Fig. 3: Non-linear reflectance behavior of ferrihydrite-basalt intimate mixtures.
From: Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
a Ferrihydrite-basalt mixture VNIR spectra with increasing amounts of wt. % ferrihydrite and b same spectra normalized at 1 µm and truncated to highlight variations in the extended visible region. The 0 wt. % spectrum represents the pure hyperfine basalt sample. At least 10 wt. % of ferrihydrite is needed to produce the prominent red slope in the visible (0.5–0.7 µm) wavelength range. These spectra suggest that more than 20 wt. % ferrihydrite (i.e. 1:3 weight ratio) may be needed to fit the Martian spectrum of dust. Already at a 1:1 ratio of ferrihydrite to basalt, the NIR (1.3–2.7 µm) increase in reflectance present in spectra of pure ferrihydrite is not observed, nor are the absorptions due to water near ~1.4 and ~1.9 µm. Source data are provided as a Source Data file.
