NIR-MID Reflectance and Emissivity Study at Different Temperatures of Sodium Carbonate Minerals: Spectra Characterization and Implication for Remote Sensing Identification

Recent satellite observations and the deconvolution of remote sensing data have shown the existence of various carbonate minerals in different solar system bodies. Emissivity, from 403 to 803 K, and reflectance spectra at 300 and 193 K of selected carbonates minerals with different ratios of Na:Ca and water amounts were respectively collected at 3–20 μm and at 1–16 μm. All reflectance spectra show absorption features in the 1.9–2.5 and 3.4–4.0 μm areas due to overtone and a combination of CO32− and fundamental vibrational models at ≈9.09, 11.35, 7.06, and 14.7 μm. The increase of the Na:Ca ratio in anhydrous samples produces a shift of the absorption features in the 3.4–4.0 μm area toward shorter wavelengths, and the peak at 3.9 μm doubles in the presence of a CO32− oxygen group shared with two cations in minerals having more complex structures. The comparison of the bands at ≈669 and ≈794 cm−1 in the emissivity spectra collected at high temperatures indicates that around 600 K, phase transitions occurred in natrite and thermonatrite. The reflectance spectra measured at 193 K reveal a fine structure compared to spectra collected at room temperature. The comparison of laboratory results with the spectrum of Ceres’s brightest crater Occator from the Dawn mission, taken as a case study, showed how the anhydrous samples, shortite and nyerereite, studied in this work can also be hypothesized for Ceres’s surface beyond that already suggested (trona, natrite, thermonatrite).

Organic Matter and Associated Minerals on the Dwarf Planet Ceres

Ceres is the largest object in the main belt and it is also the most water-rich body in the inner solar system besides the Earth. The discoveries made by the Dawn Mission revealed that the composition of Ceres includes organic material, with a component of carbon globally present and also a high quantity of localized aliphatic organics in specific areas. The inferred mineralogy of Ceres indicates the long-term activity of a large body of liquid water that produced the alteration minerals discovered on its surface, including ammonia-bearing minerals. To explain the presence of ammonium in the phyllosilicates, Ceres must have accreted organic matter, ammonia, water and carbon present in the protoplanetary formation region. It is conceivable that Ceres may have also processed and transformed its own original organic matter that could have been modified by the pervasive hydrothermal alteration. The coexistence of phyllosilicates, magnetite, carbonates, salts, organics and a high carbon content point to rock–water alteration playing an important role in promoting widespread carbon occurrence.

Characterization of NH4-montmorillonite coexisting with NH4Cl salt at different aggregation states. Application to Ceres.

<p>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 NH<sub>4</sub><sup>+</sup> 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.</p> <p>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<sup>-5</sup> 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<sup>-8</sup> 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 NH<sub>4</sub><sup>+ </sup>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 NH<sub>4</sub><sup>+</sup> IR band was measured after just 1 day of being exposed to vacuum conditions.</p> <p>Acknowledgments</p> <p>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”.</p> <p>References</p> <p>[1] De Sanctis et al.,  Space Sci. Rev. 216, 60, 2020</p> <p>[2] Raponi et al., Icarus 320, 83,  2019</p> <p>[3] Borden and Giese, Clays Clay Miner. 49, 444, 2001</p> <p>[4] Mateo-Marti et al., Life 9, 72, 2019</p>

Laboratory Investigations Coupled to VIR/Dawn Observations to Quantify the Large Concentrations of Organic Matter on Ceres

Organic matter directly observed at the surface of an inner planetary body is quite infrequent due to the usual low abundance of such matter and the limitation of the infrared technique. Fortuitously, the Dawn mission has revealed, thanks to the Visible and InfraRed mapping spectrometer (VIR), large areas rich in organic matter at the surface of Ceres, near Ernutet crater. The origin of the organic matter and its abundance in association with minerals, as indicated by the low altitude VIR data, remains unclear, but multiple lines of evidence support an endogenous origin. Here, we report an experimental investigation to determine the abundance of the aliphatic carbon signature observed on Ceres. We produced relevant analogues containing ammoniated-phyllosilicates, carbonates, aliphatic carbons (coals), and magnetite or amorphous carbon as darkening agents, and measured their reflectance by infrared spectroscopy. Measurements of these organic-rich analogues were directly compared to the VIR spectra taken from different locations around Ernutet crater. We found that the absolute reflectance of our analogues is at least two orders of magnitude higher than Ceres, but the depths of absorption bands match nicely the ones of the organic-rich Ceres spectra. The choices of the different components are discussed in comparison with VIR data. Relative abundances of the components are extrapolated from the spectra and mixture composition, considering that the differences in reflectance level is mainly due to optical effects. Absorption bands of Ceres’ organic-rich spectra are best reproduced by around 20 wt.% of carbon (a third being aliphatic carbons), in association with around 20 wt.% of carbonates, 15 wt.% of ammoniated-phyllosilicate, 20 wt.% of Mg-phyllosilicates, and 25 wt.% of darkening agent. Results also highlight the pertinence to use laboratory analogues in addition to models for planetary surface characterization. Such large quantities of organic materials near Ernutet crater, in addition to the amorphous carbon suspected on a global scale, requires a concentration mechanism whose nature is still unknown but that could potentially be relevant to other large volatile-rich bodies.

Special Crater Types on Vesta and Ceres as Revealed by Dawn

The exploration of two small planetary bodies by the Dawn mission revealed multifaced surfaces showing a diverse geology and surface features. Impact crater are the most distinctive features on these planetary bodies. The surfaces of asteroid Vesta and the dwarf planet Ceres reveal craters with an individual appearance as caused by different formation processes. Special topographic and subsurface conditions on both bodies have led to the development of special crater types. This chapter present the three most characteristic crater forms fund on both bodies. Asymmetric craters are found on both bodies, whereas ring-mold craters and floor-fractured craters are only visible on Ceres.

The large-scale troughs on Asteroid 4 Vesta are opening-mode fractures

<p>The Dawn mission at Asteroid 4 Vesta revealed two sets of enormous linear structures. Both sets are troughs—linear, negative-relief landforms—with one spanning around at least two-thirds of the equator and the other set incompletely preserved in the northern hemisphere. A previous study evaluated the cross-sectional geometries of the troughs and interpreted them as analogous to grabens, which are landforms caused by normal faults. However, for the troughs to be large-scale opening-mode fractures, i.e., joints, was heretofore not considered. To distinguish between normal faulting and jointing, we investigated the map patterns, cross-sectional geometries, and variations of relief and width along the length of these troughs. Relief and width are meaningful measurands that causally relate to the vertical displacement of faults or aperture of joints, respectively. Their distributions along the trough length should thus reveal differences in fracturing behavior. In addition, we derived strength-depth profiles to characterize the rheologic structure of Vesta’s lithosphere and determine the predicted fracturing behavior in its brittle regime.</p><p>We mapped all large-scale troughs on Vesta, including four equatorial and two northern troughs, and no map patterns diagnostic for faulting were identified. The troughs are bounded by scalloped rims and mainly show V- and bowl shapes in cross-section. The variation of reliefs of the two-opposing trough-bounding scarps reveals that the relief maxima for each of the investigated troughs are located off-center, and at different locations along the trough they bound. In contrast, we found that both the individual and cumulative variations in trough width have their maxima near the center of the trough. These map patterns and geomorphologic characteristics are largely inconsistent with the mechanics of graben formation but instead point to an origin by opening-mode fracturing. Moreover, our calculations of lithospheric strength evolution that enable assessments of fracturing behavior reveal that Vesta’s lithosphere has been dominated by a thick brittle portion throughout its history. Solutions to the Coulomb criterion considering a range of strengths properties of intact to fractured basaltic materials are in support of jointing as the major fracturing mode in at least the upper ~14 km of Vesta’s lithosphere.</p>

Dwarf planet (1) Ceres surface bluing due to high porosity resulting from sublimation

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.

Organic Material on Ceres: Insights from Visible and Infrared Space Observations

The NASA/Dawn mission has acquired unprecedented measurements of the surface of the dwarf planet Ceres, the composition of which is a mixture of ultra-carbonaceous material, phyllosilicates, carbonates, organics, Fe-oxides, and volatiles as determined by remote sensing instruments including the VIR imaging spectrometer. We performed a refined analysis merging visible and infrared observations of Ceres’ surface for the first time. The overall shape of the combined spectrum suggests another type of silicate not previously considered, and we confirmed a large abundance of carbon material. More importantly, by analyzing the local spectra of the organic-rich region of the Ernutet crater, we identified a reddening in the visible range, strongly correlated to the aliphatic signature at 3.4 µm. Similar reddening was found in the bright material making up Cerealia Facula in the Occator crater. This implies that organic material might be present in the source of the faculae, where brines and organics are mixed in an environment that may be favorable for prebiotic chemistry.