Aerosol Dust Optical Properties from Martian Analogs in the UV-vis-NIR

1. Introduction

Aerosol dust particles are an important component in planetary atmospheres, influencing energy balance, thermal structure, weather systems, and remote observations. Despite their importance, radiative-transfer models and climate simulations for planets are often limited by assumptions of oversimplified particle shapes and fixed complex refractive indices for aerosol dust, which hinders the interpretation of observational data. To address this gap, we present an experimentally validated database of aerosol dust optical properties derived from Martian dust analogs. The work combines laboratory measurements with advanced light-scattering modeling and provides a transferable methodology for studying aerosols in a variety of planetary environments.

2. Sample Preparation and Characterization

We selected three spectrally and mineralogically distinctive Martian dust analogs representative of different Martian terrains: JSC Mars-1, MGS-1, and MMS-2 (see Figure 1). The samples were processed at Instituto de Cerámica y Vidrio (ICV) to produce narrow particle size distributions in the geometric optics domain for the retrieval of complex refractive indices, as well as narrow size distributions representative of airborne dust aerosols for the computation of optical properties. Detailed morphological and compositional characterization was performed using SEM imaging, laser diffraction particle sizing, and X-ray diffraction (XRD) (see Martikainen et al. 2023).

Figure 1: The JSC Mars-1, MGS-1, and MMS-2 analogs.

 

3. Experimental Data

Diffuse reflectance spectra were measured from 200 to 2000 nm using a Varian Cary 5000 UV-vis-NIR spectrophotometer for samples with narrow particle size distributions in the geometric optics domain (Martikainen et al. 2023). The scattering matrix measurements at 488 and 640 nm were carried out at the Cosmic Dust Laboratory (CODULAB, Muñoz et al. 2010) of the Instituto de Astrofísica de Andalucía (IAA), covering scattering angles from 3° to 177° using analogs with narrow size distributions representative of airborne dust (Martikainen et al. 2024). The measurements include the phase function (F₁₁), degree of linear polarization (−F₁₂/F₁₁), and, where possible, additional scattering matrix elements (F₂₂, F₃₃, F₃₄, F₄₄). Together, these datasets provide the experimental basis for retrieving the complex refractive indices and computing bulk optical properties for radiative-transfer modeling.

4. Light-scattering modeling

The complex refractive indices were retrieved using the SIRIS4 ray-tracing model (Muinonen et al. 2009), which accounts for the irregular shapes of real dust grains. The model was applied to the measured diffuse reflectance spectra and corresponding narrow particle size distributions in the geometric optics domain. In this regime, the scattering behaviour of large particles is highly sensitive to the complex refractive index, allowing it to be reliably constrained from the measurements.

Bulk optical properties were computed using the TAMUdust2020 database of irregular hexahedral particles (Saito et al. 2021), based on the retrieved complex refractive indices and narrow size distributions representative of airborne dust. The modeled phase matrices were compared with measurements at 488 and 640 nm to validate both the retrieved complex refractive indices and the hexahedra particle model. The final set of calculated properties includes single-scattering albedo, extinction efficiency, extinction cross-section, asymmetry factor, and phase matrix elements across the 200–2000 nm spectral range.

5. Broader Applications
While the database is based on Martian dust analogs, both the experimental approach and the modeling framework are designed to be transferable. Similar methods can be applied to characterize aerosols in other planetary atmospheres, such as those of Venus or exoplanets. Our work demonstrates the value of combining controlled laboratory measurements with advanced numerical scattering models to improve the realism of aerosol dust optical data. Moreover, our derived optical properties reproduce the key spectral features of observed Martian regolith. This confirms the physical relevance of the results and supports their use in forward modeling and data interpretation.

6. Conclusions
We provide a validated, multi-wavelength database of aerosol dust optical properties derived from Martian dust analogs (Martikainen et al. 2025), intended for use in radiative-transfer modeling and observational retrievals across the planetary sciences. The methodology is designed for adaptation to other planetary atmospheres and dust compositions. The full dataset is available at the Granada-Amsterdam Light Scattering Database (Muñoz et al. 2025).

References: Martikainen et al. (2023), ApJS 268; Martikainen et al. (2024), ApJS 273; Martikainen et al.(2025), MNRAS 537; Muñoz et al. 2011, JQSRT 111; Muñoz et al. (2025), JQSRT 331; Muinonen et al. (2009), JQSRT 110; Saito et al. (2021), JAS 78.