WATER VAPOR SATURATION AND ICE CLOUD OCCURRENCE IN THE
ATMOSPHERE OF MARS
L. Poncin1,2, A. Kleinböhl1, D. M. Kass1, R. T. Clancy3, S. Aoki4, A. C. Vandaele5, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA (armin.kleinboehl@jpl.nasa.gov), 2École
Supérieure des Techniques Aéronautiques et de Construction Automobile, Paris, France, 3Space Science Institute, Boulder, CO, USA, 4Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan
5
Royal Belgian Institute for Space Aeronomy, Brussels, Belgium.

Introduction:
The vertical distribution of water vapor and its
relation to ice cloud occurrence in the Martian atmosphere has been a longstanding question. Saturation and cloud formation are major factors that constrain the vertical distribution of water vapor. The
altitude at which water vapor saturation occurs is
largely controlled by the thermal structure of the atmosphere and varies with season and in response to
dust storm occurrence. The vertical distribution of
water vapor has recently gained additional interest
with the recognition that middle atmospheric water
vapor can photolyze, leading to the formation of hydrogen, which in turn can propagate to the upper
atmosphere and escape. Hence the transport of water
vapor to the middle atmosphere influences the hydrogen escape rate and quantifying its variation
might help explain this contribution to hydrogen escape and consequently water loss from Mars.
The saturation state of water vapor in the Martian
middle atmosphere allows us to better understand the
mechanisms behind the planet's water cycle and how
efficient the ascent of water vapor to the upper atmosphere is. It is widely assumed that water vapor in
the presence of water ice clouds usually is in saturation or near saturation, and that supersaturation is a
rare behavior in the cold Martian atmosphere. However, recently this view has been challenged as findings from the Atmospheric Chemistry Suite (ACS)
onboard the ExoMars Trace Gas Orbiter (TGO) suggest supersaturation even in the presence of dust and
ice clouds [1]. ACS observed widespread
supersaturation over a large vertical range, with saturation ratios frequently exceeding a value of 10 and
even reaching values over 100. This suggests that
even though condensation nuclei are present in the
middle atmosphere, the process of condensation
would have to be overpowered by a process that
would allow for water vapor to become supersaturated in cloudy conditions, such as rapid drops in temperature and/or rises in water concentration.
In this paper, we study water vapor saturation in
the presence or proximity of clouds in the Martian
atmosphere by intercomparisons of water vapor pro-

files, derived from measurements by the Compact
Reconnaissance Imaging Spectrometer for Mars
(CRISM) onboard the Mars Reconnaissance Orbiter
(MRO) and by the Nadir and Occultation for MArs
Discovery (NOMAD) instrument onboard TGO, with
saturated water profiles, derived from temperature
measurements by the Mars Climate Sounder (MCS)
onboard MRO. We find that the atmosphere above
cloud layers is largely subsaturated, suggesting that
cloud formation is fairly instantaneous upon the temperature dropping below the frost point and high
levels of supersaturation are not required to form
water ice clouds. We propose a schematic model of
cloud evolution in which small cloud particles are
formed rapidly in slightly supersaturated regions, and
then fall and grow such that ice opacity is still observed even in the subsaturated regions below. Detailed findings of this study have been presented by
Poncin et al. [2].

Coordinated observations between CRISM
and MCS:
Part of this study was motivated by the publication of seasonal zonal averages of water vapor content from CRISM derived from O2(Δg) dayglow
emission rates [3] between Ls=30° and Ls=360°
spanning from MY29 to MY33. Operating from the
same spacecraft as CRISM, MCS was able to perform limb/on-planet measurements in coordination
with most CRISM limb measurements. CRISM data
are provided as zonal averages between 6 and 54 km
altitude above the areoid and between 90°S and
90°N in latitude. Each individual MCS temperature
profile is used to estimate water vapor volume mixing ratio at saturation [4] in the presence or proximity of clouds, as diagnosed by the MCS water ice profile. Then zonal and subseasonal averages of water
vapor saturation ratio is derived from CRISM ambient water vapor and MCS saturated water vapor.

Coordinated observations between NOMAD
and MCS:
In addition to comparisons with water vapor derived from CRISM we evaluate comparisons with

water vapor as measured by the NOMAD instrument
in solar occultation geometry from ExoMars TGO
[5]. Part of these intercomparisons is based on serendipitous co-located observations between MCS
and NOMAD during the global dust event in 2018.
In addition, MCS performed coordinated observations with NOMAD from February 15 to March 7,
2019, in which MCS actively targeted the tangent
point of a solar occultation measurement by
NOMAD. NOMAD data as individual profiles between -20 and 120 km altitude above the areoid were
selected as families of measurements by looking for
coordinated MCS measurements that fall within a
time and space window centered on each NOMAD
measurement. Each individual MCS temperature
profile is used to estimate water vapor volume mixing ratio at saturation [3] in the presence or proximity of clouds, as diagnosed by the MCS water ice profile. Then vertical averages of water vapor saturation
ratio are derived for each family of co-located measurements from NOMAD ambient water vapor and
MCS saturated water vapor.

Results:
CRISM/MCS. The CRISM part of the study allows us to evaluate the saturation state of the atmosphere and its variation with season. We separate the
clear and dusty seasons averaged between MY29 and
MY33 into the two Ls-ranges Ls=30°-140° and
Ls=140°-360° [2]. For this abstract we focus on a
subset of data during the aphelion season spanning
Ls=60°-100° (Fig. 1). In this season a good correspondence of the overall morphology of the water
vapor distribution in the two datasets can be observed, especially in cloud occurrence as shown in
Fig. 1 (circle-shaped points). In the aphelion season

Figure 1: Correlations between saturated water vapor
mixing ratio from MCS and ambient water vapor
mixing ratio from CRISM for Ls=60°-100°.

CRISM ambient water vapor is usually near saturation or slightly supersaturated in the presence of
clouds.
The dusty season is characterized by a rise of the
hygropause, with the region of cloud occurrence almost reaching the top of the considered altitude
range of 54 km. Higher atmospheric temperatures
lead to an increase in MCS saturated water vapor,
which tends to be somewhat higher than the CRISM
zonal averages so during the dustier perihelion season the atmosphere appears largely subsaturated. In
some Ls-ranges of the dusty season, temperatures and
water vapor mixing ratios in the southern high latitudes are somewhat higher but still suggest some
level of supersaturation.

Figure 2 : Average profiles of temperature, water vapor, water ice opacity, and water saturation
ratio for a southern high latitude band in southern summer (L s=338.9°-347.3°).

NOMAD/MCS. The NOMAD part of the study
allows us to evaluate the saturation state of the atmosphere during the dusty season in detail. We separate the dataset into seven families of measurements
retrieved during the decay phase of the MY34 GDS
(around Ls=240°) and after the MY34 C storm
(around Ls=340°) [2].
For this abstract we focus on one family after the
MY34 C storm (Fig. 2). We observe that NOMAD
ambient water vapor is often close to saturation in
the presence of water ice clouds between 10 and 40
km, with the ambient water vapor mixing ratio following the saturated mixing ratio over a large vertical extent. However, during the decay phase of the
GDS the atmosphere is more dynamic and MCS observes that water ice clouds reach higher altitudes
between 50 and 70 km.
Some supersaturation is observed in each profile,
slightly above the peak in ice opacity, reaching no
more than a factor of 2 with the exception of a single
family where a supersaturation ratio of about 5 is
reached at the beginning of the decay phase of the
GDS. Above the top of the cloud, the atmosphere is
also largely subsaturated as temperature increases
slightly above the peak in ice opacity, suggesting that
extended regions of supersaturation are not required
to form clouds.

largely subsaturated, suggesting that cloud
formation is fairly instantaneous upon the
temperature dropping below the frost point.
Large amounts of supersaturation are not required to form water ice clouds.
We propose a schematic model of cloud formation and evolution in Fig. 3 that illustrates a qualitative explanation regarding the saturation state of
the atmosphere in the presence and proximity of
cloud occurrence as observed by MCS, NOMAD and
CRISM.

Figure 3: Schematic summarizing the saturation state
of the atmosphere in the presence of cloud occurrence
and the evolution of water ice particles.

Conlusions:
We have evaluated water vapor saturation in the
Martian atmosphere in the presence or proximity of
water ice clouds by comparing profile averages of
ambient water vapor derived from CRISM and
NOMAD measurements with profile averages of
saturated water derived from MCS temperature profiles. We come to the following conclusions:








During the aphelion season the atmosphere is
close to saturation in the presence of clouds.
Supersaturation ratios reach values of no
more than 2 to 3 towards the top of the cloud
layer.
During the perihelion season water vapor is
close to saturation or somewhat subsaturated
in the presence of clouds, with some
supersaturation towards the top of the
clouds. Regions towards the bottom of the
clouds are often subsaturated.
Based on the examples studied during the
Global Dust Storm the region of cloud occurrence shifts to higher altitudes but the
overall structure of supersaturated vs.
subsaturated regions does not change markedly.
The atmosphere above cloud layers appears

When the atmospheric temperature drops below
the frost point at high altitudes, small water ice particles start to form. No large supersaturations are required for this to happen, suggesting that cloud condensation nuclei, most likely in the form of small
dust particles, are sufficiently abundant to enable
nucleation. The water ice particles grow in size due
to constant exposition to water vapor on their surfaces in conditions of saturation or supersaturation and
through coagulation by colliding with other ice particles. As they grow the particles will fall due to gravity while consuming the available water vapor on
their way through the atmosphere. The suggested
structure in ice particle size is qualitatively consistent
with recent results [6, 7] that found a general trend of
decreasing ice particle size with altitude. As the particles reach warmer and more subsaturated regions of
the atmosphere, they start to sublimate. Hence the
condition of subsaturation at the bottom of water ice
clouds is likely related to ice particles falling into
these subsaturated regions and evaporating.
We suggest future coordinated measurements between MCS and the solar occultation instruments on
ExoMars TGO as well as extended intercomparisons
between the TGO instruments in order to further

constrain the saturation state of the Martian atmosphere.
Acknowledgement:
We thank Frank Daerden for fruitful discussions
and helpful suggestions on the manuscript. L. P. is
grateful for a grant from ESTACA, allowing him to
participate in a remote internship at JPL via the JPL
Visiting Student Research Program. A. K. acknowledges support from the Mars Data Analysis Program
(80NM0018F0719). We thank the MCS instrument
and MRO spacecraft operations teams for making the
coordinated observations with the CRISM and
NOMAD instruments possible. Work at the Jet Propulsion Laboratory, California Institute of Technology, is performed under contract with the National
Aeronautics and Space Administration. Copyright
2022, California Institute of Technology. All rights
reserved.

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