NOMAD ON EXOMARS TRACE GAS ORBITER: OBSERVATIONS,
CALIBRATION, AND PUBLICLY AVAILABLE DATA
I. R. Thomas, L. Trompet, Y. Willame, A. Piccialli, J. T. Erwin, A.C. Vandaele, B. Ristic, S. Robert, Z.
Flimon, F. Vanhellemont, F. Daerden, L. Neary, S. Viscardy, C. Depiesse, Royal Belgian Institute for Space
Aeronomy (BIRA-IASB), Brussels, Belgium (ian.thomas@aeronomie.be), S. Aoki, University of Tokyo, Tokyo,
Japan, M.R. Patel, Open University, Milton Keynes, U.K., and STFC Rutherford Appleton Laboratory, Oxfordshire U.K, J.J. Lopez Moreno, Instituto de Astrofisica de Andalucia (IAA/CSIC), Granada, Spain, G. Bellucci,
Istituto di Astrofisica e Planetologia Spaziali (IAPS/INAF), Rome, Italy, and the NOMAD Team

Introduction:
NOMAD is a suite of three spectrometers onboard the ExoMars 2016 Trace Gas Orbiter, designed to measure the constituents of the Martian
atmosphere in unprecedented detail [1]. The three
channels observe gas species in the 200-650 nm and
2.2-4.3 µm spectral regions, in nadir, limb and solar
occultation modes [2]. NOMAD has been operating
continuously since April 2016 and has already generated a wealth of data about the atmospheric constituents and processes on Mars.

LNO also observes the nightside of the planet occasionally, plus the Mars’ limb on both the dayside
and nightside to search for non-LTE CO2 emissions.
Recently, observations have been made of Phobos and Deimos: Phobos is certainly observable by
LNO; however Deimos is potentially too small and
too distant from TGO to be observed. Optimisation
of these observations is ongoing: systematic noise
removal, FOV pointing direction and the trade-off
between the SNR and the number of spectral regions
measured are all under investigation.

Observations:
SO channel: Typically, the SO channel operates
in the infrared in solar occultation mode, measuring
six spectral regions ~30 cm-1 wide at a resolution of
~0.15 cm-1 in each occultation [3]. The spectral regions are chosen based on the desired molecules/atmospheric constituents, isotopologues and
altitude ranges, where the lines are strong enough to
be observable but not saturated.
The channel can also operate in ‘fullscan’ mode,
where any number of spectral regions are measured
consecutively, covering the full spectral range of the
channel or a selected region, for example [4].
Absorption lines of H2O [5], CO [6], CO2 [7] and
their isotopologues, plus aerosols [8], are measured
in almost every occultation, with searches for HCl
[9], CH4 [10] and other trace gases and clouds made
regularly.

UVIS channel: This channel operates in the same
modes as SO and LNO, measuring solar occultations,
dayside + nightside nadirs and dayside + nightside
limbs, in the ultraviolet and visible spectral region
from 200-650 nm. Unlike SO and LNO, the full
spectral range is typically measured on every observation, with a spectral resolution of ~1.5 nm [14]
when operating in unbinned mode for nadirs and
limbs. For solar occultations, spectral binning is
normally performed, giving a spectral resolution of
~12nm [14].
In solar occultation and nadir modes, the channel
observes O3 [15] and dust/aerosols [16]. In limbpointing mode, airglow is observed: the green [17]
and red [18] lines have both been measured and we
are currently searching for auroral emissions in the
nightside limb observations.
The UVIS channel has also successfully observed
Phobos and Deimos; calibration and observation
optimisation of these observations is ongoing.

LNO channel: This channel measures primarily
in nadir, measuring 2-6 spectral regions ~30 cm-1
wide at a resolution of ~0.2 cm-1 [3] in each nadir
pass over the dayside of Mars when operating. The
spectral range is limited to 2.2-3.8 µm [1] and it
typically observes in nadir on one-out-of-four orbits
for thermal and lifetime considerations.
Absorption lines of H2O [11] and CO [12] are
measured in almost every nadir observation, with
searches for HDO and CH4 made regularly.
The channel occasionally operates in ‘fullscan’
mode, to probe a wider spectral range in nadir mode;
the channel can also operate in solar occultation
mode and has made some fullscans during solar occultations like the SO channel [13].

Calibration:
Recently, several calibration papers have been
published for all three channels:
 Calibration of NOMAD on ESA′s ExoMars
Trace Gas Orbiter: Part 1 – The Solar Occultation channel [4]
 Calibration of NOMAD on ESA′s ExoMars
Trace Gas Orbiter: Part 2 – The Limb, Nadir
and Occultation (LNO) channel [13]
 Calibration of NOMAD on ExoMars Trace Gas
Orbiter: Part 3 - LNO validation and instrument
stability [19]
 Calibration of the NOMAD-UVIS data [14]
 Removal of straylight from ExoMars NOMADUVIS observations [20]



The deuterium isotopic ratio of water released
from the Martian caps as measured with
TGO/NOMAD (supporting information) [21]

These papers describe in detail the latest calibration analysis, giving data users insight into how the
instrument was calibrated and important information
for understanding how NOMAD’s three channels
function for further analysis of the spectra.
Availability of Data:
At the time of writing (April 2022), work is ongoing to convert almost all NOMAD data into the
NASA PDS4 format [22]. This data will then be
available to everyone via the ESA PSA interface
[23]. The observations that will be available are as
follows:
 SO + UVIS occultations (figure 1)
 SO + LNO occultation fullscans (figure 2-3)
 LNO + UVIS day nadir (figure 4)
 UVIS night nadir
 UVIS day limb (figure 5)
 UVIS night limb
The other remaining observation modes (LNO
night nadirs; LNO day + night limbs; LNO day nadir
fullscans and LNO + UVIS Phobos/Deimos) will be
delivered to the PSA when the respective calibrations
are ready.
Each observation on the PSA has an associated
thumbnail image: examples of these can be found in
figures 1-5.
References:
[1] Vandaele, Ann Carine, et al. "Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission." Planetary and Space Science 119
(2015): 233-249.
https://doi.org/10.1016/j.pss.2015.10.003
[2] Robert, S., et al. "Expected performances of
the NOMAD/ExoMars instrument." Planetary and Space Science 124 (2016): 94-104.
https://doi.org/10.1016/j.pss.2016.03.003
[3] Liuzzi, Giuliano, et al. "Methane on Mars:
New insights into the sensitivity of CH4 with
the NOMAD/ExoMars spectrometer through
its first in-flight calibration." Icarus 321
(2019): 671-690.
https://doi.org/10.1016/j.icarus.2018.09.021
[4] Thomas, Ian R., et al. "Calibration of
NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 1–The Solar Occultation channel."
Planetary and Space Science (2021):
105411.
https://doi.org/10.1016/j.pss.2021.105411
[5] Aoki, Shohei, et al. "Water vapor vertical
profiles on Mars in dust storms observed by
TGO/NOMAD." Journal of Geophysical Re-

search: Planets 124.12 (2019): 3482-3497.
https://doi.org/10.1029/2019JE006109
[6] Yoshida, Nao et al. "Variations in vertical
CO/CO2 profiles in the Martian mesosphere
and lower thermosphere measured by the
ExoMars TGO/NOMAD: Implications of
variations in eddy diffusion coefficient",
submitted to Geophysical Research Letters
[7] Trompet, Loïc, et al., “Carbon dioxide retrievals from NOMAD-SO on ESA’s
ExoMars Trace Gas Orbiter and temperature
profiles retrievals with the hydrostatic equilibrium equation. II. Temperature variabilities in the mesosphere at Mars terminator.”,
submitted to Journal of Geophysical Research: Planets.
[8] Liuzzi, Giuliano, et al. "First detection and
thermal characterization of terminator CO2
ice clouds with ExoMars/NOMAD." Geophysical Research Letters 48.22 (2021):
e2021GL095895.
https://doi.org/10.1029/2021GL095895
[9] Korablev, Oleg, et al. "Transient HCl in the
atmosphere of Mars." Science Advances 7.7
(2021): eabe4386.
https://doi.org/10.1126/sciadv.abe4386
[10] Knutsen, Elise W., et al. "Comprehensive investigation of Mars methane and organics
with ExoMars/NOMAD." Icarus 357 (2021):
114266.
https://doi.org/10.1016/j.icarus.2020.114266
[11] Crismani, M. M. J., et al. "A global and seasonal perspective of Martian water vapor
from ExoMars/NOMAD." Journal of Geophysical Research: Planets 126.11 (2021):
e2021JE006878.
https://doi.org/10.1029/2021JE006878
[12] Smith, Michael D., et al. "The climatology of
carbon monoxide on Mars as observed by
NOMAD nadir-geometry observations."
Icarus 362 (2021): 114404.
https://doi.org/10.1016/j.icarus.2021.114404
[13] Thomas, Ian R., et al. "Calibration of
NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 2–The Limb, Nadir and Occultation (LNO) channel." Planetary and Space
Science (2021): 105410.
https://doi.org/10.1016/j.pss.2021.105410
[14] Willame, Yannick, et al. “Calibration of the
NOMAD-UVIS data”, submitted to Planetary and Space Science.
[15] Patel, M. R., et al. "ExoMars
TGO/NOMAD‐ UVIS Vertical Profiles of
Ozone: 1. Seasonal Variation and Comparison to Water." Journal of Geophysical Research: Planets 126.11 (2021):

e2021JE006837.
https://doi.org/10.1029/2021JE006837
[16] Streeter, P., et al. “Vertical aerosol distribution and mesospheric clouds from ExoMars
UVIS”, submitted to Journal of Geophysical
Research: Planets.
[17] Gérard, J-C., et al. "Detection of green line
emission in the dayside atmosphere of Mars
from NOMAD-TGO observations." Nature
Astronomy 4.11 (2020): 1049-1052.
https://doi.org/10.1038/s41550-020-1123-2
[18] Gérard, J‐ C., et al. "First observation of the
oxygen 630 nm emission in the Martian dayglow." Geophysical Research Letters 48.8
(2021): e2020GL092334.
https://doi.org/10.1029/2020GL092334
[19] Cruz Mermy, G., et al. "Calibration of
NOMAD on ExoMars Trace Gas Orbiter:
Part 3-LNO validation and instrument stability." Planetary and Space Science (2021):
105399.

https://doi.org/10.1016/j.pss.2021.105399
[20] Mason, Jonathon P., et al. "Removal of
straylight from ExoMars NOMAD-UVIS observations." Planetary and Space Science
(2022): 105432.
https://doi.org/10.1016/j.pss.2022.105432
[21] Villanueva, Geronimo, et al. “The deuterium
isotopic ratio of water released from the
Martian caps as measured with
TGO/NOMAD”, submitted to Journal of
Geophysical Research: Planets
[22] https://pds.nasa.gov/
[23] Besse, S., et al. "ESA's Planetary Science
Archive: Preserve and present reliable scientific data sets." Planetary and Space Science
150 (2018): 131-140.
https://doi.org/10.1016/j.pss.2017.07.013

Figure 1: SO (left) and UVIS (right) solar occultation spectra. Thumbnail images available on the ESA PSA.

Figure 2: SO solar occultation fullscan spectra. Thumbnail images available on the ESA PSA.

Figure 3: LNO solar occultation fullscan spectra. Thumbnail images available on the ESA PSA.

Figure 4: LNO (left) and UVIS (right) day nadir spectra. Thumbnail images available on the ESA PSA.

Figure 5: UVIS day limb altitude profile and spectra. Thumbnail images available on the ESA PSA.