THE VERTICAL STRUCTURE OF TRACE GASES IN THE ATMOSPHERE
OF MARS REVEALED BY ACS ON EXOMARS TGO.
Kevin S. Olsen, P. G. J. Irwin, Department of Physics, University of Oxford, UK (kevin.olsen@physics.ox.ac.uk),
A. Fedorova, A. Trokhimovskiy, O. Korablev, M. Luginin, D. Belyaev, A. Patrakeev, A. Shakun, Space Research Institute (IKI), Moscow, Russia, F. Lefèvre, F. Montmessin, L. Baggio, Laboratoire Atmosphères, Milieux,
Observations Spatiales (LATMOS), Paris, France, J. Alday, Open University, UK, F. Forget, E. Millour, A. Bierjon,
Laboratoire de Météorologie Dynamique (LMD), Paris, France, L. Montabone, Space Science Institute, Boulder,
USA; Laboratoire de Météorologie Dynamique (LMD), Paris, France.

Introduction
The ExoMars Trace Gas Orbiter (TGO) has been making high resolution spectroscopic measurements of the
atmosphere of Mars since April 2018. By MAMO 2022,
it will have acquired 4 Earth-years of data, which will
be just over two full years on Mars. Observations began
just before the autumnal equinox in Mars year (MY) 34,
we observed the global dust storm in 2018, then two full
perihelion and aphelion periods. MAMO will take place
over solar longitude (Ls ) 245◦ in MY 36, which is into
the third dusty season observed by TGO.
While observing the dusty season of Mars, observations made using the Atmospheric Chemistry Suite midinfrared channel (ACS MIR) revealed the first novel trace
gas found during the TGO mission: hydrogen chloride
(HCl) [1]. Over the perihelion periods, while it is spring
and summer in the southern hemisphere, the atmosphere
of Mars is characterized by lifted dust, warmer temperatures, increased water vapour content, and an elevated
hygropause. Towards the vernal equinox, the dust subsides, water content falls, and the absorption signal of
HCl vanishes in the ACS MIR data.
Over the aphelion period, the atmosphere is much
colder and dryer, except for near the surface in the northern hemisphere. Without water vapour, odd-hydrogen
chemistry is greatly reduced. The odd-hydrogen family
of gases contains the hydroxyl and hydroperoxyl radicals
(OH and HO2 ), which are formed after the photolysis
of water vapour. Reactions with these radicals is the
main pathway to removing odd-oxygen (O, O3 ) from
the atmosphere of Mars by forming a more-stable O2
bond. Therefore, over the aphelion period we are able to
observe elevated levels of ozone (O3 ) in the atmosphere
of Mars.
HCl and Ozone are both closely linked to the seasonal cycles of water. Ozone is reduced when water
vapour, and its odd-hydrogen photolysis products, is
present [2], while those same products are needed to
form HCl from free chlorine, resulting in a positive correlation between HCl and H2 O [3].
Both of these species may be closely linked to aerosols
as well. Chloride minerals in dust may be a source of
atmospheric chlorides, while water vapour condensation and cloud formation may contribute to its removal

from the atmosphere. Ozone is rapidly reduced as water vapour abundances increase, and changes in atmospheric temperature that lead to cloud formation limit
the production of odd-hydrogen (OH, HO2 ) from H2 O
photolysis.
Here we present a comparison of the vertical profiles
of H2 O, O3 , and HCl volume mixing ratios (VMRs)
with each other and other data sources: temperatures
measured with the near-infrared channel of ACS [4],
and dust and ice aerosols measured by the Mars Climate
Sounder [5].

ACS MIR
ACS MIR is a cross-dispersion spectrometer on TGO
that operates in solar-occultation mode. It consists of an
optical telescope that makes direct observations of the
sun, an echelle grating whose diffraction orders are in the
infrared, and a secondary, steerable grating that separates
overlapping diffraction orders. The secondary grating is
steerable, and its position determines the instantaneous
spectral range for a series of solar occultation observations. The absorption features of the HCl 1← 0 band
are measurable between 2710–3020 cm−1 , which is observed using grating positions 11 (2680–2950 cm−1 )
and 12 (2920–3210 cm−1 ). The absorption features of
the 003←000 vibration-rotation band of O3 are measurable between 3010–3060 cm−1 in position 12. Position
12 also features several strong water vapour absorption
lines.
As TGO goes into, or comes out of, the shadow of
Mars, ACS MIR makes a series of observations of the
sun while the limb of the atmosphere lies between the
spacecraft and the sun. Spectra are obtained at unique
tangent heights every 2–4 km. By comparing the observations through the atmosphere with those made above
the atmosphere, we can determine how much sunlight
was absorbed by the atmosphere along the line-of-sight
(LOS). ACS MIR uses a 2D detector and the vertical
field of view of the fore-optics allow ∼ 12 spectra to
be acquired at each tangent height. By analyzing each
of the detector rows and then taking the average of the
retrieved VMR vertical profiles, we can greatly improve
the accuracy of our results and the sensitivity of the re-

Figure 1: Schematic of possible surface-atmosphere interactions involving chlorine. 1 Dust-bearing minerals are lofted into the
Mars atmosphere; 2 atmospheric warming caused by the dust raises the hygropause and increases the water vapour content; 3
hydrated dust aerosols may make chlorine ions available for reaction; 4 reactions between Cl− and HO2 or OH will form HCl; 5
HCl will break down in sunlight; 6 free Cl will either re-form HCl, or form a bond with oxygen, eventually leading to perchlorate
formation; 7 HCl may adhere to dust or ice aerosols and return to the surface. Volcanic emissions cannot be ruled out (1b).

trievals to trace gases, or low VMRs towards the limits
of our sensitivity. Instrument details can be found in
[6, 7].
Spectral fitting is done using the JPL Gas Fitting
Software (GGG) [8, 9] which uses a non-linear LevenbergMarquardt least squares method to compare the measurements to a computed spectrum. Absorption coefficients are calculated line-by-line using the HITRAN2020
line list [10], with line broadening parameters in a CO2 rich atmosphere for HCl from [11] and for H2 O from
[12]. Temperature and pressure are measured from simultaneous measurements of CO2 made with ACS NIR
along the same LOS. VMR vertical profiles are retrieved
from the spectral fits by inverting the matrix of slant path
abundances with the matrix of ray paths through atmospheric layers. For details about these retrievals, see
[13, 3].
HCl
HCl was first observed in the atmosphere of Mars using ACS MIR shortly after the onset of the 2018 global
dust storm [1]. It appeared simultaneously in both hemispheres, but with larger VMRs in the south. The southern VMRs fell quickly after solar longitude (Ls ) ∼ 300◦
as the atmosphere above 30 km began to cool, shortly
before the late-season ‘C’ dust storm. This behaviour
was repeated in MY 36 [3].
While HCl was a target gas of the ExoMars mission

because of its link to active volcanism, the nature of
atmospheric HCl does not seem to support such an origin. Other trace gases associated with active volcanism,
such those bearing sulphur, have not been detected by
ACS [14, 15]. Some sort of surface emission, maybe
related to magma pockets, or to clathrates experiencing
seasonal thawing, remains a possibility. In the northern
summer, around Ls 110◦ , two ACS MIR observations
featured distinct HCl absorptions below 10 km [3].
On the other hand, we have posited that HCl may
be derived from aerosol minerals lofted from the surface, which would be a novel surface-atmosphere interaction with implications for our interpretation of chloride and perchlorate reservoirs [16, 17]. Figure 1 shows
a schematic of possible HCl pathways. First, dust loaded
with chloride-bearing minerals, possibly NaCl, is lofted
into the atmosphere. The atmosphere warms, the hygropause elevates, and conditions become suitable for
the dust aerosols to become hydrated. This allows for
reactions between chlorine ions and HO2 (or OH) to
form HCl. HCl will rapidly photolyze, but the free chlorine should also rapidly find odd-hydrogen molecules
to reform HCl. A ClO bond may be formed, eventually leading to perchlorate formation and surface deposition [18], but this may not be rapid enough for the
observed seasonal behaviour. Another possibility is that
HCl leaves the gas phase after adhering to dust or ice
aerosols.

1

1

Ozone

Odd-oxygen chemistry plays a crucial role in the atmosphere of Mars. CO2 photolysis leads to the the formation of atomic oxygen and carbon monoxide (CO).
Atomic oxygen will lead to O3 production via a threebody reaction with O2 , and will become the dominant
form of reactive odd-oxygen in the lower atmosphere.
Unlike O, O3 is also directly measurable via remote
sensing.
It was first detected by the Mariner ultraviolet spectrometers [19, 20] and its seasonal behaviour along the
vertical was more recently examined with the ultraviolet
component of the NOMAD instrument on TGO [21].
Observations of O3 on Mars have been dominantly performed in the ultraviolet range, but with the sensitivity
of ACS MIR, we have been able to identify its spectral
signature in the mid-infrared as well [22].
Ozone is not visible at mid-to-low altitudes or in the
southern hemisphere while it is spring or summer in the
south (over the perihelion). This is due to the abundance
of water vapour in the atmosphere that results from the
perihelion dust activity. At high northern latitudes the
TGO observations are made around the region of polar
night (though not within). Rarely, observations of ∼
200 ppbv O3 are made below 10 km. These may be
indicative of dynamic excursions of polar air from within
the polar vortex.
Around the aphelion, when it is fall and winter in the
southern hemisphere, the atmosphere is much cooler and
dryer. This allows for frequent observations of O3 across
Mars. In the time periods surrounding the equinoxes, the
atmosphere is fairly symmetric and 200–500 ppbv O3 is
seen from around 30 km to near the surface. In the
southern hemisphere, this behaviour persists throughout
the year. In the north, where it is summer, there is
more water vapour present in the lower atmosphere and
its abundance increases from ∼ 20 km to the surface,
leading to an O3 VMR that is reduced below 20 km.
Around the equator, the aphelion cloud belt is formed.
As the atmosphere cools and water vapour condenses,
odd-oxygen is no longer suppressed by HO2 , and we
observe an equatorial ozone layer centred just below
40 km.

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OZONE

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