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Retrieval and seasonal variations of aerosols in the Martian atmosphere from the NOMAD instrument on board the Trace Gas orbiter

par Flimon, Zachary
Président du jury Clarisse, Lieven
Promoteur Bauduin, Sophie
Co-Promoteur Robert, Séverine
Publication Non publié, 2025-12-16

Thèse de doctorat

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Résumé :

Mars is a terrestrial planet with a mass and a much thinner atmosphere compared to Earth. Although its rotation period is similar to Earth’s day, its revolution around the Sun takes nearly twice as long. The planet’s elliptical orbit results in strong seasonal variations in its atmosphere. Martian aerosols composed of dust, H$_2$O ice, and CO$_2$ ice clouds are strongly influenced by these seasonal cycles.Dust is a permanent component of the Martian atmosphere. Depending on the season, dust storms can occur frequently, ranging from local to regional and, in some cases, global events. Dust plays a key role in the Martian climate system by absorbing solar radiation and heating the atmosphere, thereby affecting circulation and temperature profiles. Understanding its properties and seasonal variations is therefore essential. Although H$_2$O and CO$_2$ ice clouds are less widespread, they also influence the thermal structure of the atmosphere by reflecting sunlight and emitting infrared radiation. These ice clouds often form on dust particles, linking their seasonal behavior to that of dust.This work focuses on retrieving the optical properties of aerosols, specifically extinction and particle size, using solar occultation measurements from the Nadir and Occultation for Mars Discovery (NOMAD) instrument aboard the ExoMars Trace Gas Orbiter (2016–present). NOMAD consists of three channels: UVIS (Ultraviolet–Visible, 200–650 nm), SO (Solar Occultation, 2.3-4.3 \textmu{}m), and LNO (Limb Nadir and Occultation, 2.2-3.8 \textmu{}m). In solar occultation, we can probe the vertical structure of the atmosphere and derive vertical opacity profiles. In the first part of this work, we use the UVIS channel alone to study aerosol optical properties. Although UVIS cannot determine aerosol composition, its large number of spectral points and low noise level make it well suited for retrieving sub-micron particles, with strong sensitivity at high altitudes.We then studied aerosols using the SO channel, which allows us to retrieve aerosol composition as well as larger particle sizes at lower altitudes, where the atmosphere is more optically thick.Finally, to take advantage of the simultaneous measurements from both channels, we merged the spectra from UVIS and SO and retrieve aerosol properties jointly. This combined approach improves vertical coverage and provides both size and composition information in a single, consistent retrieval.Because the UVIS and SO channels were not originally designed for aerosol studies, a dedicated pre-processing and merging methodology was developed to combine their measurements into a single spectrum. This approach allowed us to exploit the complementary sensitivities of both instruments.The broad wavelength coverage of NOMAD enables us to distinguish between different aerosol types. In the UV the aerosols cannot be separated due to the lack of absorption features. In the infrared, dust exhibits a relatively featureless spectrum, while H$_2$O and CO$_2$ ices show distinct absorption bands.In this work, we focus on the optical properties and climatology of dust and H$_2$O ice aerosols. Our methodology is designed to retrieve aerosols, which primarily affect the baseline of the spectra in both NOMAD UVIS and SO. The approach can be applied to each instrument separately or to their combined spectra, and we describe the different parameters and limitations associated with each case. Because aerosols behave differently in the UV and IR, and because they modify the spectral baseline in distinct ways, different strategies are required to retrieve their contributions efficiently.In particular, the specific absorption features of H$_2$O ice in the infrared require us to include the SO spectral region in the retrieval to retrieve the composition. Using the observed spectra from UVIS and SO either separately or jointly we retrieve particle size,number density using Mie theory. Only with the SO channel alone or combined with UVIS we can retrieve composition (dust or H$_2$O ice). A key aspect of our methodology is that we simultaneously retrieved both dust and H$_2$O ice for all observations. We identified several cases showing a mixture of the two aerosol types within the same observation.Using the combined UVIS and SO dataset, along with the simultaneous retrieval of dust and H$_2$O ice, we produced a global climatology spanning mid–Martian Year (MY) 34 to MY 37. This climatology captures the seasonal evolution of aerosols, including dust storm activity and H$_2$O ice distributions during all Martian seasons. Our results are consistent with previous datasets and reveal that dust and H$_2$O ice can coexist at specific altitudes, providing new insights into their coupled behavior in the Martian atmosphere.