Aerosol Climatology on Mars as Observed by NOMAD UVIS on ExoMars TGO

The Nadir and Occultation for MArs Discovery spectrometer on board Trace Gas Orbiter began science operations in April 2018, providing infrared and ultraviolet-visible spectra of the Martian atmosphere. This paper explores the application of the UVIS channel in solar occultation to study aerosols. We have developed a retrieval scheme that allows us to study the size and extinction of the aerosol as a function of altitude. Results from mid-MY 34 to the end of MY 36 are reported and discussed. Particle size is retrieved using a Mie code with log-normal distribution with an effective radius (r_eff), 0.1–0.8 μm and an effective variance (v_eff) equal 0.1. In this work, we show the presence of aerosol-detached layers, characterized by a local increase in particle size and extinction. These detached layers can be composed of dust, H₂O ice, or CO₂ ice. CO₂ ice clouds can be detected up to 80 km, while H₂O ice clouds are usually more present around 30–50 km. Symmetry of the particle effective radius between the northern and southern regions is observed. During northern/southern winter, the atmospheric aerosols are composed of smaller particles below 0.6 μm but in summer, the size increases to micron-sized particles that are larger than the sensitivity of UVIS. This increase in size in the northern/southern summer is not correlated with a similar increase in extinction. Our data support the previously observed relationship between water vapor and aerosol, especially the formation of high-altitude water ice cloud during the global dust storm event.