Fig. 1: Identification of ferrihydrite in the Martian dust.
From: Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
a Ochre hue in the light-toned regions of Mars (as observed on 2021-08-14 by the Emirates Exploration Imager; R = 635, G = 546 and B = 437 nm), b ferrihydrite-basalt (1:2 ratio) laboratory hyperfine ( < 1 µm) mixture acquired under ambient conditions in a sample dish, and c comparison of an orbital spectrum of Martian dust (from CRISM image FRT00009591) to the spectrum of the ferrihydrite-basalt mixture. The steep increase in reflectance near 0.5 µm is due to the presence of ferric iron and its electron pair transition absorption, which dominates the UV range and extends into the blue wavelengths. The NIR spectral bands at 1.41 and 1.93 µm due to bound H2O in ferrihydrite are not observed in spectra of these hyperfine mixture samples. The characteristic NIR increase in reflectance (1–2.5 µm) in spectra of pure ferrihydrite (see Fig. 3) is also not observed in our mixture spectra, likely due to nonlinear spectral mixing with the basalt powder. The 3-µm band may be due to chemically bound water in both the Martian dust and the lab sample. Source data are provided as a Source Data file.
