atmospheric dust particles, water ice crystals also absorb and scatter incoming solar radiation, thus impacting the atmospheric structure and temperature (Wilson et al., 2007, 2008; Haberle et al., 2011; Madeleine et al., 2012; Navarro et al., 2014). In addition, the formation of clouds affects the ability of water (or hydrogen) to be further mobilized and to escape from the planet. However, more observational data are needed to better characterize the properties of water ice clouds in order to understand and model the evolution of the Martian atmosphere (Vals et al., 2018).

The formation of clouds in the atmosphere depends on several factors, such as the presence of water vapor, the pressure and temperature conditions, and the availability of condensation nuclei (Michelangeli et al., 1993; Montmessin et al., 2004). However, as water may exist in a supersaturated state in the Martian atmosphere (Maltagliati et al., 2011; Fedorova et al., 2020; Poncin et al., 2022), simply considering the freezing point for the condensation of atmospheric water is problematic. In addition, accurately predicting the distribution and the properties of airborne dust particles also remains a challenging aspect in current Global Climate Models (GCM) (Forget et al., 2014; C. Wang et al., 2018; D’Aversa et al., 2022). Thus, accumulating additional observations of the distribution and properties of the water ice crystals in the Martian atmosphere is required to further constrain the present-day clouds cycle, and its relationship to the Martian climate and water cycle.
The Atmospheric Chemistry Suite (ACS) instrument is a set of three spectrometers onboard the ExoMars Trace Gas Orbiter (TGO) ESA-Roscosmos spacecraft, which has been conducting science operations since March 2018 (Korablev et al., 2018, 2019; Vandaele et al., 2019). The Mid-InfraRed (MIR) channel is a high-resolution, cross-dispersion echelle spectrometer dedicated to solar occultation (hereafter ”SO”) geometry. Each observation covers a ∼300 nm wide spectral interval selected between 2.3 and 4.2 µm, which is set by rotating the secondary grating to one of the 10 positions (Trokhimovskiy et al., 2015; Korablev et al., 2018). The cross-dispersion optical scheme produces 10 to 21 stacked diffraction orders. The number and separation between the displayed orders depend on the secondary grating position. Considering the instantaneous angular field of view of the detector and its displacement during the integration time for each spectrum, ACS-MIR provides a sampling of the Martian atmosphere with a vertical resolution of ∼2.5 km.
Even though the primary objective of ACS and NOMAD is the study of the atmospheric trace gases in the Martian atmosphere, they also provide a unique dataset for the study of the distribution and properties of Martian aerosols (Stcherbinine et al., 2020; Luginin et al., 2020; Liuzzi et al., 2020, 2021; Streeter et al., 2022). In this paper we use ACS-MIR observations acquired in position 12 (i.e., ∼3.1–3.4 µm spectral range) to retrieve the properties of the Martian water ice clouds from the 3 µm water ice absorption band. Indeed, water ice atmospheric particles exhibit a specific signature due to the O-H stretching and bending that enables the distinction between water ice and bound water in dust. And as the depth and shape of the absorption feature depend on both the abundance and the sizes of the ice crystals, we can retrieve information on these quantities (Vincendon et al., 2011; Guzewich et al., 2014; Clancy et al., 2019). The methodology has already been developed and applied to observations conducted during the global dust storm (hereafter ”GDS”) of Martian Year (MY) 34. These efforts revealed very high altitude water ice hazes (up to 100 km) and large ice crystals (effective radius reff ∼1.5–2 µm) up to 65 km during the storm (Stcherbinine et al., 2020). ACS observations now cover two MY, one with and one without a GDS, for all latitudes, allowing us to observe the seasonal and spatial evolution of the Martian water ice clouds. In addition, observations acquired during MY 35 during the same Ls period when the GDS occurs in MY 34 also offer a reference to be compared to MY 34 GDS observations.
First, we briefly describe in Section 2 the ACS-MIR dataset and the methods used in this study. Sections 3 & 4 present the results of the annual monitoring of water ice clouds with their particle sizes and opacity, and Section 5 compares the retrieved vertical profiles of water ice clouds with results of GCM simulations from the Mars Planetary Climate Model (PCM) (Forget et al., 2022). Finally, Section 6 summarizes the main points of this study.