Fig. 4: Comparison of observations of Martian dust to lab spectra and RMSE values.

a Comparison of ChemCam, CRISM, OMEGA, and lab ferrihydrite-basalt (1:2) mixture spectra. Mars spectra comparison with the b hematite-basalt (1:2) mixture spectrum, c akaganeite-basalt (1:2) mixture spectrum, and d goethite-basalt (1:2) mixture spectrum. e Bar plot of RMSE values derived between each instrument observation and laboratory spectra. RMSE values indicate that ferrihydrite provides the best fit (low RMSE values) to these Martian spectra. Observations of Martian dust, especially in the ChemCam spectrum, show a small shoulder at ~600 nm, likely due to the ⁶A₁ → ⁴T₂ band at slightly longer wavelengths. Our synthetic ferrihydrite sample lacks this feature but it has been observed in natural ferrihydrite samples⁵⁹ (see also our Block Island and Azores natural ferrihydrites, Fig. 5). An alternative explanation is that there may be minor amounts of other iron (oxy)hydroxide phases present in the dust (Supplementary Fig. 9). The RMSE analysis was conducted over the range 500–840 nm, due to the limitation of the ChemCam data spectral range. All spectra shown were normalized at 800 nm.