SB3

Ceres, dwarf planet of the main asteroid belt, is considered a relic ocean world since the Dawn mission discovered evidences of aqueous alteration and cryovolcanic activity [1]. Unexpectedly, a variety of ammonium-rich minerals were identified on its surface, including phyllosilicates, carbonates, and chlorides [2]. Although from the Dawn’s VIR spectroscopic data it was not possible to specify the exact type of phyllosilicates observed, montmorillonite is considered a good candidate owing to its ability to incorporate NH₄⁺ in its interlayers [3]. Ammonium-rich phases are usually found at greater distances from the Sun. Hence, the study on their stability at environmental conditions relevant to Ceres’ interior and of its regolith can help elucidate certain ambiguities concerning the provenance of its precursor materials.

In this study, it was investigated the changes in the spectroscopic signatures of the clay mineral montmorillonite after (a) being immersed in ammonium chloride aqueous solution and, subsequently, (b) washed with deionized water. After each treatment, samples were submitted to different environmental conditions relevant to the surface of Ceres. For one experiment, they were frozen overnight at 193 K, and then subjected to 10^-5 bar for up to 4 days in a Telstar Cryodos lyophilizer. For the other, they were placed inside the Planetary Atmospheres and Surfaces Chamber (PASC) [4] for 1 day at 100 K and 5.10^-8 bar. The combination of different techniques, i.e., Raman and IR spectroscopies, XRD, and SEM/EDX, assisted the assignment of the bands to each particular molecule. In this regard, the signatures of the mineral external surface were distinguished from the interlayered NH₄⁺ cations. The degree of compaction of the samples resulted crucial on their stability and spectroscopic response, being stiff smectites more resistant to low temperatures and vacuum conditions. In ground clay minerals, a decrease in the basal space with a redshift of the interlayered NH₄⁺ IR band was measured after just 1 day of being exposed to vacuum conditions.

Acknowledgments

This work was supported by the Spanish MINECO projects ESP2017-89053-C2-1-P and PID2019-107442RB-C32, and the AEI project MDM‐2017‐0737 Unidad de Excelencia “María de Maeztu”.

References

[1] De Sanctis et al.,  Space Sci. Rev. 216, 60, 2020

[2] Raponi et al., Icarus 320, 83,  2019

[3] Borden and Giese, Clays Clay Miner. 49, 444, 2001

[4] Mateo-Marti et al., Life 9, 72, 2019

How to cite: Munoz-Iglesias, V., Fernández-Sampedro, M., Gil-Lozano, C., J. Bonales, L., Ercilla Herrero, O., Valles González, M. P., Mateo-Martí, E., and Prieto-Ballesteros, O.: Characterization of NH4-montmorillonite coexisting with NH4Cl salt at different aggregation states. Application to Ceres., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-11, https://doi.org/10.5194/epsc2021-11, 2021.

Introduction

The VIR spectrometer on board the NASA’s Dawn spacecraft allowed providing important clues  the mineralogical composition of the Ceres regolith (De Sanctis et al., 2015) and of the bright areas widespread across its surface (Palomba et al., 2019; Carrozzo et al., 2018). Some bright spots are thought to be the result of phenomena like cryovolcanism (Ruesch et al., 2016; Russell et al., 2016) and post-impact hydrothermal activities (Bowling et al., 2016). The study of Ceres bright areas is important to understand in more detail the mineralogical composition of the subsurface materials that could host water ice (Prettyman et al., 2017; Schmidt et al., 2017) or have been under aqueous alteration (De Sanctis et al., 2016). 

In this study different bright areas of Haulani crater (e.g. Southern floor, i.e. ROI3 and North-east crater wall, i.e. ROI4) on Ceres have been studied by creating different analogue mixtures and comparing them with Dawn VIR data. The end-members have been identified based on previous studies (Tosi et al. 2018, 2019) and the analogue mixtures have been produced with grain size 50-100 µm for two bright crater regions. The spectra of two initial analogue mixtures have been acquired in the VIS-NIR spectral range (0.35-4.5 µm) at low temperature, i.e. from 200 K to 300 K similar to Haulani by using the Cold Spectroscopy Facility (CSS; https://cold-spectro.sshade.eu) (IPAG, France).

Scientific goals and method

The main scientific objectives of this study are: 1) the study of two different bright areas of Haulani crater (hereafter called ROI3 and ROI4, Figure 1) on Ceres in order to study the mineralogy  of the sub-surface materials starting from results inferred by Dawn VIR; 2) the identification of the end-members of mineral mixtures of bright areas and production of endmembers and analogue mixtures  with grain size 50-100 µm; 3) the acquisition of reflectance spectra of end-members and analogue mixtures in the VIS-NIR spectral range (0.35-4.5 µm); 4) the analysis of appropriate spectral parameters of reflectance spectra and comparison with those obtained by VIR data.

In particular, spectral parameters of mixtures will be estimated, focusing on Band Center (BC), Band Depth (BD), Full Width Half at Maximum (FWHM) of bands, reflectance level and spectral slopes (estimated between 1.2 and 1.9 µm). The spectral parameters of analogue mixtures have been compared with the VIR data corresponding to the selected area in order to constrain their mineralogical composition.

Data analysis and conclusion

Two analogue mixtures (50-100 µm), here called A3-1 and A3-2 have been produced by using the end-members Antigorite (Mg-phyllosilicate); NH₄-montmorillonite (ammoniated phyllosilicate); anhydrous Sodium Carbonate (Na-carbonate); Graphite (dark component), Illite (Phyllosilicates) to simulate the two bright crater regions (Figure 1, i.e. southern floor and red spot or ROI3, i.e. north-east inner crater wall or ROI4). Reflectance spectra of the two mixtures have been acquired in the VIS-NIR spectral range (0.35-4.5 µm) at cold temperature, i.e., from 200 K to 300 K (phase angle of 30°) with the SHINE spectro-gonio-radiometer equipped with the CARBONIR simulation chamber (sample in inner cell filled with few mbar of pure N₂ gas) at the Cold Spectroscopy Facility (CSS) in IPAG, France (Figure 1, Right). Finally, the analysis of spectral parameters of the reflectance spectra (mainly relative to the absorption bands at 2.7, 3.1, 3.4 µm) and the comparison with VIR data have been performed. The acquired spectra have been finally converted in radiance factor. 

A first analysis shows that the Mixture A3-1 and A3-2 are not well representative due to the high amount of dark components (up to 86 % for A3-1) and missing Na-carbonate bands (for A3-2). Thus, the A3-2 has been modified (by producing the intermediate mixture) and by reaching 9 % for Na-carbonate, 32 % of dark component (i.e. carbon black) and 25 % of NH4-Montmorillonite in the final mixture named as A3-8. Finally, graphite and NH4-montmorillonite have been added to the A3-8 mixture, obtaining the last mixture A3-9. Thanks to carbon black the reflectance level compared with Haulani spectra is more similar. The analysed mixture were heated in the furnace in air at 120°C for 2 hours before each measurement and then placed in the sample holder under vacuum to remove the adsorbed H₂O.

The mixtures with a reflectance spectrum similar to the spectra of ROI3 and ROI4 have been analysed in detail. By the spectral analysis, the Mixture A3-8 shows the most representative reflectance spectrum for the Haulani’s areas of interest (even if the difference in the reflectance level is probably due to opaque end-member composition) and exhibits BD values for the 2.7, 3.1 and 3.4  µm bands that are the closest one to the ROI3 and ROI4. The width of the 3.1 µm band (3.1FWHM) of A3-8 has a value similar  to the ROI4 (about 0.15). In particular, the 2.7 BD is about 13% lower than ROI3 and ROI4, the 3.1BD is 5-9% higher while the 3.4BD has the same value of ROI4 and 11% lower than ROI3. A more in-depth analysis of the data is in progress.

Besides, in order to better reproduce Haulani areas some improvements may be performed, e.g., by adding a low amount of hydrous natrite , e.g. 2-8%, to assess the role of this component found in Haulani bright areas and how it contributes to the 2.7 µm spectral band.    

References

Carrozzo, F.G., et al., 2018. Nature, Sci. Adv. 4 (3); De Sanctis. M. C. et al., 2015. Nature 528, pp. 241-244; De Sanctis, M.C., et al., 2016. Nature 536. Issue 7614, 54–57; Palomba, E., et al., 2019. Icarus 320, 202–212; Prettyman T. H., et al., 2017. Science 355:55–59; Ruesch, O., et al., 2016. Science 353 (6303); Russell, C.T., et al., 2016. Science 353 (6303), 1008–1010; Schmidt B. E. et al., 2017. Nature Geoscience 10:338–343; Tosi, F. et al., 2018. M&P Sci. 53, Nr.9, pp. 1902-1924; Tosi, F. et al., 2019. Icarus 318, pp.170-187.

How to cite: Dirri, F., Galiano, A., Longobardo, A., Palomba, E., Schmitt, B., Beck, P., Poch, O., and Brissaud, O.: VIS-NIR reflectance analysis of analogue mixtures representative of Haulani crater on Ceres to assess the mineralogical composition of bright areas, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-397, https://doi.org/10.5194/epsc2021-397, 2021.

Summary

Ceres is the largest object in the Solar System main belt. Clearly, Ceres experienced extensive water-related processes and geochemical differentiation and nowadays it is a body with a complex geological and chemical history [1]. Its surface is characterized by dark materials, phyllosilicates, ammonium-bearing minerals, carbonates, water ice, and salts. In addition to a global presence of carbon bearing chemistry, local concentration of aliphatic organics has been detected by Dawn [2].

In this context, we have started a series of laboratory spectroscopy measurements targeted to study the physicochemical interactions between organic material and minerals possibly present on Ceres. The goal is to understand the transformations induced on these samples by ultraviolet radiation, neutral atoms, and fast ions, under experimental conditions that simulate the environment of Ceres. The spectroscopic data obtained in laboratory experiments allow, through the comparison with the observations of the VIR spectrometer aboard the Dawn mission, to clarify the nature and origin of organic material identified on Ceres. 

Introduction

Organic material in the minor bodies of the Solar System is an important component to understand planetary evolution and, eventually, the origin of life. Nevertheless, our knowledge on the subject is still limited. Recently the Dawn mission, thanks to the data collected by the Italian instrument VIR [3], showed clear evidence of a high amount of aliphatic organic material on the surface of Ceres [4, 5, 6] (Fig 1). This evidence has raised new questions about the origin and preservation of this material, especially when considering its high estimated abundance and the mineralogical context.

Fig 1 Ceres spectra of the organic-rich area in Ernutet crater (label “Organics”); of a background organic-poor area from a region southeast of Ernutet (label “Background”); and of Occator bright material (label “Carbonate”) [4].

In order to understand the organic chemical species and in particular their abundance on Ceres, laboratory studies were performed [7]. The importance of having a direct comparison between laboratory and remote sensing data can provide a further investigation clues to shed light on the origin and evolution of Ceres. Through this project, we intend to study, through dedicated experiments, the interaction between minerals, water, and organic concerning the environmental conditions of Ceres. Making a synergistic use of complementary and indispensable skills present within INAF (Italian National Institute of Astrophysics) laboratories we investigated a complex issue such as that concerning the origin and preservation of organic molecules on planetary surfaces. Within INAF, complementary and unique realities coexist which, thanks to joint and coordinated work, can give a new interpretation of the physical-chemical processes active on Ceres.

Project development and results

In this study, we prepare mixtures of materials resembling the Ceres surface composition [8, 9] adding organic molecules in order to:

(i) understand how organic molecules behave and eventually degrade on Ceres, in particular, how aliphatic molecules degrade by energetic processing with fast ions (keV-MeV) and UV photons [10, 11]. Moreover, the physico-chemical properties of the materials exposed to a flux of neutral atoms are investigated [12, 13].

(ii) evaluate the interaction between ammoniated minerals and simple organic molecules that may lead to the synthesis of complex compounds. In the presence of ultraviolet (UV) radiation, these minerals present on the surface of Ceres can show photocatalytic effects accelerating the photo-reactions, which generally destroy the original organic molecule and in the synthesis of new complex organic molecules [14].

(iii) evaluate the role of minerals in the protection or degradation of organic compounds. Some studies indicated a fundamental role of clays in the catalysis and preservation of organic materials [15]. Ceres is rich in clays and other hydrated minerals, making the interactions with the observed organics of particular interest.

The project is carried out by several INAF institutes and laboratories. In detail: INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali prepared the analog mineral mixtures taking into account the compositional information gained by VIR observations.  INAF - Osservatorio Astrofisico di Arcetri subsequently doped the mixture with several organic investigating UV photostability in Ceres analog conditions and the influence of temperature. INAF -Osservatorio Astronomico di Capodimonte studied irradiation with atoms and temperature effect while INAF - Osservatorio Astrofisico di Catania performed irradiation with fast ions. Finally, results of laboratory measurements were compared with data obtained by VIR instrument onboard Dawn mission.

Acknowledgements
This work is support by INAF Main Stream programme, grant  1.05.01.86.08 Evoluzione ed alterazione del materiale organico su Cerere (ref. Maria Cristina De Sanctis).

References

[1] De Sanctis et al., 2016, Nature 536, 54–57

[2] Marchi et al., Nature Astr., 2019

[3] De Sanctis et al., 2011, Space Science Reviews 163, 329-369.

[4] De Sanctis et al., science 2017 355, 719

[5] Pieters et al., 2018, Meteoritics and Planetary Science 53 (9), 1983-1998

[6] De Sanctis et al., 2019, 482 (2), 2407–2421

[7] Vinogradoff et al., 2021

[8] Ferrari et al., 2019, Icarus 321, 522-530

[9]De Angelis et al., 2021 JGR Planets doi: 10.1029/2020JE006696

[10] Baratta et al. 2002, A&A, 384, 343-349

[11] Brucato et al. 2006, A&A, 455, 395-399

[12] Mennella et al. 2003, ApJ, 587, 727-738

[13] Palumbo et al 2004, Ad. Sp. Res., 33, 49-56

[14] Fornaro et al. 2013, Icarus, 226(1), 1068–1085

[15] Fornaro et al 2018, Astrobiology, 18, 989-1007

How to cite: Poggiali, G., De Sanctis, M. C., Brucato, J. R., Ferrari, M., De Angelis, S., Palumbo, M. E., Baratta, G., Mennella, V., Fulvio, D., Popa, C., Strazzulla, G., and Sciré, C.: Evolution and alteration of organic material on Ceres, a pathway towards the understanding of complex geological and chemical history of a wet small body, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-723, https://doi.org/10.5194/epsc2021-723, 2021.

Several lines of evidence indicate that most of the smaller asteroids (< 1 km) consist of granular material loosely bound together primarily by self-gravity; these are commonly called rubble piles [1]. While the strength of these rubble piles is valuable information on their origin and fate, it is still debated in the literature [2]. Therefore, we have started a laboratory measurement campaign on simulated asteroid regolith, studying the impact of several factors on material strength, such as grain size, size mixtures, and surface properties. In the work presented here, we focus on fine-coarse mixtures and the influence of the fraction of fines on the sample strength. Computer simulations suggest that the increase in the ratio of fine grains to coarse grains will increase the strength of the sample in all configurations [3].  In a series of table-top measurements, we have determined sample compression and shear strengths for various fine-coarse mixtures. We used confined setups (less than 10cm in length) to measure the strength of the material in constricted environments such as an asteroid’s core and unconfined setups (greater than 10cm in length) to simulate open environments such as the surface of an asteroid.

Using CI Orgeuil high fidelity asteroid soil simulant [4], we performed three measurement types to determine the strength of our samples. Samples of regolith were created by measuring percentage by volume amounts of both coarse and fine grains into the measurement container. We prepared coarse grains in two size distributions, mm-sized (Figure 1) and cm-sized. The fine fraction was composed of grains sieved between 100 and 250 µm. A shear box setup was used to obtain shear yield measurements which in turn provided values for the Angle of Internal Friction (AIF), bulk cohesion, and tensile strength of the samples. A compression setup was used to measure values for the Young’s Modulus (YM) in both confined and unconfined samples. The third setup measured the Angle Of Repose (AOR), the steepest angle of descent relative to the horizontal plane to which a material can pile before collapse. From the AOR, we determined the coefficient of friction of each sample.

For compression and AOR measurements, we find that the strength of the coarse grain samples increases with the addition of a fine fraction (Figure 2, left). These findings are intuitive and support the results from computer simulations. However, we find that the increase of the fine fraction in a sample of coarse grains does not consistently increase the sample shear strength. With increasing fine fractions, the AIF and bulk cohesion (Figure 2, right) of the mixed samples decrease (until a point of saturation). This could be indicative of the fine grains acting as a lubricant as the larger grains move across each other, aiding rolling and reducing interlocking strength.

Our findings suggest that in the case of the surface of an asteroid, the presence of fine grains does indeed increase the strength of coarse regolith material.  However, fine grains in the regolith sublayers or the asteroid interior will reduce material strength due to grain interlocking and ease disruption. Therefore, rubble piles that are depleted in fine grains will have higher internal strength compared to those composed of grain size distributions that include sub-mm sized particles.

[1] Walsh, K.J., 2018. Rubble pile asteroids. Annual Review of Astronomy and Astrophysics, 56, pp.593-624.

[2] Holsapple, K., 2020. Main Belt Asteroid Histories: Simulations of erosion, cratering, catastrophic dispersions, spins, binaries and tumblers. arXiv preprint arXiv:2012.15300.

[3] Sánchez, P. and Scheeres, D.J., 2014. The strength of regolith and rubble pile asteroids. Meteoritics & Planetary Science, 49(5), pp.788-811.

[4] Metzger, P.T., Britt, D.T., Covey, S., Schultz, C., Cannon, K.M., Grossman, K.D., Mantovani, J.G. and Mueller, R.P., 2019. Measuring the fidelity of asteroid regolith and cobble simulants. Icarus, 321, pp.632-646.

How to cite: Cox, C., Brisset, J., Partida, A., Madison, A., and Bitcon, O.: Mechanical properties of fine-coarse grain mixtures of asteroid regolith, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-454, https://doi.org/10.5194/epsc2021-454, 2021.

Ammonium minerals have been proposed to be present in variable percentage on icy planetary bodies such as Enceladus, Ceres, Pluto and its satellites and their presence is evident also in others celestial bodies. The presence of these compounds is related to the raise of ammonium (NH₄⁺) salts from the interior of the icy planetary body, where oceans are likely located, due to cryovolcanism activity mixed with ice. De Sanctis et al. (2020) suggest that the presence of ammonium bicarbonate and/or ammonium chlorides on Ceres’s surface is a trace of the recent ascent of deep brine. In fact, some authors (e.g., Castillo-Rogez, 2020) proposed the presence of an early ocean on the subsurface of Ceres as deep brine. The Virgil Fossae on Pluto show evidence of recent cryovolcanism activity and ammonia spectral signature (Cruikshank et al., 2019) revealed by the analysis of data collected by New Horizons from the Linear Etalon Imaging Spectral Array (LEISA) (Schmitt et al., 2017). Recently, several studies (Cook et al., 2018; Dalle Ore et al., 2019) modelled the surface composition of Nix, Hydra and Kerberos, Pluto’s moons, using ammoniated salts as end members: NH₄Cl, NH₄NO₃ and (NH₄)₂CO₃. The identification of these minerals on the surface can give information about internal composition/dynamics and potential habitability of icy bodies. Among the tested samples, several hydrated and anhydrous ammonium compounds undergo phase transitions under specific temperature conditions. For these reasons, these minerals at cryogenic conditions can experience variations in resistivity, electrical conductivity as well as  other mechanical properties, that can affect the internal dynamics, cryovolcanism and buoyancy of celestial bodies.

This study focuses on a series of selected minerals, as sulfates, phosphates, aluminates and borates, and is aimed at understanding how (1) different anionic groups, (2) the amount of water, (3) the occurrence of low temperature phase transitions and (4) different grain-size affect the absorption bands parameters of the ammonium bearing minerals. As the experimental data to interpret the remote sensing data, available nowadays for these systems, are usually restricted to small spectral ranges and collected only at room temperature, we performed reflectance spectroscopy analyses in the near-infrared (NIR) region (1-5 μm) in a temperature range from 298K to 60K with specific temperature steps for samples characterised by phase transitions. The reflectance spectra of the samples were measured under cryogenic conditions representative of real planetary surfaces. In addition, ammonium compounds were sieved in three different grain size ranges: 36-80, 80-125 and 125-150 μm. Each grain-size was measured at room temperature. X-ray diffraction analyses were performed on the samples before and after thermal treatments.

We measured natural and synthetic ammonium bearing minerals; sal-ammoniac NH₄Cl, mascagnite (NH₄)₂SO₄, ammonium bicarbonate (NH₄)HCO₃, ammonium nitrate NH₄NO₃, ammonium carbonate (NH₄)₂CO₃, ammonium phosphate monobasic (NH₄)H₂PO₄, larderellite (NH₄)B₅O₇(OH)₂·H₂O, struvite(NH₄)MgPO₄·6H₂O and tschermigite (NH₄)Al(SO₄)₂·12H₂O using using the SHINE spectro-gonio radiometer of the Cold Spectroscopy Facility (https://cold-spectro.sshade.eu) at IPAG equipped with a simulation chamber to control the temperature of the minerals. Some of them undergo structural transformations at cryogenic temperature: i.e., mascagnite shows phase transitions at 223 K which involve a changing from space group Pnam at room temperature to Pna2₁, with ferroelectric behaviour related to a stronger hydrogen bond, whereas tscermigite presents low temperature transitions at 76 K.

Reflectance spectra of anhydrous samples show well defined absorption features in the 1-2.5 µm range due to NH₄⁺ groups overtones and combinations. The bands located at 1.3 (2ν₃ + ν₄) and 1.56 (2ν₃) µm could be useful to discriminate these salts. The reflectance spectra of water-rich samples show H₂O fundamental absorption features, overlapped to the NH₄⁺bands, in the area from 1 to 2.8 μm and over 3 μm the spectra show a minor number of peaks. The parameters (area, depth and FWHM) of several absorption bands change in relation to the low temperature conditions and different grain size. In detail, the low temperature spectra compared to the room temperature ones reveal fine structure displaying more specific and defined absorption bands. The different granulometry affects mainly the bands area and depth. Moreover, we notice as the grain size becoming larger, the value of area and FWHM (full width half maximum) increase. Samples mascagnite (NH₄)₂SO₄, sal-ammoniac NH₄Cl, ammonium phosphate (NH₄)H₂PO₄, tschermigite (NH₄)Al(SO₄)₂·12(H₂O) and ammonium nitrate NH₄NO₃ are characterized by phase transitions at low temperature and in some cases showed clear and very interesting spectral bands variations during cooling, indicating that a phase transition occurred. In these minerals, the detected phase transitions are characterized by a progressive deepening and shift toward shorter wavelength whit an abrupt change in depth of the sensitive bands.

In the analysed temperature range, like that can be found on the surface of large icy bodies, the ammonium minerals undergo different evolutions. In some cases, phase transformations generate important variations in the structural configuration that reflect on the characteristics of the band’s parameters and shape. In this scenario, the behaviour of the studied minerals at low temperature is very interesting for the remote sensing identification. These collected cryogenic data, with a carefully analysis of NH₄⁺ absorption features, could be used to the detection of these salts on the surfaces of planetary bodies. The presence of ammonium minerals in the ice shell could influence the dynamics of icy satellites, especially if they are subject to phase transformations.

• Castillo-Rogez, J. (2020). Future exploration of Ceres as an ocean world. Nature Astronomy, 4(8), 732-734.
• Cook, J. C. et al., (2018). Composition of Pluto’s small satellites: Analysis of New Horizons spectral images. Icarus, 315, 30-45.
• Cruikshank, D. P. et al., (2019). Recent cryovolcanism in virgil fossae on Pluto. Icarus, 330, 155-168.
• Dalle Ore, C. M. et al., (2019). Detection of ammonia on Pluto’s surface in a region of geologically recent tectonism. Science advances, 5(5), eaav5731.
• De Sanctis, M. C et al., (2020). Fresh emplacement of hydrated sodium chloride on Ceres from ascending salty fluids. Nature Astronomy, 4(8), 786-793.
• Schmitt, B. et al., Physical state and distribution of materials at the surface of Pluto from New Horizons LEISA imaging spectrometer. Icarus, 287, 229-260.

How to cite: Fastelli, M., Comodi, P., Zucchini, A., Schmitt, B., Beck, P., and Poch, O.: VIS-NIR analysis at low temperature and different grain size of ammonium bearing minerals: a tool to improve the knowledge of icy planetary bodies surface., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-313, https://doi.org/10.5194/epsc2021-313, 2021.

Introduction

The determination of surface properties of Solar System rocky bodies is a fundamental step in the interpretation of remote-sensing data from planetary missions. Estimating the surface temperature from remotely sensed spectroscopic data data is generally performed by applying models and inversions to infrared spectra. Such models have been used in the past to estimate the surface temperature of planetary bodies like Mars and Ceres (e.g. Pollack et al., 1990; Tosi et al., 2016). In some specific case, namely water ice, the absorption bands in the near infrared range have been calibrated in position (Fink and Larson, 1975; Grundy and Schmitt, 1998; Mastrapa et al., 2009) and their shift with respect to laboratory P-T conditions can be used to infer the surface temperature of icy satellites if these are relatively free from contaminants, or of ice outcrops on smaller bodies (see e.g. Raponi et al., 2018, for Ceres). Other non-water-ice materials can be in principle used as temperature proxy, useful for an independent estimation of the surface temperature of rocky bodies. Phyllosilicates for example are characterized by diagnostic OH absorption bands in the 1-2.8 μm-region, and these have been detected on Mars (Bibring et al., 2005; Ehlmann and Edwards, 2014) as well as on asteroids (De Sanctis et al., 2015; Hamilton et al., 2019; Kitazato et al., 2019) by telescopic and space missions. Nevertheless, the absorption of water hiddens or strongly affects the structural OH absorption in the IR, thus other non-hydrated materials are needed in order to be related to temperature changes. Carbonates, also detected on bodies such as Mars (Ehlmann et al., 2009) or Ceres (De Sanctis et al., 2016) are good candidates for such a study, because they are characterized by a number of absorption bands that may be little to no affected by water; in particular the bands at 3.4-4 μm are outside the broad 3-μm water band. In previous laboratory studies carbonates reflectance spectra in the IR have been acquired at room temperature (Harner et al., 2015). In other laboratory studies a correlation between band position and temperature change has been showed for 3.4 and 4-μm bands (De Angelis et al., 2018) regarding the natrite, although measurements were carried out at low spectral resolution.

Methods

In this work we studied the IR spectral reflectance of a number of different carbonates, in the 3.2-4.6-μm range, at high spectral sampling and resolution, in a wide temperature range from 270 K down to 60 K. This set of measurements is part of a larger project, which aims at investigating also other classes of anhydrous minerals at high spectral resolution, in order to identify other valuable temperature proxies that can be useful in the interpretation of remote-sensing data from current and future planetary missions. We acquired spectra on six different types of carbonates, namely: calcite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), siderite (FeCO₃), natrite (Na₂CO₃), malachite (Cu₂(CO₃)(OH)₂). These materials cover a wide range of carbonates with different cations (Ca²⁺, Mg²⁺, Fe²⁺, Na⁺, Cu²⁺) thus allowing studying also the band variability due to the different chemical environment. Spectra were acquired with the setups of the Cold Surfaces Spectroscopy (CSS; https://cold-spectro.sshade.eu) facility at the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG); measurements were performed with the SHINE Spectro-Gonio-Radiometer facility (Brissaud et al., 2004) equipped with a simulation chamber to control the sample temperature. The spectral sampling was 3 nm, corresponding to a spectral resolution < 8 nm. This facility uses a monochromator as a light dispersion element. All spectra were acquired at standard conditions of illumination (i = 30°) and emission (e = 0°). Spectralon and Infragold (Labsphere ©) were used as reference targets.

The materials have been analyzed in the form of fine powders, each mineral having been ground and dry-sieved at grain size below 50 μm.

Preliminary results and Conclusions

Spectra of a dolomite sample are shown in Fig.1. Several changes can be seen in the spectra as the temperature is lowered from 270 to 60 K. In Fig.1A the overall 3.2-4.6 μm-spectra are shown. The reflectance level at continuum becomes higher for wavelengths beyond 3.5 μm; below 3.3 μm a decrease of reflectance could be related to some minor amount of water contained in the sample, or to frost condensation in the chamber. In Fig.1B a closeup on the first minimum of the carbonate 3.4-μm band is shown. The position of the first minimum, located at about 3.31 μm at 270 K, seems to shift towards shorter wavelengths as the temperature decreases, by roughly 6-7 nm. In Fig. 1C a closeup on the 4-4.2-μm region is displayed. A very weak band, that is quite unrecognizable at 270 K, becomes clearer and definite as the temperature decreases to 60 K, located at 4.15 μm.

All these subtle changes in spectral bands positions as well as small bands appearing at low temperatures could be in principle detectable by high-resolution spectrometers.

Future work will deal with the detailed spectral analysis of band parameters of all the carbonate samples, as well as with the investigation of other anhydrous materials.