Retrieving scattering properties of Martian dust analogues by modelling light scattering

We applied advanced light-scattering models on laboratory measurements to retrieve the scattering properties of Martian dust analogues JSC Mars-1, MMS-2, and MGS-1. The retrieval was performed in two parts: first, we computed the optical constants of the three samples. Then, the obtained values were used together with two different scattering databases to model the measured scattering matrices of the samples.

Optical constants are needed from modelling single-scattering within the atmospheric dust to simulating the global Martian climate. Reliable complex refractive indices of Martian dust are difficult to find in the literature. Previous studies by Wolff et al. (2009) & (2010) have obtained optical constants from analysing the observed dust storm spectra at 258-2900 nm region by using cylindrical particle shapes. However, these values could not be supported with direct laboratory measurements. In this work, we retrieved the complex refractive indices of the three Martian dust analogues at UV-vis-NIR wavelengths by using the measured size distributions, diffuse reflectance spectra, and an advanced light-scattering model with realistic particle shapes. 

The scattering properties of the analogues were retrieved by utilizing the derived refractive indices, measured scattering matrices, and two databases of single scattering properties of triaxial ellipsoids (Meng et al. 2010) and hexahedral shapes (Saito et al. 2021) The databases have been developed by a combination of three computational methods: the T-matrix method, the discrete dipole approximation (DDA), and an improved geometric optics method. Each database consists of pre-calculated scattering properties over a broad range of complex refractive indices, particle sizes, particle shapes, and wavelengths. The modelled scattering matrices were then compared with those obtained using spherical shapes. Finally, the best-fit model gives us physical properties of the particles such as cross sections, single scattering albedos, extinction efficiencies, asymmetry factors, and Legendre polynomials used to mathematically calculate the phase functions.

 

References: Wolff et al. (2009), JGR Planets, 114, E00D04 ; Wolff et al. (2010), Icarus, 208, 143-155 ; Meng et al. (2010), J. Atmos. Sci., 41.5: 501-512 ; Saito et al. (2021), J. Atmos. Sci., 78, 2089-2111.