¹Vlastimil Vojáček,¹Jiří Borovička,¹Pavel Spurný,¹David Čapek
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.104882]
¹Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65, Ondřejov, Czech Republic

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¹A.Galiano,¹F.Dirri,^1,2E.Palomba,¹ A.Longobardo,³B.Schmitt,³P.Beck
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113692]
¹INAF-IAPS, Rome, Italy
²SSDC-ASI, Rome, Italy
³Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
Copyright Elsevier

The dwarf planet Ceres is an airless body composed of Mg-phyllosilicates, NH₄-phyllosilicates, Mg/Ca-carbonates and a dark component. The subsurface of Ceres, investigated by the material composing the peak of complex craters (ccp, crater central peak material; Galiano et al., 2019), reveals a composition similar to the surface, with an increasing abundance of phyllosilicates in the interior. A moderate trend between age of craters’ formation and spectral slope of ccps suggests that younger ccps show a negative/blue slope and older ccps are characterized by positive/red slope. To investigate the causes of different spectral slope in ccps, different grain-sized Ceres analogue mixtures were produced and spectrally analysed. First, the end-members of the Ceres surface (using the antigorite as Mg-phyllosilicate, the NH₄-montmorillonite as NH₄-phyllosilicate, the dolomite as carbonate and the graphite as dark component), were mixed, obtaining mixtures with different relative abundance, and identifying the mixture with the reflectance spectrum most similar to the average Ceres spectrum. The selected mixture was reproduced with grain size of 0–25 μm, 25–50 μm and 50–100 μm. The three mixtures were heated and spectrally analysed, both with an acquisition temperature of 300 K (room temperature) and 200 K (typical for surface Ceres temperature during VIR observations).

The best analogue Ceres spectrum is coincident with a mixture composed of 18 M% (mass percentage) of Dolomite, 18 M% of Graphite, 36 M% of Antigorite and 28 M% of NH₄-montmorillonite, after experiencing a heating process.

The heating process produces: 1) a darkening and reddening of spectrum, as consequence of the devolatilization of OH group in phyllosilicates and a more dominant effect of opaque phase; 2) a deepening in the intensity of the 3.4 and 4.0 μm band, as well as the 2.7 and the 3.1 μm band, likely due to the loss of absorbed atmospheric water; 3) narrowing of 3.1 μm band and the shift of band center toward longer wavelength (i.e. at 3.06 μm) coincident with mean Ceres spectrum, related to the loss of absorbed atmospheric water.

The analysis of the best Ceres analogue mixture, reproduced at different grain size and after heating process, reveals a weakening of 2.7, 3.1, 3.4 and 4.0 absorption bands in coarser samples, likely related to large size of dark grains which reduce the spectral contrast. Furthermore, spectra of coarser mixtures are more red-sloped, suggesting that this trend is more affected by the dark component.

The best analogue Ceres mixture produced in this work is almost coincident with the mean spectrum of Haulani ccp, the youngest ccps on Ceres and therefore representative of less altered material on Ceres.

The redder spectral slope observed in the older ccps is probably the consequence of the space weathering effects on the original material composing the peak.

¹M.PineauaL.Le Deit,²B.Chauviré,³J.Carter,¹B.Rondeau,¹N.Mangold
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113706]
¹Laboratoire de Planétologie et Géodynamique, CNRS UMR 6112, Université de Nantes, Université d’Angers, Nantes, France
²Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, Grenoble, France
³Institut d’Astrophysique Spatiale, CNRS UMR 8617, Université Paris-Sud, Orsay, France
Copyright Elsevier

Hydrated silica detected on the martian surface, from both orbital and in-situ data, is an indicator of past aqueous conditions. On Earth, several near infrared (NIR) spectral criteria can be used to discriminate silica phases (e.g. opal-A, opal-CT and chalcedony) and their formation processes. We have applied these spectral criteria to Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data in order to investigate the geological origin of hydrated silica on Mars. We used two spectral criteria: (i) the crystallinity spectral criteria on the 1.4- and 1.9 μm absorption bands to distinguish between amorphous (opal-A and hydrated glasses) and more crystalline (opal-CT and chalcedony) varieties of silica, and (ii) the Concavity-Ratio-Criterion (CRC) to differentiate opals of hydrothermal origin from weathering origin. We first adapted the CRC measurements on terrestrial samples to make them comparable to CRISM measurements on Mars: we resampled our terrestrial spectra down to the CRISM resolution, and tested the martian pressure effect on spectral signatures. Then, we selected several areas over nine sites where hydrated silica has been detected on Mars, on the basis of good quality detections. Our results show that two main types of spectra can be distinguished, and these are consistent with two distinct geomorphological contexts proposed by Sun and Milliken (2018): amorphous and/or dehydrated silica-bearing bedrock deposits, and more crystalline and/or hydrated silica-bearing aeolian deposits. The concavity criterion also indicates silica origins that are in agreement with most of the hypothesized geological origins proposed in the literature. Although these results need further strengthening, they are promising for the use of NIR signatures as means of investigating the processes of hydrated silica on Mars.