Plain Language Summary
Water vapor plays an important role in the weather and climate on Mars,
even though little of it remains today. The behavior of water vapor has
been studied for decades, yet how water vapor varies with altitude,
especially close to the surface, remains an open question. In this
study, we use measurements from two instruments on the Mars Express
satellite to learn about the near-surface water vapor. By combining
measurements from the SPICAM and PFS spectrometers, a composite
full-year climatology is assembled. We measure the total amount of water
vapor with great accuracy, and also obtain information about the
vertical distribution. The north polar cap is studied in detail during
early summer, when part of the polar ice cap sublimates into water vapor
and is transported south. The results are compared to model data from
the Mars Climate Database, and significant differences between the
observations and the model are identified. The total water content is
found to be smaller than model estimates, while observations indicate
that more water than expected is confined near the surface. This
suggests that some aspects of the atmospheric transport processes are
not currently fully understood.
1 Introduction
Water vapor on Mars was first detected in 1963 with the use of a
ground-based telescope which observed eleven near-infrared absorption
lines (Spinrad et al., 1963). Since then, numerous observatories,
ground-based, Earth-orbiting, Mars-orbiting, landers and rovers, have
observed the highly volatile trace gas. Even as a minor atmospheric
constituent, water vapor plays a major role in shaping the climate on
Mars (along with the CO2 and dust cycles). Water
controls the stability of the atmosphere, as H2O
photolysis supplies hydroxyl radicals, the main oxidant of the Martian
photochemical cycle (e.g. McElroy and Donahue (1972)), and impacts the
radiative equilibrium through cloud formation (Madeleine et al., 2012).
The Mars Atmospheric Water Detector (MAWD) instruments on the Viking
orbiters provided evidence that the Northern polar cap is the primary
source of atmospheric water, and also indicated a strong north–south
asymmetry in the atmospheric water abundance (Farmer et al., 1976;
Jakosky & Farmer, 1982). The most complete climatology, upon which
modern Martian water climatology is based, was obtained by the Mars
Global Surveyor mission and its Thermal Emission Spectrometer (Smith,
2002, 2004). A revised retrieval scheme on TES observations provide an
annual reference water vapor cycle with column abundance maximum at high
latitudes during midsummer in both hemispheres, reaching a peak of ∼60
pr-μm on average in the north, and ∼25 pr-μm in the south (Pankine et
al., 2010). Low water abundances are observed during fall and winter at
middle and high latitudes of both hemispheres. General circulation
models along with TES observations indicate that water from the southern
summer maximum is transported to the Northern Hemisphere (NH) more
efficiently than the reverse process (Montmessin et al., 2004).
One of the main objectives of the Mars Express (MEX) orbiter is to study
the water cycle on Mars. Three spectrometers onboard MEX can measure the
water vapor abundance in different spectral bands: The Planetary Fourier
Spectrometer (PFS), The Observatoire pour la Minéralogie, l’Eau, les
Glaces, et l’Activité (OMEGA) and SPectroscopy for the Investigation of
the Characteristics of the Atmosphere of Mars (SPICAM). For the purpose
of this study, PFS was selected for its coverage of water vapor
diagnostic features in the thermal infrared (TIR) domain, while SPICAM
was chosen over OMEGA to cover the near infrared (NIR) due to its higher
spectral resolution, and the presence of CO2 bands near
the 2.6 µm water feature for OMEGA.
1.1 Water vapor vertical distribution
Until recently, knowledge of the near-surface H2O
profile on Mars mostly relied on general circulation models. The
vertical distribution of water vapor has been inferred from nadir
measurements (Fouchet et al., 2007; Pankine & Tamppari, 2015) and
measured directly by solar occultation viewing geometry with SPICAM on
Mars-Express since 2004, and with the ExoMars Trace gas Orbiter (TGO)
and its infrared spectrometers NOMAD and ACS since 2018. SPICAM
occultation campaigns were not a primary focus of the spacecraft and are
therefore not performed very often, whereas TGO, with its orbit adapted
for occultation measurements with good vertical and temporal resolution,
allows the study of dynamical behavior of water distribution including
escape processes in great detail. With this technique, new knowledge has
been obtained on the vertical distribution of water in the upper
atmosphere as a result of supersaturation above the hygropause, and the
occurrence of high altitude water during dust storms (e.g. (Aoki et al.,
2019; Fedorova et al., 2020)). SPICAM solar occultations were also used
to produce a climatology of vertical distribution covering the Martian
years (MY) 27 to 34, that encompassed two global dust events (Fedorova
et al., 2021). With solar occultation measurements one can obtain very
fine vertical resolution, nevertheless, measurements below 10 km are
relatively sparse as aerosol loading in the lower atmosphere leads to
high opacities which reduces the transmittance significantly. The lower
limit for observation is typically 5-10 km for dust-free conditions, and
as high as 20-30 km during the dusty perihelion season (e.g., Aoki et
al., (2019)). Only under very clear conditions will solar occultation
observations be able to probe below 10 km, however such conditions
mostly occur at high latitudes. As a result, information about the
low-atmosphere water vapor profile remains exceptional.
Below 10 km, surface-atmosphere interactions such as convection, frost
sublimation and deposition are expected to be the main forcers on the
vertical distribution, while above 10 km water ice clouds are thought to
be dominant (Montmessin et al., 2004; Richardson, 2002). Below the
saturation level, controversy exists regarding whether water vapor is
well mixed with CO2, or distributed in a more complex
manner. Davies (1979) used Viking Orbiter 1 data to directly probe the
location of water vapor with altitude for the first time. He found that
H2O vertical distribution was indistinguishable from the
dust vertical distribution and was well mixed up to about 10 km, which
has been commonly assumed since. It is also argued that water is either
confined to, or reduced in the lower atmosphere, depending on season and
location. This ties in with to which extent there is exchange of water
vapor between the atmosphere and the regolith.
Adsorption of CO2 by the Martian regolith was first
suggested by Davis et al., (1969), and the theory was later expanded
upon to include water vapor by Fanale and Cannon (1971), whose
adsorption isotherm expression has been widely used since (although
found to require modification by Savijärvi and Harri (2021)). Using data
from Viking 1 and 2, Jakosky et al., (1997) showed a nocturnal depletion
of atmospheric water vapor, suggesting a diurnal exchange cycle between
the porous regolith and the atmosphere. Similar results were obtained
with the thermal and electrical conductivity probe on the Phoenix lander
by Zent et al., (2010) and Fischer et al.,(2019). It was found that the
layer that experiences a diurnal exchange of water with the surface was
0.5–1 km deep (Tamppari et al., 2010). This phenomenon was again
confirmed by Harri et al., (2014) and Martínez et al., (2017), who used
the REMS-H device on Curiosity rover to derive water vapor volume mixing
ratios. Savijärvi and Harri (2021) found that regolith exchange is
largely indifferent to surface properties, and that diurnal
adsorption/desorption generates approximately 1% variation in the
column abundance, which matches Earth analogue measurements very well.
The results of Fouchet et al., (2007) indicate that the vertical
distribution is controlled by an intermediate state where the water is
controlled by atmospheric saturation on one hand, and confined to a
surface layer on the other, pointing to significant regolith-atmosphere
exchange processes. This result is inferred by investigating the
correlation of water columns and pressure, and was not observed
directly. Maltagliati et al., (2011) and Trokhimovskiy et al,. (2015)
also attempted to discern a diurnal exchange process between atmosphere
and regolith, but found no evidence of local time variation in
H2O abundances. Thus, the extent of exchange between
regolith and atmosphere remains an open question.
1.2 Spectral synergy
When observing an atmosphere in nadir viewing geometry, the outcome is
normally a column abundance value of the target species. However, it is
possible to obtain information about the vertical distribution of the
species by combining multiple spectral domains in the retrieval process.
This approach is commonly referred to as a spectral synergy, and was
developed for Earth observation by Pan et al., (1995, 1998), who
predicted higher sensitivity to near-surface layers of CO if near and
thermal infrared spectral bands were combined. This was later confirmed
by Edwards et al., (2009), who demonstrated that combining NIR and TIR
measurements in a common retrieval allowed for a significantly higher
sensitivity in the troposphere. The method has also been used to
increase near-surface sensitivity to other gasses such as
CO2 (Christi & Stephens, 2004), O3(Landgraf & Hasekamp, 2007) and CH4 (Razavi et al.,
2009).
TIR measurements are mostly sensitive to the middle atmosphere (at the
origin of the photon emission) where the temperature contrast of the
atmosphere with respect to the surface is high. NIR measurements on the
other hand are sensitive to any molecule present along the column as the
technique relies on solar photons traversing the entire atmosphere back
and forth. Although Trokhimovskiy et al., (2015) indicate the NIR
technique is mostly sensitive to the atmosphere below 30 km, it is only
true from a mixing ratio perspective, which favors the denser layers of
the atmosphere. In other words, any given change in H2O
mixing ratio will be easier to sense in the bottom of the profile as
pressure and number density is continuously increasing towards the
surface. If seen from a number of molecules perspective, the NIR
inversion technique has no preference to a particular position of the
column, unless this portion concentrates more water molecules at a
specific location. One must note however that dust modulates this
assertion. At high dust opacity, part of the incoming flux does not
reach the surface and is sent back to space without sampling the entire
column. Only in such cases will the NIR technique become altitude
dependent.
This difference in sensitivity of NIR and TIR can be viewed as a
difference in the shape and peak altitude of the weighting function of
water vapor retrieval in a particular wavelength domain, and has been
advocated to explain the dispersion of H2O column
abundance values retrieved by the various instruments of MEX (Tschimmel
et al., 2008). On the other hand, the difference in sensitivity can also
be considered a way to offer simultaneous access to different regions of
the atmosphere, leading to the derivation of more than a single
parameter representative of the whole column, as is usually the case
with instruments that study water vapor using nadir observations. In
fact, combining two spectral domains increases the degree of freedom of
the signal (DOF). The DOF gives an estimate of the number of independent
bits of information in an atmospheric measurement (Rodgers, 2000), and a
DOF higher than 1 indicates the presence of some amount of profile shape
information.
If attempting to retrieve vertical information with only one instrument,
one could argue that as the single instrument is primarily sensitive to
a specific altitude region, the obtained vertical confinement is not a
“real” partitioning. Instead, the obtained partitioning might be a
product of a lack of sensitivity to other, and perhaps wetter, altitude
regions, thus producing an artificial vertical partitioning. This
problem is avoided with the use of a spectral synergy, as each
wavelength interval is susceptible to emission/absorption signatures in
separate regions, and therefore obtains information from different
altitudes.
This consideration led Montmessin and Ferron (2019) to investigate the
potential for a synergistic retrieval of water vapor in the Martian
atmosphere using MEX, as the spacecraft constitutes the only asset at
Mars observing water in both NIR (SPICAM, OMEGA, PFS) and TIR (PFS)
spectral intervals. Despite their differences in field-of-view, sampling
and coverage, SPICAM (NIR) and PFS (TIR) were selected for this study as
the two have the most extensive records of water vapor retrievals on
Mars among the MEX instruments (Fedorova et al., 2006; Fouchet et al.,
2007; Giuranna et al., 2019; Trokhimovskiy et al., 2015). As Montmessin
and Ferron (2019) concluded on the promising potential for a synergistic
retrieval of water vapor on Mars with MEX, this work is intended to
follow-up on this earlier study and present the analysis of a
multi-annual dataset covering the period from MY 27 to 34.
The first part of the manuscript provides an overview of the instruments
used in this study (Section 2), and continues in Section 3 with an
outline of the synergistic retrieval method, including a description of
the selection of measurements within the dataset. The results are
presented in Section 4, where in 4.1 a complete synergistic column
abundance climatology is presented, followed by a comparison of the
column abundance between the synergy, the model and the single spectral
domain retrievals are made, before the vertical and spatial distribution
is elaborated upon. A discussion of the results and how they compare to
previous works follow in Section 5, and Section 6 concludes the findings
of this study.
2 Instruments
The Mars Express mission was launched in June 2003, and began nominal
science operations in mid-January 2004 (Chicarro et al., 2004),
corresponding to the very end of MY 26. From a quasi-polar orbit with a
period of 7.5 hours, MEX has a particularly detailed view of the polar
caps at the sublimation onset. With three instruments able to measure
the atmospheric water vapor content (OMEGA, PFS, SPICAM), either in the
solar reflected or in the thermal component, MEX has delivered a vast
amount of valuable data with complete global and seasonal coverage. The
PFS and SPICAM instruments cover the thermal and near-infrared domains,
respectively, within which water vapor possesses diagnostic
emission/absorption signatures. As each spectral interval provides a
distinct sensitivity along the vertical, observations of the same
species in separate wavelength regions provide constraints on the
vertical distribution.
The measurements used in the following analysis were retrieved from
nadir observations, and were selected according to a number of criteria
to ensure satisfactory quality of every individual measurement,
sufficient geographical and seasonal coverages, and a minimum error of
radiative transfer modeling due to surface inhomogeneity (Montmessin &
Ferron, 2019). For a detailed description on the selection and averaging
processes used for the creation of a dataset compatible with a
synergistic extraction of water vapor, the reader is referred to
Montmessin and Ferron (2019).
2.1 Mars Express PFS
The Planetary Fourier Spectrometer is an infrared spectrometer with two
wavelength channels optimized for atmospheric sensing. The short
wavelength channel covers the range 1700-8200 cm-1with a full width at half maximum (FWHM) of the instantaneous field of
view (FOV) of 1.6°, while the long wavelength channel spans the 250-1700
cm-1 (5.88-40 µm) with a FWHM FOV of 2.8°, which at an
altitude of 250 km corresponds to a 12 km diameter surface footprint.
Both channels have a spectral resolution of 1.3 cm-1.
Only the long wavelength channel was utilized for this work. For further
details, see Formisano et al., (2005) and Giuranna et al., (2005).
For the synergistic approach, several windows in the long wavelength
channel were selected. The windows from 8-10 µm and 19-25 µm were used
to obtain surface temperature and dust model properties, the region at
12-19 µm is dominated by the absorption of the 15 µm CO2vibrational transition which was used to retrieve atmospheric
temperature profiles, while the 20-35 µm thermal emission band was used
to retrieve the water vapor abundance, henceforth referred to as TIR.
The PFS spectrum is used to retrieve several parameters, such as surface
temperature, dust properties and water vapor column abundance. Because
of this, a high signal-to-noise ratio (SNR) is required, and one
individual spectrum obtained with PFS is not satisfactory. Therefore,
the retrievals were performed on the average of nine consecutive
spectra. The total time passed between the acquisition of the first
spectrum to the last of the nine to be averaged is 108 seconds, as it
takes 4.5 seconds to acquire a single PFS interferogram and the
repetition time is 8.5 seconds (Fouchet et al., 2007).
After years of operation, an issue with PFS caused the interferogram
peak to not always be centered. The instrument line-shape used here (a
sinc function with 1.3 cm-1 FWHM) is then not optimal,
and could lead to biased water vapor retrievals, with a tendency of
being too low. This issue started around orbit 6000 (MY 29), became
particularly relevant after orbit 7500 (MY 30), but data obtained in MY
32 and after are less affected. In an effort to largely avoid this
problem, we exclude all measurements during MY 30 and MY 31 from further
analysis.
2.2 Mars Express SPICAM
The SPICAM UV-IR instrument (Spectroscopy for the Investigation of the
Characteristics of the Atmosphere of Mars) is a dual-channel
spectrometer designed to study the Martian atmosphere from top to bottom
(Bertaux et al., 2006). In this study, only the IR channel was utilized
working in the spectral range of 1-1.7 µm with a spectral resolution of
3.5-4.0 cm-1, a complete description of which can be
found in Korablev et al., (2006).
In nadir viewing geometry, the IR channel has an instantaneous FOV of
1°, corresponding to a 4 km footprint on the surface when the spacecraft
is near the 300 km pericenter of its orbit. The incoming flux is
separated into two detectors, where detector 1 was used for this work as
it provides significantly higher performance in nadir. The wavelength
interval 1.34-1.43 µm is defined as the NIR range for the synergy, as it
covers the strong water absorption band at 1.38 µm. Averages of ten
SPICAM-IR spectra are demonstrated to have a SNR sufficient for reliable
retrievals of water vapor column abundances (Fedorova et al., 2006;
Trokhimovskiy et al., 2015). For the sake of the synergy, the SPICAM
observation closest in time to the center PFS spectrum is selected, and
averaged together with the seven previous and following spectra. The 15
spectrum average has a FOV similar to that of the nine PFS spectrum
average. Together, the SPICAM and PFS average spectra constitute a
co-located observation.
3 Data set and retrieval
The first time a synergistic retrieval method was tested on a planet
other than Earth, atmospheric H2O on Mars was retrieved
by combining measurements from PFS and SPICAM on Mars Express, probing
the TIR and NIR spectral intervals respectively (Montmessin & Ferron,
2019). In a nadir viewing geometry, retrievals have traditionally
returned a single item of information regarding the target species:
either the column-integrated abundance (CIA) in the case of NIR, or the
middle atmosphere concentration in the case of TIR, leaving open the
question of how the species is distributed along the vertical and in
particular whether and how it might interact with the surface. By taking
advantage of multiple spectral regions, it is possible to increase the
DOF for the signal, and thereby resolve the vertical partitioning of
water vapor on Mars.
In the earlier demonstration of the synergy method applied to Martian
water vapor, a subset of 449 co-located observations from 133 orbits
distributed through MY 27 were presented (Montmessin & Ferron, 2019),
showcasing that the synergy brings additional robustness to the
retrieval of water vapor column abundance, and provides insight into the
vertical distribution of water vapor. In this study, we expand on those
findings, and conduct a comprehensive analysis of the complete
synergistic dataset available from MEX, which contains nearly 200 000
measurements.
The dataset presented here consists of co-located observations taken
over 1379 individual orbits distributed across seven Mars years from Ls
334° of MY 26 to Ls 297° of MY 34, with no measurements from MY 30-31.
The geographical and seasonal coverage is highly variable from year to
year, several being quite sparsely covered. Some sparsity is due to
operational constraints, as not all instruments can be concurrently
active, while most is due to the requirement of co-located measurements
from both SPICAM and PFS.
3.1 Synergistic retrieval routine
The synergistic approach requires a set of co-located PFS and SPICAM
observations on which to apply the retrieval method. To obtain a
satisfying PFS SNR for the fitting of multiple parameters, nine
consecutive spectra are averaged together. The SPICAM observation
closest in time to the central PFS spectrum is then selected and
averaged with the seven observations prior to it and the seven after it,
resulting in a combined FOV similar to that of the nine combined PFS
observations. A screening process is conducted on this set of co-located
observations, the details of which can be found in Montmessin and Ferron
(2019). The simultaneous inversion of H2O follows the
approach outlined in Montmessin and Ferron (2019), and will only be
briefly described here.
A priori values of the water vapor and temperature profiles are
extracted from the Mars Climate Database (MCD) based on the general
circulation model developed at the Laboratoire de Météorologie Dynamique
(LMD GCM) (Forget et al., 1999; Millour et al., 2018) with an
uncertainty of the water equal to the abundance values. MCD version 5.3
is used. For each year the corresponding scenario is chosen, except for
MY 34, which is not yet included (the version used was last updated on
11/01/2019). A composite scenario was therefore built for MY 34 by
combining the scenario of MY 33 with the standard MCD dust storm
scenario 4 and the warm and dusty scenario 7 (for the intervals
Ls=180°-200° and Ls=200°-220° respectively).
Temperature and aerosol parameters are retrieved individually from the
PFS average spectra, which are then injected into the synergistic
routine. The overall spectral fitting procedure uses the HITRAN 2012
spectroscopic database (Rothman et al., 2013) as a baseline for the
computation of absorption coefficients of H2O and
CO2, and then relies on a Bayesian approach that
consists in maximizing the probability that a given retrieval satisfies
both the observed averaged spectra and falls within a range of plausible
values specified by prior assumptions on the value and its dispersion.
The weight of the prior assumption in the retrieval is dictated by its
prior uncertainty, which is set equal to the prior water vapor column
value.
Water vapor is inferred from the set of combined NIR and TIR spectra, by
a simultaneous inversion from both spectral domains. In practice, the
algorithm adjusts the water vapor abundance along the vertical profile
at nine altitude points separated by 2.5 km from ground to 10 km, and by
5 km from 10 to 30 km. All points are correlated with a Gaussian kernel,
such that the points are less strongly correlated when the distance
between them is increasing. The results include a posterior covariance
matrix, from which the DOF can be calculated from the sum of the trace
of the matrix. The DOF normally fluctuates around 1 when the retrieval
includes a single spectral domain (NIR or TIR), which implies only one
independent parameter can be inferred from a water vapor measurement
(e.g. the CIA), while with a higher DOF some information of the water
vapor vertical distribution can be obtained.
Some example spectra are shown in Figure 1, where the selected NIR and
TIR spectral intervals include strong diagnostic features of water
vapor. The co-located observations are from early summer of MY 27, at
high latitudes. The corresponding vertical profile obtained from the
synergistic retrieval performed on the spectra is also shown and
compared to the MCD prior profile, along with the averaging kernel and
the posterior-to-prior error ratio by altitude for the synergy and each
single spectral domain retrieval. The post-to-prior profile shows at
which altitudes added information is coming from, indicating that the
synergy is more sensitive to the lower atmosphere than both PFS and
SPICAM.
We quantify the amount of information added by the synergy at each
altitude level by comparing the synergistically retrieved error profiles
to the MCD prior error profiles, shown in the bottom right panel of
Figure 1. In this way, we demonstrate that the synergy does not simply
reproduce the prior when calculating vertical profiles, and that for the
lower atmosphere, the synergy brings more information than the single
spectral domain retrievals. The MCD prior and the retrieved vertical
mixing ratio profiles are close to identical above 15 km, but start to
deviate below this, where the synergy provides a significant amount of
added information.