Martian water ice clouds during the 2018 global dust storm as observed by the ACS-MIR channel onboard the Trace Gas Orbiter

TL;DR

This study uses ACS-MIR observations to analyze water ice clouds on Mars during the 2018 global dust storm, revealing increased altitude, smaller particle sizes, and the presence of large particles at high altitudes during the storm.

Contribution

It introduces a new detection method for small water ice particles and provides detailed characterization of cloud properties during a major dust storm on Mars.

Findings

Water ice cloud maximum altitude increased during the GDS.

Small water ice particles were more frequent during the GDS.

High altitude, large water ice particles were observed during the GDS.

Abstract

The Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) ESA-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 - 4.2 $\mu$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 $\leq$ 2 $\mu$m, and applied it to observations gathered between $L_s=165^\circ$ and $L_s=243^\circ$. We observe a shift in water ice clouds maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small-sized ($r_\mathrm{eff} \leq 0.3$ $\mu$m) water ice clouds are more frequent during MY34 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 dataset. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of km for certain sizes. We notably notice some large water ice particles ($r_\mathrm{eff}\geq1.5$ $\mu$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.