Introduction
The water vapor vertical distribution in the Martian atmosphere is driven by microphysics, radiative transfer, dynamics and chemistry. Temperature is one of main parameters controlling the saturation of water, which governs cloud formation, and subsequently the vertical distribution of water. Because clouds have a radiative effect (see e.g., Navarro et al., 2014), they generate in turn a feedback loop that impacts climate overall. The cold aphelion season is characterized by a low hygropause that produces clouds and blocks water vapor below 10–20 km, thereby regulating the advection of water from the Northern to Southern Hemisphere (Clancy et al., 1996; Montmessin et al., 2004). In the warm dusty perihelion season, higher atmospheric temperatures induce higher hygropause, with water vapor extending up to 60–80 km (Clancy et al., 1996; Montmessin et al., 2004;Trokhimovskiy et al., 2015a; Fedorova et al., 2021). Recent observations found out that global dust storms have a strong impact on the water vertical distribution. During these events, water can reach up to 100 km, as was observed during the MY28 Global Dust Storm (GDS) (Fedorova et al., 2018; Heavens et al., 2018) and MY34 (Aoki et al., 2019; Fedorova et al., 2020, 2021, Belyaev et al., 2021). Such events constitute a quasi-direct source of hydrogen in the upper atmosphere, accelerating the escape of hydrogen to space (Clarke, 2014; Chaffin et al., 2014, 2017; Krasnopolsky, 2019; Shaposhnikov et al., 2019). In situ sampling by the Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ions Mass Spectrometer (NGIMS) even showed that water molecules were propelled up to an altitude of 150 km (Stone et al., 2020), producing one of the most spectacular illustration of the tight and fast coupling between the lower and the upper atmosphere. The link between dust and hydrogen escape has been explored thanks to simultaneous measurements of dust, temperature, ice, water and hydrogen during the regional C-storm of MY34 (Ls 310 and 320°). This isolated dust event boosted planetary H loss by a factor of 5 to 10 (Chaffin et al., 2021). Highly sensitive Trace Gas Orbiter (TGO) measurements were able to profile water up to 120 km during perihelion in MY34 and MY35, and during the MY34 GDS (Belyaev et al., 2021). Together with the multi-annual SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) survey of H2O vertical distribution (Fedorova et al., 2021), such kind of dataset has proven insightful to address the contribution of the dusty perihelion season and that of the MY34 GDS in enhancing the production of H atoms that eventually make their way to the upper atmosphere (Belyaev et al.,2021).
Supersaturation reflects the ability that water has to penetrate the cold trap associated with cloud formation and propagate above it. It also reveals inefficient condensation process, that may be attributed to a lack of condensation nuclei (CN) or higher resistance to diffusive growth. The first evidence of supersaturated water vapor in the Martian atmosphere was produced by SPICAM on Mars Express. Maltagliati et al. (2011) observed supersaturation reaching 2 to 10 at 30–40 km in the aphelion season, from Ls=50° to 120° in both hemispheres using SPICAM water profiles and collocated Mars Climate Sounder (MCS) temperatures corrected for local time. The observed supersaturated state was then explained by a possible lack of CN due to the scavenging effect; that is, a cleaning of the atmosphere of dust captured by forming ice particles and then dragged to lower altitudes by sedimentation. Yet, Fedorova et al. (2014), analyzing the same set of observations have shown that aerosols, and thus CN, was present in the atmosphere for latitudes below 60N at 30-50 km but mostly with submicron particle size <0.1 µm. Author estimated that the critical saturation ratio for such particles can vary from 2 to 4 for low temperatures of the middle atmosphere that is roughly consistent with Maltagliati et al. (2011). Clancy et al. (2017) used 1.27 µm O2(1Δg) dayglow profiles from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) limb observations and the Laboratoire de Météorologie Dynamique (LMD) Mars Global Climate Model (GCM) to constrain water profiles and estimate a saturation ratio for Ls=60–140° and 200–310° seasons. A high (>2.2) saturation ratio was found in northern mid-to-high latitudes at 20-40 km, in rough agreement with SPICAM findings. In contrast, Clancy et al. (2017) reported no supersaturation in the southern summer season, when SPICAM found supersaturation of 2–3 times at 80 km in presence of clouds (Maltagliati et al., 2013).
However, these earlier discoveries of supersaturation had to rely on separate temperature estimates and the first simultaneous water vapor and temperature profiles were produced by TGO and ACS (Korablev et al., 2018; Fedorova et al., 2020) experiment dedicated to occultation measurements and vertical profiling of atmosphere. TGO began its science phase in march 2018 of 2018 (Ls=163° of MY34) shortly before the GDS. ACS observed the saturation state on the global scale during the dusty season and definitely showed that supersaturation occurs even when clouds are present. The supersaturation was detected in both hemispheres above 60–80 km from Ls=210 to 340° and in high latitudes in the lower atmosphere, below 20–30 km.
Poncin et al. (2022) have investigated water vapor saturation in the presence or proximity of water ice clouds for co-located observations of temperature and ice clouds by MCS and water vapor profiles by CRISM and Nadir and Occultation for MArs Discovery (NOMAD) on TGO at various times during the Martian year. During the aphelion season, water was close to saturation in the presence of clouds with supersaturation of 2-3 at the top of the cloud layer. During the perihelion season, the supersaturation up to 1.5 was observed in high southern latitudes near the top of the clouds. Measurements during the GDS of 2018 suggest that supersaturation did not exceed 5. Based on these measurements, a schematic model of cloud evolution was proposed (Poncin et al., 2022) with small particles formed rapidly in supersaturated regions and then growing and falling explaining why water ice was observed even in the subsaturated region below.
Water ice clouds play a key role in the radiative transfer of the Martian atmosphere, impacting its circulation and thermal structure (Madeleine et al., 2012). Therefore, detailed cloud microphysics has become an important component of Mars’ Global Climate models (Montmessin et al., 2004, Navarro et al., 2014; Shaposhnikov et al., 2019; 2022). The first time supersaturation was simulated by a climate model, it had to include a cloud microphysics module accounting for nucleation of dust particles, water ice particle growth/sublimation, and scavenging (Navarro et al., 2014). The model found reasonable agreement with SPICAM observations with regard to the supersaturation level predicted above the hygropause. At Ls=210°-240° and 330°-360° studied by authors, temperature clearly controlled the boundary of supersaturation and its values reached 1000 in some cases but the absolute value of water mixing ratio in this region was less than 1 ppmv and negligible compared to total water.
Following TGO observations, Holmes et al., (2021a) studied global variations of water vapor and saturation state in the dusty season of MY34 including the GDS. Using the cloud model of Navarro et al. (2014), they assimilated observed temperature and water fields from a variety of instruments in Mars’ climate model. Discrete layers of supersaturated water were found at all latitudes during the dusty season of MY34. They found evidence of water supersaturation above 60 km for most of the GDS period. The GDS and the southern summer regional C- storm (Ls from 310° to 320°) forced water to be supersaturated at altitudes where it can break up and then produce hydrogen atoms that eventually escape to space. The model reanalysis indicates that it happens at all latitudes where ACS observed.
Compared to Fedorova et al. (2020), we present here the full set water vapor saturation state obtained by Near-InfraRed (NIR) ACS spectrometer throughout two Martian years from Ls=163° of MY34 to Ls=170° of MY36, while Fedorova et al. (2020) stopped their analysis at Ls = 355° of MY34. Since this first publication, the calibration of the instrument and the retrieval procedure and pipeline have been improved as described in Section 2 where an overview of the ACS NIR spectrometer, retrievals and observation coverage are presented as well as observations of H2O clouds with mid-infrared channel of ACS. Section 3 provides an overview of results focusing on the seasonal, latitudinal, local time and interannual variations. Section 4 discusses the correlation with previous measurements.