Aerosol nadir retrieval from NOMAD/UVIS on board Exomars TGO
Y. Willame, A. C. Vandaele, J. Erwin, A. Picciali, C. Depiesse, F. Daerden, L. Neary, I. Thomas, B. Ristic,
Royal Belgian Institute for Space Aeronomy (IASB-BIRA), 1180 Brussels, Belgium; M. J. Wolff, Space Science
Institute, Boulder, CO, USA; J. Mason, M. R. Patel, School of Physical Sciences, The Open University, Milton
Keynes, UK; G. Bellucci, Istituto di Astrofisica e Planetologia Spaziali (IAPS/INAF), Rome, Italy; J. J LopezMoreno, Instituto de Astrofisica de Andalucia (IAA/CSIC), Granada, Spain.

Introduction:
Aerosols are key components of Martian
radiative transfer. Airborne dust is ubiquitous on the
planet and influences the climate by absorbing
shortwave radiation, resulting in a local warming of
the atmosphere. Dust loading follow a cycle where it
is present in lower concentration during the “cooler”
aphelion season, and shows a repeatable pattern from
year to year [Smith 2009]. While for the second part
of the year, i.e. during the “warmer” perihelion season, the dust loading is larger. The warmer temperatures favor the formation of many local dust storms,
some of can grow to a regional scale or even become global. The period LS = 200-300° is sometimes
called the “dust storm season” and the formation of
these storms can arise differently each year, introducing an important interannual variability [Smith
2008].
Ice clouds are related to the water cycle. They
form due to adiabatic cooling of upward where the
water vapor condenses on dust particles where the
temperature is low enough. These clouds can be
observed having several forms, such as: 1) the cloud
belt forming during “cooler” aphelion around the
equator [Smith 2004, 2009], responsible for the
asymmetry in the water vapor transport from one
hemisphere to the other [Clancy et al., 1996]; 2) the
polar hoods that appears above the polar regions
during the winter [Benson et al., 2010, 2011]; 3) and
the orographic clouds that are present above tallest
volcanoes for a large part of the year often [Benson
et al., 2003, 2006].

The UVIS instrument and data:
The NOMAD (“Nadir and Occultation for MArs
Discovery”) spectrometer suite on board the
ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the composition of Mars' atmosphere using a suite of three spectrometers operating in the UV-visible and infrared. NOMAD is a
spectrometer operating in ultraviolet, visible and
infrared (near-IR) wavelengths covering large parts
of the 0.2-4.3 µm spectral range [Vandaele et al.,
2018].
The UV-visible “UVIS” instrument covers the
spectral range from 200 to 650 nm and can perform
solar occultation, nadir and limb observations [Patel

et al., 2017]. The main purpose of UVIS is dedicated
to the analysis and monitoring of ozone, dust and ice
clouds.
In the present work, we have used the
radiometrically calibrated data [Willame et al.,
2022], including a step for straylight removal [Mason
et al., 2022]. UVIS ultraviolet measurements suffer
from a significant NIR contamination.

Methodology:
Nadir UV measurements are useful to study these
dust and ice clouds, and allow one to derive
climatologies to analyze their cycles (annual, diurnal). We have used a similar methodology as in
[Willame et al., 2017] to simultaneously retrieve the
dust and ice cloud optical depth (OD), ozone and
surface reflectance from UVIS nadir measurements
between 220 and 320 nm. The present work focuses
on the results of the aerosol retrieval. We use the
LIDORT radiative transfer code [Spurr et al., 2001,
2017] with the aerosol properties from [Wolff et al.,
2009, 2010] for dust and from [Wolff et al 2010,
2019] for ice clouds.
Ice clouds are very bright and reflective in the
UV, especially compared to dust and surface, both of
which are quite absorbing. However, it is not always
an easy task to disentangle the three respective contributions due to a lack of diagnostic spectra. The
presence of a relatively thick cloud, overlying dust,
generally produces a large increase of the UV signal
and can be easily retrieved from wavelengths around
300-320 nm, where the impact of dust is weak. However, an increase of the surface reflectance can produces a similar effect to that of a cloud, making these
two parameters often not independent, and thus not
fitted simultaneously. The contribution from an ice
surface is large and, as a result, we do not retrieve ice
clouds when such a surface is possibly present (predicted by GEM-GCM [Daerden et al., 2019; Neary
& Daerden., 2018]). The surface ice albedo is fitted
as Lambertian in that case. The contribution from
regolith surface is significantly smaller and the
choice of fitting ice cloud OD or surface reflectance
depends on whether a cloud is obviously present: if a
cloud is present, cloud OD is fitted and the surface
albedo is kept fixed, else the surface reflectance is
fitted using the Hapke formalism [Hapke 2005]. The
sensitivity to dust is maximized around 220-230 nm,

outside of the Ozone Hartley absorption (maximum
around 255 nm).

Preliminary results:
In the present work, we will present preliminary
results of UV retrievals from the nadir geometry
observations. We will present and discuss spatial and
seasonal distribution of ice clouds and dust.

Acknowledgement:
The NOMAD experiment is led by the Royal Belgian
Institute for Space Aeronomy (IASB-BIRA), assisted by
Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS),
and the United Kingdom (Open University). This project
acknowledges funding by the Belgian Science Policy
Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401,
4000121493), by the Spanish Ministry of Science and
Innovation (MCIU), and by European funds under grants
PGC2018-101836-B-I00
and
ESP2017-87143-R
(MINECO/FEDER), 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-HH.0. This work was supported by the Belgian
Fonds de la Recherche Scientifique – FNRS under grant
number 30442502 (ET_HOME). This project has received
funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No
101004052 (RoadMap project). The IAA/CSIC team
acknowledges financial support from the State Agency for
Research of the Spanish MCIU through the ‘Center of
Excellence Severo Ochoa’ award for the Instituto de
Astrofísica de Andalucía (SEV-2017-0709) and funding
by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER,
EU). US investigators were supported by the National
Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency.

Bibliography:
Benson et al., 2003. The seasonal behavior of
water ice clouds in the Tharsis and Valles Marineris
regions of Mars: Mars Orbiter Camera observations.
Icarus 165, 34–52.
Benson et al., 2006. Interannual variability of water ice clouds over major martian volcanoes observed
by MOC. Icarus 184, 365–371.
Benson et al., 2010. Mars' south polar hood as
observed by the Mars climate sounder. J. Geophys.
Res (Planets) 115, 12015.
Benson et al., 2011. Mars' north polar hood as
observed by the Mars Climate Sounder. J. Geophys.
Res. (Planets) 116, 3008.
Clancy et al., 1996. Water vapor saturation at
low altitudes around Mars Aphelion: a key to Mars
climate? Icarus 122, 36–62.
Daerden et al., 2019. Mars atmospheric chemistry simulations with the GEM-Mars general
circulation model. Icarus, Volume 326, Pages

197 224.
Hapke, B., 2005. Theory of reflectance and
emittance spectroscopy. In: Topics in 880 Remote
Sensing, Cambridge University Press.
Mason et al., 2022. Removal of straylight from
ExoMars NOMAD-UVIS observations, Planetary
and Space Science, 2022, 105432, Neary &
Daerden, 2018. The GEM-Mars general circulation
model for Mars: Description and evaluation. Volume
300, Pages 458 476
Smith, M.D., 2004. Interannual variability in TES
atmospheric observations of Mars during 1999–
2003. Icarus 167, 148–165.
Smith, M.D., 2008. Spacecraft observations of the
Martian Atmosphere. Annu. Rev. Earth Planet. Sci.
36, 191–219.
Smith, M.D., 2009. THEMIS observations of
Mars aerosol optical depth from 2002–2008. Icarus
202, 444–452.
Spurr et al., 2001. A Linearized Discrete Ordinate Radiative Transfer Model for Atmospheric
Remote Sensing Retrieval. J Quant Spectrosc Radiat
Transf , Volume 68, Pages 689 735.
Spurr & Christi, 2019. The LIDORT and
VLIDORT Linearized Scalar and Vector Discrete
Ordinate Radiative Transfer Models: Updates in the
Last 10 Years. In: Kokhanovsky, A. (eds) Springer
Series in Light Scattering. Springer Series in Light
Scattering. Springer, Cham.
Willame et al., 2017. Retrieving cloud, dust and
ozone abundances in the Martian atmosphere using
SPICAM/UV nadir spectra. Planetary and Space
Science, Volume 142.
Willame et al., 2022. Calibration of the NOMAD
UVIS data. Planetary and Space Science, In revision.
Wolff et al., 2009. Wavelength dependence of
dust aerosol single scattering albedo as observed by
the compact reconnaissance imaging spectrometer. J.
Geophys. Res. (Planets) 114.
Wolff et al., 2010. Ultraviolet dust aerosol properties as observed by MARCI. Icarus 208, 143–155.
Wolff et al., 2019. Mapping water ice clouds on
Mars with MRO/MARCI. Icarus, Volume 332, Pages
24 49.