Martian aerosol Climatology on Mars as Observed by NOMAD UVIS on ExoMars TGO

The NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite onboard the ExoMars Trace Gas Orbiter (TGO) is composed of three spectrometers. In this work, we will use the UVIS channel in occultation mode. An aerosol climatology had been produced covering the second half of MY 34 up to the end of MY 36. Aerosols are an important part of the Martian atmosphere and have a strong relationship with the atmospheric temperature. They are composed of dust, H2O ice, and CO2 ice. Dust is the main aerosol and has a significant contribution to the radiative transfer budget, as it absorbs solar radiation, leading to local heating of the atmosphere. Dust is confined to lower altitudes during the aphelion season and can reach higher altitudes during the perihelion, especially during dust storms that frequently arise on Mars during this period. The ice clouds are more present during the aphelion when the temperature is colder and follow a seasonal pattern. Several types of clouds can be found throughout the year, contrary to the dust they reflect the sunlight and cool locally the atmosphere. Using only the spectral range of UVIS dust, H2O ice, and CO2 ice cannot be differentiated because the three aerosols have similar spectral features in the UV-visible. Dust represents most of the aerosols present in the atmosphere, therefore only dust refractive indices are used in this work. Detection of CO2 and water ice will be investigated in future work using the infrared channel of NOMAD. Nevertheless, we presented a way of indirectly recognizing the composition of the aerosols using indirect parameters such as the temperature or comparison with other datasets.It is possible to distinguish the particle size between 0.1 to 0.8 µm with confidence. When the particles are larger it is not possible to retrieve the precise size. In conclusion we present a climatology of Martian aerosols, including vertical extinction profiles as well as vertical profiles of particle size distributions. The seasonal cycle of the dust is observed and recurring structures over different Martian years such as dust storms or ice clouds are detected. We also present a comparison with water vapor profiles and aerosol profiles during regional dust storms, we showed that the water vapor during the storm could condense to water ice due to the presence of dust condensation nuclei at high altitudes. The thermal and dynamical structure of the atmosphere, and chemical species are all sensitive to the aerosol’s abundance and size.