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: Quartz Crystals Microbalances (QCMs) are widely used sensors for monitoring and characterizing dust and particles deposition processes in different planetary environments and measuring material contamination coming from outgassing sources in space, in support to other instruments (e.g. spectrometers). Furthermore, they are capable of detecting and measuring the presence of volatile com-pounds of astrobiological interest such as water and organics. These measurements can be particularly relevant when performed on primitive asteroids or comets, or on targets of potential astrobiological interests, e.g. Mars [1]. These sensors convert mass changes into fundamental resonance frequency variations, according to Sauerbrey equation [2]. The VISTA (Volatile In Situ Thermogravimetry Analyser) instrument is a QCM-based device able to perform measurements of abundance of volatiles and dust particles in planetary and asteroidal environments. The instrument can characterize the planetary regolith from 5-10 µm to sub-µm particles and monitor the contamination processes on board satellites (cubesat, etc.) caused by molecular outgassing. VISTA is one of the two scientific payloads of MILANI CubeSat, developed by Tyvak International that leads a consortium composed by entities and institutions from Italy, Czech Republic and Finland, in the framework of the Hera program of the European Space Agency (ESA). Hera, due to launch in 2024, is the ESA part of the Asteroid Impact & Deflection Assessment (AIDA) international collaboration with NASA, who is responsible for the Double Asteroid Redirection Test (DART) kinetic impactor spacecraft. The main objective of MILANI is the study of the binary asteroid system Didymos-Dimorphos, characterizing the asteroid with a dust sensor (VISTA) and a spectrometer (ASPECT). In this work, the calibration operations and the performance tests, to assess VISTA capability of characterizing volatile compounds and simulant contaminations in vacuum chamber at low temperatures. The sensor has been developed by an Italian Consortium composed by three Research Institutes: INAF-IAPS (National Institute of Astrophysics - Institute for Space Astro-physics and Planetology), CNR-IIA (National Council of Research – Institute of Atmospheric Pollution) and Politecnico di Milano and led by INAF-IAPS. Figure 1. VISTA EQM developed for ESA Hera space mission. Working Principle: The instrument core is a QCM whose frequency variations directly depends on the deposited sample mass on the crystal surface during sublimation, condensation and absorption/desorption processes. The instrument consists of: 1) two quartz crystals mounted in a sandwich-like configuration; 2) a Thermal Control System (TCS), composed by two integrated heaters and a Thermoelectric Cooler (TEC); 3) a Proximity Electronics (PE). VISTA is also capable of performing Thermo-Gravimetric Analysis, which is a technique used to monitor thermal processes involving volatile compounds, e.g. deposition/sublimation and absorption/desorption. It can also monitor particles lower than 5-10 µm and sub-µm particles [1]. Calibration: The QCM frequency can change not only due to the mass deposition/release, but also due to the variation of environmental parameters, such as temperature and pressure. In order to disentangle frequency variations due to mass deposition and environmental parameters, the sensor is calibrated by measuring the frequency as a function of temperature. According to literature [3], the frequency-temperature curve follows a third-degree polynomial (Figure 2). Figure 2. QCM frequency as a function of temperature in vacuum. Performance tests: VISTA capability to detect contaminant depositions and to monitor the accumulation and desorption processes is verified by placing an effusion cell containing an organic compound, used as a contamination source, placed in the Field Of View (FOV) of the sensing crystal and heated up to 100°C. The experimental setup is shown in Figure 3. Figure 3. VISTA EQM experimental setup for contamination simulation. The QCM is connected with four screws with a copper U shape and in contact with a cold sink set to -10°C to help the molecules condensation on the crystal surface. The frequency is monitored during the tests and the deposited flux can be retrieved (Figure 4) at each temperature set point. Figure 4. Deposition test from +50°C to +100°C. Two methods can be used to retrieve the enthalpy of sublimation ΔHsub by using the deposition rates, i.e. the Van’t Hoff relation [4] or Langmuir relation [5]. Thus, by measuring two different deposition rates, k1 and k2, at two different close temperatures T1 and T2, it is possible to obtain the compound ΔHsub by means of Van’t Hoff relation or throughout the temperature range by using Langmuir relation. TGA cycles are then performed by heating the crystals by means of the built-in heaters. After the heating cycles, the frequency returns to its initial value, thus indicating that all the deposited mass desorbed during the test (Figure 5). Figure 5. TGA cycles from +15°C to +50°C and from +40°C to +70°C. The desorption rates from crystals surface can be used as well to calculate the ΔHsub and compare it with the ΔHsub results obtained during the depositions/contamination processes. References: [1] E. Palomba et al (2016), OLEB, 46(2-3; [2] G. Sauerbrey (1959), Z. Phys., 155, 206-222; [3] D. Salt (1987); [4] S.W. Benson et al., 1968; [5] I. Langmuir, 1913.

1. Introduction Hayabusa2 – Japanese Aerospace Exploration Agency Mission investigated the asteroid (162173) Ryugu and on 06 December 2020 successfully returned to the Earth ~5.4 g of collected materials. Indeed, the first touch-down operation (TD1) sampled surface material on 21st February 2019; after the formation of a 15 m artificial crater with the SCI operation (Saiki et al., 2013), the TD2 occurred on 11st July 2019 and collected samples in the vicinity of crater, probably also subsurface material excavated by the impact. Ryugu appears to be a rubble-pile asteroid due to the low density (about 1.2 g/cm3) which suggests a formation by re-accumulation of fragments generated by the catastrophic destruction of the parent body (Watanabe et al., 2019). NIRS3 spectra of Ryugu surface acquired during Hayabusa2 mission show a strong features at 2.7 μm (indicating a OH-rich content ascribed to Mg-rich phyllosilicates) (Kitazato et al., 2019) and a secondary absorption at about 2.8 μm, strongly correlated with the 2.7 μm band (Galiano et al., 2020). This work includes the Carbonaceous Chondrite meteorites (CCs) identification and selection in grains from ~1 to 4mm to simulate the Ryugu surface and analogue mixtures production by using different end-members: a dark component (graphite, carbon, etc.), Mg-rich phyllosilicates (i.e. Antigorite for OH stretching band at 2.7 μm) and Fe-rich dark component (e.g. Cronstedtite) to reproduce the 0.7 and 2.8 μm bands of Fe-bearing phyllosilicates. Four analogue mixtures have been sieved in the grain size 100-200 μm and 200-300 μm and named: Mixture#3, Mixture#4, Mixture#5 and Mixture#6. The spectra of the samples have been acquired by means of Bruker Vertex 80 FT-IR Spectrometer (incidence angle of 30° and emission angle of 0°) in the spectral range 2-4 μm (spectral resolution of 4 cm-1) and then compared with average Ryugu spectrum. The spectral slope (in the 2.0-2.5 μm range), Band Depth and Band Center of the feature at 2.7μm and reflectance at 2.1μm have been analysed. 2. CCs meteorites analysis The acquired meteorites spectra (fragments) compared with Ryugu ones are reported in Figure 1. The results show as the reflectance (at 2.1μm) values of meteorites (0.042 Murray, 0.032 Nagoya and 0.030 Nagoya) can be comparable with Ryugu reflectance level, i.e. 0.0296 as well as the BC (2.731 Murray, 2.749 Nagoya and 2.716 Nagoya and 2.719 of Ryugu spectra). Differently, the meteorites BD is from four to eight times more intense than Ryugu one. Figure 1. Meteorites spectra and comparison with Ryugu (smoothed with 61 channels). 3. Analogue mixtures analysis The analogue mixtures (about 0.1g) have been produced by considering the grain size of 100-200 µm and 200-300 µm (Figure 2). Each end-member has been analysed in the spectral range of interest, for example the Antigorite shows the bands at 2.3 µm (due to combination of Mg-O-H bending and O-H stretching) and at 2.7 µm (O-H stretching). Figure 2. Percentage composition of Ryugu analogue mixtures. The Figure 3 at left shows the spectra of Mixture #3 and #4 while at right the Mixture #5 and #6 compared with the mean spectrum of Ryugu (obtained by NIRS3 data) multiplied by 7 for a better comparison. The Mixture#3 does not show the contribution of OH bands due to high percentage of dark component, i.e. 42.5%. Thus, the Mixture#4 has been produced by increasing the Mg-rich phyllosilicates, reducing the dark component and introduce the Cronstedtite (Fe- bearing phyllosilicate). So that, it is important to introduce an additional dark component, i.e. the Cronstedtite (Fe- bearing phyllosilicate) which can help to decrease the reflectance level and to contribute to shallow 2.7 µm band due to OH. Figure 3. Analogue mixtures compared with average Ryugu spectrum. 4. Preliminary results and future aims The mixtures #4, #5, #6 showed a negative spectral slope due to the high percentage of Antigorite (up to 60%) responsible of the 2.7 μm band while 3 μm spectral band has been ascribed to the adsorbed atmospheric water. Furthermore, the Cronstedtite included in the Mixture#6 gave a contribution to the dark component equal to 32.5% by avoiding negative effects on 2.7 μm spectral band that remains to be well defined. Preliminary analysis exhibit a good comparison between Ryugu BC and analogue mixtures BC (due to the presence of Mg-phyllosilicate) whereas the BD in spectra of analogue mixtures is deeper than the Ryugu one. The Ryugu reflectance is two-three times lower than the mixtures reflectance and characterized by a positive spectral slope, contrarily to the mixtures’ spectra which have a negative spectral slope. Starting from Mixture#6, next step will aim at reducing the reflectance level and decrease the OH band contribution, as well as producing a positive spectral slope. By using thermal controlled setup in vacuum chamber at different temperatures (e.g. 100, 200, 300, 400°C), a thermal metamorphism can be simulated and new spectra will be acquired. In the future, laser and ion irradiations can be considered to simulate the micrometeorite impacts and solar wind processes which help to decrease the spectral level and increase the spectral slope, as well as to reduce the analogue mixtures spectral bands. References: Galiano A., et. al. 2020, Icarus, v. 351, 113959; Kitazato, K. et al. 2019, Science 364, 6437: 272-275; Saiki, T., et al. 2013, Acta Astronautica, v. 84, 227-236; Tatsumi, E. et. al., 2021, Nat Commun 12, 5837; Watanabe, S. et al., 2019, Science 364, 6437: 268-272

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

Introduction: The Solar System dust is the smallest Solar System building block, and therefore its study allows understanding the early stages of the Solar System formation. The comprehension of the Solar System formation and evolution processes is also constrained by the inventory and spatial distribution of volatiles in our System. In particular, the delivery process of volatiles of astrobiological interest (water and organics) to the Earth is a key issue to understand the life formation in our planet. Asteroids and comets may have been the source of these volatiles to the terrestrial planets’ atmospheres and to the Earth’s oceans. The boundary between asteroids and comets is not well defined, and many main-belt asteroids are probably volatile-rich and would become cometary if they were moved to the inner Solar System. Piezoelectric Crystal Microbalances (PCMs) are widely used sensors to monitor dust and particles deposition processes in space and to characterize material outgassing in vacuum. This kind of sensors converts mass changes into fundamental resonance frequency variations, according to Sauerbrey equation [1]. In this work we present VISTA (Volatile in-Situ Thermogravimeter Analyser), one of the two scientific payloads on board MILANI Cubesat, as part of the ESA Hera Program. The MILANI Cubesat development is led by Tyvak International, prime contractor of a consortium composed by entities and institutions from Italy, Czech Republic and Finland. The main MILANI objective is executing a scientific mission aiming at studying the binary asteroid system Didymos-Dimorphos, characterizing the asteroid with a dust sensor, i.e. VISTA, and a spectrometer, i.e. ASPECT. VISTA is developed by an Italian Consortium composed by three Research Institutes: INAF-IAPS (National Institute of Astrophysics - Institute for Space Astrophysics and Planetology), CNR-IIA (National Council of Research – Institute of Atmospheric Pollution) and Politecnico di Milano. HERA Mission goals: Hera is a planetary defence mission under development at the European Space Agency (ESA), launching in October 2024. Hera is the European contribution to the international Asteroid Impact Deflection Assessment (AIDA) cooperation, the first planetary defence mission, in collaboration with NASA, who is responsible for DART (Double Asteroid Redirection Test) kinetic impactor spacecraft. Hera will travel to a binary asteroid system, the Didymos-Dimorphos pair of near-Earth asteroids, and will study and characterize the asteroid system after the DART impact in September 2022. In the framework of Hera mission, VISTA will accomplish the following scientific goals: 1) detect the presence of dust particles smaller than 10 µm (residual dust particles from the impact and suspended dust in the binary system or coming from dust levitation process); 2) characterization of volatiles (e.g. water) and light organics (e.g. low carbon chain compounds) by using Thermo-Gravimetric Analysis (TGA) cycles (the desorption rates ad specific temperatures are used to characterize e volatiles and organics desorbed from the sensor surface); 3) molecular contamination monitoring in support to other Cubesat instruments and ASPECT spectrometer, coming from outgassing processes on-board the spacecraft, that usually happen in the first days or weeks in orbit. VISTA Heritage: The Consortium has a considerable heritage in the design, manufacturing and testing of PCM-based instrumentation both for laboratory and space applications coming from different ITT-Emits ESA Projects: 1) CAM (Contamination Assessment Microbalance), developed for “Evaluation of an in-situ Molecular Contamination Sensor for space use” (2014-2016) (Figure 1); 2) CAMLAB (Contamination Assessment Microbalance for LABoratory) developed for “Development of a European Quart Crystal Microbalance” (2017-2019) (Figure 2). Figure 1. PCM Engineering Model developed during CAM-ESA Project for space applications. Figure 2. PCM breadboard developed during CAMLAB-ESA Project for laboratory applications. Working principle: PCMs exploit the piezoelectric properties of quartz crystals, as mass deposition on the sensing area of the instrument induces variations of the crystal resonance frequency. The core of VISTA is composed of: 1) two quartz crystals mounted in a sandwich-like configuration (one sensing crystal and the other reference crystal); 2) a Thermal Control System (TCS), composed by two integrated heaters and a Thermoelectric Cooler (TEC); 3) a Proximity Electronics (PE). The instrument is also capable of performing Thermo-Gravimetric Analysis, a technique used to monitor thermal processes involving volatile compounds, e.g absorption/desorption and deposition/sublimation, by means of the built-in heaters. The TEC is used to cool down the sensor and enhance the condensation of particles on the microbalance. VISTA is capable of monitoring particles lower than 5-10 µm and sub-µm particles. VISTA technical characteristics are shown in Table 1. Table 1. VISTA technical characteristics. The sublimated compounds can be characterized by calculating the enthalpy of sublimation ΔHsub, which can be retrieved with two methods: 1) by considering the deposition rates and using Van’t Hoff relation [3]; 2) by using Langmuir relation [4]. Conclusions and future works: In this work, the VISTA instrument working principle and heritage were presented. Qualification activities are currently ongoing on the VISTA Engineering Qualification Model (EQM), shown in Figure 3, in order to qualify the sensor within the expected mechanical and thermal environment and evaluate the instrument performances in a representative environment, prior integration on the MILANI Cubesat. The next phase of the research activities will be the manufacturing, integration and testing of VISTA Flight Model (FM), that will launch in 2024. Figure 3. VISTA EQM developed for MILANI cubesat/HERA Space Mission. References: [1] G. Sauerbrey 1959, Z. Phys., 155, 206-222; [2] Palomba E. (2016), OLEB, 46 (2-3); [3] S.W. Benson et al. 1968; [4] I. Langmuir, 1913.

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.