Characterisation of Martian dust aerosol phase function from sky radiance measurements by MSL engineering cameras

Abstract

Dust is the main driver of Mars' atmospheric variability. The determination of Martian dust aerosol properties is of high relevance for radiative modelling and calculating its weather forcing. In particular, the light scattering behaviour at intermediate and large scattering angles can provide valuable information regarding the airborne dust particle shape. The angular distribution of sky brightness observed by the Mars Science Laboratory engineering cameras (Navcam and Hazcam) is used here to characterise the atmospheric dust single scattering phase function and to constrain the shape of the particles. An iterative radiative transfer based retrieval method was implemented in order to determine the aerosol modelling parameters which best reproduce the observed sky radiance as a function of the scattering angle in the solar almucantar plane. The aerosol models considered in this study for retrieving dust radiative properties were an analytical three term Double Henyey-Greenstein (DHG) phase function, T-matrix calculations for cylindrical particles with different diameter-to-length (D/L) aspect ratios and experimental phase functions from laboratory measurements of several Martian dust analogue samples. Results of this study returned mean DHG phase function parameter values g1 = 0.889 ± 0.098, g2 = 0.094 ± 0.250, α = 0.743 ± 0.106; generating a phase function with an asymmetry parameter of g = 0.673 ± 0.081 (in line with Wolff et al., 2009). Although differences were observed during the low opacity aphelion season (lower forward scattering values, presence of a peak in the backward region) compared to the rest of the year, no clear evidences of seasonal behaviour or interannual variability were derived. The obtained average D/L aspect ratios for T-matrix calculated cylindrical particles were 0.70 ± 0.20 and 1.90 ± 0.20 (similar to Wolff et al., 2001), and the best fitting Martian dust analogue corresponded to the basalt sample (in agreement with Dabrowska et al., 2015).