A 3.5 full Martian Years dust and water ice climatology from Nomad-SO/TGO observations

Introduction

The Martian atmospheric aerosols are mainly composed of mineral dust and/or water ice. Water ice clouds affect both the radiative balance [1] and the water cycle [2], while mineral dust is globally the most abundant component, modifying the thermal structure and atmospheric dynamics [3,4]. It can affect observations from both orbiting satellites and rovers on ground, especially at the dusty season around perihelion, where many storms are initiated at different temporal and spatial scales [5].

Both aerosol types are related, as dust can serve as a base for water ice cloud formation [5]. Therefore, a comprehensive understanding of the Martian atmosphere requires a simultaneous climatology of both components, such as the one presented in this work.

Dataset and methodology

NOMAD is a three spectrometers suite on board the Trace Gas Orbiter (TGO). It has been routinely observing the Martian atmosphere since April 2018 [6], providing detailed atmospheric transmission vertical profiles. Data acquired from its solar occultation (SO) channel are used to retrieve aerosol properties and their vertical distributions in the infrared at high vertical resolution. This is achieved by combining multiple diffraction orders (i.e., wavelength intervals), selected to maximize coverage of the SO spectral range while consistently including an order near 3 μm. This spectral region is key to distinguish between dust and water ice aerosols [6].

Our retrieval strategy builds on [7] and is thoroughly described there. The inferred aerosol characteristics using this methodology are size (effective radius and variance) and composition (proportions of mineral dust and water ice). These magnitudes are used to derive other aerosol magnitudes such as number density, mass loading and mass mixing ratio.

We processed ~3000 SO observations from Ls = 194° (MY34) to the end of MY37, significantly extending the dataset in [7], focused only on the first year of NOMAD observations. We have also developed a scheme to identify cases with mixed aerosol components (dust and ice) from those where there is a single dominant component, and the secondary one is only adding noise to the results.

 

Results

In contrast to [7], due to our extended dataset we can study latitudinal and interannual comparisons between the different Martian Years, being able to detect the global patterns that govern the distribution of dust and water ice clouds.

We report an average maximum altitude of aerosol detection around 60 km near perihelion (Ls ~ 270°) and 40 km near aphelion (Ls ~ 90°). Variations within these values are sometimes significant, and linked to the latitude of the observations. Our results reveal features such as water ice clouds and signatures of both regional and global dust storms. All four Martian Years show systematic differences of their aerosol properties between aphelion and perihelion, which is consistent with the previous work by [7].

Lastly, a comparison of our results with similar datasets from NOMAD-UVIS [8] and ACS [9] has been performed. This comparison focuses on comparing the extinctions and effective radius retrieved, as well as their seasonal and latitudinal distribution. We have found that NOMAD-UVIS retrieves in average smaller particle sizes than SO, which is more sensitive lo larger sizes. Therefore, both wavelength ranges, ultraviolet and infrared, are complementary to produce a complete climatology of Martian aerosols. This combined information is important for improving climate models, which rely on a priori assumptions about aerosol microphysics, composition, and optical properties.

 

References:

 

• Wilson, R.J, et. al., 2008.
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• Michael Battalio and Huiqun Wang, 2021.
• Smith, M.D. et. al, 2019.
• Määttänen, A. and Montmessin, F., 2021
• Vandaele, A, C. et. al, 2018
• Stolzenbach, A. et. al, 2023.
• Flimon, Z., et. al, 2025.
• Schterbinine, A. et. al, 2022