AN OVERVIEW OF THE MARS ATMOSPHERIC STATE AS OBSERVED
BY EMIRS
Michael D. Smith, NASA Goddard Space Flight Center, Greenbelt, MD, USA (Michael.D.Smith@nasa.gov),
K. Badri, E. AlTunaiji, Mohammed Bin Rashid Space Center, Dubai, UAE, S. A. Atwood, LASP, University
of Colorado, Boulder, CO, USA. C. S. Edwards, N. Smith, C. Wolfe, Northern Arizona University, Flagstaff,
AZ, USA, M. J. Wolff, Space Science Institute, Boulder, CO, USA, P. R. Christensen, S. Anwar, Arizona State
University, Tempe, AZ, USA.

Introduction: Thermal infrared spectra taken by
The Emirates Mars Infrared Spectrometer (EMIRS)
(Edwards et al., 2021) on-board the Emirates Mars
Mission (EMM) spacecraft (Amiri et al., 2021) are
well suited for the retrieval of surface temperatures,
the atmospheric temperature profile from the surface
to ~50 km, and the column abundance of dust aerosols, water ice clouds, and water vapor. A constrained linear inversion retrieval routine that includes multiple scattering has been developed and
optimized for this purpose. The unique, high-altitude
orbit of EMM enables sampling of all local times
over a wide range of latitudes and longitudes over a
short sub-seasonal timescale of less than two weeks.
Here, we present an overview of the retrieval algorithm and first atmospheric science results from observations taken by EMIRS over the first Earth year
of EMM Science Phase operations.
EMIRS Data: EMIRS is a thermal infrared
spectrometer that observes Mars at wavelengths between ~100 and 1600 cm-1 (~100 and 6 µm) at a
spectral resolution of 5 or 10 cm-1. From its 55-hour
period orbit that varies between 20,000 and 43,000
km altitude, EMIRS raster scans the disk of Mars
~20 times during each orbit to provide a global, synoptic view of Mars that samples all local times, day
and night. Over the course of approximately 4 orbits
(or 10 days), sufficient observations are taken to provide a broad sampling of all local times at nearly all
latitudes and longitudes. Although the typical footprint is relatively large (~100–300 km) this spatial
resolution is consistent with modern global circulation models and is sufficient to provide a detailed
global view of the current climate state.
Retrieval Overview: We follow the constrained
linear inversion algorithm of Conrath et al. (2000)
and Smith et al. (2006) to retrieve atmospheric state
parameters that best match the observed spectra of
Mars from EMIRS. The radiative transfer model
includes a discrete ordinates treatment of multiple
scattering (e.g., Goody & Yung, 1989; Thomas &
Stamnes, 1999) to accurately model dust and water
ice cloud aerosols, and it accounts for the absorptions from CO2 and water vapor gases using the
HITRAN database (Gordon et al., 2022) and the

correlated-k approximation (Lacis & Oinas, 1991).
Given that the spectral signatures of gases and
aerosols are relatively well separated in the spectral
range observed by EMIRS, the retrieval is performed
sequentially for the atmospheric temperature profile,
the column optical depths of dust and water ice aerosol, and the column abundance of water vapor. This
sequence can be iterated to obtain a self-consistent
solution.
There are a number of assumptions that must be
made to perform the retrieval. The surface pressure
cannot be reliably retrieved with these data and so is
instead taken from the Mars Climate Database
(MCD) (Forget et al., 1999; Millour et al., 2018).
Aerosol optical properties are taken from the work
by Wolff et al. (2006). Perhaps most importantly, we
must assume a vertical distribution for the aerosols
and water vapor. Dust is assumed to follow a
Conrath profile (Conrath, 1975). The Conrath- parameter is chosen so that the top of the dust layer
varies as a function of season and latitude ranging
from 1 (aphelion, high latitudes) to 5 scale heights
(perihelion, low latitudes) based on prior observations (e.g., Heavens et al., 2011; Smith et al., 2013).
Water ice clouds are placed at the water condensation level, while water vapor is well-mixed up to its
condensation level.
In these nadir geometry observations, a thermal
contrast between the surface and the atmosphere is
required in order to observe the absorption (or emission) features from dust, water ice clouds, and water
vapor. This thermal contrast becomes vanishingly
small near dawn and dusk, which limits our ability to
perform the aerosol and water vapor retrievals at
those local times. The uncertainty in retrieved parameters is directly related to this thermal contrast,
so we compute an estimate for the uncertainty in
each retrieved parameter for every retrieval since this
uncertainty varies greatly with season, latitude, and
local time.
Results: Figure 1 provides an overview of the retrieval results for EMIRS observations taken between
24 May 2021 (Ls=49°) and 24 February 2022
(Ls=180°). The gap between Ls=100° and 120° was
caused by the combination of solar conjunction and
the spacecraft entering safe mode. Shown are dust
extinction column optical depth (referenced to 1075

cm-1), atmospheric temperature (at 0.5 mbar or ~25
km altitude), water ice cloud extinction column optical depth (referenced to 825 cm-1), and the column
abundance of water vapor (pr-µm).
The expected aphelion-season variations and interrelations between the four quantities (e.g., Smith,
2004) are evident in Fig. 1. Dust exhibits its annual
minimum optical depth, but there was an early season regional dust storm beginning at Ls=153° (see
also Badri et al., 2022 for more details). Atmospheric
temperatures become warmer as the season progresses and they respond rapidly to the onset of the regional dust storm. The aphelion season water ice
cloud belt dominates the third panel of Figure 1 with
additional polar clouds appearing later in the season
(see also Atwood et al., 2022 for more details). Finally, the high-latitude northern hemisphere summer
maximum and subsequent equatorward transport of
water vapor is well documented by the EMIRS retrievals.

Hemisphere Spring (Ls=55°–70°) and just before
Northern Hemisphere Autumn equinox (Ls=160°–
175°) retrieved from EMIRS afternoon observations.
These retrievals include thermal variations caused by
the general circulation, tides, waves, and radiative
processes (see also, Fan et al., 2022; Young et al.,
2022).

Figure 2: Latitude-pressure cross-sections of
atmospheric temperatures retrieved from
EMIRS observations for two different seasons.

Figure 1: The seasonal and latitudinal variation
of (top) dust column-integrated optical depth,
(second) atmospheric temperature at 0.5 mbar
(~25 km), (third) water ice cloud columnintegrated optical depth, and (bottom) water
vapor column abundance (pr-µm) as retrieved
from EMIRS observations.

Figure 2 shows atmospheric temperature crosssections for two different seasons during Northern-

Summary: The unique orbit of the Emirates
Mars Mission enables nearly the entire atmosphere
of Mars to be sampled over all local times on timescales as short as 10 days. The thermal infrared spectra observed by the EMIRS instrument are well suited for the retrieval of atmospheric temperature profiles and the column-integrated quantities of dust and
water ice aerosol optical depth and of water vapor
abundance. The thermal structure and the spatial
variations of aerosols and water vapor were as expected for the aphelion season that has so far been
observed. Clear diurnal variations in atmospheric
temperatures and water ice cloud opacity are evident
in the retrievals (see Atwood et al., 2022; Fan et al.,
2022 for details). Detailed analysis of these retrieval
results will improve our understanding of the underlying physical processes while also helping to validate and tune GCM models. Systematic EMIRS observations continue as the part of the baseline set of

ongoing EMM observations promising exciting new
information as we enter the dusty perihelion season.

References:
Amiri, H.E. S. et al., 2021. Space Sci. Reviews,
218:4, doi:10.1007/s11214-021-00868-x.
Atwood, S.A., et al., 2022. The diurnal and seasonal
variation of water ice clouds as observed by
EMIRS. 7th Mars Atmosphere Modeling and Observations conference, Paris.
Badri, K., et al., 2022. The diurnal and seasonal
variation of dust aerosol as observed by EMIRS.
7th Mars Atmosphere Modeling and Observations
conference, Paris.
Conrath, B.J., 1975. Icarus, 24, 36–46.
Conrath, B.J. et al, 2000. J. Geophys. Res., 105, E4,
9509–9519.
Edwards, C.S. et al., 2021. Space Sci. Reviews,
217:77, doi:10.1007/s11214-021-00848-1.
Fan, S. et al., 2022. Migrating thermal tides in the
Martian atmosphere during aphelion season observed by EMM/EMIRS. 7th Mars Atmosphere
Modeling and Observations conference, Paris.
Forget, F. et al., 1999. J. Geophys. Res., 104, E10,
24155–24176.
Goody, R.M. & Yung, Y.L., 1989. Atmospheric Radiation: Theoretical Basis. Oxford Univ. Press.
Gordon, I.E., et al., 2022. JQSRT, 277,
doi:10.1016/j.jqsrt.2021.107949.
Heavens, N.G., et al., 2011. J. Geophys. Res., 116,
E04003, doi:10.1029/2010JE003691.
Lacis, A.A. & Oinas, V., 1991. J. Geophys. Res., 96,
9027–9063.
Millour, E. et al., 2018. “From Mars Express to
ExoMars” workshop. ESAC Madrid, Spain.
https://www.cosmos.esa.int/documents/1499429/
1583871/Millour_E.pdf
Smith, M.D., 2004. Icarus, 167, 148–165.
Smith, M.D., et al., 2006. J. Geophys. Res., 111,
E12S13, doi:10.1029/2006JE002770.
Smith, M.D., et al., 2013. J. Geophys. Res., 118, 1–
14, doi:10.1002/jgre.20047.
Thomas, G.E. & Stamnes, K., 1999. Radiative
Transfer in the Atmosphere and Ocean, Cambridge Univ. Press.
Wolff, M.J., et al., 2006. J. Geophys. Res., 111,
E12S17, doi:10.1029/2006JE002786.
Young, R. et al., 2022. Atmospheric data assimilation with the Emirates Mars Mission. 7th Mars
Atmosphere Modeling and Observations conference, Paris.