VARIABILITY IN MARTIAN CO AS OBSERVED BY NOMAD-SO FROM
FIRST YEAR OF TGO OPERATION.
Ashimananda Modak, (ashim@iaa.es), M. A. López-Valverde, A. Brines, A. Stolzenbach, B. Funke, F.
González-Galindo, Instituto de Astrofı́sica de Andalucia, Granada, Spain, S. Aoki, Belgian Royal Institute for
Space Aeronomy, Brussels, Belgium, Japan Aerospace Exploration Agency (JAXA), Japan, I. Thomas, Belgian Royal
Institute for Space Aeronomy, Brussels, Belgium, G. Liuzzi, NASA Goddard Space Flight Center, USA, G. Villanueva,
NASA Goddard Space Flight Center, USA, J. Erwin, Belgian Royal Institute for Space Aeronomy, Brussels, Belgium,
J. J. Lopez-Moreno, Instituto de Astrofı́sica de Andalucia, Granada, Spain, N. Yoshida, Graduate School of Science,
Tohoku University, Sendai, Japan, U. Grabowski, Karlsruhe Institute of Technology, Institute of Meteorology and
Climate Research, Karlsruhe, Germany, F. Forget, Laboratoire de Météorologie Dynamique, IPSL, Paris, France, F.
Daerden, Belgian Royal Institute for Space Aeronomy, Brussels, Belgium, B. Ristic, Belgian Royal Institute for Space
Aeronomy, Brussels, Belgium, G. Bellucci, Institute for Space Astrophysics and Planetology, Italy, M. Patel, Open
University, Milton Keynes, UK, L. Trompet, Belgian Royal Institute for Space Aeronomy, Brussels, Belgium, A.C.
Vandaele, Belgian Royal Institute for Space Aeronomy, Brussels, Belgium.

Introduction

Data Processing and Retrieval

Mars’ atmosphere is primarily a CO2 atmosphere with
an amount of ∼ 95% and the rest of the ∼ 5% of the
atmospheric mass is composed of trace gases such as
O2 , H2 , CO, H2 O, O3 , OH, HO2 , etc. During the daytime
exposure to the sunlight, CO2 breaks into CO and O
and is recycled via a chemical reaction of CO with OH
[6]. Like any other chemically active trace species,
the measurement of CO is important to characterize and
understand the chemical processes in the atmosphere.
In addition, CO is also important for being a dynamical
tracer due to its long lifetime [4].
Columnar measurements of CO have been carried out
by few spaceborne instruments such as CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
[10], PFS (Planetary Fourier Spectroscopy) [1], OMEGA
(Observatoire pour la Minéralogie, l’Eau, les Glaces et
l’Activité) [2], and more recently by the LNO(Limb
Nadir and solar Occultation) channel of the NOMAD(Nadir
and Occultation for MArs Disovery instrument) instrument on board TGO (Trace Gas Orbiter)[11]. The
columnar measurements revealed the seasonal and latitudinal distribution of CO. One of the important results
confirmed by these studies is the increase in the relative
abundance of CO, or volume mixing ratio (VMR), during the polar winter. This is resulting from the known
seasonal change in CO2 , the main atmospheric species
at Mars. These measurements also established a global
average of 800 ppm for CO mixing ratios.
However, the observations of vertical profiles of CO
have become possible in a systematic manner thanks
for the TGO Exomars 2016 mission. We report here
new results of CO vertical profiles obtained for the first
tie from the SO channel of the NOMAD instrument on
board TGO. Largely based on a recent work submitted
to JGR-Planets [8], this study covers the first year of
TGO science operations, which extended for the last
two seasons of MY34.

The NOMAD instrument is a suite of three spectrometers
SO, LNO and UVIS (ultraviolet and visible spectrometer). Two of them SO and LNO operate in the infrared
and the third one, UVIS, operates in the UV [13]. Our
objective here is to retrieve carbon monoxide from the
SO observations. The SO spectrometer integrates an
echelle grating in littrow configuration with an Acousto
Optical Tunable Filter (AOTF) for selecting the desired
diffraction orders [9]. The NOMAD-SO channel covers a wavelength range of 2.3 - 4.3 µm. In this spectral
range, five different diffraction orders, internally named
as 186 - 191, contain strong CO lines and are suitable for
CO retrievals. We only used order 190 in this work.
The SO measurements are level 1 transmittance data
shown in the left panel of Figure 1. The calibrated
transmittances suffer from diverse imprecissions and
systematics, in particular from small spectral shifts and
broad spectral bendings. We developed a pre-processing
or cleaning method to correct for spectral shifts and
bending effects, based on precise line-by-line simulations including a climatological atmospheric state and
incorporating the SO instrumental response. This preproccessing is applied to small regions, or spectral microwindows (MW), within each diffraction order. The
bending is corrected in each MW while the spectral shift
correction is assumed to be linear across the order and
therefore requires several MWs. In order 190 we used
4 MWs. Our line-by-line calculations were performed
with the KOPRA model (Karlsruhe Optimized and Precise Radiative transfer Algorithm) [12].
The cleaned spectra are used as inputs in the inversion. The inversion is performed with a retrieval control
program (RCP) which uses Levenberg-Marquardt minimization method to solve for CO using the forward
model KOPRA iteratively [5]. The a-priori temperature
and pressure are are not obtained from a climatology
but from a previous retrieval of the same Nomad/SO

Figure 1: Right panel: Level 1 calibrated transmittance spectra
from 1 scan. Each spectrum is shown with a different color
and corresponds to a different tangent altitude. Vertical lines
indicate CO line positions from Hitran 2016, for reference.
It can apprecieate a residual spectral bending and a spectral
shift. Left panel: Same transmittances after the cleaning (not
applied to the edges of the order).

scan performed by our team [7]. The a-priori The apriori CO profile is taken from the LMD-GCM (Mars
Global Climate Model developed at the Laboratoire de
Météorologie Dynamique) simulations.
An example retrieval is shown in the Figure 2 together with few diagnostic plots. In the Figure four
panels, from the left, show CO, averaging kernel, percentage error and vertical resolution respectively. The
averaging kernel describes the mapping of the measurements onto the retrieval grid and the vertical resolution
is described by FWHM (Full Width at Half Maxima)
of the averaging kernel. The errors of the retrievals are
typically within 10 - 15% and the vertical resolutions are
better than 5km below the altitude 80km.

Figure 3: The effect of the reference atmosphere on the retrieved CO. First and second panels from left: reference temperature and pressure from LMD-GCM and SO retrieval respectively. Third and fourth panels: the volume mixing ratio
and number density of CO retrived with temperature and pressure from GCM (black) and SO (red).

one with a-priori temperature and pressure from GCM
(plotted in black) and another with a-priori temperature
and pressure retrieved from a different diffraction order(plotted in red)[7]. The retrieved VMR and number
density of CO with SO T/P are shown in red curves in the
panels c and d while those with GCM T/P are in black.
The VMR values retrieved using GCM T/P are notably higher than the values retrieved with retrieved temperature and pressures. Both pressure and temperature
affect the inversion of CO. The VMR is affected by both
while the absolute CO (panel d) is possibly affected most
by temperature. In both cases, retrieved absolute number density values are similar above 40km but below, the
values differ. Below 40km, lower values of CO density
are retrieved for the warmer, i.e., the SO temperature.

CO Vertical Profiles and Variability

Figure 2: Retrieved CO VMR profile for one particular scan
(left panel) and three profile’s diagnostics: averaging kernel
rows, retrieval error in percentage, and vertical resolution in
km. The data below 25 km is ignored due to the large dust
opacity.

Temperature Dependence of the Retrievals
One of the main challenges for the CO retrieval is its
dependence on temperature and pressure. To illustrate
this dependence, Figure3 shows results of two retrievals,

Here we present, briefly, the results obtained in the retrieval from the SO observation of order 190. Due to the
dependence on temperature and pressure, we only have
attempted to retrieve CO from scans with simultaneous
temperature retrievals. These were performed by our
team previously, from NOMAD/SO data in order 149.
There are 275 such scans for MY34, among which 200
were successful. All the retrieved profiles are shown in
panels c and d of Figure 4. Panel c is corresponding to
the Northern hemisphere and d to the Southern. Panels a and b show the location of the profiles in c and d
respectively.
CO is a long-lived species affected by both photochemistry and long range transport. The photochemical
production of CO occurs at high altitudes while its chemical loses are most effective at low altitudes. We observe
increasing CO VMR with altitudes as a general trend in
our retrieved CO VMR vertical profiles. Particularly,
high values and large gradients are observed in the alti-

tudes above 60km.

Figure 4: All the retrieved CO profiles plotted against season and altitude in panel c for the Northern hemisphere and
in panel d for the Southern hemisphere. The panels a and b
respectively, indicate the latitude and season of each profile

The dataset extends from April 2018 to March 2019,
i.e., Northern Autumn and Winter MY34, including the
MY34 global dust storm (GDS). In the work by [8] we
study the variability in CO in detail. We will highlight some results here, like the CO at high altitudes.
This is described by the iso-contour indicating a value
log10 (CO(in VMR)) = 3.5 (orange color). The high altitude CO seems to downwell and reaches, successively,
to low altitudes over Northern hemisphere (panel c, Figure 4). The downwelling reaches about 60km during the
Southern summer at Ls ∼ 275◦ . While over the Southern
hemisphere, this iso-contour seems to retreat towards altitudes higher than 80km. However, the downwelling of
CO seems to increase during the end of the year over
both hemispheres.
The variability of the CO VMR below 60km altogether reflects a different seasonality. During GDS, the
CO of this region depletes due to warming of the atmosphere which in turn facilitates the atmosphere to hold
more water vapor. The increase in the water vapor produces an increase in OH radicals which is normally the
primary photochemical destruction of CO at these altitudes. On contrary, during the decay phase of the GDS
(Ls = 210◦ −225◦ ), in the presence of high optical depth
an increase in the low altitude ∼ 20km CO is observed.
This increase is puzzling, when combined with the CO
behavior during the peak of the GDS. It suggests that the
photochemical destruction by OH and the H2 O photolysis are not the primary sources of CO loses, or that their
effects are very non-linear with the dust opacity, or that
different processes affecting CO are at work during the
peak and decay phases of the GDS.
A depletion in the CO VMR, is again observed during
the Southern summer over both the hemispheres. Over
the Northern hemisphere, the depletion can be observed
(Figure 4) in the period Ls = 275◦ − 300◦ while over
Southern hemisphere it extends till Ls = 325◦ . During
the Southern summer the well known phenomena of
polar cap melting occurs which releases the trapped CO2
and water vapor into the atmosphere. CO being a longlived trace species may respond to both the causes.
The effect of Southern summer on CO distribution

reduces and a general increase in VMR values of all the
observed profiles found after Ls = 325◦ . Over the Northern hemisphere, for this period, most of the profiles are
located in the high latitudes and for these profiles, high
CO values are retrieved for all the altitudes. However,
over the Southern hemisphere, though the high values
of CO are retrieved, a latitudinal gradient is also visible
from the Figure 4.
Figure 5 shows the latitudinal variation for two selected periods corresponding to the onset of the GDS
and to the Southern summer. The above panels a and b
show the latitudinal distribution of the CO profiles and
the panels c and d show the colocated GCM-simulated
values. During the onset of the GDS, Martian dynamics
is dominated by two Hadley cells extending from the
equator to the high latitudes in both the hemispheres.
The effect of the hemispheric Hadley circulation can be
seen clearly in the CO distribution of LMD-GCM as well
as in the observation. The increase in the low altitude
CO over the high Southern latitudes (Figure 4) is due to
this effect.
Another interesting result can be observed near the summer solstice season, regarding the latitudinal variation
of CO. It is the clear pattern of the global Hadly cell typical of the dynamical regime of this period. The global
Hadley cell [3] seems to extend from the Southern latitudes towards the Northern latitudes and downwells near
the Lat = 60◦ .

Figure 5: Latitudinal distribution of the CO profiles for the
periods as indicated in the panels. The above panels (a and b)
show the distribution of the retrieved CO and the below panels
(c and d) show the corresponding distribution of CO profiles
simulated by GCM. The color bar indicates the VMR values
of CO.

Acknowledgments
The IAA/CSIC team acknowledges financial support
from the State Agency for Research of the Spanish MCI
through the "Center of Excellence Severe Ochoa" award
(DEV-2017-0709) and funding by grant PGC2018-101836B-100 (MCI/AEI/FEDER, EU). This project acknowledges funding by the Belgian Science Policy Office

REFERENCES

(BELLS), with the financial and contractual coordination by the ESAU Prod ex Office (PEA 4000103401,
4000121493) as well as by UK Space Agency through
grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1
and Italian Space Agency through grant 2018-2-HHS.0.
US investigators were supported by the National Aeronautics and Space Administration.
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