A 3 Martian Year climatology of aerosols with ExoMars TGO-NOMAD: seasonal cycles and new insights

The Nadir and Occultation for MArs Discovery (NOMAD) spectrometer has been collecting Mars observations since 2018, providing a massive amount of information regarding its atmospheric composition, its vertical structure and bridging the gap between the previous knowledge of the lower atmosphere and the data from other missions (e.g., MAVEN) regarding atmospheric escape. The capability of the Solar Occultation (SO) channel to map the vertical structure of the atmosphere with a very high (>1000) signal-to-noise ratio, at a very high spectral resolution (>17000) and a high vertical sampling (0.5 to 2 km) is valuable in many contexts, ranging from the search for trace species in the lower atmosphere (10 to 40 km) to mapping the isotopic composition of the main atmospheric constituents (H₂O, CO₂, CO) or exploring the vertical structure of dust, water ice and CO₂ ice clouds.

Aerosols are some of the main drivers of the Martian climate, and the study of their spatial distribution and microphysical properties can advance our knowledge of their impact on the climate of the planet and on their formation and dynamics. This work will show the extension of previous investigations focused on dust, water ice and CO₂ ice using NOMAD data, by presenting the mapping of these atmospheric components on a global scale over 3 Martian Years (MY34 Ls 160 to MY37 Ls 170). The acquisition by NOMAD of several diffraction orders during a single occultation allows in fact to obtain spectrally broad information that can be used to map dust and water ice vertical distributions and particle sizes. The information content of NOMAD data about particle sizes of water ice has been demonstrated to be particularly high and to give important information about the nucleation processes of water ice. NOMAD data can also be used to look for CO₂ ice by combining broad spectral information with localized CO₂ ice features at 3600 and 3710 cm^-1, which are well identifiable in the NOMAD spectra.

Besides presenting the climatology of aerosols, we will illustrate specific features occurring during the Martian Year and their repeatability; more specifically, we will look into the differences between MY 34, characterized by a Global Dust Storm, and following years, to highlight the impact of dust-induced heating over cloud formation. We will also give some insights into CO₂ ice cloud formation, which was confirmed to be surprisingly heterogeneous compared to results obtained before TGO operations.