A new model for Mars atmospheric dust based upon analysis of ultraviolet through infrared observations from Mariner 9, Viking, and Phobos

We propose key modifications to the Toon et al. (1977) model of the particle size distribution and composition of Mars atmospheric dust, based on a variety of spacecraft and wavelength observations of the dust. A much broader (reffvariance ∼ 0.8 μm), smaller particle size (rmode ∼ 0.02 μm) distribution coupled with a “palagonite-like” composition is argued to fit the complete ultraviolet-to-30-μm absorption properties of the dust better than the montmorillonite-basalt, reffvariance = 0.4 μm, rmode = 0.40 dust model of Toon et al. Mariner 9 (infrared interferometer spectrometer) IRIS spectra of high atmospheric dust opacities during the 1971–1972 Mars global dust storm are analyzed in terms of the Toon et al. dust model, and a Hawaiian palagonite sample (Roush et al., 1991) with two different size distribution models incorporating smaller dust particle sizes. Viking Infrared Thermal Mapper (IRTM) emission-phase-function (EPF) observations at 9 μm are analyzed to retrieve 9-μm dust opacities coincident with solar band dust opacities obtained from the same EPF sequences (Clancy and Lee, 1991). These EPF dust opacities provide an independent measurement of the visible/9-μm extinction opacity ratio (≥2) for Mars atmospheric dust, which is consistent with a previous measurement by Martin (1986). Model values for the visible/9-μm opacity ratio and the ultraviolet and visible single-scattering albedos are calculated for the palagonite model with the smaller particle size distributions and compared to the same properties for the Toon et al. model of dust. The montmorillonite model of the dust is found to fit the detailed shape of the dust 9-μm absorption well. However, it predicts structured, deep absorptions at 20 μm which are not observed and requires a separate ultraviolet-visible absorbing component to match the observed behavior of the dust in this wavelength region. The modeled palagonite does not match the 8- to 9-μm absorption presented by the dust in the IRIS spectra, probably due to its low SiO2 content (31%). However, it does provide consistent levels of ultraviolet/visible absorption, 9- to 12-μm absorption, and a lack of structured absorption at 20 μm. The ratios of dust extinction opacities at visible, 9 μm, and 30 μm are strongly affected by the dust particle size distribution. The Toon et al. dust size distribution (rmode = 0.40, reffvariance = 0.4 μm, rcwμ = 2.7 μm) predicts the correct ratio of the 9- to 30-μm opacity, but underpredicts the visible/9-μm opacity ratio considerably (1 versus ≥2). A similar particle distribution width with smaller particle sizes (rmode = 0.17, reffvariance = 0.4 μm, rcwμ = 1.2 μm) will fit the observed visible/9-μm opacity ratio, but overpredicts the observed 9-μm/30-μm opacity ratio. A smaller and much broader particle size distribution (rmode = 0.02, reffvariance = 0.8 μm, rcwμ = 1.8 μm) can fit both dust opacity ratios. Overall, the nanocrystalline structure of palagonite coupled with a smaller, broader distribution of dust particle sizes provides a more consistent fit than the Toon et al. model of the dust to the IRIS spectra, the observed visible/9-μm dust opacity ratio, the Phobos occultation measurements of dust particle sizes (Chassefiere et al., 1992), and the weakness of surface near IR absorptions expected for clay minerals (Clark, 1992; Bell and Crisp, 1993).