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.

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   

3200 Phaethon is a near-Earth asteroid known to be actively emitting dust and gas when near its perihelion, and it is the target of the Japanese DESTINY+ mission. Several visible–near-IR spectra were obtained of Phaethon during 2003–2017, which have been collected together and analyzed with respect to its shape model. Using a thermal model, we examine the diurnal temperature variations across Phaethon’s surface (ΔT ∼ 600 K). We find different solar radiation heating between the north and south regions, with very high surface temperatures in the south region (max peak ∼ 1050 K), while a lower temperature is found in the north region (a mean value of 760 K). Our analysis of the photometrically corrected slope suggests a latitudinal trend in surface composition. We propose that the northern region of Phaethon may be linked to unusual chondrites and naturally heated Y-82162 samples (63–125 μm and/or chips), while the southern region shows similarities to Ivuna CI chondrites heated to 700°C (0–125 μm). The equatorial region appears to include a mixture of materials from both hemispheres.

We present Fourier transform infrared (FTIR) and Raman spectroscopic analyses of two particles (A0226 and C0242) returned from the C-type asteroid Ryugu by the Hayabusa2 mission. These particles, recovered from two different stratigraphic depths during the touchdown operations, provide a unique opportunity to investigate the compositional heterogeneity, alteration history, and space weathering processes of a primitive asteroid. All particles exhibit the 2.7 μ m OH absorption feature, consistent with phyllosilicate hydration. However, the subsurface particle C0242 shows a significantly deeper OH band and stronger spectral signatures of carbonates and organic matter, suggesting the preservation of primitive material compared to the surface of the A0226 particle, which is distinctly affected by space weathering. To characterize compositional diversity, we applied unsupervised machine learning methods (Uniform Manifold Approximation and Projection for dimensionality reduction and Hierarchical Density-Based Spatial Clustering of Applications with Noise for cluster detection), identifying five spectral clusters linked to variations in their surface evolution and surface texture. This provides an objective framework to resolve spectral heterogeneity beyond median analyses. Raman spectroscopy of a fragment of A0226 reveals broad D and G bands typical of disordered carbonaceous matter, with spectral variations attributable to space weathering effects, consistent with FTIR analyses. These results, integrated with Hayabusa2 remote sensing data, support a model of stratified regolith evolution on Ryugu shaped by impact gardening, mass wasting, and surface irradiation. By modeling the reduction of the 2.7 μ m band over time, we estimate surface exposure of 10 ⁵ yr for C0242 and 10 ⁶ for A0226.

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...