CLIMATOLOGY OF CO AND O2 ON MARS BASED ON TWO MARTIAN
YEARS OF ACS TGO OCCULTATION MEASUREMENTS
A. A. Fedorova, A. Trokhimovskiy, O. Korablev, D.A. Belyaev, N. Ignatiev, Space Research Institute (IKI),
Moscow, Russia (fedorova@iki.rssi.ru), F. Lefèvre, F. Montmessin, LATMOS, Guyancourt, France, K. S.
Olsen, AOPP, Department of Physics, University of Oxford, UK, J. Alday, The Open University, UK, F.
Forget, Laboratoire de Meteorologie Dynamique (LMD), Paris, France, A. Lomakin, A. Patrakeev, IKI,
Moscow, Russia.
The first CO vertical profiles were inferred from
ACS MIR at altitudes 20–120 km and Ls=164–220°
Introduction:
The molecular oxygen (O2) and carbon oxide
before and during the global dust storm (GDS) of
(CO) are minor constituents of the Martian
Mars year (MY) 34. They showed a prominent
atmosphere with the annual mean mixing ratio of
depletion in the CO mixing ratio up to 100 km,
(1560 ± 60 ppm) and (673 ± 2.6 ppm), respectively
pointing to the importance of CO oxidation during
(Krasnopolsky, 2017). Both are non-condensable
wetter GDS conditions (Olsen et al., 2021).
species and their mixing ratio responds to the
In this work we present results of the long-term
condensation and sublimation of CO2 from the polar
monitoring of the vertical distribution of O 2 and CO
caps, resulting in seasonal variations of its
for two Martian years from Ls=163° of MY34 to the
abundance in polar regions.
Ls=180° of MY36.
The O2 column-averaged mixing ratio was
obtained by several ground-based observations as
Results:
well as by Herschel orbiting observatory. Recently,
We study seasonal and spatial variations of both
measurements of the O2 near-surface mixing ratio by
species
and their ratio during the TGO measurements
SAM on the Mars Science Laboratory Rover showed
as
well
as
interannual variability.
significant seasonal and interannual variability
unexpected for non-condensable trace gases (Trainer
We report the averaged mixing ratio for CO of
et al, 2019).
~950 ppm and for O2 of~1900 ppm at altitudes 0–40
The CO abundance on Mars has been measured
km and latitudes from 45°S to 45°N with averaged
by high-resolution ground-based spectroscopy from
ratio O2/CO~2.
infrared to microwave range and by nadir
observations from Mars’ orbit with OMEGA and
PFS/Mars-Express,
CRISM/MRO
and
NOMAD/TGO. These observations have shown
strong seasonal variations of CO, especially in high
latitudes with a global average of ~800 ppmv
(Bouche et al., 2021, Smith et al., 2018, 2021). In
situ measurements by SAM/Curiosity resulted in an
even lower value of 580 ppmv (Trainer et al., 2019).
While the global seasonal trends at least for CO
are relatively well understood, the vertical
distribution of these species was poorly documented
before the arrival of TGO at Mars.
ACS solar occultation observations:
In 2018, the ExoMars TGO began its science
phase by observing Mars’ atmosphere from its orbit.
The Atmospheric Chemistry Suite (ACS) (Korablev
et al., 2018) can sound the vertical structure of the
atmosphere in solar occultation mode. ACS includes
three high-resolution infrared spectrometers: NIR
(near-infrared, 0.7-1.7 m), MIR (middle infrared,
2.3-4.2 m) and TIRVIM (thermal infrared, 0.7-17
m). All three channels measure the vertical
distribution of CO in three spectroscopic bands: 1.57
μm (NIR), 2.3 μm (MIR and TIRVIM) and 4.7 μm
(TIRVIM). The NIR channel also provide the
measurement of the O2 density at altitudes of 0-50
km based on 0.76 µm band by solar occultation.

Figure 1. The averaged values of CO from ACS profiles
measured within ±45° latitude range and below 35 km.
Blue points, purple and green triangles are individual NIR,
MIR and TIRVIM occultations, respectively. Purple stars
are the results of Olsen et al., 2021 from MIR position 7
(integrated below 40 km). Blue diamonds with error bars
are averages of NIR data binned within 5° of Ls. Black
squares are GCM model results corresponding to the NIR
averages. Grey crosses are averages of PFS data (Bouche
et al., 2021). All error bars are standard deviations.

We found a strong enrichment of both species
near the surface during the southern winter and
spring in middle and high southern latitudes with a
layer of 3000-4000 ppm (for CO) at 10–20 km

corresponding to local depletion of CO2. The GCM
does not predict the enrichment in this period, both
in terms of absolute value and vertical extent. This
indicates that the breakup of the polar vortex
enriched in CO and O2 and the subsequent mixing
with midlatitude air occur too early with the settings
of the LMD GCM.
At equinoxes, both in the northern and southern
spring, we found an increase of CO mixing ratio
above 50 km to 3000–4000 ppmv explained by the
downwelling flux of the Hadley circulation on Mars.
Comparison with the general circulation chemical
model has shown it tends to overestimate the
intensity of this process, bringing too much CO from
its region of production in the high atmosphere.
The minimum of CO observed in the
southern summer in the high and middle southern
latitudes has average VMRs of 700–750 ppmv in the
low atmosphere and agrees well with nadir
measurements by CRISM/MRO and PFS/MEX even
they have a lower values of 400-700 ppmv and ~600
ppmv, respectively, in the same period. The CO
profiles calculated in southern summer by the LMD
GCM are in broad agreement with ACS.
The observations during the two Martian years
allow us to study the interannual variability for the
year with the global dust storm (MY34) and year
without GDS (MY35). We observed the depletion of
the CO mixing ratio at 30-40% both in the northern
and southern hemispheres during the global dust
storm of MY34 compared with the calm MY35. The
decrease of the CO mixing ratio in MY34 can
indicate the response of CO2 production rate to the
increase of water vapor abundance in the atmosphere
for the same period that suggests an impact of HOx
chemistry on the CO abundance.
References:
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Variability of Carbon Monoxide (CO) in the Martian
Atmosphere From PFS/MEX Observations. Journal
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https://doi.org/10.1029/2020JE006480
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https://doi.org/10.1029/2019JE006175