The Martian Moons eXploration (MMX) mission of the Japan Aerospace Exploration Agency is equipped with different cameras and one spectrometer [K. Kuramoto et al., Earth, Planets Space 74, 12 (2022)] called the MMX InfraRed Spectrometer (MIRS). The instrument is devoted to investigate the origin of the Martian moons, Phobos and Deimos, as well as the Martian atmosphere and surface [M.A. Barucci et al., Earth, Planets Space 73, 211 (2021) and M. A. Barucci et al., Prog. Earth Planet. Sci. 12, 69 (2025)]. It is designed to identify spectral features attributed to ice, dust, water vapor, minerals and organics, all detectable within the [0.9-3.6] μm wavelength range of the instrument. MIRS itself is composed of different parts, namely, the electronic box and the OBOX (optical box), which are both linked by an electronic harness. This paper presents the second part of the MIRS calibration. The first paper [G. Poggiali et al., Rev. Sci. Instrum. 97, 054501 (2026)] aimed to explain the calibration plan, present the materiel and the list of measurements performed, and demonstrate MIRS' compliance with the scientific and technical objectives. This paper details the methodology used to extract the spectral registration, correct data from distortion, verify the radiometric curve against the theoretical optical one and enhance it, and extract the pixel-to-pixel response of the instrument. Effect of the OBOX temperature and the scanner orientation will also be discussed as the current caveats, which will be addressed using inflight calibration during the cruise and the main phases of the MMX mission.

The MMX InfraRed Spectrometer (MIRS) is a spectro-imager on board the Japan Aerospace Exploration Agency Martian Moons eXploration mission, set to launch in 2026, to investigate the origin of the Martian moons, Phobos and Deimos, as well as the Martian atmosphere and surface. MIRS, operating in the 0.9-3.6 μm wavelength range, is designed to identify and map minerals, ices, and organic compounds on the Martian moons, while also monitoring water vapor and dust in Mars's atmosphere. This paper details the ground calibration and performance evaluation of the MIRS Flight Model, conducted at the Laboratory for Instrumentation and Research in Astrophysics at the Paris Observatory during the thermal-vacuum test campaign at the end of 2023. A detailed description of the apparatus and the procedures used during the campaign is provided. The calibration campaign tested the instrument's thermal response and radiometric performance, ensuring compliance with stringent mission requirements. The tests demonstrated MIRS's capability to deliver high-resolution spectral data, fulfilling critical scientific and technical objectives. The preliminary results indicate MIRS's readiness for in-flight operations and its potential to contribute significantly to the understanding of the Mars system.

MIRS (MMX InfraRed Spectrometer) is a push-broom imaging spectrometer onboard of the JAXA sample return MMX mission. It has been built by the French laboratory LESIA, today LIRA (Laboratory for Instrumentation and Research in Astrophysics) of Paris Observatory-PSL in collaboration with five other French laboratories, collaboration and financial support of CNES and close collaboration with JAXA and MELCO. MIRS, designed to accomplish the MMX scientific objectives, has been built to be adapted on MMX Exploration Module. MIRS will remotely observe the Martian system for three years. MIRS will observe Phobos, Deimos and Mars in the spectral range 0.9–3.6 µm to characterize surface composition of the satellites and investigate Martian atmospheric variations. An overview of the MIRS Flight Model is presented as well as the data processing and the expected results.

Surface porosity and texture has been found to be an important property for small bodies. Some asteroids and comets can exhibit an extremely high surface porosity in the first millimeter layer. This layer may be produced by various processes and maintained by the lack of an atmosphere. However, the influence of porosity on the spectro-photometric properties of small body surfaces is not yet fully understood. In this study, we looked into the effect of the texture on the spectro-photometric properties of Phobos regolith spectroscopic simulants. Macro-and micro-porosity were created by mixing the simulants with ultra-pure water, producing ice-dust particles, and then sublimating the water. The sublimation of the water ice enabled the production of porous and rough powdered simulants with significant micro-and macro-porosity associated with macro-roughness. The reflectance spectroscopic properties in the visible and near-infrared (0.5-4.2 mu m) demonstrate a brightening of the porous samples in comparison to the compact ones. One simulant exhibits a bluing of the spectral slope after increasing porosity, which is likely linked to the presence of expandable phyllosilicates. In the mid-infrared range, a contrast increase of the 10 mu m emissivity-related plateau due to silicates is observed. This spectral feature is typically observed as a 10 mu m emissivity plateau on some asteroids, making the mid-infrared region important for assessing mineralogy and surface texture. Photometry reveals a modification of the phase reddening behavior between the compact powder and the sublimation residue for both simulants. However, the observed behavior is different between the simulants, suggesting that the phase reddening may be dependent on the composition of the simulants. The phase curves of the sublimation residues exhibit a higher contribution of forward scattering. The derivation of the Hapke parameters indicates an increase in roughness for the porous sample, but no significant modification of the opposition effect. The modifications of the spectrophotometric properties observed in this experiment are definitely due to the textural changes obtained after sublimation, which depend on the initial composition of the simulants. This study aims to provide new insights into the understanding of porosity by using two Phobos simulants in the context of the upcoming JAXA/Martian Moons eXploration mission. We suggest that the Phobos blue unit may be due to the presence of a highly porous layer, rather than only to space-weathering processes, as often postulated.

The leading hemisphere of Saturn’s synchronous moon Iapetus is covered by a low-albedo material, contrasting with its bright trailing hemisphere. This dichotomy is also apparent in radar and microwave radiometry observations, which are sensitive to the properties of the near subsurface. To better understand the regional properties of Iapetus and their variations with depth, we assemble the microwave spectra of its leading and trailing hemispheres. Pre-existing data are combined with new millimetric and centimetric observations acquired with the IRAM 30-meter dish, IRAM NOEMA interferometer, and VLA interferometer. These data, interpreted with the help of a model with vertically uniform thermal properties, reveal complex variations in structure and/or composition with depth on the leading side. Meanwhile, the trailing side emissivity is found to be especially low at all observed frequencies, indicating efficient scattering processes on subsurface structures, as observed on Saturn’s other icy moons. We also report the first observations of Saturn’s retrograde moon Phoebe at these frequencies, which has an emissivity higher than that of the trailing hemisphere of Iapetus and similar to its dark leading side, consistent with the theory that Phoebe is the source of the dark material on Iapetus.

At first order, the physical temperature of Titan’s surface can be regarded as nearly constant and predictable. Due to the low incident solar flux reaching its surface (1/1000 of what Earth receives) and the high thermal inertia of its atmosphere, diurnal, seasonal (including latitudinal) and altitudinal variations of temperature are limited as well as the effect of surface albedo (Lorenz et al., 1999). Voyager 1 radio-occultation measurements indeed show no diurnal effect and point to lapse rates in the lower atmosphere smaller than 1.5 K/km (McKay et al 1997). Voyager infrared observations indicate a pole-to-equator temperature contrast of 2-3 K (Flasar et al., 1981; 1998).The Cassini mission (2004-2017) somewhat confirmed these predictions and first measurements. On board the Cassini spacecraft, two instruments were able to measure the physical temperature of Titan’s surface: the Cassini’s Composite IR Spectrometer (CIRS) through a spectral window of low opacity in the thermal IR and the Cassini radar used as a microwave radiometer. Both instruments monitored the surface brightness temperature at their respective wavelengths (19 microns and 2.2 cm, respectively) during the almost two Titan’s seasons of the Cassini mission. Interestingly, these two instruments probe different depths; the very surface for CIRS and at least several decimeters in the lands for the microwave radiometer (Janssen et al., 2016), much more in the lakes (Mastrogiuseppe et al., 2014). Combining these datasets thus provides insights into the vertical variations of the thermal and physical properties of the surface.From the analysis of CIRS dataset, Cottini et al. (2012) report a diurnal signal of 1-1.5 K indicative of a thermal inertia of 300-600 MKS while Jennings et al. (2009; 2016) investigate seasonal changes confirming a constant maximum temperature of 93.65 +/- 0.15 K (as measured by the Huygens probe at 10.3°S latitude, Fluchignoni et al., 2005) and a variation of the latitude at which this maximum occurs following the sub-solar latitude (which moved from 24°S to 23°N between 2004 and 2017). Jennings et al. (2016) also found a 2-4 K equator-to-pole difference and note a delay in the northern warming at the end of the mission, as summer was on its way. This later was interpreted as a cooling effect of both the lakes and the surrounding moist lands as CIRS observations show no difference in the thermal behavior of these two types of terrains (within measurement uncertainty of about 0.5 K).From the radiometry dataset, Janssen et. (2016) find latitude-dependent seasonal temperature variations smaller than those measured by CIRS by a factor of 0.87 +/- 0.05 in relative amplitude which is consistent with a penetration depth of 40 cm-1 m in organic sands. The difference with CIRS observations is slightly more pronounced in the northern hemisphere likely owing to the presence of lakes and seas in which microwaves penetrate deeper than in dry lands. In the North pole, Le Gall et al. (2016) also report the hint of a slower than expected rise in temperature in the second largest sea of Titan, Ligeia Mare, toward the end of the mission. Any diurnal effect was neglected considering that the radiometer would probe much deeper depths than the diurnal skin depth (Lorenz et al., 2003).In this work, in order to investigate further the seasonal variations of Titan’s temperature, we present the analysis of the high-resolution radiometry observations recorded in the northern pole from 2007 to 2017. This analysis demonstrates that the seas warm more slowly than their surrounding lands and are therefore responsible for the global lag in summer warming observed in Titan’s high northern latitudes both by CIRS and the Cassini radiometer. This cooling effect could be due to the high thermal inertia of liquid hydrocarbons, their high transparency (which leads microwaves to sense the coldness buried from last winter, Le Gall et al., 2016) and/or methane evaporation (Mitri et al., 2007).In addition, we present for the first time the analysis of the 118 distant observations of Titan collected during the course of the Cassini mission. These observations were designed for the computation of the disk-integrated brightness temperature of Titan. Though unresolved, they provide clues on the seasonal and longitudinal variations of Titan’s surface thermal emission. In particular, they clearly show Xanadu, a large-scale low emissivity/radar-bright feature on the leading side of the satellite. The analysis of this dataset reveals a possible diurnal component of amplitude 0.6 K and peaking at 4 pm in Titan’s radar-dark terrains and of amplitude 0.8 K and peaking at 2 pm in Xanadu. Unfortunately, such a signal cannot be isolated in high resolution observations because of the way data are calibrated (Janssen et al., 2016). If confirmed, this detection would bring a further argument for a smaller than expected electrical skin depth (i.e., a more absorptive subsurface) in most of Titan’s equatorial lands and/or a larger diurnal thermal skin depth (i.e. a higher thermal inertia), especially in Titan’s radar-dark dune terrains.To conclude, monitoring surface temperature brings key insights into the surface properties and its coupling with the atmosphere. Cassini findings provide a global context for the future observations of DraGMet, the geophysics and meteorology package on board the Dragonfly quadcopter (Lorenz et al., 2018) which will investigate the ground thermal properties and record temperature variations in parallel with the atmosphere humidity, the ground moisture and wind speed.How to cite: Le Gall, A., Bonnefoy, L., Sultana, R., Janssen, M., Lorenz, R., and Tokano, T.: Evidence of diurnal variations of Titan’s near-surface temperature and of a cooling effect of the northern seas from the Cassini radar/radiometer, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-618, 2020

IntroductionThe two martian moons, Phobos and Deimos, will be visited by the sample return JAXA mission MMX in 2027 (launch planned in October 2026). They orbits Mars at respectively 9400 and Deimos at 23000km, with low inclinations and eccentricities. Their heavily processed surfaces harbour a fine regolith [1]. Visible and Near-Infrared (Vis-NIR) spectral readings reveal dark and red flat spectra[2,3]. These characteristics have spawned competing hypotheses regarding their formation:- They could be remnants of D-type asteroids, captured by Mars[3].- They may have formed from the aftermath of an impact between Mars and a protoplanet[4].While the former hypothesis aligns with spectral similarities observed in D-type asteroids, it fails in explaining the moons' peculiar orbits. Conversely, the latter resolves the orbital puzzle but fails in elucidating their spectral characteristics.To unravel the origins of Phobos and Deimos, the Martian Moons Explorer (MMX) mission[5] aims to scrutinise the moons' compositions. The MMX Infrared Spectrometer (MIRS) will observe the two moons in the range of 0.9 to 3.6µm[6]. However, at a distance of 1.5AU, the moons' surfaces emit a significant thermal flux, especially beyond 2µm, necessitating correction in spectral measurements.We present a model adapted from previous works[7] to characterise the thermal behaviour of the Martian moons in preparation of MIRS data interpretation. From their properties—such as albedo, inertia, and thermal conductivity—alongside the incoming flux, we aim to compute their surface temperatures accurately, aiding in the interpretation of MIRS spectral observations.Illumination at the moons’ surfaceGiven the absence of an atmosphere and the moons small size, their surface temperatures are dictated by the absorbed flux. Consequently, the initial step in computing their physical surface temperatures involves discerning the incident flux.We developed a model that computes the flux reaching the surface of the moons over one orbit around the Sun. The model relies on the SPICE/NAIF toolkit to computes the distance and the viewing incidence and reflection angles at the moons surfaces. It takes into account for the eclipses when Phobos or Deimos enters Mars' shadow cone, and for the reflected and emitted light on Mars in the Vis-NIR.Additionally, our model incorporates the shape model provided by[8] to compute illumination angles on the moons' surfaces. Furthermore, it utilizes the Mars albedo map from[9].In the model, the incidence flux at a specific point of the surface hence varies over a year with: -solar distance,-eclipses,-moon orientation,-Mars thermal emission.Fig.1: Incident flux at Phobos surface on the 28/07/2005.Fig.1 illustrates the incident flux map at Phobos surface at a specific date (here on the 28/07/2005 at 09:27).It clearly displays the sub-solar point at 175°E, and the sub-Mars point (at 0°E) where a second peak of flux arises as a consequence of the reflected and emitted light on Mars (resp. ~26W.m-2$ and ~11W.m-2). Solving heat equationsTo retrieve the surface temperature, we adapted a thermal model [7] to Phobos and Deimos.This model solves the 1-D time-dependent heat equation for each point of the incident flux map previously computed.It takes as input the thermal properties of the surface:-thermal inertia -porosity-bulk densityIt does not take into account for convection, but considering the absence of atmosphere the model is still applicable. However, it does not consider radiation, which may reveal itself important for high porosity at the surface, where conduction is reduced to punctual contacts between grains, giving more importance to heat transfer by radiation. Fig.2 presents a temperature overview at Phobos surface, computed from the incident flux map shown in Fig.1, for an inertia of 100J.m-2.s-1/2.K-1 a porosity of 0.9 and a density of 1300kg.m-3. The temperature range computed for the period of observation is similar to the results of [10]. The sub-solar point is clearly visible and reaches 295K, whereas the boreal pole is much cooler at 140K.In this simulation, the influence of Mars was taken into account.Fig.2: Phobos temperature map surface the same day. Influence of Mars: the case of Phobos Computing the surface temperature with and without Mars, allow to determine its influence on Phobos temperature.Fig.3 illustrates the ΔT-map on Phobos surface on the hemisphere pointing directly to Mars. It results in an increase of almost 3.5K of the surface temperature at the sub-Mars point. This effect is far from negligible, as even this slight temperature variation lead to a significant increase of 26% in the flux emitted by Phobos' surface at 2.5µm, and may be even more important in some configurations where the polar region will be illuminated only by Mars.Fig.3: Mars contribution on Phobos surface temperature, up to 3.5K. PerspectivesWe show the importance of a model that takes into account for Mars’s reflection and emission.However, this model is still under development and currently does not consider the infrared self-heating of the moons. This factor could significantly alter temperatures within craters, which might be crucial for correcting MIRS spectra, particularly in heavily cratered regions.The temporal albedo variations of Mars (i.e. the dust storms) will also be added in the model as they will completely remove the thermal emission of Mars surface, and increase the albedo. AcknowledgmentsThis work is funded by the DIM ORIGIN (Ile de France).References[1] Thomas, 1979[2] Fraeman+, 2012[3] Fraeman+, 2014[4] Craddock, 2011[5] Kuramoto+, 2022[6] Barucci+, 2021[7] Ferrari & Leyrat, 2006[8] Willner+, 2014[9] Christensen+, 2001[10] Kuzmin & Zabalueva, 2003

Introduction Small bodies of the Solar System are residues of the epoch of formation of the planetary system. The most primitive of these bodies are likely comets, that should provide the best recording of the chemical composition and mineralogy of the protoplanetary disk. P- and D-type asteroids present featureless reddish spectra similar to the comets nuclei ([10]; [8]) and are believed to be somehow related to comets. Mid-infrared spectroscopic observations have led to the suggestion that these objects are covered by a porous layer constituted of submicrometer-sized (hyperfine) grains ([2]; [11]). Radiative transfer models have been used to retrieve composition from cometary dust emission as well as asteroid mid-IR spectra ([3]; [4]), but laboratory simulations on specific analogues that could also give indication regarding their compositionand their surface texture (porosity, roughness) are currently lacking. This study aims to explore the emission spectral features of hyperfine and hyperporous powders with decreasing grain size in order to compare laboratory simulations to observational data of the comet C/1995 Hale-Bopp and the D-type Trojan asteroid (624) Hektor. Methods Using a specific grinding and sieving protocol [9] we were able to produce large quantities of powder of different composition (olivine and smectite) at decreasing grain size. Grain size has been quantified using SEM imaging. The finest powders obtained have average grain size below one micron. In order to explore the effect of porosity on emission spectra, we produced hyperporous surfaces (with a porosity larger than 99 %) by sublimating under vacuum mixtures of water ice particles containing the mineral powders, following the protocol described in [7]. We also simulated porosity in our samples by mixing the mineral powders (1 vol%) with potassium bromide (KBr, 99 vol%), which is non-absorbing in the Vis-IR. The powders were brought to the DLR in Berlin to perform emissivity measurements at the PSL [5]. During the measurements, samples are heated from the bottom of the sample holder. Measurements at several temperatures were obtained. Emission spectra in the mid-infrared region were measured on the different samples. Reflectance was measured after direct emissivity measurements to observe chemical/mineralogical changes during the heating process. Results Figure 1 presents normalized emissivity spectra between 8 and 13 µm of the hyperfine powder of olivine, the hyperporous and hyperfine smectite powder, the KBr-diluted olivine powder, and observations of D-type asteroid Hektor and comet Hale-Bopp ([2]; [1]). Both observations of Hektor and comet Hale-Bopp exhibit a notable emission feature in the region 9-12 µm. Powder of submicroscopic olivine does not show the silicates emission features around 10 µm as well as the hyperporous smectite sample. These two spectra are very flat on the whole spectral range studied here. The spectrum of the KBr-diluted powder however shows a strong feature around 10 µm. Discussion Emissivity of D-type objects resembles to features observed for cometary dust tails ([2]; [10]). This may seem surprising at first since cometary dust tail may not be optical thick, while the surface of an asteroid is. The presence of such an emissivity feature has been interpreted by the presence of high-porosity, based on reflectance measurement of mixture of silicates with KBr. In the present work, we produced a hyperporous and hyperfine grained sample to simulate the presence of porosity without using KBr, which showed that such sample is featureless and has an emissivity close to 1 (whether emissivity is directly measured or estimated by Kirchoff's law). This means that porosity solely cannot explain the presence of emissivity features of silicates on small bodies. Emissivity of samples with porosity simulated using KBr and with real porosity are therefore very different in our results. This could be explained by the fact that, while KBr is non-absorbing, its real optical index is higher than 1. Using KBr will increase reflectance outside of where the silicate absorbs and therefore decrease emissivity, thereby producing the observed emission contrast. So alternative processes have to be proposed to explain the presence of emission feature. A first one is that, somehow similar to KBr, a brightening constituent is present in the material of D-type asteroids. Potential candidates are salts, responsible for the 3.2 µm signature on comet 67P/Churyumov-Gerasimenko and possibly on some asteroids including Trojans [6]. Another possibility that needs to be investigated is the presence of a temperature gradient. In our experiment the temperature at the top of the sample is lower than at the bottom, and there is no emissivity feature in the measured spectra. However, when observing the illuminated side of an object, the temperature gradient is in the other direction (the top surface is warmer). If only a few layers of hot surface grains are producing the emission signature, they may emit like an optically thin layer, similarly to cometary dust tails. The two possiblities will be investigated further. Acknowledgements This work was funded by the European Research Council under the SOLARYS grant agreement ERC-CoG2017-771691. Visit to DLR was supported by the Europlanet 2020 RI Program (H2020), grant N° 654208. References [1] Crovisier,et al., (1997). ESA SP,(419):137–140. [2] Emery, J. et al., 2006. Icarus, 182(2):496 – 512. Results from the Mars Express ASPERA-3 Investigation. [3] Gicquel, A. et al., (2012). Astronomy & Astrophysics, 542:A119. [4] Licandro, J., et al., (2011). Astronomy and Astrophysics, 525(13):1–7. [5] Maturilli, A. et al., (2019). The newly improved set-up at the Planetary Spectroscopy Laboratory (PSL). [6] Poch, O. et al., (2020). Science, 367 (6483). [7] Poch, O. (2016). Icarus, 267:154–173. [8] Raponi, A. et al., (2020). Nature Astronomy, 4:500–505. [9] Rousseau, B. et al., (2018). Icarus, 306:306–318. [10] Vernazza, P. and Beck, P. (2016). Composition of Solar System small bodies. [11] Vernazza, P. et al., (2012). Icarus, 221(2):1162–1172. How to cite: Sultana, R., Beck, P., Poch, O., Schmitt, B., Maturilli, A., Alemanno, G., and Helbert, J.: Mid-IR emissivity of hyperfine small bodies analogues, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-847, 2020

Introduction: The hyper-spectral imaging spectrometer MIRS [1] is part of the Martian Moon eXploration (MMX, [2]) probe, scheduled to be launched toward the Martian system in 2026. MIRS will observe Phobos and Deimos’ surfaces in the 0.9-3.6 μm spectral range to bring new constraints on their surface composition. Up to now, VNIR observations of the two satellites revealed red spectra, with only weak absorption features at about 1 μm [3] and 1.9 [4] probably linked with mafic minerals; as well as weak ones at 0.65 and 2.8 μm [5], possibly due to structural OH- in a desiccated phyllosilicate such as nontronite, or by OH- implanted from solar wind. CRISM spectra also revealed an absorption at 3.2 μm whose origin remains unclear [6], possibly linked either to calibration artifacts or mineral species (e.g., organic compounds or goethite [7]). Thermal infrared observations (10-35 μm) show several spectral features consistent with feldspars/feldspathoids, along with phyllosilicates [8] for which, biotite and antigorite provide very good spectral matches.For air-less bodies, like Phobos and Deimos, the optical/chemical/mineralogical properties of the surface can be modified by space weathering effects that include micrometeoritic bombardment and charged particle irradiation. Previous studies have shown that micrometeoritic bombardment simulation can result in darkening and/or reddening of the reflectance spectra, reduce the intensity of particular absorption bands or slightly shift the position and width of absorption bands (e.g., [9, 10, 11]).In this experimental study, we explore the effect of micrometeoritic bombardment on some mineral phases relevant to Phobos and Deimos surfaces, in order to prepare future investigations of the moons surface with the MIRS. Method: Two samples were selected including one iron-bearing phyllosilicate (nontronite) and one iron oxyhydroxide (goethite). To simulate micrometeorite impacts on these samples, we performed pulse-laser shock experiments (figure 1) using the LIBS suite of the SuperCam spare [12, 13] from the IRAP laboratory (Toulouse, France). The samples are located in a vacuum chamber, enabling to reach a pressure of around 10-3 mbar and approach the low oxygen fugacity conditions at the surface of the martian moons. The Nd: YAG laser beam of SuperCam delivers energy pulse of 10.7 mJ at 1064 nm, with a pulse duration of 4 ns and a laser spot of ~300 μm in diameter. A matrix of 9x9 shots was realized on each sample. We carried out grids of 1 shot and 3 laser shots repeated at the same location. Before and after irradiation, the reflectance spectra of the samples were acquired from 0.5 to 3.6 μm using the SHADOWS spectrogoniometer at IPAG (Grenoble, France) with an illumination spot of ~1.3x1.7mm in diameter [14].Figure 1: Nontronite pellet irradiated showing the laser pits for 9x9 matrix of 1 and 3 shots at the same location. The image on the left shows a zoomed-in version of the 3 shots grid. Preliminary results: A comparison between unaltered and irradiated samples is presented in figure 2. For irradiated nontronite, the VNIR part of the reflectance spectra displays a decrease in the overall intensity as the number of shots increases. The strength of the absorption band linked to the Fe3+ electronic transition near 0.65 μm is reduced by 10.6% and 20.5%, after simulated irradiation of 1 and 3 shots respectively. In addition, we observed a slight shift of 10 nm of the band center after 3 shots (0.64 μm). Similar observation holds for the Fe3+ associated absorption near 0.97 μm, with the same band depth reduction (10.0% and 20.8%), and a similar small shift of 10 nm toward lower wavelength after 3 shots. Conversely, we notice no shift of the 1.43 μm (H2O/OH features), 1.92 μm (H2O features), and 2.29 μm (Fe3+-OH vibrations) band positions. If the strength of the 1.43 μm decreases with an increasing shot number (5.3% and 18.9%), for the 1.92 and 2.29 μm, 1 shot produces no significant change, and 3 shots slightly reduces the depth of the bands (respectively 7.0% and 5.9 %). The effect of irradiation on goethite is much more drastic. Spectra show flattening and darkening in the visible–near-infrared range. The iron band at 0.66 μm is reduced by 72.5% after 1 shot, and by 90.8% after 3 shots with a shift of 20 nm toward lower wavelength for the latter. The 0.97 μm absorption band broadens after irradiation, and the minimum shifts to 1.04 and 1.0 μm after 1 and 3 shots. Absorption bands linked to water and hydroxyl seem to have mostly disappeared, or strongly reduced as for the H2O/OH features at 1.43 and 3.1 μm corresponding to the stretching mode of the hydroxyl groups. In this area, there is an upturn at 3.13 μm for unaltered goethite, which begins at 3.2 μm after irradiation.Figure 2: Spectra of nontronite and goethite minerals before and after laser ablation experiments. Summary and perspective: Our experiments show different behaviors between nontronite and goethite after laser shock alteration. In the case of nontronite, the absorption bands linked to Fe3+ are more impacted by irradiation than H2O/OH features. Conversely, in the case of goethite, the two iron bands at 0.65 and 0.97 μm are strongly impacted but remain visible whereas H2O/OH features mostly disappear. This result suggests that the observation of H2O/OH features related to goethite on airless bodies exposed to micrometeorite impacts like Phobos and Deimos, is unlikely.Additional samples will be studied in the future using the same protocol, in particular other phyllosilicates (biotite, antigorite and montmorillonite), as well as an unweathered basalt, to investigate further the effect of micrometeorite bombardment into mineral phase signatures. References :[1] Barucci et al., EPS, 2021[2] Kuramoto et al., EPS, 2022[3] Murchie et al., JGR:P, 1999[4] Gendrin, Langevin & Erard, JGR:P, 2005[5] Fraeman et al., Icarus, 2014[6] David et al., submitted[7] Beck et al., A&A, 2011[8] Giuranna et al., PSS, 2011[9] Pieters et al., MPS, 1998[10] Donaldson Hanna et al. JGR:P, 2017[11] Matsuoka et al., AJL, 2020[12] Maurice et al., SSR, 2021[13] Wiens et al., SSR, 2021[14] Potin et al., AO, 2018

Context and motivationUnder its thick atmosphere, Titan hosts a unique variety of geomorphological features [1] including indications of evidence of fluvial and/or pluvial activity [2,3,4]. Rivers, canyons, and alluvial fans have been mapped by the RADAR on board the Cassini probe which explored the cronian System from 2004 to 2017 [5]. Operating at a wavelength of 2.2-cm, the Cassini RADAR measured the variations of the surface normalized backscatter cross section coefficient (NRCS or σ°) in its active mode, while assessing the surface’s 2.2-cm emissivity (e) in its passive mode [6]. By virtue of the Kirchhoff's law of thermal radiation, these two quantities (σ° and e) are expected to be anti-correlated. However, they can sometimes be correlated, meaning they are sensitive to different properties of the surface/subsurface and can therefore bring complementary information. Typically, σ° is very sensitive to surface roughness while e is tightly related to near-subsurface composition and temperature (through brightness temperature).Investigating the relationship between σ° and e may highlight regions with peculiar structural and/or composition properties or temperature anomalies. In particular it has been proposed as a way to detect (cryo-)volcanic hot spots on Titan and Venus [7].For this work, we have analysed the relationship between σ° and e considering all active and passive data acquired by the Cassini RADAR during the mission. This investigation has led to the detection 10 areas for which σ° and e are highly correlated. Relying on the geomorphological interpretation of these areas as well as on a combined radar and radiometry model we propose an explanation for this correlation which involves the presence of sediment deposition layers.MethodsBy the end of the Cassini mission, the Cassini RADAR had recorded simultaneously the backscatter σ° and brightness temperature Tb of about 65% of Titan’s at a resolution of, at best, 5km (the size of the real-aperture footprint on the ground). Tb is easily converted into emissivity thanks to the surface temperature model derived from the Composite InfraRed Spectrometer (CIRS) observations [8]. We developed a code investigating the correlation between e and σ° and in particular to search for areas where these two quantities are spatially correlated during at least 15 consecutive footprints. We have thus identified ten regions of interest (RoI) where the correlation coefficient is greater than 0.9 (Fig. 1b). Most of them (eight) are located in the austral hemisphere, and seven in the polar region (Fig. 1a).Figure 1: a) Geographical location of the RoIs where e and σ° are spatially correlated (correlation coefficient >0.9) overlaid on a global VIMS mosaic of Titan’s surface [9]. b) Scatterplot of σ° versus e for the T59 swath (black dots) and the RoIs identified in this swath (blue dots).Geomorphological analysis: The case of T592For each RoI, we generated a comprehensive set of maps (Fig. 2 for the T592 RoI, highlighted in red on Fig. 1a), including a SAR image (Fig. 2a), an emissivity map (Fig. 2a), and a geomorphological analysis of the RoI and adjacent terrains (Fig. 2c). Terrain units were discerned from SAR images, building upon the framework established by [1] and tailored to the specific scale of this study. Figure 2: Cassini SAR image a), emissivity map b) and geomorphological analysis c) of a portion of the T59 RADAR swath where a RoI was identified.In the case of T592 RoI, the vicinity of the correlation zone includes labyrinthic terrains, plains and rivers flowing into a large radar-dark region of low altitude (according to topography data) identified as a possible dry paleolake. The RoI is centred on a triangularly-shaped radar-dark feature of dimension 19x47km2, crossed by a canyon (digged by rivers) and tagged as an alluvial fan. A similar but smaller triangular dark feature can be seen on the west despite not being detected.Comparison to a combined radar and radiometer modelConsistent with our geomorphological analysis, we propose that the correlation between e and σ° could be explained by the presence of two superimposed layers with the bottom layer having a lower permittivity and a larger roughness than the top layer. Top layer could be made of fine-grained organic sediments deposited over a rough substrate of water ice or solidified organics. Such hypothesis is supported by the two-layer radar and radiometry model we developed based on [9,10] for e and [11] for σ° (Fig. 3). Variations along the correlation slope would be simply due to variations of the thickness of the upper layer from one radar/radiometry footprint to the other.Figure 3: a) 2-layer model displaying the roughness of the upper and lower layers and the distance between the interfaces (i.e. upper layer thickness). The top layer has a permittivity of 2+0.001j and the bottom layer of 3+0.0001j. d) Scatterplot of the simulated emissivities and σ° with the computed correlation coefficient.Discussion and PerspectivesWe have found that, among the ten RoIs detected as exhibiting a strong positive correlation between Cassini active and passive RADAR dataset, six are geomorphologically consistent with areas of sediment deposition as they exhibit features due to fluvial activity in an ancient lake system. In this paper, we will further describe these regions and how the joint analysis of active and passive radar data can serve as a tool for detecting them. References[1] Lopes+2020[2] Stofan+2007[3] Malska+2016[4] Malaska+2020[5] Elachi+2005[6] Paganelli+2008[7] Lorenz+2016[8] Jennings+2019[9] Le Mouelic+2008[10] Ulaby & Long2014[11] Le Gall+2016[11] Hayes+2

There are various indications that the most primitive small bodies (P, D-type asteroids, comets) have surfaces made of intimate mixtures of opaque minerals and other components (silicates, carbonaceous compounds, etc.) in the form of sub-micrometre-sized grains, smaller than the wavelength at which they are observed, so-called hyperfine grains. Here, we investigate how the Vis-NIR-MIR spectral and V-band polarimetric properties of surfaces made of hyperfine grains are influenced by the relative abundance of such hyperfine materials, having strongly different optical indexes. Mixtures of grains of olivine and iron sulfide (or anthracite), as analogues of silicates and opaque minerals present on small bodies, were prepared at different proportions. The measurements reveal that these mixtures of hyperfine grains have spectral and polarimetric Vis-NIR properties varying in strongly nonlinear ways. When present at even a few percent, opaque components dominate the Vis-NIR spectral and polarimetric properties, and mask the silicate bands at these wavelengths. The Vis-NIR spectral slope ranges from red (positive slope), for pure opaque material, to blue (negative slope) as the proportion of silicates increases, which is reminiscent of the range of spectral slopes observed on P, D, X, C- and B-types asteroids. The spectra of the darkest mixtures in the Vis-NIR exhibit the absorption bands of Si-O in olivine around 10 m in the MIR, which is observed in emission for several small bodies. This work shows that both the contrasted optical indexes of the components, and the dispersion or aggregation (depending on their relative proportions) of their hyperfine grains, induce different light scattering regimes in the Vis-NIR and MIR, as observed for primitive small bodies. The optical separation of hyperfine grains seems to be a major parameter controlling the optical properties of these objects.

While Saturn's main airless moons are all composed largely of water ice, their respective thermal histories and near environments have led to different regolith compositions and structures. Part of this history is recorded in their subsurface which can be probed by microwaves. Using a combined thermal and radiative transfer model, we here investigate all distant observations acquired in the passive mode of the RADAR on board the Cassini spacecraft (2004–2017) at 2.2-cm wavelength. The joint analysis of the derived disk-integrated emissivities and published radar albedos provides new insights into the purity and maturity of the regolith of Saturn's icy moons. We find that satellite-to-satellite variations and large-scale regional anomalies in microwave signatures primarily reflect different degrees of contamination of the regolith by non-ice compounds. To a lesser extent, they may also point to different concentrations of scatterers in the subsurface; these scatterers must be made of ice and/or void rather than of non-ice contaminants. Enceladus appears to have the cleanest regolith likely due to the geological youth of its surface. Observations also suggest that the current heat flux emanating from this moon is not confined to the South Pole Terrain. In the inner system, the degree of purity of the satellites' regoliths decreases from Enceladus outward likely due to the decrease of the E-ring influx. In the outer system, Phoebe's ring mantles Iapetus' leading hemisphere with a decimetric layer of optically-dark and microwave-absorbent dust. Dione is surprisingly less radar-bright and more emissive than expected from both the observed general trend and the current understanding of its geological history. Another question remains outstanding: why are Saturnian moons, and to a lesser extent Jovian moons, so radar-bright at centimetric wavelengths? Current models assuming purely-random scattering in their subsurface fail to simultaneously reproduce active and passive microwave observations, especially for Saturn's inner moons. This may be due the presence of organized and especially efficient backscattering structures in their subsurface. The challenge is now to identify structures that are geologically plausible.

The Cassini mission explored Saturn’s system from 2004 to 2017. On its board, a RADAR operating at a wavelength of 2.2 cm (13.78 GHz), had been initially designed for the exploration of the surface of Titan but also regularly turned its antenna towards the main airless icy satellites of Saturn (Elachi et al., 2004). In addition to its active mode, the Cassini RADAR included a passive (or radiometry) mode designed to record the thermal emission from the targeted surfaces at 2.2-cm. The scientific objectives of Cassini RADAR icy satellite observations were to provide constraints on the thermal, physical and compositional properties of the first few meters below the surface of the investigated objects. Doing so, it brings insights into the degree of purity and maturity of their water-ice regolith which are both indicative of their geological activity and interaction with their environment. The RADAR dataset acquired on icy moons has already proved to be very fruitful bringing light to notable differences among Saturn’s mid-sized satellites (Ostro et al., 2006; 2010; Le Gall et al., 2019). However, it has not been fully analyzed yet. Following the final analysis of Cassini RADAR active observations of Saturn’s icy moons described in Le Gall et al. (2019) and expanding and improving upon the work of Ostro et al. (2006; 2010), we here present the analysis of all Cassini distant passive RADAR observations of these objects. This represents a total of 63 observations collected during 4 flybys of Mimas, 10 of Enceladus, 3 of Tethys, 6 of Dione, 9 of Rhea, 3 of Iapetus, 1 of Phoebe. Most of Cassini RADAR icy satellite observations were distant i.e., occurred at ranges where the antenna beamwidth is comparable to or greater than the apparent angular extent of the target’s disk and were thus primarily designed to provide disk-integrated quantities: hemispheric-averaged radar albedos in the RADAR active mode and disk-integrated brightness temperatures in the passive mode. We here present the reduction of all available Cassini passive radiometry data with the goal of providing a range of possible values for the disk-averaged 2.2-cm emissivity of Saturn’s main airless satellites (separating their leading and trailing sides if relevant). These latter are obtained as a function of their possible thermal and electrical properties using a combined thermal and radiative transfer model (Le Gall et al., 2012; Bonnefoy et al., 2020). As an example, Fig. 1 displays the emissivity values obtained for Mimas from 4 distant radiometry observations. These values are shown as a function the assumed thermal inertia and ratio of electrical and thermal skin depths of Mimas’s near-surface. For all sets of parameters they are very low, as low as 0.5. For comparison, the disk 2.2-cm emissivity of Iapetus (Le Gall et al., 2014), Phoebe and Titan (see Sultana et al., this conference) is close to 0.9. Mimas low emissivity is indicative of subsurface mostly made of pure water ice and where volume scattering is very efficient maybe due to a highly fractured structure. We find that Enceladus and Tethys also exhibit low emissive surfaces and that, as a general rule, moon-to-moon and hemispheric emissivity variations seems to reflect variations in the moon interaction with Saturn’s dust rings, namely the E-ring for Enceladus and its neighbours and Phoebe’s ring further away from Saturn (Iapetus and Phoebe). The derived emissivities will be analysed in light of the (active) radar albedos measured on the same hemispheres. Both active and passive microwave observations will be compared to several combined emissivity-backscatter models thus providing further clues on the physical properties of the icy moons. Their implications in terms of surface geology and evolution will be discussed. Fig. 1: 2.2-cm emissivity of Mimas’s surface derived from Cassini distant radiometry observations and a combined thermal and radiative transfer model as a function of Mimas subsurface thermal and electrical properties (namely its thermal inertia and the ratio of its electrical and thermal skin depths). The sub-spacecraft point of each distant observation is indicated on an ISS map of the satellite.

Introduction Solar System small bodies are presumed relics from the eve of the Solar System, as they were the first objects to accrete inside the protoplanetary disk. The P-/D-type asteroids are particularly interesting because of the similarity of their spectra, in the visible (Vis) and near infrared (NIR) wavelengths, with cometary nuclei, suggesting that they are the most primitive types of small bodies [1-3]. In the mid-infrared (MIR), emission spectra of both P-/D-type asteroids surfaces and cometary comae display a signature around 10 µm due to the fundamental mode of vibrations of Si-O in silicates [2, 4-5]. P-/D-type asteroids are among the low-albedo class, and are characterized by linear polarimetric phase curves in V-band with a minimum of polarization around 1.2 ± 0.4 % and an inversion angle around 8.5 ± 2.5° [6]. There are various indications that the surface of these primitive small bodies are covered by so-called “hyperfine” grains, with individual grain sizes smaller than the wavelength at which they are observed (< 1 µm) [7]. The spectral characteristics of these objects in the Vis-NIR and MIR have also been attributed to their peculiar micro-texture and grain size [4, 5]. Here, we investigate how the Vis-NIR spectra, MIR spectra and Vis polarimetric phase curves of surfaces made of hyperfine grains are influenced by the relative abundance of materials having strongly different optical indexes. Methods We used olivine and iron sulphide (a mixture of pyrrhotite and troilite; labelled as “FeS”) having contrasted optical indexes. Following a dedicated grinding protocol, we have produced grains of average diameter ranging from 0.3 to 0.6 µm, as imaged by electron microscopy [8]. Mixtures were produced by mixing the two powders in a mortar manually with the pestle for about 10 min. Reflectance spectra in the Vis-NIR range were obtained at IPAG with the SHADOWS instrument [9] (emergence e=30º, incidence i=0º). MIR reflectance spectra were obtained using a Brucker Vertex 70V FT-IR spectrometer equipped with a reflectance kit A513/QA. Because small bodies MIR observations are emissivity spectra, we show on Figure 1b the experimental spectra as “1 - reflectance” to approximate their emissivity spectra according to the Kirchhoff’s law. Polarimetric phase curves were measured at the University of Bern with the POLICES instrument at 530 nm [10]. Results Figure 1a presents the evolution of normalized reflectance spectra of olivine-FeS mixtures with decreasing volume concentration of olivine. We observe a general decrease of reflectance, associated to a modification of the spectral slope, as the concentration of FeS (opaque in the Vis) increases, with a blueing followed by a reddening of the spectra (Fig. 1a). The measurements reveal that mixtures of hyperfine grains made of two components with contrasted optical indexes have spectral and polarimetric properties which varies in strongly nonlinear way in the Vis-NIR (Fig. 1a,b, Fig. 2). In the MIR, while the spectra of the endmembers are relatively flat, the spectra of mixtures containing high concentrations of FeS exhibit the absorption bands of Si-O in the olivine around 10 µm. Discussion Spectra of mixtures of olivine and iron sulphide (or anthracite, not shown here) exhibit an emissivity feature in the MIR and various degree of bluing or reddening in the Vis-NIR, as observed on several small bodies (Fig. 1). Moreover, the same mixtures exhibiting the 10-µm feature also have a polarimetric phase curve similar to P-/D-type asteroids (Fig. 2). Spectra of pure olivine, or mixtures with high concentration of olivine exhibit spectral features mainly due to reflectivity effects. As the concentration of FeS grains increases, these features are progressively replaced by the absorption bands of olivine material, maximum around 10 to 11 µm. In these cases, the olivine grains are well dispersed in a matrix of FeS grains which diffuse the light and enable photons to escape the sample after some absorption by olivine grains. A similar mechanism was pointed out with mixtures in KBr [5], but while mixtures with KBr are very reflective, mixtures with FeS have a much lower reflectance, compatible with the low reflectance and high emissivity values observed on small bodies. The resemblance between mid-IR spectra of our hyperfine mixtures and P-/D-type asteroids emission feature (Fig. 1d) implies that elevated porosity is not a requirement for the presence of a silicate signatures at 10 µm. We show here that a relatively compact surface (porosity of the order of 50 %) exhibits similar mid-IR feature as cometary dust tails. An interpretation that can be proposed is that in both cases an optical separation of olivine grains occurred, whether by vacuum in the case of comae, or by optically featureless grains in the case of P-/D-type asteroids. Finally, we note that some mixtures of hyperfine grains (such as olivine-FeS 10:90 vol%) exhibit altogether a red spectral slope in the Vis-NIR, a 10-µm feature in the MIR, and a V-band polarimetric phase curve similar to P-/D-type asteroids, reinforcing the hypothesize that these bodies are made of powdery mixtures of hyperfine grains. Acknowledgments: We acknowledge funding from the European Research Council (ERC) (SOLARYS ERC-CoG2017_771691) References: [1] Capaccioni et al. (2015) Science 347, 6620. [2] Vernazza and Beck (2017) in Planetesimals, Cambridge Univ. Press. [3] Poch et al. (2020) Science 367, 6483. [4] Emery et al. (2006) Icarus 182, 496-512. [5] Vernazza et al. (2012) Icarus 221, 1162–1172. [6] Belskaya et al. (2017) Icarus 284, 30–42. [7] Levasseur-Regourd et al. (2018) Space Sci Rev 214. [8] Sultana et al. (2021) Icarus 35, 11412. [7] Mustard and Hays (1997) Icarus 125, 145-163. [9] Potin et al. (2018) App. Optics 57, 28. [10] Poch, (2018) JGR Planets, 123.

* Introduction During its 13 years of mission in the cronian system, Cassini performed 127 flybys of Titan. On board the spacecraft, the RADAR, operating as a (passive) radiometer, recorded the 2.2-cm thermal emission from the surface. All resolved radiometry observations were combined to build a global map of Titan's surface brightness temperature [4]. In the calibration process, this data was corrected to a common season and the possibility of diurnal signal was ignored. In this work, we focus on the unresolved radiometry observations of Titan as they represent an opportunity to search for potential seasonal and diurnal signatures. This data was acquired from long range i.e., in a configuration where the antenna beam footprint is commensurate or larger than Titan’s disk ([6], [11]). It has been reduced to compute Titan’s disk-integrated brightness temperature. 118 of them are available; they were collected for different sub-spacecraft points (Figure 1 (a)), at different local hours and epochs of the cronian year. Figure 1: (a) ISS map of Titan’s surface showing the locations of the sub-spacecraft points of the 118 distant radiometry observations of Titan. Xanadu regio is highlighted in red. (b) Tbdisk as a function of Titan’s longitude as measured by the Cassini radiometer. * Longitudinal variations Titan’s surface brightness temperature depends both on the surface emissivity and the effective physical temperature of the surface. At first order, this latter can be regarded as nearly constant and predictable. The main source of variations in the disk-integrated brightness temperature Tbdisk derived from distant radiometry observation is related to spatial variations of the surface emissivity. Indeed, Tbdisk latitudinal variations clearly show the signature of Xanadu, a large-scale low emissivity/radar-bright feature on the leading side of the satellite ([12], [1], [13], [7]) (Figure 1 (a)).When 25% of the observable disk contains Xanadu, the disk-integrated brightness temperature is reduced by more than 1 K. For the study of Tbdisk variations as a function of time (hour or season), we therefore only consider data that were acquired on disk containing less than 1% of Xanadu. * Monitoring seasonal changes Cassini arrived at Saturn as it was in the late northern winter (July 2004) and the mission ended just after the northern summer solstice (Sept. 2017). Distant radiometry observations were collected regularly all along the 13 years of observations of the Cassini mission that is during half a cronian year. As such, they provide a valuable dataset to investigate the seasonal variations of Titan’s surface temperature, if any. As expected no significant variations are observed for data centred on the equatorial regions, where the seasonal effects are limited. Figure 2 displays the Tbdisk of observations centred on Titan’s poles as a function of the epoch of the year. It clearly shows the warming of the north pole and the cooling of the south one, consistent with CIRS observations [5]. This implies that the radiometer probing depth is smaller than the seasonal thermal skin depth. By comparison to a radiative transfer model in a wet sand sub-surface, we estimate the probing depth to be of at most a few meters. The comparison of the CIRS and RADAR datasets also allow to better estimate the disk-integrated emissivity of Titan’s surface: 0.93±0.01 excluding Xanadu, and 0.92±0.01. Figure 2: Disk-integrated brightness temperature of Titan’s poles as a function of time derived from CIRS and Cassini RADAR radiometer measurements. Measurements in the southern hemisphere are plotted in blue, and in red for the northern. * Absence of diurnal signal From the analysis of CIRS dataset, Cottini et al., 2012 [3] report a diurnal signal of 1-1.5K indicative of a thermal inertia of 300-600MKS. However, Cassini distant radiometry observations show no specific variations with the local hour of the sub-spacecraft point. This implies that the radiometer probe deeper depths than the diurnal thermal skin depth. By comparison to a radiative transfer model in a wet sand subsurface, we estimate the probing depth to be of at least a few tens of centimeters. * Comparison with Earth-based observations Cassini distant radiometry observations can be directly compared to ground-based observations. In particular, the disk-integrated brightness temperature of Titan was measured at 3.5 cm from the Very Large Array (VLA) radio-telescope in 1992 ([8], [2]). This dataset was used to produce the light curve displayed in Figure 3 which exhibits a puzzling minimum of Tbdisk around 0° longitude, therefore shifted by more than 90° with respect to the location of Xanadu and the light curve derived from Cassini data. This discrepancy will be discussed during the presentation as well as comparison with other earth-base observations. Figure 3: Light curve at 2.2 and 3.5 cm measured respectively by Cassini and the VLA. * Emissivities in Saturn's system Among all major cronian satellites, Titan exhibits the highest emissivity. When the innermost moons interacting with Saturn's E-ring have present lower emissivities (0.5-0.8), Titan's is much closer to Pheobe and Iapetus trailing values (0.8-0.9) suggesting a near sub-surface where the ice is less present (see Le Gall et al., this conference, [10]). * Conclusions With the good time and spatial sampling that Cassini distant observation provides, we monitored the regional and temporal variations of Tbdisk on Titan, giving insights on its surface and the interaction with its atmosphere. * Acknowledgments We sincerely thank the Institut Universitaire de France for funding this work. * References [1] Barnes et al., (2005). Science, DOI:10.1126/science.1117075. [2] Butler and Gurwell, (2004). AAS/DPS [3] Cottini et al., (2012). Planetary and Space Science, DOI:10.1016/j.pss.2011.03.015. [4] Janssen et al., (2016). Icarus, DOI:10.1016/j.icarus.2015.09.027. [5] Jennings et al., (2016). The Astrophysical Journal, DOI:10.3847/2041-8205/816/1/L17. [6] Le Gall et al., (2014). Icarus, DOI:10.1016/j.icarus.2013.06.009. [7] Lemmon et al., (1993). Icarus, DOI:10.1006/icar.1993.1074. [8] Muhleman et al., (1993). 25:25.01. AAS/DPS [10] Ostro et al., (2006). Icarus, DOI:10.1016/j.icarus.2006.02.019. [11] Ostro et al., (2010). Icarus, DOI:10.1016/j.icarus.2009.07.041. [12] Radebaugh et al., (2011). Icarus, DOI:10.1016/j.icarus.2010.07.022. [13] Smith et al., (1996). Icarus, DOI:10.1006/icar.1996.0023.

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

The composition of Solar System surfaces can be inferred through reflectance and emission spectroscopy, by comparing these observations to laboratory measurements and radiative transfer models. While several populations of objects appear to be covered by sub-micrometre sized particles (D < 1 mu m) (referred to as hyperfine), there are limited studies on reflectance and emission of particulate surfaces composed of particles smaller than the visible and infrared wavelengths. We have undertaken an effort to determine the reflectance of hyperfine particulate surfaces in conjunction with high-porosity, in order to simulate the physical state of cometary surfaces and their related asteroids (P- and D-types). In this work, we present a technique developed to produce hyperfine particles of astrophysical relevant materials (silicates, sulphides, macromolecular organics). This technique is used to prepare hyperfine powders that were measured in reflectance in the 0.4-2.6 mu m range. These powders were then included in water ice particles, sublimated under vacuum, in order to produce a hyperporous sample of hyperfine material (refers as to sublimation residue). When grinded below one micrometre, the four materials studied (olivine, smectite, pyroxene and amorphous silica), show strong decrease of their absorption features together with a blueing of the spectra. This "small grain degeneracy" implies that surfaces covered by hyperfine grains should show only shallow absorption features if any (in the case of moderately absorbing particles as studied here). These two effects, decrease of band depth and spectral blueing, appear magnified when the grains are incorporated in the hyperporous residue. We interpret the distinct behaviour between hyperporous and more compact surfaces by the distancing of individual grains and a decrease in the size of the elemental scatterers. This work implies that hyperfine grains are unabundant at the surfaces of 5- or V-type asteroids, and that the blue nature of B-type may be related to a physical effect rather than a compositional effect.

The Dawn mission found that the dominant colour variation on the surface of dwarf planet Ceres is a change of the visible spectral slope, where fresh impact craters are surrounded by blue (negative spectral-sloped) ejecta. The origin of this colour variation is still a mystery. Here we investigate a scenario in which an impact mixes the phyllosilicates present on the surface of Ceres with the water ice just below. In our experiment, Ceres analogue material is suspended in liquid water to create intimately mixed ice particles, which are sublimated under conditions approximating those on Ceres. The sublimation residue has a highly porous, foam-like structure made of phyllosilicates that scattered light in similar blue fashion as the Ceres surface. Our experiment provides a mechanism for the blue colour of fresh craters that can naturally emerge from the Ceres environment.