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.