TRANSPORT AND ESCAPE OF WATER: TOWARD A MORE COMPLETE
INTERPRETATION WITH TGO NOMAD AND MAVEN NGIMS.
S. W. Stone, G. L. Villanueva, G. Liuzzi, M. Benna, P. R. Mahaffy, M. K. Elrod, NASA Goddard Space Flight
Center, Greenbelt, MD, USA, R. V. Yelle, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA.

Introduction
The present-day transport, destruction, and subsequent
loss to space of H2 O in the Martian atmosphere has
important implications for our understanding of the evolution of the Martian atmosphere and climate through
time.[1] On Earth, most H2 O is trapped relatively close
to the surface by an efficient hygropause. In contrast, on
Mars the hygropause is often transgressed by significant
amounts of H2 O transported vertically from the lower atmosphere around perihelion, corresponding to southern
hemisphere summer, during regional dust storms which
occur every Martian year, and during global dust storms
which occur roughly every ten years.[2–8] A portion of
this H2 O progresses into the upper atmosphere, where it
is quickly destroyed by ion-neutral reactions to produce
H that can escape to space.[7, 9] This relatively rapid
transport of H into the upper atmosphere in the form of
H2 O is supplemented by the steady flow of H2 . H2 is produced in the lower atmosphere by the photolysis of H2 O
and subsequent odd-hydrogen reactions. The H2 is then
transported vertically via diffusion past the hygropause
and into the thermosphere, where it is destroyed by ionneutral reactions in a manner similar to H2 O, producing
H.
The imprint of escape on the atmosphere is observed
in the isotope ratios of escaping elements. For example,
the preferential escape of H over heavier D via the Jeans
mechanism[10, 11] leads to the enrichment of the atmosphere in D relative to H.[12–14] A number of other
processes are responsible for isotopic fractionation in
the atmosphere, including preferential condensation of
HDO relative to H2 O and the distinct photolysis rates of
molecular isotopologues.[13–17] Using measurements
of the H2 O abundance and various isotope ratios in the
lower, middle, and upper atmosphere, this work aims to
construct a more complete understanding of the transport of H2 O into the upper atmosphere, its condensation
and destruction, and its escape to space.

Methods
Upper atmospheric H2 O densities are calculated from
Mars Atmosphere and Volatile EvolutioN (MAVEN)
Neutral Gas and Ion Mass Spectrometer (NGIMS) ion
measurements assuming photochemical equilibrium (Figure 1).[7] NGIMS measurements of atmospheric H2 O
at mass per charge (𝑚/𝑧) 18 are not possible due to

Figure 1: The H2 O abundance calculated from MAVEN
NGIMS data on a single orbit. In this region of the atmosphere, the mixing ratio increases with height due to diffusive
separation.

background signal of H2 O synthesized from reactive atmospheric species once inside the instrument. Thus,
the H2 O abundance is calculated from NGIMS ion measurements using photochemical equilibrium calculations
based on the production and loss reactions of H3 O+ :
𝛼1

H3 O+ + e − −−−→ OH + H + H
𝛼2

−−−→ H2 O + H
𝛼3

−−−→ OH + H2
𝛼4

−−−→ O + H2 + H
𝑘1

HCO+ + H2 O −−−→ H3 O+ + CO
𝑘2

H2 O+ + H2 O −−−→ H3 O+ + OH

This allows us to construct a simple equation for the H2 O
abundance,
[H2 O] = (𝛼1 + 𝛼2 + 𝛼3 + 𝛼4 )

[H3 O+ ] [e − ]
.
𝑘 1 [HCO+ ] + 𝑘 2 [H2 O+ ]

The species on the righthand side of the equation are
measured by the ion mode of NGIMS and the reaction
rates are obtained from the University of Manchester

Stone, et al.: Water Transport and Escape
10

2

uses the latest line broadening coefficients for H2 O and
HDO in a CO2 atmosphere.[22–24] The a priori atmosphere used by PSG is generated by the Global Environmental Multiscale general circulation model for Mars
(GEM-Mars).[25]

H 2 O Mixing Ratio (ppm)

MY 32
MY 33
MY 34
Fitted Model

10 1

Results and Discussion

10 0

0°

30°

60°

90°

120° 150° 180° 210° 2400° 270° 300° 330° 360°

LS

Figure 2: Seasonal Variation of H2 O in the upper atmosphere
of Mars as observed by MAVEN NGIMS.[7] The H2 O abundance in the upper atmosphere peaks during southern summer
( 𝐿 S ∼270°) and significant enhancements in the H2 O abundance are observed during dust storms (black boxes), both regional (MYs 32 and 33) and global (MY 34). The black line
is a sine fit to the data, excluding the dust storms.

Institute of Science and Technology (UMIST) Database
for Astrochemistry (UDfA).[18]
ExoMars Trace Gas Orbiter (TGO) Nadir and Occultation for MArs Discovery (NOMAD) measurements
of H2 O, HDO, and water ice in the lower and middle atmosphere are obtained using the Solar Occultation (SO) channel in the infrared between wavelengths
of 2.2 µm and 4.3 µm.[14, 19] During an occultation,
the atmosphere is sensed by pointing the instrument at
the Sun and measuring the absorption of solar photons
by molecules and aerosols along the tangential line of
sight. Such measurements are obtained at increasing or
decreasing altitudes along the dawn or dusk terminator
of Mars to probe the vertical variation of the absorbing
molecules and aerosols. The NOMAD SO channel is an
echelle spectrometer with an acousto-optical tunable filter (AOTF) which limits the observed signal to a single
diffraction order. Calibration of the instrument through
characterization of the AOTF response functions, grating
parameters, and instrument line shape function is necessary for accurate interpretation of spectral features.
Detailed information on these quantities is currently under review.[20] The H2 O absorption features used here
are the bands in the 2.2 µm and 3.3 µm region (NOMAD
SO diffraction orders 133 to 170). For HDO, the 𝜈1
band at 3.7 µm is used (orders 119 to 121). Retrieval
of molecular abundances from the NOMAD occultations is performed by the Planetary Spectrum Generator
(PSG) using a layer-by-layer and line-by-line method in
a spherical refractive geometry.[21] Molecular line lists
are obtained from the HITRAN-2020 database and PSG

The combination of the TGO NOMAD and MAVEN
NGIMS data sets enables a more complete interpretation of the transport of H2 O through the atmosphere of
Mars and its eventual escape as H and O. The H2 O abundance can now be retrieved nearly continuously from the
lower atmosphere to the thermosphere. In addition, various isotope ratios can be retrieved from the two data sets
in the lower, middle, and upper atmosphere, supporting the study of the relative importance of condensation,
photolysis, and escape in the determination of these ratios in the atmosphere over various timescales.
Using NGIMS and NOMAD data spanning several
Mars years, the seasonal variation of the H2 O abundance
in the upper atmosphere is obtained (Figures 2–4). The
NGIMS data have indicated that more H2 O is transported
into the upper atmosphere during southern summer than
during northern summer and that regional and global
dust storms rapidly loft even more H2 O into the upper atmosphere (Figures 2 and 3).[7] Similarly, these
variations are observed in the lower and middle atmosphere using NOMAD (Figure 4).[14] Once combined
with NOMAD retrievals of HDO and water ice particle size,[14, 19] these H2 O measurements are powerful
indicators of the role of condensation in the fractionation of hydrogen isotopes in water at altitudes between
10 km and 60 km. Isotopic fractionation of H2 O also
occurs through its photolytic destruction in the middle
atmosphere.[13] Here, 1D photochemical modeling and
NGIMS H2 O abundances in the lower thermosphere can
aid in our understanding of the importance of photolysis and ion-neutral reactions with respect to escape and
the long term evolution of the D/H ratio on Mars. Additional context may be provided by the isotope ratios
measured by NGIMS in the upper atmosphere. Finally,
observations in the UV made by the Imaging Ultraviolet
Spectrograph (IUVS) on MAVEN track the abundance
and variation of escaping H and D, providing a view of
the end result of these processes.[26, 27]
References
[1] Haberle, et al. (Eds.). 2017. The Atmosphere and
Climate of Mars. Cambridge University Press. [2] Chaffin, et al. 2014. Geophys. Res. Lett., 41(2), 314–320.
[3] Clarke, et al. 2014. Geophys. Res. Lett., 41(22),
8013–8020. [4] Chaffin, et al. 2017. Nat. Geosci., 10(3),
174–178. [5] Vandaele, et al. 2019. Nature, 568(7753),
521–525. [6] Fedorova, et al. 2020. Science, 367 (6475),

2.5

180

Altitude (km)

2
1.5

160
1
0.5

140

0
-0.5

120
0°

30°

60°

90°

log 10 H 2O Mixing Ratio (ppm)

Stone, et al.: Water Transport and Escape

120° 150° 180° 210° 240° 270° 300° 330° 360°

LS
Figure 3: Seasonal variability of the H2 O in the upper atmosphere of Mars as calculated from the MAVEN NGIMS data.

Figure 4: Seasonal variability of the H2 O abundance in the southern hemisphere as measured by TGO NOMAD. See reference [14].

297–300. [7] Stone, et al. 2020. Science, 370(6518),
824–831. [8] Chaffin, et al. 2021. Nat. Astron., 5,
1036–1042. [9] Krasnopolsky. 2019. Icarus, 321, 62–
70. [10] Johnson, et al. 2008. Space Sci. Rev., 139(1-4),
355–397. [11] Gronoff, et al. 2020. J. Geophys. Res.
Space Phys., 125(8). [12] Webster, et al. 2013. Science, 341(6143), 260–263. [13] Alday, et al. 2021. Nat.
Astron., 5(9), 943–950. [14] Villanueva, et al. 2021.
Sci. Adv., 7 (7), eabc8843. [15] Bertaux, et al. 2001.
J. Geophys. Res. Planets, 106(E12), 32879–32884.
[16] Montmessin, et al. 2005. J. Geophys. Res. Planets,
110, E03006. [17] Villanueva, et al. 2015. Science,
348(6231), 218–221. [18] McElroy, et al. 2013. Astron.

& Astrophys., 550, A36. [19] Liuzzi, et al. 2020. J. Geophys. Res. Planets, 125(4). [20] Villanueva, et al. 2022.
Geophys. Res. Lett., under review. [21] Villanueva,
et al. 2018. J. Quant. Spectrosc. Radiat. Transf., 217,
86–104. [22] Gordon, et al. 2022. J. Quant. Spectrosc.
Radiat. Transf., 277, 107949. [23] Devi, et al. 2017.
J. Quant. Spectrosc. Radiat. Transf., 203, 158–174.
[24] Régalia, et al. 2019. J. Quant. Spectrosc. Radiat. Transf., 231, 126–135. [25] Neary, et al. 2018.
Icarus, 300, 458–476. [26] Chaffin, et al. 2018. J. Geophys. Res. Planets, 123(8), 2192–2210. [27] Mayyasi,
et al. 2019. J. Geophys. Res. Space Phys., 124(3),
2152–2164.