MODAL MINERALOGY OF SEITAH UNIT IN JEZERO CRATER (MARS) RETRIEVED FROM NONLINEAR UNMIXING ANALYSES OF IRS/SUPERCAM.

NONLINEAR UNMIXING ANALYSES OF IRS/SUPERCAM. F. Poulet1, C. Royer2, P. Beck3, L. Mandon2, C. Quantin-Nataf4, J.R. Johnson5, O. Beyssac6, O. Forni7, A. Cousin7, F. Montmessin8, C. Pilorget1, S. Le Mouélic9, E. Dehouck4, A. Brown10, J. Tarnas11, K. Benzerara6, T. Fouchet2, S. Maurice6, R.C. Wiens12, and The SuperCam team 1IAS, Orsay (francois.poulet@ias.u-psud.fr), 2LESIA, Meudon, 3IPAG, Grenoble, 4LGL-TPE, Lyon, 5JHUAPL, Laurel 6IMPMC, Paris, 7IRAP, Toulouse, 8LATMOS, Paris, 9LPG, Nantes, 10Plancius Research, Severna Park, 11JPL, Pasadena, 12LANL, Los Alamos Introduction and motivation: The Perseverance rover performed in-situ mineralogical and geological investigations of an unique olivine-bearing unit of the Jezero crater floor (so-called Séı́tah also referred as to the Crater Floor fractured 1 (Cf-f-1)) using the multiple techniques of the SuperCam instrument [1,2]. Data acquired by its infrared spectrometer (IRS) [3] revealed the presence of various signatures including a distinct Fe2+ signature at shorter wavelengths typical of olivine, an absorption near 1.9 μm indicating the presence of water as well as several signatures in the 2.282. 5 μm wavelength range consistent with the presence of multiple alteration phases such as Fe/Mg phyllosilicates and possible Fe/Mgcarbonates in some locations [4]. A key question is what was the geological environment of alteration? The mineral assemblage, modal mineralogy and rock texture can provide important constraints on the formation environment and alteration sequences. In NIR spectral data of planetary surfaces, multiple scattering dominates and spectral mixing is nonlinear. Radiative transfer models predict that the systematics can be linearized by converting reflectance data to single scattering albedo [5, 6], and applications of such radiative transfer models to laboratory data show that more accurate understanding of geologic processes is gained relative to linear mixture models [7,8]. Previous works have also demonstrated that radiative transfer modeling can be applied to NIR spectroscopic data from Mars orbiters to provide quantitative estimation of bulk mineralogy for hydrated terrains [9-11]. We here present the modal mineralogy of some olivinerich, carbonate-bearing rocks observed in Jezero crater. Method: Modal mineralogy is derived using a nonlinear unmixing modeling based on the Shkuratov radiative transfer model [6]. This modeling technique uses a selected set of optical constants for reproducing spectra. The IRS spectra (FOV=1.15 mrad corresponding to ~a few mm2 targets for typical in situ observations) are fitted in the 1.30–2.50 μm wavelength range using a simplex minimization algorithm to retrieve the abundance and grain size of each mineral included in the assemblage. The quality of the fit is evaluated by the value of the residual mean squared (RMS). The Shkuratov model simulates the reflectance of a particulate surface (also referred to as an intimate mixture) from a a priori set of minerals. The selection of minerals to be included (through their complex indices of refraction) in the modeling is thus critical. Primary minerals such as olivine, pyroxenes (both highand low-calcium) and plagioclase are considered as potential endmembers and hence are routinely added. On the other hand, it is not possible to add per default all the potential alteration minerals (Table 1) due both to computation time and efficiency of the convergence process. Each spectrum is then examined to identify a potential metal-OH or carbonate signature and to discriminate the different altered phases. The iterative deconvolution of most of the IRS spectra of the abraded targets of Séı́tah requires a complex assemblage with up to 8 endmembers (olivine, 1 or 2 pyroxenes, one carbonate, 3 or 4 hydrated phases). Fe/Mg phyllosilicates (Nontronite, Saponite, Montmorillonite, Serpentine, Talc) Carbonate (Magnesite, Siderite)