CONSTRAINING THE ODD-HYDROGEN CHEMISTRY IN THE
ATMOSPHERE OF MARS WITH THE EXOMARS TRACE GAS ORBITER
J. Alday, J. A. Holmes, M. R. Patel, J. P. Mason, School of Physical Sciences, The Open University, Milton
Keynes, UK (juan.alday@open.ac.uk), A. Trokhimovskiy, A. A. Fedorova, D. A. Belyaev, O. Korablev,
Space Research Institute (IKI), Moscow, Russia, F. Lefèvre, F. Montmessin, L. Baggio, A. S. Braude,
LATMOS, Guyancourt, France, K. S. Olsen, P. G. J. Irwin, AOPP, Department of Physics, University of Oxford, UK, C. F. Wilson, European Space Research and Technology Centre (ESTEC), Noordwijk, Netherlands.
al., 2013).
Introduction:
Odd-hydrogen (HOx) species such as OH and
HO2 have a key role in regulating the chemistry of
the atmosphere of Mars. This role was first evidenced by the slow recombination rate of the
photolysis products of CO2 (i.e., CO + O), which
required a catalytic pathway to maintain a 95% CO2
atmosphere on Mars. McElroy & Donahue (1972)
and Parkinson & Hunten (1972) demonstrated that
the stability of CO2 in the Martian atmosphere holds
due to reactions of its photolysis products with HOx
(CO + OH  CO2 + H), evidencing the importance
of these photochemical species in the composition of
the atmosphere.
The photochemistry of the martian atmosphere is
frequently studied using remote sensing observations
of O3 (e.g., (Patel et al. (2021); Perrier et al. (2006)),
CO (e.g., Olsen et al. (2021); Smith et al. (2021))
and H2O2 (e.g., Clancy et al. (2004); Encrenaz et al.
(2008)), all of which are highly sensitive to the
abundance of odd-hydrogen and often used as tracers
of the HOx chemistry.
Current numerical models of the atmosphere of
Mars are able to explain the seasonal and latitudinal
variations of these measured chemical tracers, generally showing good qualitative agreements between
the measurements and the expectations (e.g., Holmes
et al. (2018); Lefèvre (2004)). However, underestimation of the absolute abundances of O3 and CO
suggests that the model representation of HOx oxidation is too efficient, which could be the result of
some missing physical or chemical processes in the
models, such as the uptake of HOx on water ice
clouds (Lefèvre et al., 2021; Lefèvre et al., 2008).
While the abundance of HOx therefore seems
crucial to understand the photochemical processes in
the martian atmosphere, there is little information
from measurements. Clancy et al. (2013) reported the
first and only detection of OH on Mars, which consisted of infrared nightglow emissions in the polar
winter atmosphere using from CRISM/MRO. On the
other hand, HO2 has never been detected in the martian atmosphere, with the most constraining upper
limit (198 ppbv) being well above the expected
abundances in 3D chemical models (Villanueva et

In this study, we search for infrared spectral signatures of OH and HO2 in the atmosphere of Mars
using solar occultation measurements by the Atmospheric Chemistry Suite (ACS) onboard the ExoMars
Trace Gas Orbiter (TGO). In addition, we analyse
the measurements in context with the expected abundances from the Mars Global Circulation Model
(GCM) used by the modelling group at the Open
University (hereafter OU-GCM).
ACS solar occultation observations:
ACS is one of the four instruments that comprise
the payload of the ExoMars Trace Gas Orbiter. It
consists of a suite of three spectrometers sampling
different spectral ranges in the infrared: the nearinfrared (NIR: 0.7-1.7 m), the mid-infrared (MIR:
2.3-4.2 m) and the thermal-infrared (TIRVIM: 0.717 m) (Korablev et al., 2018).
In this work we use the MIR channel, which
measures high-resolution spectra (/  20,00030,000) and is dedicated to solar occultation observations. In this type of measurements, the Sun is continuously monitored from a tangent altitude of 270
km until it is occulted by the surface. The normalisation of the spectra using the reference solar signal
measured at high altitudes (>120 km) allows us to
generate a set of transmission spectra at different
tangent heights. Solar occultations are especially
suitable for the detection of trace gases in planetary
atmospheres thanks to the use of the Sun as the light
source, which results in high signal-to-noise ratio
(SNR), and the long path sampled by the observations.
The dataset analysed in this work comprises all
ACS MIR observations made using secondary grating position 3, which measures eight diffraction orders (206 to 213) encompassing a spectral range between 2.78-2.9 µm (3448-3597 cm-1). Up to October
2021, there are 151 observations made using this
specific position and covering a range between LS =
160 in Martian Year 35 (MY35) and LS = 60 in
MY36.
Radiative transfer analysis:
The radiative transfer analysis of the observations

is performed using the NEMESIS algorithm (Irwin et
al., 2008), which has previously been applied to the
retrieval of isotopic ratios in the atmosphere from
ACS solar occultations (e.g., Alday et al. (2021)).
The spectral range sampled by ACS MIR secondary
grating position 3 includes absorption features by
CO2, H2O, OH and HO2, whose absorption cross
sections are computed with line-by-line modelling
using the spectroscopic parameters from the 2020
edition of the HITRAN database (Gordon et al.,
2022). In the case of the H2O absorption, the pressure-broadening coefficients are updated based on
the laboratory measurements of Devi et al. (2017)
and Régalia et al. (2019), suitable for the broadening
of the absorption lines in a CO2-dominated atmosphere.
Figure 1 shows a forward model indicating the
contribution of different gases to the spectra. The
biggest challenge for the detection of HOx in this
spectral range is the removal of the absorption by
CO2, H2O and dust to isolate the weak spectral features of OH and HO2. While some of the strongest
features of HOx in the measured range are found for
wavenumbers  > 3500 cm-1, the absorption of CO2
in that spectral range is much stronger than for  <
3500 cm-1, which directly impacts the sensitivity of
the measurements to these species. Therefore, we
select five spectral windows in diffraction orders
206, 207 and 208 to perform our retrievals, as shown
in Figure 1.

The retrieval scheme is performed in two steps.
First of all, vertical profiles of pressure and temperature are derived from the CO2 absorption based on
the hydrostatic equilibrium equation and a known
volume mixing ratio, fitting these features together
with the parameters describing the instrument line
shape (Alday et al., 2019). Once the pressure and
temperature profiles have been calculated, these are

fixed, and a new set of retrievals is run to determine
the volume mixing ratios of H2O, OH and HO2. The
retrieved HOx abundances are then evaluated based
on the ratio between the retrieved abundance and
uncertainty  = [HOx]/[HOx]err, where  > 3 indicates a detection. On the other hand, when  < 3, we
define the 3 upper limit using the retrieved uncertainties (UL = 3[HOx]err).
Comparison with the OU-GCM:
Once the retrieved dataset for the HOx species
from the ACS observations is generated, the results
will be compared with the expected abundances from
the 3D chemical OU-GCM. This model shares
physical parameterisations with the GCM from the
Laboratoire de Mètèorologie Dynamique (LMD)
(Forget et al., 1999), including the latest update of
the photochemical module (Lefèvre et al., 2021).
In addition, the OU-GCM allows the assimilation
of spacecraft data into the model. This combination
of measurements and model constraints provides the
best estimations for the HOx abundance with the current physical and chemical parameterisations. The
sets of assimilated data are similar to those in
Holmes et al. (2022): temperature profiles and dust
column from the Mars Climate Sounder on the Mars
Reconnaissance Orbiter (MCS/MRO); water and
ozone profiles, as well as water and carbon monoxide column abundances from the NOMAD instrument on the ExoMars TGO; temperature and water
vapour profiles from ACS/TGO.

As these chemical species are tightly related to
the HOx chemistry, their assimilation provides useful
constraints on the abundance of OH and HO2 during
the time period encompassed by the observations.
These predicted model abundances will be compared
with the results from the ACS dataset to evaluate if
the observations can meaningfully constrain Mars’
photochemical models. In addition, the expectations

from the model will also be used to maximise the
sensitivity of the ACS measurements by averaging
the spectra where the expected abundances in the
model are highest.
Summary and conclusions:
Despite the key role odd-hydrogen species have
in regulating the chemistry of the atmosphere of
Mars, there is little direct information on the abundance of these from measurements. In this study, we
aim to constrain the abundances of OH and HO2 using solar occultation measurements from the ACS
instrument on the ExoMars TGO. These measurements are ideal for the detection of trace species in
the atmosphere of Mars thanks to the geometry of the
observations.
We search for spectral signatures of these species
using measurements made between 2.78-2.9 µm
(3448-3597 cm-1) between LS = 160 in MY35 and LS
= 60 in MY36. These measurements allow the retrieval of pressure, temperature and water vapour
volume mixing ratios during the whole period. In
addition, the spectral signatures from OH and HO2
allow the detection or derivation of upper limits at
different altitudes, locations and time periods.
The results from these measurements will be
compared with the expectations from the assimilation
runs from the OU-GCM to test whether the measurements can meaningfully constrain Mars’ photochemical models. In additions, these expectations
from the model will also be used to establish a sensitive search at the times/locations when the abundance
of HOx is highest.
References:
Alday et al. (2019). Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board
the ExoMars Trace Gas Orbiter. A&A, 630, A91.
doi/10.1051/0004-6361/201936234
Alday et al (2021). Isotopic fractionation of water
and its photolytic products in the atmosphere of
Mars. Nature Astronomy. doi/10.1038/s41550-02101389-x
Clancy et al. (2004). A measurement of the 362 GHz
absorption line of Mars atmospheric H2O2. Icarus,
168(1), 116–121. doi/10.1016/j.icarus.2003.12.003
Clancy et al. (2013). First detection of Mars atmospheric hydroxyl: CRISM Near-IR measurement versus LMD GCM simulation of OH Meinel band emission in the Mars polar winter atmosphere. Icarus,
226(1), 272–281. doi/10.1016/j.icarus.2013.05.035
Devi et al. (2017). Line parameters for CO2- and

self-broadening in the ν3 band of HD16O. JQSRT,
203, 158–174. doi/10.1016/j.jqsrt.2017.02.020
Encrenaz et al. (2008). Simultaneous mapping of
H2O and H2O2 on Mars from infrared highresolution imaging spectroscopy. Icarus, 195(2),
547–556. doi/10.1016/j.icarus.2008.01.022
Forget et al. (1999). Improved general circulation
models of the Martian atmosphere from the surface
to above 80 km. JGR: Planets, 104(E10), 24155–
24175. doi/10.1029/1999JE001025
Gordon et al. (2022). The HITRAN2020 molecular
spectroscopic database. JQSRT, 277, 107949.
doi/10.1016/j.jqsrt.2021.107949
Holmes et al. (2018). A reanalysis of ozone on Mars
from assimilation of SPICAM observations. Icarus,
302, 308–318. doi/10.1016/j.icarus.2017.11.026
Holmes et al. (2022). Global variations in water vapour and saturation state throughout the Mars Year
34 dusty season. In review at JGP: Planets.
Irwin et al. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. JQSRT,
109(6), 1136–1150. doi/10.1016/j.jqsrt.2007.11.006
Korablev et al. (2018). The Atmospheric Chemistry
Suite (ACS) of Three Spectrometers for the ExoMars
2016 Trace Gas Orbiter. Space Science Reviews,
214(1), 7. doi/10.1007/s11214-017-0437-6
Lefèvre (2004). Three-dimensional modeling of
ozone on Mars. JGR: Planets, 109(E7), E07004.
doi/10.1029/2004JE002268
Lefèvre et al. (2021). Relationship Between the
Ozone and Water Vapor Columns on Mars as Observed by SPICAM and Calculated by a Global Climate Model. JGR: Planets, 126(4).
doi/10.1029/2021JE006838
Lefèvre et al. (2008). Heterogeneous chemistry in the
atmosphere of Mars. Nature, 454(7207), 971–975.
doi/10.1038/nature07116
McElroy & Donahue (1972). Stability of the Martian
Atmosphere. Science, 177(4053), 986–988.
doi/10.1126/science.177.4053.986
Olsen et al. (2021). The vertical structure of CO in
the Martian atmosphere from the ExoMars Trace Gas
Orbiter. Nature Geoscience, 14, 67–71.
doi/10.1038/s41561-020-00678-w
Parkinson & Hunten (1972). Spectroscopy and
Aeronomy of O2 on Mars. Journal of the Atmospheric Sciences, 29(7), 1380–1390. doi/10.1175/15200469(1972)029<1380:SAAOOO>2.0.CO;2

Patel et al. (2021). ExoMars TGO/NOMAD-UVIS
Vertical Profiles of Ozone: 1. Seasonal Variation and
Comparison to Water. JGR: Planets, 126(11),
e2021JE006837.
Perrier et al. (2006). Global distribution of total
ozone on Mars from SPICAM/MEX UV measurements. JGR, 111(E9), E09S06.
doi.org/10.1029/2006JE002681
Régalia et al. (2019). Laboratory measurements and
calculations of line shape parameters of the H2O–
CO2 collision system. JQSRT, 231, 126–135.
doi/10.1016/j.jqsrt.2019.04.012
Smith et al. (2021). The climatology of carbon monoxide on Mars as observed by NOMAD nadirgeometry observations. Icarus, 362, 114404.
doi/10.1016/j.icarus.2021.114404
Villanueva et al. (2013). A sensitive search for organics (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4),
hydroperoxyl (HO2), nitrogen compounds (N2O,
NH3, HCN) and chlorine species (HCl, CH3Cl) on
Mars using ground-based high-resolution infrared
spectroscopy. Icarus, 223(1), 11–27.
doi/10.1016/j.icarus.2012.11.013