Fig. 8: Orbital observations of Martian surface dust and comparison to laboratory spectrogoniometer measurements.
From: Detection of ferrihydrite in Martian red dust records ancient cold and wet conditions on Mars
a Phase curve reflectance ratio in two CaSSIS filters (NIR-940 nm, BLU-479 nm) of observations of dust in Arabia Terra and b same ratios for two sieved synthetic ferrihydrite size fractions. The observed arch in both plots is known as the phase reddening and bluing effect. The color ratios of both the <1 µm size fraction and Martian observations exhibit maxima at approximately 40° phase angle, followed by a distinct decrease at larger angles. In contrast, the <11 µm size fraction lacks a clear inflection point, suggesting particle size-dependent variations in light scattering properties. These observations imply that particle size and/or physical particle properties of Martian dust are similar to <1 µm ferrihydrite particles. Optical depth overall is low in (a) implying that atmospheric effects are not the cause of the trends observed here. Similar reflectance ratio curves are observed for other dusty regions of Mars125, which suggests homogenous mixing of dust on a global scale. Source data are provided as a Source Data file.
