Since the first spectroscopic detection by Mariner 9 (Hanel et al., 1972), water ice clouds have been extensively studied because of their connection to the Martian water cycle (Clancy et al., 1996; Madeleine et al., 2012; Smith et al., 2013; Clancy et al., 2017; Guzewich & Smith, 2019). Clouds scatter and absorb incoming solar radiation, thus impacting atmospheric structure and temperature. As a result, water ice cloud particles can modify the global circulation of the Martian atmosphere (Wilson et al., 2008). Clouds are also a major actor in the inter-hemispheric water exchange (Clancy et al., 1996). As the evolution of our understanding of the martian climate shows an increasing role of water ice clouds, there is a growing need to better characterize the properties of water ice aerosols in order to better understand and model Mars climate and weather (Richardson, 2002; Montmessin et al., 2004; Navarro et al., 2014). Recent studies have notably reveal how more precise vertical representation of water-related atmospheric phenomena may impact water cycle modeling (Vals et al., 2018). Precise observational constraints about the actual microphysiccal properties of clouds as a function of altitude, notably particle size, are thus of interest to better characterize the whole water cycle.

Planetary-scale storms, characterized by widespread lifting and transport of dust particles, modify the thermodynamics and circulation of the Martian atmosphere on a global scale, and subsequently affect the water cycle.

While regional dust storms are recurrent phenomena on Mars, few of them evolve into events that encircle the whole planet to become global dust storms (hereafter ”GDS”).

Such events are erratic, with an average of one occurrence every three to four Martian years (Zurek & Martin, 1993; Clancy et al., 2000; D.

M.

Kass et al., 2016).

The first two GDSs of the 21th century occurred 6 Earth years apart, in 2001 and 2007, corresponding to Martian Years (hereafter ”MY”) 25 and 28 (e.g.

Wang & Richardson, 2015).

Two large (regional) dust storms developed in 2018 (MY 34): one in the Northern hemisphere starting at Ls = 181◦, and the other in the Southern hemisphere at Ls = 188◦.

They subsequently merged to become a GDS at Ls = 193◦, lasting until Ls = 250◦ (Guzewich et al., 2019; S´anchez-Lavega et al., 2019; Smith, 2019).

This GDS occured near the Northern fall equinox, however, it should be noted that there is no unique timing for the onset and development of GDSs on Mars.

For instance, the MY 28 GDS, occured significantly later, at the end of the so-called storm season (from Ls = 260◦
to Ls = 310◦) (Wang & Richardson, 2015).

This variability implies that a variety of scenarios dictate the formation of GDSs, requiring individual assessments of GDS’s.

The MY 28 dust storm has been shown to have profoundly influenced the water vapor atmospheric distribution, elevating water well above the troposphere (>60 km) (Fedorova et al., 2018) while pushing up the hygropause altitude (Heavens et al., 2018).

A rapid increase of the H2O and HDO abundances at altitudes between 40 and 80 km (Vandaele et al., 2019) was observed during MY 34 storm, and recently reproduced in a GCM simulation (Neary et al., 2019).

This large augmentation of the high altitude water content is believed to boost hydrogen, or nearly equivalently, water escape from Mars (Chaffin et al., 2017; Heavens et al., 2018; Fedorova et al., 2020).

However, the formation of clouds impacts the ability of water (or hydrogen) to escape as it confines (in theory) a significant fraction of water below the cloud level.

Thus, understanding the extent to which high altitude water vapor condenses to form clouds during GDSs is important in assessing the possible propagation pathways of water vapor up to the exobase (Neary et al., 2019).

Because water vapor can exist in a supersaturated state in the Martian atmosphere (Maltagliati et al., 2011) and has been observed in a large amount during the MY 34 GDS (Fedorova et al., 2020), simply supposing that the freezing point can be used to set the limits where water is free to be mobilized is likely to be problematic.