WATER VAPOR VERTICAL DISTRIBUTION DURING PERIHELION
SEASON OF MARTIAN YEARS 34 AND 35 OBSERVED WITH NOMAD-SO.
A. Brines, (adrianbm@iaa.es), M. A. López-Valverde, A. Stolzenbach, A. Modak, B. Funke, F. G. Galindo,
J.J. Lopez-Moreno, IAA/CSIC, Granada, Spain, S. Aoki, Japan Aerospace Exploration Agency (JAXA), Japan,
G.L. Villanueva, G. Liuzzi, NASA Goddard Space Flight Center, USA, American University, Washington DC, USA,
I.R. Thomas, J.T. Erwin, F. Daerden, L. Trompet, B. Ristic, A.C. Vandaele, Royal Belgian Institute for Space
Aeronomy, Belgium, U. Grabowski, Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research,
Karlsruhe, Germany, F. Forget, Laboratoire de Météorologie Dynamique, IPSL, Paris, France, M.R. Patel, Open
University, UK, G. Bellucci, Istituto di Astrofisica e Planetologia, Italy.

Introduction
In the Martian atmosphere, water vapor is crucial in
most of the chemical and radiative processes, playing
a significant role in the planet’s climate. Recent
observations have proven that in the past, Mars was
wetter than today, meaning that processes like the escape
of water to the space have driven the planet to its
current state. Also, previous works have pointed out
how important is the vertical distribution of the water
in the atmosphere for the evolution of the planet and
for the understanding of the physical and chemical
processes driving the water cycle [1]. However, despite
its importance, there are many open debates, in particular
those devoted to its vertical distribution.
Since April 2018, when Nadir and Occultation for
MArs Discovery (NOMAD) onboard the ExoMars 2016
Trace Gas Orbiter (TGO) started its science phase, we
are capable of sounding the Martian atmosphere with
an unprecedented vertical resolution using the solar
occultation technique.
Recent studies with the Atmospheric Chemistry
Suite (ACS), also onboard TGO, and NOMAD revealed
an enhancement of water vapor at high altitudes duirng
dust events such as the Global Dust Storm during MY34
[2, 3, 4]. Other works focused on the hydrogen escape
[5, 6] have suggested that the dust enhancement has a
strong effect on the escape processes.
Here we present vertical profiles of water vapor from
solar occultation observations by NOMAD, analyzing
the water distribution and its seasonal and latitudinal
variability during the first half of the perihelion season
of two consecutive Martian Years (MY), allowing an
extensive and direct comparison between MY34 and
MY35.

Figure 1: NOMAD observations analyzed during the selected
period with SO diffraction orders 134 and 168.
The
observations during MY34 are shown in the top panel and
during MY35 in the bottom panel. Color indicates the Local
Solar Time.

between 2.3 and 4.3 µm (2320-4350 cm−1 ) and uses
an echelle grating with a density of ∼4 lines/mm and
has a spectral resolution of λ/∆ λ ' 20000. The
sampling time of this channel is ∼1 s allowing a
vertical sampling in the observations of ∼1 km. An
Acousto-Optical Tunable Filter (AOTF) is used to select
different spectral windows of a width of ∼30 cm−1
corresponding to the desired diffraction orders to be
used during the observation. The AOTF quick change
from one diffraction order to another allows to observe
the atmosphere at a given altitude through 6 different
spectral ranges within 1 second.
For this study we have selected two subsets of
measurements in MY34 and MY35 covering the same
seasonal period between the solar longitudes LS = 180◦
and LS = 270◦ , corresponding to the first half of the
perihelion season. We have analyzed a total of 962
observations taken at the NOMAD-SO diffraction order
134 (3011 - 3035 cm−1 ) and 168 (3775 - 3805 cm−1 )
and its latitudinal coverage is shown in Figure 1.

NOMAD-SO measurements
NOMAD [7, 8] is an infrared spectrometer working
in the spectral range between 0.2 to 4.3 µm. It has two
IR spectral channels (SO and LNO) and one operating
in the UV-visible spectral range (UVIS; 200-650 nm).
The SO (Solar Occultation) channel covers the range

Methodology
Here we use Level 1 SO data provided by the
NOMAD calibration team that are processed with an inhouse-develpoed software to evaluate and to correct for

residual bending in the continuum and for a remaining
spectral shift [9]. To do so, we use the radiative
transfer model KOPRA (Karlsruhe Optimized Radiative
transfer Algorithm described in [10]) to generate a
radiative forward model. The instrumental response
of the AOTF and the Instrumental Line Shape (ILS)
is included into the forward model using the latest
calibration proposed by the NOMAD consortium and
described in [5]. The reference atmosphere needed
by the forward model is obtained from the Mars
Global Climate Model developed at the Laboratoire de
Météorologie Dynamique (LMD-MGCM) [11]. For
each observation, a temperature and density profile
was extracted from the model at the exact location
and time of the occultation path at 50 km altitude.
For the inversion of the vertical profiles, we use the
Retrieval Control Program (RCP) developed at Institut
für Meteoriologie und Klimaforschung (IMK) which
incorporates the KOPRA forward model. After the
inversion of each scan, we combine the retrieve profiles
of the simultaneous scans of diffraction orders 134 and
168. Since the intensity of the water absorption lines in
these two orders is noticeably different (S134 ∼ 10−21
and S168 ∼ 10−19 cm−1 /(molec·cm−2 ) we decide to use
the order 134 for the lower atmosphere and the order
168 for the upper atmosphere (above 60 km) to avoid
the saturation of the absorption lines in the order 168 at
low altitudes. Figure 2 shows the retrieved water vapor
at one particular observation as an example. Also it
shows the vertical resolution and the averaging kernels
of the inversion.
With these profiles, we generate different subproducts to study different aspects of the Martian
atmosphere.

Figure 2: Performance of the retrieval of the data taken
at the observation 20180928 043918 1p0a SO A I. Panel (a)
shows the retrieved water vapor, panel (b) shows the vertical
resolution obtained for the order 134 (blue) and 168 (orange).
Bottom panels (c) and (d) show the the averaging kernels of
the retrievals for orders 134 (c) and 168 (d).

Results and discussion
Here we present detailed maps showing the vertical
distribution of the water vapor abundance and an
estimation of the saturation ratio (1). To do so, we
use the expression for the saturation pressure over water
ice (2) derived by [12].
S=

µH 2 O
Ptot
= µH 2 O
µsat
Psat

Psat (T ) = exp 9.550426 −

5723.265
T

(1)

+ 3.53068 · ln(T ) − 0.00728332T




(2)

110 < T < 273K

The strong activity Global Dust Storm took place
during the MY34 at LS = 190◦ − 205◦ . Figure 3
shows an intense peak of water vapor at that period,
with abundances about 150 ppm reaching the upper
mesosphere. This strong increase is not observed
during the same period of MY35, where water vapor
does not exceed abundances of 50 ppm above 50 km

Figure 3: Vertical distribution of the retrieved water vapor.
Panels (a) and (b) show the latitude of the of the observations
(top) and the seasonal variation of the water vapor (bottom)
at northern and southern hemispheres respectively for MY34.
Panels (c) and (d) show the same information for MY35.

2

used for the retrievals, but with an averaged 13% larger
H2 O in our retrievals. The comparison is shown in
Figure 5.

Conclusions
We present water vapor vertical distributions during
the first half of the Martian perihelion season from two
consecutive Martian Years with a vertical sampling of
1 km and a mean vertical resolution of 4-6 km. We
confirm a strong impact of the 2018 Global Dust Storm
in the water vapor abundances during MY34 and a
high contrast of the hygropause in dusty and non-dusty
conditions. Also atmospheric supersaturation events
with presence of water ice at mesospheric altitudes are
reported.

Figure 4: Same as Figure 3 but showing the estimated
saturation ratio derived from the retrieved water vapor profiles.

Acknowledgments
revealing a strong contrast of the hygropause in dusty
and non-dusty conditions. In Figure 4 we show the
estimation for the saturation ratio calculated using LMDMGCM temperatures. We observe multiple layers of
supersaturation during both martian years at different
altitude ranges and also several cases where water ice
could be expected [13] in a supersaturated atmospheric
environment. We will describe the H2 O distribution and
saturation ratios in detail.

The IAA/CSIC team acknowledges financial support
from the State Agency for Research of the Spanish MCI
through the Center of Excellence Severe Ochoa award
(DEV-2017-0709) and funding by grant PGC2018101836-B-100 (MCI/AEI/FEDER, EU). This project
acknowledges funding by the Belgian Science Policy
Office (BELLS), with the financial and contractual
coordination by the ESAU Prod ex Office (PEA
4000103401,4000121493) as well as by UK Space
Agency through grants ST/V002295/1, ST/V005332/1
and ST/S00145X/1 and Italian Space Agency through
grant 2018-2-HHS.0. US investigators were supported
by the National Aeronautics and Space Administration.

Comparison and validation

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