Global vertical distribution of water vapor on Mars:
Results from 2 Mars years of ExoMars-TGO/NOMAD science operations
S. Aoki, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Japan
(shohei.aoki@edu.k.u-tokyo.ac.jp), A. C. Vandaele, F. Daerden, I. R. Thomas, L. Trompet, J. T. Erwin, L.
Neary, S. Robert, A. Piccialli, B. Ristic, Royal Belgian Institute for Space Aeronomy, Brussels, Belgium, G. L.
Villanueva, G. Liuzzi, M. D. Smith, NASA Goddard Space Flight Center, 8800 Greenbelt Rd., Greenbelt,
20771 MD, USA, M. A. Lopez-Valverde, A. Brines, J. J. Lopez-Moreno, Instituto de Astrofísica de Andalucía, Glorieta de la Astronomia, Granada, Spain, R. T. Clancy, Space Science Institute, Boulder, USA, N. Yoshida, Tohoku University, Sendai, Japan, J. A. Holmes, M. R. Patel, School of Physical Science, The Open University, Milton Keynes, UK, J. Whiteway, Centre for Research in Earth and Space Science, York University,
Toronto, Canada, G. Bellucci, Istituto di Astrofisica e Planetologia Spaziali, IAPS-INAF, Rome, Italy.

Introduction:
Two instruments onboard ExoMars Trace Gas
Orbiter (TGO), Nadir and Occultation for Mars Discovery (NOMAD) and Atmospheric Chemistry Suite
(ACS) are unique platforms to provide the global
vertical distribution of trace gases (such as water
vapor, carbon monoxide, hydrogen chloride, etc) on
Mars from their daily solar occultation measurements
(Vandaele et al., 2018: Korablev et al., 2018). The
science operation of TGO has started since April
2018 and we have collected measurements for more
than 2 Mars years. Here, we present the results on the
vertical profiles of water vapor from the measurements by the NOMAD instrument. A full paper describing these results, Aoki et al. (2022), is currently
under review.
Dataset and analysis:
NOMAD instrument is a spectrometer working in
the spectral range between 0.2 and 4.3 μm with 3
spectral channels (2 out of 3 are in the infrared range,
and the other one is in the UV-Visible range). For the
analysis of water vapor presented in this study, the
data taken by solar occultation measurements in the
infrared channel is used. The infrared channel is
composed by echelle grating and Acousto Optical
Tunable Filter (AOTF), which enables us to achieve
relatively high spectral resolution (λ/dλ~17,000).
AOTF can instantaneously switch the observing diffraction orders, which allows us to acquire a spectrum less than every 1 second. It corresponds to the
vertical sampling of less than 1 km altitudes from
surface to 250 km altitude. Typically, we observe 5
or 6 different diffraction orders (i.e., spectral windows) in a single occultation. Water vapor lines are
available in diffraction order 134, 136 (3011-3035
cm-1, 3056-3080 cm-1) and order 167, 168, 169
(3752-3782 cm-1, 3775-3805 cm-1, 3797-3828 cm-1).
The water lines in the orders 134-136 are weaker and
not heavily saturated even in the deep atmosphere so
that the vertical profiles of water vapor can be retrieved from surface up to only ~90 km altitude. The
water lines in the latter spectral windows (orders
167-169) are stronger, so that the water vapor density

can be retrieved from surface up to higher altitude,
~120 km. We have analyzed the NOMAD solar occultation data taken between April 2018 and September 2022, which corresponds to Ls~160° in MY
34 and Ls~106° in MY 36. We are continuing to
process the data to cover 2 Mars years. Fig. 1 shows
the coverage of the dataset of the lower diffraction
orders (the top panel) and the higher diffraction orders (the bottom panel). The total number of
occultations are 5472 and 6471 for the lower and
higher orders, respectively. The coverage of both
spectral windows is similar since these diffraction
orders are regularly observed.
The retrievals are performed with Asimut
radiative transfer code (Vandaele et al., 2006). We
treat each spectrum individually by assuming a constant volume mixing ratio of water vapor along the
line of sight (i.e., retrievals of water vapor column
density along the line of sight for each spectrum).
The calibration of the solar occultation channel of
NOMAD infrared has been significantly improved
recently (Villanueva et al., 2022), which allows us to
perform robust retrievals. We have also compared
different methodologies and retrievals algorithms for
the water retrievals (COPRA code developed at IAA,
and PSG code developed at NASA-GSFC) and the
comparison shows a good agreement (Brines et al.,
2022).
Results and discussion:
Figure 2 and 3 show the retrieved seasonal variation of the water vapor vertical profiles from the
lower orders (Fig. 2) and the higher orders (Fig. 3).
For each figure, the small top panels show the latitude of the measurements (same as Fig. 1), and the
middle and bottom panels exhibit the water vapor
vertical profiles in the northern and southern hemispheres, respectively. As shown in Fig. 2 and 3, the
results from the lower and hither orders show a very
good agreement.
Fig. 2 and Fig. 3 illustrate a strong contrast between aphelion and perihelion water vertical distributions. In the aphelion periods, the water vapor is confined into very low altitudes (below 10-40 km). In

contrast, in the perihelion periods, water vapor
reaches very high altitude (above 100 km). It can be
explained by the fact that transport of water vapor by
the global circulation is limited by the formation of
ice clouds in the aphelion periods, whereas water is
more effectively transported to the higher altitude
and winter hemisphere in the perihelion periods because its vertical extent is less constrained by cloud
formation in the warm southern summer atmosphere.
In addition to the ones in the perihelion periods,
the increase of water vapor in the middle/upper atmosphere appeared around Ls=190° in MY 34 and
Ls=320°-330° in MY 34 and MY 35 are also remarkable. These periods correspond to the occurrences of the global dust storm (Ls=190° in MY 34)
and annual strong regional dust storms (Ls=320°330°). As shown in the earlier studies (e.g., Aoki et
al., 2019; Neary et al., 2020), water vapor can reach
higher altitudes when a strong dust storm occurs because the atmosphere heats up due to the presence of
dust and it prevents water from condensing as ice
clouds. These results also confirm that dust storms
and the southern summer are the main events for
water vapor to reach middle/high altitude as shown
by earlier studies (Fedorova et al., 2020; Villanueva
et al., 2021).

Figure-2: Vertical profiles of water vapor in the
unit of volume mixing ratio retrieved from orders
134 and 136 of NOMAD measurements. The top,
middle, and bottom panels show the latitude and local solar time of the measurements, water vapor vertical profiles in the northern hemisphere, and southern hemisphere, respectively.

Figures:

Figure-3: Same as Fig. 2 but retrieved from orders 167-169 of the NOMAD measurements.
Bibliography:
Figure-1: Coverage of the solar occultation
measurements by NOMAD used in this study. The
top panel is the data taken in orders 134 and 136, and
the bottom panel is orders 167-169. X-axis is solar
longitude (from MY 34 to MY 36), and Y-axis is
latitudes. The difference in color indicates the local
time.

Aoki, S., et al. (2019), Water vapor vertical profiles on Mars in dust storms observed by
TGO/NOMAD. J. Geophys. Res.: Planets, 124,
3482-3497, doi:10.1029/2019JE006109.
Aoki, S., et al. (2022), Global vertical distribution of water vapor on Mars: Results from 3.5
years of ExoMars-TGO/NOMAD science operations.
J. Geophys. Res.: Planets, under review.
Brines, S., et al. (2022), Water vapor vertical distribution on Mars during perihelion season of MY34
and MY35 with ExoMars-TGO/NOMAD observations. J. Geophys. Res.: Planets, under review.
Fedorova, A. A., et al. (2020), Stormy water on

Mars: The distribution and saturation of atmospheric
water during the dusty season. Science, Vol. 367,
Issue
6475,
pp.
297-300,
doi:
10.1126/science.aay9522.
Korablev, O., et al. (2018), The Atmospheric
Chemistry Suite (ACS) of Three Spectrometers for
the ExoMars 2016 Trace Gas Orbiter, Space Science
Reviews, 214, 1, article id. 7, 62, doi:
10.1007/s11214-017-0437-6.
Neary L., et al., (2020), Explanation for the increase in high altitude water on Mars observed by
NOMAD during the 2018 global dust storm,
Geophys. Res. Lett., 47, e2019GL084354,
doi:10.1029/2019GL084354.
Vandaele, A. C., et al., (2006), Simulation and
retrieval of atmospheric spectra using ASIMUT,
paper presented at Atmospheric Science Conference,
Eur. Space Agency, Frascati, Italy.
Vandaele, A. C., et al., (2018), NOMAD, an integrated suite of three spectrometers for the ExoMars
Trace Gas mission: technical description, science
objectives and expected performance, Space Science
Reviews, 214, 5, article id. 80, 47 pp, doi:
10.1007/s11214-018-0517-2.
Villanueva, G., L. et al., (2021). Water heavily
fractionated with altitude on Mars as revealed by
ExoMars/NOMAD, Science Advances, Vol. 7, no. 7,
eabc8843, DOI: 10.1126/sciadv.abc8843
Villanueva, G., L. et al., (2022). The deuterium
isotopic ratio of water released from the Martian
caps as measured with TGO/NOMAD, Geophysical
Research Letters, under review