Investigating trace gases in the Martian atmosphere using the ExoMars Trace
Gas Orbiter Part 1: Analysis of ESA PSA NOMAD SO Channel Data
George H. Cann , University College London, London, UK (george.cann.15@ucl.ac.uk). Ingo Waldmann,
Dave Walton, Jan-Peter Muller, University College London, London, UK.
Introduction:
In 2016, the European Space Agency’s (ESA)
ExoMars Trace Gas Orbiter (TGO) was launched to
investigate the nature of methane, on Mars, utilising
the Nadir and Occultation for MArs Discovery
(NOMAD), Vandaele et al. (2015) [1], and Atmospheric Chemistry Suite (ACS), Korablev et al. (2018)
[2], instruments, both of which have the sensitivity to
make the detection, Liuzzi et al. (2019) [3]. However, the arrival of the TGO and subsequent science
mission has detected no CH4, with upper limits of
0.05 ppbv, Korablev et al. (2019) [4], 0.06 ppbv
Knutsen et al. (2021) [5], and 0.02 ppbv Montmessin
et al. (2021) [6] derived.
In contrast, NASA’s Curiosity Sample Analysis
at Mars Tunable Laser Spectrometer instrument
(SAM-TLS) Mahaffy et al. (2012) [7] has made multiple measurements of CH4, including measuring an
elevated CH4 background of 7.2 ± 2.1 ppbv CH4
over a 60-sol period in 2013, Webster et al. (2015)
[8]. Subsequently, Webster et al. (2018) [9], using
SAM-TLS determined a mean CH4 abundance of
0.41 ± 0.16 ppbv, as well as a repeatable seasonal
variation from 0.24-0.65 ppbv. Moreover, on the 20th
June 2019 a 19.5 ± 0.18 ppbv CH4 emission in Gale
Crater was reported by SAM-TLS, Moores et al.
(2019) [10]. Furthermore, the Planetary Fourier
Spectrometer (PFS) onboard Mars Express detected
15.5 ± 2.5 ppbv of CH4, above Gale Crater, one day
after SAM-TLS independently detected a CH4 spike
of 5.78 ± 2.27 ppbv, Giuranna et al. (2019) [11].
Since then, PFS has detected no CH4.
As part of a novel search for Martian trace-gases,
such as CH4, all publicly available ESA Planetary
Science Archive (PSA) NOMAD Solar Occultation
(SO) calibrated Level 3 (L3) data has been processed
to calculate ≈ 79,000 maps of transmittance residuals
(TR) against wavenumber and tangent height, covering all available NOMAD diffraction orders, from
21st April 2018 to 31st December 2019. These representations, provided as an open dataset may be useful
in identifying new trace-gas species, highlighting
which spectra are worthy of performing retrievals
and/or contain systematic errors, as well as identifying missing absorption lines in forward models.
Method:
The following procedure provides the general
method used for calculating the transmittance residuals, steps 1-5. Extending the method, in steps 6-9,
enables the creation of diffraction order averages of
NOMAD SO observations.

1. Use PDS4 Tools PDS SBN (2021) [15] to
read all NOMAD SO L3 calibrated transmittance
spectra .xml/.tab files for a particular diffraction
order.
2. For each NOMAD solar occultation assign all
values for transmittance, transmittance errors, wavenumbers, and starting tangent point altitude.
3. For a particular NOMAD SO bin, loop through
all measurements of the occultation and perform a set
of 5th degree polynomial fits using numpy
polyfit to all spectra.
4. Subtract the appropriate polynomial fit, utilising numpy poly1d with the wavenumber grid of
the observation, from the transmittance spectra, creating a set of residual spectra - note that these residuals are bin specific.
5. Repeat step 4. for all bins and create the bin
averaged set of spectra, i.e. observations indexed 14, 5-8, etc. are averaged. Note that certain filters
must be applied, as the total number of observations,
n within a solar occultation does not necessarily satisfy n mod4 = 0.
6. A set of rectangular bivariate spline fits is performed on the transmittance minus polynomial baseline values, over a rectangular mesh of wavenumber
and
tangent
heights,
using
the
scipy
RectBivariateSpline module. The order of
the bivariate spline is taken as a 5th degree polynomial, in both wavenumber and tangent height directions.
7. Evaluate the bivariate spline using the ev
method on a meshed grid.
8. Export the 2D arrays of transmittance residual
values as a .npy files and repeat or all other solar
occultations.
9. Iteratively open all .npy files and average the
2D arrays of transmittance residual values.
All code and data associated with this research
will be made open source to the community on.
Results:
Extending from individual representations we
will present diffraction order averages of NOMAD
SO observations calculated for all publicly available
ESA PSA NOMAD SO L3 data. Averaging spectra
improves the signal-to-noise ratio of observations,
Robert et al. (2016) [16], Liuzzi et al. (2019) [3] and
has been applied in multiple studies searching for
trace gases, such as CH4, in the Martian atmosphere,
such as Giuranna et al. (2019) [11], Formisano et al.
(2004) [19] and Geminale et al. (2011) [22].
Figures 1-4 show the average transmittance re-

siduals vs wavenumber and tangent height, for diffraction orders 133, 134, 135 and 136, orders of particular interest in the search for CH4. Figures 1-4 are
annotated with suggestions on the cause(s) of particular spectral lines from comparisons against
TauREx [23] and Planetary Spectrum Generator
(PSG) simulations, Villanueva et al. (2018) [16] and
Villanueva et al. (2022) [17].

O3 are potentially retrievable.

Figure 4: Diffraction Order 136. Plot of average
TR, for 724 solar occultation observations. Robert et
al. (2016) [18] suggests CH4, and C2H4 are potentially retrievable. PSG simulations suggest that if present CH4, CH3Cl, C2H4, H2O and O3 are potentially
retrievable within 136.
Figure 1: Diffraction Order 133. Plot of average
TR, for 355 solar occultation observations. PSG
[16][17] simulations suggest that if present H2O, O3,
CO2, CH4, C2H4, C2H6, and CH3Cl are potentially
retrievable within 133. Robert et al. (2016) [18] suggests CH4, C2H4, and C2H6 are potentially retrievable.

Figure 2: Diffraction Order 134. Plot of the average transmittance residual, for 2418 solar occultation observations.

Figure 3: Diffraction Order 135. Plot of average
TR, for 194 solar occultation observations. PSG
[16][17] simulations suggest that if present H2O, O3,
CO2, CH4, and C2H4 are potentially retrievable within 135. Robert et al. (2016) [18] suggests CH4, and

Figure 2 of diffraction order 134 from 3010-3036
cm−1 shows no clear evidence of CH4, although evidence for the magnetic dipole of CO2, between 30163021 cm-1, Trokhimovskiy et al. (2020) [14] is. Figure 4 of diffraction order 136 from 3054-3080 cm−1
shows a feature between 3070-3075 cm−1 that appears to correspond to CH3Cl and/or C2H4 in PSG
[16][17] simulations. Alternatively, this could be an
instrument issue, H2O/CO2 side order contributions,
or an unknown line. Furthermore, from individual
representations of diffraction order 129, evidence for
HCl, Korablev et al. (2021) [12], Olsen et al. (2021)
[13] and in diffraction order 134, evidence for the
magnetic dipole of CO2, Trokhimovskiy et al. (2020)
[14], has been identified. Note that the applicability
of averaging spectra should be considered for highly
variable species such as H2O.
Conclusions:
A full spectral assessment of all publicly available NOMAD SO channel transmittance data from
21st April 2018 to 31st December 2019 has been performed. Suggestions on the causes of particular lines
are given, however, the source of many lines in
NOMAD SO channel transmittance residuals are
challenging to establish. The cause of many lines
should be investigated by future research.
This research has found no clear evidence for the
abundances of CH4 on Mars as reported by
Formisano et al. (2004) [19], Krasnopolsky et al.
[20], Mumma et al. (2009) [21], Webster et al.
(2015) [5] and Webster et al. (2018) [9]. This research instead supports the levels of CH4 reported by
Korablev et al. (2019) [4] and Knutsen et al. (2021)
[5]. Furthermore, from individual transmittance residual representations evidence for HCl, Korablev et
al. (2021) [12], Olsen et al. (2021) [13] and the magnetic dipole of CO2, Trokhimovskiy et al. (2020)
[14], has been identified.

[11] Giuranna et al. (2019), Independent confirmation of a methane spike on Mars and a source region
east of Gale Crater, Nature Geoscience, 12, 326-332.
Acknowledgements:
We would like to thank the UK Space Agency for
their support of this studentship through the Aurora
Science Programme, STFC ST/P002528/1. We
would also like to acknowledge the achievements of
the NOMAD experiment. The NOMAD experiment
is led by BIRA-IASB and assisted by Co-PI teams
from Spain (IAA-CSIC), Italy (INAF-IAPS), and the
United Kingdom (Open University).
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