Retrieval and seasonal variations of aerosols in the Martian atmosphere from the NOMAD instrument on board the Trace Gas orbiter
Zachary Flimon (LPAP/STAR/ULiège)

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, H2O ice, and CO2 ice clouds are strongly influenced by these seasonal cycles. 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 μm), and LNO (Limb Nadir and Occultation, 2.2-3.8μm). 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. We then studied aerosols using the SO channel, which allows us to retrieve aerosol composition as well as larger particle sizes at lower altitudes. 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. Using the combined UVIS and SO dataset, 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 H2O ice distributions during all Martian seasons. Our results are consistent with previous datasets and reveal that dust and H2O ice can coexist at specific altitudes, providing new insights into their coupled behavior in the Martian atmosphere.