets, 127, e2021JE007083.

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.

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.

The OMEGA spectrometer on board Mars Express acquired several observations of the Martian limb that are still largely unexploited. Here, we explore the information content of these data in terms of abundance and size of the Martian atmospheric dust. We approach the dust retrievals by applying a full spherical multiple scattering Monte Carlo 1D radiative transfer code to model the 0.5–2.5 μm spectral range (VNIR and SWIR OMEGA channels), in order to derive dust effective radius and number density variation with altitude, between approx. 8 and 50 km. This is the first time this approach is applied to OMEGA limb data; therefore, we only present three study cases where water ice is below the detectability level, in order to focus on the methodological issues, assumptions, and performances before future more extensive applications. The model fully includes multiple scattering effects, which are known to be responsible for the coupling between the limb spectra taken at different altitudes and the surface. An extended three-dimensional modelling of the surface reflectance, acting as surface-shine for limb spectra, is developed. The VNIR channel is found useful in reducing the degeneracy of the radiative transfer solutions. The derived dust vertical distributions, yielding dust effective radii of the order of 0.85 ± 0.15 μm (corresponding to a modal radius rm of ~0.3 μm) between 15 and 30 km are quite in agreement with Global Circulation Models (GCMs), but the abundances lie about one order of magnitude above model predictions, as previously reported from comparison between models and Mars Climate Sounder (MCS) data. An overall agreement is in fact found with MCS data, and in one case the OMEGA-retrieved dust is compatible with a local storm rising from the Hellas basin. Such storms are not well represented in the Mars Climate Database, which provides monthly mean statistics. Our results demonstrate the capability of OMEGA limb data to quantitatively contribute to Martian dust studies, despite the complex and slow radiative transfer computation scheme required to accurately model multiple scattering in the probed spectral range. The desirable application of the retrieval method to the whole OMEGA limb dataset, also expected to help assess the occurrence of local dust storms, requires further work aimed to include water ice aerosols and possibly thermal emission. Being the first attempt to use a Monte Carlo modelling approach to OMEGA limb data, this work represents a useful benchmark for validating faster, although less accurate, radiative transfer models.

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

In this work we analyze ExoMars/TGO NOMAD spectrometer [1] nadir data. We exploit both the ultraviolet-visible UVIS channel and the infrared LNO channel to obtain information about Martian dust densities and grains sizes. For the analysis we apply the MITRA radiative transfer tool [2,3,4] to all the spatially and temporally coincident UVIS and LNO 2018 data, also covering a global dust storm. We investigate possible correlations between the results and the local time of the observations.IntroductionStudies focused on Mars 2018 global-scale dust event, as observed by the instruments on board the TGO spacecraft, demonstrate that Martian trace gases abundance and distribution is strongly affected by the presence of dust [5,6,15]. Airborne dust drives Mars’ thermal structure and climate [7] by heating and cooling the lower atmosphere through absorption at VIS-NIR wavelengths [8] and emission in the IR range respectively [9,10,8,11]. These mechanisms affect, for example, the water-ice clouds formation and can drive convection, leading to variations of water vapor abundances [6,16]. In general, the understanding of dust properties is mandatory for the investigation of Martian trace gases vertical distribution.Instrument and ObservationsThe NOMAD spectrometer has three channels covering the ultraviolet/visible (UVIS channel) and the infrared (LNO and SO channels) spectral ranges. While the instrument main focus is the study of trace gases, it can also be exploited to study atmospheric particulate. In this work, we take advantage of the combined UVIS and LNO channels data, acquired in nadir geometry, to study Martian dust microphysical properties. We analyze the whole 2018 dataset taking into account all observations that are spatially and temporally coincident among the two NOMAD channels, in order to build a dataset spanning from the ultraviolet/visible to the infrared spectral range.MethodWe use the MITRA radiative transfer model and inversion algorithm to retrieve dust microphysical properties from the selected dataset. The use of the combined ultraviolet/visible and infrared ranges, covered by UVIS and LNO NOMAD channels together, is required in order to separate the information related to the dust density from that of the grains sizes. However, if no coincident LNO observations are available for a certain UVIS orbit, we still apply the MITRA tool to the UVIS spectral range alone to obtain the dust integrated optical depth. We use only LNO orders covering the wavelength range 2.20-2.65 μm, since they are mostly affected by narrow and isolated gaseous absorption lines and, hence, they can be exploited to study the intensity of the spectral continuum. We take the temperature-pressure profiles from the Mars Climate Database (MCD, [12]) and use the dust optical constants from [13,14]. Surface albedo spectra are derived using an approach based on the SAS method [15] applied to the MEx/OMEGA dataset. The results of the retrieval are investigated for the identification of eventual trends of density and grains sizes with local time or other observing parameters.SummaryThe presented method allows to study Martian dust microphysical properties taking advantage of the combined UVIS and LNO NOMAD channels datasets. Given the reduced spatial coverage of the coincident data, we focus our analysis on the investigation of possible trends in the results with the observations local time. Once validated on 2018 data, the method is planned to be extended to the whole NOMAD dataset.AcknowledgementsExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-BI00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency.References:[1]Neefs, E., et al, 2015. Appl. Opt. 54, 28, 8494-8520.[2]Oliva, F., et al, 2016. Icarus 278, 215-237.[3]Sindoni, G., et al, 2013. EPSC2013.[4]Oliva, F., et al, 2018. Icarus 300, 1-11.[5]Daerden, F., et al, 2019. Icarus 326, 197-224.[6]Vandaele, A.C., et al, 2019. Nature 568, 521-525.[7]Kahre, M.A., et al, 2008. Icarus 195, 576-597.[8]Korablev, O. ,et al, 2005. Adv. Space Res. 35, 21–30.[9]Gierasch, P.G., Goody, R.M., 1972. J. Atmos. Sci. 29, 400–402.[10]Pollack, J.,et al, 1979. J. Geophys. Res. 84, 2929–2945.[11]Määttänen, A., et al, 2009. Icarus 201, 504-516.[12]Millour, E., et al, 2018. From Mars to Exomars, 2018.[13]Wolff, M.J., et al, 2009. J. Geophys. Res., 114, E9. [14]Wolff, M.J., et al, 2010. Icarus, 208.[15]Geminale, A., et al, 2015. Icarus 253, 51-65.[16]Aoki, S., et al. 2019. J. Geophys. Res.: Planets,124, 3482-3497.

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.

Impact craters of different geological ages, sizes and morphologies are not only the most obvious surface features on Ceres’ surface. The investigation of their spectral properties in combination with Ceres’ geology and topography reveals not only lateral compositional variations in Ceres’ surface material but also possible stratigraphic differences within Ceres’ crust. Spectral properties of impact craters with different ages do show distinct trends implying variations with increasing exposure duration of the impact material onto Ceres’ surface. Local concentrations of H2O ice and carbonates are associated with the youngest, either recently emplaced or excavated, surface deposits. On the contrary, regionally higher amounts of ammoniated phyllosilicates originate from deeper regions of Ceres’ crust and strengthen the theory of ammonia being a primordial constituent of Ceres. The blue spectral slope, clearly associated with relatively weak absorptions of OH-bearing and/or ammoniated phyllosilicates, is limited to fresh impact material. Either, the blue spectral slope diminishes slowly with increasing geologic age due to space weathering processes, or shortly as a result of gravitation-induced slumping, forming a fine and loosely consolidated regolith.

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.

This work takes advantage of the NOMAD spectrometer observations, on board the 2016 ExoMars Trace Gas Orbiter. ExoMars is an ESA-Roscosmos joint mission consisting of a rover and an orbiter (Trace Gas Orbiter - TGO). The Nadir and Occultation for Mars Discovery (NOMAD) is one of the four instruments on board TGO. The instrument is a suite of three spectrometers designed to observe the atmosphere and the surface of Mars in the UV, visible and IR. For this study, the Limb, Nadir and Occultation (LNO) channel, operating in the IR, is selected [1][2]. We focus on specific signatures in the [2.3 - 3.8 μm] range of NOMAD-LNO in order to study the possible detection of clouds at these wavelengths in the infrared.For this study, we have selected the order 169 ([2611.8 nm - 2632.7 nm]) located in the vicinity of 2.7 µm CO2/H2O ices absorption band. We search for the presence of ice clouds in MY 34 (LS = 150° - 360°) and MY 35 for observations with a solar zenith angle below 80 degrees. The detection method is adapted from Bellucci et al., 2019 [3] and L. Ruiz Lozano et al., 2020 [4]. The initial results indicate a number of detections in the Tharsis region consistent with the known ‘W’ clouds [6][7]. Finally, these results will be compared with the NOMAD-UVIS observations ([230 nm - 310 nm]) obtained at the same TGO orbits.AcknowledgementsThe NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME) and the FRIA, with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0. References[1] A.C. Vandaele et al., 2015. Optical and radiometric models of the NOMAD instrument part I: the UVIS channel. Optics Express, 23(23):30028–30042.[2] E. Neefs et al., 2015. NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 1—design, manufacturing and testing of the infrared channels. Applied optics, 54(28):8494–8520.[3] G. Bellucci et al., 2019. TGO/NOMAD Nadir observations during the 2018 global dust storm event, EPSC-DPS 2019[4] L. Ruiz Lozano et al., 2020. Use of TGO-NOMAD nadir observations for ice detection, EPSC Abstracts, Vol. 14, Virtual EPSC 2020, EPSC2020-748. [5] M. Vincendon, et al., 2011. New near‐IR observations of mesospheric CO2 and H2O clouds on Mars, J. Geophys. Res., 116, E00J02, doi:10.1029/2011JE003827.[6] J. L. Benson, et al., 2003. The seasonal behavior of water ice clouds in the Tharsis and Valles Marineris regions of Mars: Mars Orbiter Camera Observations, Icarus, Volume 165, Issue 1, 2003, Pages 34-52, ISSN 0019-1035, https://doi.org/10.1016/S0019-1035(03)00175-1.

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

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

Data acquired at Ceres by the visible channel of the Visible and InfraRed mapping spectrometer (VIR) on board the NASA Dawn spacecraft are affected by the temperatures of both the visible (VIS) and the infrared (IR) sensors, which are respectively a CCD and a HgCdTe array. The variations of the visible channel temperatures measured during the sessions of acquisitions are correlated with variations in the spectral slope and shape for all the mission phases. The infrared channel (IR) temperature is more stable during the acquisitions, nonetheless it is characterized by a bi-modal distribution whether the cryocooler (and therefore the IR channel) is used or not during the visible channel operations. When the infrared channel temperature is high (175K, i.e. not in use and with crycooler off), an additional negative slope and a distortion are observed in the spectra of the visible channel. We developed an empirical correction based on a reference spectrum for the whole data set; it is designed to correct the two issues related to the sensor temperatures that we have identified. The reference spectrum is calculated to be representative of the global Ceres' surface. It is also made of data acquired when the visible and infrared channel temperatures are equal to the ones measured during an observation of the Arcturus star by VIR, which is consistent with several ground-based observations. The developed correction allows reliable analysis and mapping to be performed by minimizing the artifacts induced by fluctuations of the VIS temperature. Thanks to this correction, a direct comparison between different mission phases during which VIR experienced different visible and infrared channel temperatures is now possible., Comment: The following article has been accepted by Review of Scientific Instruments on 24 November 2019. After it is published, it will be found at this link: https://publishing.aip.org/resources/librarians/products/journals/

Thermal inertia is a key information to quantify the physical status of a planetary surface. We derive the thermal inertia of the surface of Ceres using spatially resolved data from the Dawn mission. For each location, this quantity can be constrained by comparing theoretical and observed diurnal temperature profiles from retrieved temperatures. We calculated Ceres's surface theoretical temperatures with a thermophysical model that provides temperature as a function of thermal conductivity and roughness, and we determined the values of those parameters for which the best fit with the observed data is obtained. Our results suggest that the area of crater Haulani displays thermal inertia values (up to 130-140 J·m-2·s-½·K-1) substantially higher than the very low to low values (from 1-15 to 50-60 J·m-2·s-½·K-1) derived for the overall surface of Ceres. The results are more ambiguous for the bright faculae located in the floor of crater Occator.

Context.Particulate surfaces exhibit a surge of reflectance at low phase angles, a phenomenon referred to as the opposition effect (OE). Two mechanisms are recognized as responsible for the OE: shadow hiding (SH) and coherent backscattering. The latter is typically characterized by a small angular width of a few degrees at most and according to the theoretical prediction should exhibit wavelength and albedo dependence.Aims.We characterize the OE on the surface of Ceres using Dawn Visible InfraRed mapping spectrometer hyperspectral images at low phase angles. Furthermore, this dataset, coupled with previous observations, allows us to perform a complete spectrophotometric modeling at visual-to-infrared (VIS-IR) wavelengths (0.465–4.05μm) in the broad phase angle range ≈0°−132°.Methods.We applied Hapke’s theory to the average phase curve for Ceres. Disk-resolved properties of the OE were investigated through an empirical model.Results.Across the investigated phase angle interval, Ceres’ average phase curve exhibits a smaller back-scattering contribution for increasing wavelengths. This determines a progressive spectral reddening at larger phase angles that we hypothesize as being related to the effect of submicron roughness on the grain surface. In the OE region, the shape of the phase curves is fairly constant across the VIS range and no sharp opposition surge at very small phase angles (α< 2°) can be recognized. This would suggest a major contribution from SH to Ceres’ OE. Assuming SH as the dominant mechanism, from the OE angular width we infer a high surface porosity (≈0.9), which appears in good qualitative agreement with Ceres’ low thermal inertia. Thanks to the OE observations we derive Ceres’ VIS-IR geometric albedo with a reference value at 0.55μm of 0.098 ± 0.007. Mapping of the VIS normal albedo and OE angular width across a portion of the surface of Ceres does not reveal a spatial correlation between these quantities, consistent with SH dominating in theα= 0°−7° interval. The comparison of Ceres’V-band magnitude curve with that of other asteroids indicates that Ceres’ OE is typical of a low-albedo object and compatible with the C-class type.

This work describes the correction method applied to the dataset acquired at the asteroid (4) Vesta by the visible channel of the visible and infrared mapping spectrometer. The rising detector temperature during data acquisitions in the visible wavelengths leads to a spectral slope increase over the whole spectral range. This limits the accuracy of the studies of the Vesta surface in this wavelength range. Here, we detail an empirical method to correct for the visible detector temperature dependency while taking into account the specificity of the Vesta dataset., Comment: 6 pages, 5 figures and 1 table excluding supplementary material

The surface and internal structure of Ceres show evidence of a global process of aqueous alteration, indicating the existence of an ocean in the past. However, it is not clear whether part of this ocean is still present and whether residual fluids are still circulating in the dwarf planet. These fluids may be exposed in a geologically young surface, and the most promising site to verify the occurrence of present fluids on Ceres is Cerealia Facula dome, in Occator crater. This very young facula exhibits minerals that are relatively rare in our Solar System, the formation of which requires the presence of liquid water in combination with hydrothermal activity. Here we report the discovery of hydrated sodium chloride on Cerealia Facula. These newly identified chloride salts are concentrated on the top of the dome, close to a system of radial fractures. The spatial distribution of the hydrated phase suggests that chloride salts are the solid residue of deep brines that reached the surface only recently, or are still ascending. These salts are very efficient in maintaining Ceres’s warm internal temperature and lowering the eutectic temperature of the brines, in which case ascending salty fluids may exist in Ceres today. Spectroscopic data obtained at high spatial resolution from Dawn detected the presence of fresh chloride salts at Cerealia Facula on Ceres. The spatial distribution of the hydration level of these salts suggests that they surfaced a maximum of a few centuries ago and that the upwelling of salty fluids may still be active.

Ceres is the largest object in the main belt and is a wet body with a complex geological and chemical history. Its surface is composed of opaque materials, phyllosilicates, ammoniated-bearing minerals, carbonates, water ice, and salts. Recently, aliphatic organics, whose origin is still uncertain, have also been detected on the Ceres surface by the imaging spectrometer Visible and Infrared Spectrometer (VIR) on board Dawn. Here, using VIR spectra, acquired with a high spatial resolution (< 100 m), we have analysed the organic-rich areas, their spectral characteristics, and the associated mineralogy. The new VIR spectra in the 3.2-3.6 µm spectral range have been compared to organic standard compounds measured in the laboratory, to investigate more deeply the nature of the organic matter on Ceres. The mineralogy appears quite complex, and the organic matter is often associated with a larger amount of ammoniated phases and carbonates. The detailed spectral analysis, as well as the laboratory comparison, reveal the presence of a band at 2.99-3 μm, likely associated with organic or inorganic NH-compounds (amine or salts), and the organic matter may be characterized by a rather low abundance of oxygen atoms. Spectral models of the organic material indicate that hundreds of meters-size areas can be very rich in aliphatic organics, with an estimated organic amount larger than the quantity normally found in CC meteorites. The mineralogical context and the retrieved amounts suggest that the aliphatic organic is likely a Ceres' endogenous product, and we formulate a hypothesis for the formation and evolution of the organic matter inside Ceres.

NASA’s Dawn spacecraft acquired images and hyperspectral data of Ceres by means of FC and VIR instruments, and identified some widespread bright areas or bright spots (BS). The most peculiar BS is inside Occator crater and it is characterized by spectral properties very dissimilar from the rest of Ceres’ surface. To perform a mineralogical analysis, absorption bands in reflectance spectra must be properly isolated by removal of continuum, and related descriptors (such as band centers and band depths) can be computed. The method for continuum removal must be applicable to all Ceres spectra, relative to different areas, so that a comparison among spectral parameters can be made and mineralogical interpretation can be achieved. This work focuses on the definition of the most appropriate continuum to isolate absorption bands located at ∼3.1, ∼3.4 and ∼4.0 µm. The ∼3.1 µm feature is related to ammoniated phyllosilicates, while the ∼3.4 and ∼4.0 µm absorption bands are indicative of carbonates. Thermal emission affects the continuum for these bands in the VIR spectral range, which extends up to 5.1 µm, moreover all thermal-removed reliable data stop at 4.2 µm. This implies that the shoulder of bands at longer wavelength cannot be identified. We therefore defined alternative continua, i.e., a linear and two polynomial ones, able to describe spectra of any area (i.e. bright or dark) and regardless of spatial resolution. We found that the linear definition satisfies these requirements best. For the first time, we performed an error evaluation on band depths and band centers introduced by the applied method, which is relevant for comparison of spectral parameters of Ceres regions and to better interpret mineralogy and photometric effects.

Thermal inertia is a key information to quantify the physical status of a planetary surface; it can be retrieved by comparison between theoretical and observed temperature diurnal profiles. We have calculated the surface temperature for a set of locations on Ceres' surface with a thermophysical model that provides temperature as a function of thermal conductivity and roughness, and we have determined the values of those parameters for which the best fit with the observed data is obtained. The observed temperatures have been retrieved form spatially-resolved data from the Dawn mission. In our previous work [Rognini et al., 2019], we have found that the average thermal inertia for the overall surface of Ceres is low (from 1 to 15 to 60 ​J ​m−2 ​s−½ K−1), as expected according to the general trend observed in the Solar System for atmosphere-less bodies, while the thermal inertia of the very bright faculae found in the floor of the Occator crater could not be well defined. Using more recently acquired VIR high resolution data we find that the central part of the Cerealia facula displays a thermal anomaly ( ~ 10 K above the average) compatible with a higher thermal inertia with respect to the surrounding regions, while the Vinalia facula does not display any consequently could have a grain size comparable with the Ceres’ surface average.

Ahuna Mons is the highest mountain on Ceres. A unique complex in terms of size, shape, and morphology, Ahuna is bordered by flanks of the talus around its summit. Recent work by Ruesch et al. [2016] based on Dawn's Framing Camera images shed light on the possible origin of Ahuna Mons. According to Ruesch et al. [2016], Ahuna Mons is formed by a volcanic process involving the ascent of cryomagma and extrusion onto the surface followed by dome development and subsequent spreading. Here we analyzed in detail the composition of Ahuna Mons, using data acquired by the Visible and InfraRed spectrometer aboard Dawn. The spectral analysis reveals a relatively high abundance of carbonates and a non-homogeneous variation in carbonates composition and abundance along Ahuna's flanks, associated with a lower amount of the Ceres's ubiquitous NH4-phyllosilicates over a large portion of the flanks. The grain size is coarser on the flanks than in the surrounding regions, suggesting the presence of fresher material, also compatible with a larger abundance of carbonates. Thermal variations are seen in Ahuna, supporting the evidence of different compactness of the surface regolith in specific locations. Results of the spectral analysis are consistent with a possible cryovolcanic origin which exposed fresher material that slid down on the flanks.

A variety of hydrothermal environments have been documented in terrestrial impact structures. Due to both past water interactions and meteoritic bombardment on the surface of Mars, several authors have predicted various scenarios that include the formation of hydrothermal systems. Geological and mineralogical evidence of past hydrothermal activity have only recently been found on Mars. Here, we present a geological and mineralogical study of the Auki Crater using the spectral and visible imagery data acquired by the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), CTX (Context Camera) and HiRISE (High Resolution Imaging Science Experiment) instruments on board the NASA MRO mission. The Auki Crater is a complex crater that is ∼38 km in diameter located in Tyrrhena Terra (96.8°E and 15.7°S) and shows a correlation between its mineralogy and morphology. The presence of minerals, such as smectite, silica, zeolite, serpentine, carbonate and chlorite, associated with morphological structures, such as mounds, polygonal terrains, fractures and veins, suggests that the Auki Crater may have hosted a post impact-induced hydrothermal system. Although the distribution of hydrated minerals in and around the central uplift and the stratigraphic relationships of some morphological units could also be explained by the excavation and exhumation of carbonate-rich bedrock units as a consequence of crater formation, we favor the hypothesis of impact-induced hydrothermal circulation within fractures and subsequent mineral deposition. The hydrothermal system could have been active for a relatively long period of time after the impact, thus producing a potential transient habitable environment. It must be a spectrally neutral component to emphasize the spectral features; It is an average of spectra taken in the same column of the numerator spectra to correct the residual instrument artifacts and reduce detector noise that changes from column to column; It must be taken in the neighborhood of the area of interest to reduce most of the common mineral component. It is not always possible to satisfy all of the criteria listed above and this must be taken into account in the interpretation of the ratioed spectra. Moreover, this procedure works well if the denominator spectra have a phase similar to that of numerator spectra, but, as we will see, that is not always the case. The ratioed spectra may continue to have multiple phases that contribute to the spectrum with its spectral features (Wiseman et al., 2013). For this reason, when we compare a ratioed spectrum with those from the laboratory, it must be taken into account that more phases may continue to affect the band positions.For the geological and morphometric analyses, we used high-resolution imagery and topography from ESA Mars Express and NASA MRO (Mars Reconnaissance Orbiter) missions. In particular, HRSC (High Resolution Stereo Camera, Neukum et al., 2004) data (visible nadir image at 12.5 m/pixel and stereo-derived topography at 100 m/pixel) were used for the overall crater context, while CTX (ConTeXt, Malin et al., 2007) and HiRISE (High Resolution Imaging Science Experiment, McEwen et al., 2007) images supported the detailed analysis of the floor and central part of the crater. The latter two datasets were also used to derive high-resolution topography (down to 7 m/pixel from CTX and 1 m/pixel from HiRISE) through the NASA Stereo Pipeline software (Moratto et al., 2010). All of the data were georeferenced and co-registered using the equirectangular projection and the Mars IAU2000 reference ellipsoid. Finally, the imagery, spectral data and topography were imported into the GIS (Geographic Information System, ArcGIS v.10.2.2) environment to obtain a multitemporal/multisensor/multiscale view of the studied crater. We delineated the map units, taking into account their morphology/morphometry, surface properties, texture at different scales (e.g., relative tonal differences from visible imagery, thermal inertia, rough or smooth texture), and their internal sedimentary structure when possible (from erosional windows, crater walls or scarps). The latter approach allowed us to i) identify the main geological/geomorphological units and to ii) correlate the defined units with the mineralogical observations from CRISM (Figs. 1 and 4).

In Ceres dwarf planet, a portion of a prior ocean at shallow depth may still exist today as localized reservoirs. In this work, we constrained chemical and geophysical properties of initial aqueous...

The Ceres surface is globally composed of Mg-phyllosilicates, ammoniated clays, carbonates and dark components. To obtain a more detailed mineralogical and geological investigation, the dwarf planet surface has been divided into fifteen quadrangles. The aim of this work is to investigate the abundance of phyllosilicates and ammoniated clays in the Kerwan quadrangle, classified as Ac-H-7 and spanning from 22°S to 22°N in latitude and from 72°E to 144°E in longitude. Maps of band depth distribution at 2.7 µm and 3.1 µm have been performed and compared with a map of geometric albedo estimated at 1.2 µm. Phyllosilicates and ammoniated clays generally correlate in the Kerwan quadrangle, even if departure from this behavior is observed in the floor of Kerwan, Inamahari and Homsuk craters. The greatest abundance of ammoniated phyllosilicates is found in Rao and Kerwan ejecta, while Bonsu and Tafakula floors are the most depleted in volatile, as well as Inamahari and Dantu ejecta. Six bright spots are detected in the Kerwan quadrangle, and the one richest in carbonate is related to Dantu ejecta in the southeast region. Some younger features (such as Rao or Kerwan ejecta) show deeper band depths than older terrain, a contrast trend with respect to the entire Ceres surface. Since this correlation is observed in a few other places on other quadrangles but not on the entire Ceres surface, it is possible that recent impact events could have been masked this correlation.

Spectral parameters of Ceres measured by the Dawn/VIR imaging spectrometer are studied as a function of illumination angles, by applying a semi-empirical method based on a statistical analysis of the VIR dataset acquired up to September 2016. The study also focuses on the photometry of the Occator faculae, i.e. the brightest spots of the Ceres surface, showing an albedo up to eight times the Ceres average. The considered semi-empirical approach takes into account the small extension (and hence small dataset) of this region and lays the groundwork to apply scattering models even on such a limited area. The behavior of Ceres visible and infrared reflectance with phase angle is similar to other asteroids belonging to its same spectral class, i.e. C-type. The depth of the bands at 2.7 µm (phyllosilicates), 3.1 µm (ammonium), 3.4 µm (magnesium carbonates) and the infrared spectral slope linearly increase with phase angle, showing analogies with other asteroids and occurrence of phase reddening. The different behavior of the 3.9 µm band depth (also due to Mg carbonates), independent of illumination angles, could indicate that other carriers contribute to the 3.4 µm band and play a more important role in photometry outside the carbonate deposits. The phase function of the Occator faculae is much steeper than expected from its high albedo. Mixture of bright and dark material and larger roughness can be at the basis of this result. The phyllosilicate bands show a steeper increase with phase angle with respect to the Ceres average, due to the lower presence of dark materials, and/or again larger roughness. The absence of trends with phase angles of the two carbonate bands and of the spectral slope suggests that carbonates do not produce phase reddening.

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

Mapping Ceres’ surface composition in the Dantu region, located between 21°–66°N and 90°–180°E, offers the unique possibility to investigate changes in the surface composition related to different stratigraphic levels of Ceres’ crust. Dantu is located in a huge depression named Vendimia Planitia, which possibly represents a completely degraded impact basin formed in the beginning of Ceres’ geological history. Most parts of this depression are characterized by strong phyllosilicate absorptions, which are stronger than elsewhere on Ceres’ surface. This spectral signature possibly is related to the material emplaced at the time of the Vendimia impact event excavating material from deeper regions of Ceres’ crust. Subsequent impacts in this basin reach far deeper into Ceres’ crust than any impact events outside of Vendemia Planitia, which could explain the spectral signature of Dantu, possibly pointing to a higher concentration of ammonium-bearing phyllosilicates in Ceres’ deeper crust. Spectral differences with respect to the small fresh craters on Dantu's floor are probably related to grain size effects causing a bluish visible slope as observed by fresh impact craters on other places on Ceres. The local enrichment of carbonates in the Dantu area could also be associated with the impact event and may have been formed by additional impact-triggered and/or post-impact alteration processes.