Comprehensive intercomparison of Martian dust optical depth datasets to assess agreement with Mars Climate Sounder observations

This study provides comprehensive intercomparisons of dust optical depths derived from Mars Climate Sounder on Mars Reconnaissance Orbiter with measurements from the orbiter instruments THEMIS on Mars Odyssey, CRISM on Mars Reconnaissance Orbiter, EMIRS on the Emirates Mars Mission, and from the surface-based instruments Pancam on the Spirit and Opportunity rovers and SSI on the Phoenix lander. We demonstrate good overall agreement between these dust optical depth datasets, in particular as these measurements were taken at different wavelengths (near IR to thermal IR) and with different measurement geometries (limb, downlooking from orbit, uplooking from the surface). To draw our conclusions, we use direct comparisons of co-located measurements from these instruments, plots of the dust optical depth differences averaged by latitude and Solar longitude, as well as modified Bland–Altman analysis that takes the measurement uncertainties into account. From the Bland–Altman analysis, we find that error estimates provided by the instruments reasonably represent the variability introduced by the two datasets but do not capture offsets. Based on our investigations, we propose potential physical causes, such as surface reflectance factor estimation and thermal contrast, for some of the observed differences. We find a linear relation of the differences in dust optical depth between MCS and the other instruments with derived MCS water ice optical depth. Taking this relation into account, together with a constant parameter that accounts for general offsets between the datasets, significantly reduces differences.