Mars Years 34-38 dust properties from TGO/NOMAD UVISand LNO channels’ nadir data analysis

In this work we discuss the dust properties (column densities n_col, effective radii r_eff and optical depths τ) obtained from the analysis of the nadir data of TGO/NOMAD’s UVIS (200 – 650 nm, [1]) and LNO (2200 – 3800 nm, [2]) channels, encompassing Mars Years from 34 to 38. The combination of the two datasets increases the information content related to airborne dust scattering and extinction, hence allowing a more robust constraining of physical properties. Indeed, the separated investigation of UVIS or LNO ranges alone is prone to non-negligible biases in the results, due to the limited spectral information that only partially covers the dynamical change of the dust scattering efficiency between visual and near infrared wavelengths [3]. We perform the retrieval with the MITRA radiative transfer tool [4,3] and take advantage of surface albedo spectra retrieved from MEx/OMEGA [5] spectrometer’s data, processed with a modification of the Surface-Atmosphere-Separation (SAS) method [6,7]. Since the dust grains’ irregular shape is still largely unconstrained, we assume spherical scatterers. Moreover, we exploit the optical constants by [8]. The pipeline behind the coupling of spatially and temporally simultaneous UVIS and LNO observations involves footprint matching, ice filtering (exploiting the conditions from [9]), UVIS spectral range selection (based on the available spectral albedo information and on the presence of calibration/instrumental issues) and LNO data radiometric ad-hoc calibration (accounting for instrumental interferences of electrical origin) and spectral handling for reliably balancing the information content among the two datasets [6].

The results provide information on the integrated atmospheric column and, even if the patchiness of UVIS+LNO dataset only allows a partial coverage of the dusty seasons (Ls > 180°), they give the possibility to discuss the order of magnitude of physical properties and their trends. Extreme dust opacities (τ_2.6 ~ 10, evaluated at 2.6 μm, n_col ~ 10^-9 cm^-2) are observed during both the main (180° < Ls < 240°) and the secondary (320° < Ls < 340°) MY34 dust storms. As expected, the retrieved r_eff appear anti-correlated with τ during the storm, as they provide information only down to those altitudes where the dust layer becomes optically thick (i.e. the lowest layers characterized by larger grains are not sounded). As a result, the storm peak is characterized by smaller particles lifted at higher altitudes (r_eff ~ 1.0 μm) while larger ones are detected at the storm onset and decay phases (r_eff > 1.5 μm). While the dust scenario in MY35 and 36 appears more quiet (n_col ~ 10^-7 cm^-2), MY37 shows enhanced dust activity at Ls > 200° (n_col ~ 10^-8 cm^-2). Finally, MY38 results indicate a return to quiet conditions at Ls < 100° (n_col < 10^-8 cm^-2) with the rest of the dataset currently under processing.

Acknowledgements

ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB- BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2026-6-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canada Space Agency.

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