Properties of water ice and dust particles in the atmosphere of Mars during the 2018 global dust storm as inferred from the Atmospheric Chemistry Suite

International audienceThe properties of Martian aerosols are an integral part of the planetary climatology. Global dust storms (GDS) significantly alter spatial and vertical distributions of dust and water ice aerosols and their microphysical properties. We explored the 2018/Martian Year 34 GDS with the Atmospheric Chemistry Suite (ACS) instrument onboard the ESA‐Roscosmos Trace Gas Orbiter (TGO) mission. Solar occultation observations of Thermal InfraRed (TIRVIM) and Near InfraRed (NIR) channels in the 0.7−6 μm spectral range with >103 signal‐to‐noise ratio are used to constrain the vertical dependence and the temporal evolution of the particle properties of water ice and dust (effective radius, effective variances, number density, and mass loading) before the 2018 GDS and during its onset and decay phases. In most of the observations, the particle size of dust and water ice decreases with altitude. The effective radius of dust and water ice particles ranges from 0.1−3.5 μm and 0.1−5.5 μm, respectively. The largest aerosol particles (> 2.5 μm for dust and > 3.5 μm for water ice) are present below 10 km before the onset and during the GDS decay phase. During the peak of the GDS, dust reached altitudes of 85 km; the most frequently observed effective radius is 1−2 μm with 0.1−1 cm‐3 number density and 0.1 effective variance. Detached layers of water ice composed of 0.1−1 μm particles are systematically observed at 50−100 km during this period. Below, at 0−50 km, we see the dust mixed with the main water ice layer comprising 1−4 μm particles