Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS-MIR Channel Onboard the Trace Gas Orbiter

The Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) European Space Agency-Roscosmos mission began science operations in March 2018. ACS Mid-InfraRed (MIR) channel notably provides solar occultation observations of the Martian atmosphere in the 2.3- to 4.2-μ m spectral range. Here, we use these observations to characterize water ice clouds before and during the MY 34 Global Dust Storm (GDS). We developed a method to detect water ice clouds with mean particle size ≤ 2 μ m and applied it to observations gathered between Ls=165∘ and Ls=243∘. We observe a shift in water ice cloud maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small-sized (reff≤0.3 μ m) water ice clouds are more frequent during MY 34 compared to non-GDS years at the same season. Particle size frequently decreases with altitude, both locally within a given profile and globally in the whole data set. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of kilometers for certain sizes. We notably notice some large water ice particles (reff≥1.5 μ m) at surprisingly high altitudes during the GDS (50-70 km). These results suggest that GDS can significantly impact the formation and properties of high-altitude water ice clouds as compared to the usual perihelion dust activity.