All Figures

Figure 1: Fitting procedure for one spectrum that exhibits an Fe-OH feature at 2.29 m (spot MV2). A) Spectra of the mineral end-members used in the modeling compared to the OMEGA data spectrum (black line) to be modeled. The end-member spectra correspond to spectra of pure minerals except for the plagioclase spectrum, for which submicrometer-sized oxide grains are embedded inside the plagioclase matrix as described in Poulet & Erard (2004). B) The data spectrum (black line) is compared to three models. The best fit (red line) corresponds to a total fraction of 75% of hydrated minerals (here, nontronite and ferrihydrite). If the total fraction of hydrated minerals is forced to be diminished by 10% (blue spectrum) or increased by 10% (green spectrum), the fit is significantly degraded. The discrepancies observed in the 1.4-1.55 m (see arrow) result from an instrumental artifact that depends on the flux received by the instrument (Langevin et al. 2007). This partly explains the lack of the 1.4 m band due to presence of OH. The spikes in the 1.9-2.1 m region (see arrow) can be attributed to the atmospheric removal (CO2 gas).

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In the text

Figure 2: A) High-Resolution Stereo Camera (HRSC) mosaic of the region of Mawrth Vallis. Blue squares indicate the location of MV1-6 spots. A box delineates an enlarged portion (red) of the HRSC color RGB image shown in (B). B) Three stratigraphically distinct units can be identified: unit 1) upper whitish bright, unit 2) middle reddish, and unit 3)  lower brown. Locations for which OMEGA spectra were extracted and then modeled are also indicated by the blue pixels (real sizes). The fit results are shown for each spot.

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In the text

	Figure 3: Fit of a spectrum representative of the Noachian basement in Nili Fossae (spot NF1). A) Spectra of the end-members used for the modelings. B) The data spectrum (black line) is compared to the best-fit model (red line) and one model excluding the pyroxene as end-member (blue line).
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In the text

Figure 4: Modal analysis of the deposit NF3 associated with the central peak of an impact crater centered at 17.0N, 71.8E. A) The red cross indicates the location over a Viking image for which the spectrum was extracted. A box delineates a portion of the infrared nighttime Thermal Emission Infrared Spectrometer (THEMIS) image I16829003 shown below the Viking image. The spectrum comes from a terrain with high brightness in the THEMIS image, indicating a relatively dust-free surface. B) Spectra of the end-members. C) Spectrum of NF3 deposit compared to its best fit.

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In the text