THERMAL STRUCTURE AND WATER CLIMATOLOGY OF THE
MARTIAN
MIDDLE-UPPER
ATMOSPHERE
FROM
ACS/TGO
SPECTROSCOPY
D. A. Belyaev, Space Research Institute of the Russian Academy of Sciences (IKI), Moscow, Russia
(bdenya.iki@gmail.com), J. Alday, School of Physical Sciences, The Open University, UK, F. Montmessin, F.
Lefѐvre, LATMOS/CNRS, Paris, France, A. Fedorova, O. Korablev, A. Trokhimovskiy, A. Patrakeev, Space
Research Institute of the Russian Academy of Sciences (IKI), Moscow, Russia, K. S. Olsen, Department of
Physics, University of Oxford, UK.

Introduction:
The thermal structure of the middle (50-100 km)
and upper (above 100 km) atmosphere of Mars holds
key information relevant for climate modeling, atmospheric escape, understanding the impact of solar
activity, as well as planning spacecraft maneuvers.
The altitude range of 80-130 km hosts the layers of
mesopause, the coldest atmospheric zone, and
homopause, where the atmosphere is no longer uniformly mixed [1]. Moreover, water vapour molecules above ~80 km, when rises so highly [2-4], are
effectively exposed by photolysis from the solar
radiation that leads to H and O atoms escape.
In this paper, we report highly sensitive measurements of the vertical distribution of temperature,
CO2 and H2O densities as measured by the Atmospheric Chemistry Suite (ACS) on board ExoMars
Trace Gas Orbiter (TGO) in the regime of solar
occultation [5]. The middle-IR channel (ACS-MIR)
has been performing the experiment since April 2018
in the spectral range from 2.3 to 4.2 μm with the
resolving power exceeding 25 000. In the occultation
mode, the instrument senses CO2 absorption band
around 2.7 μm in an extremely broad altitude range,
from 20 to 180 km, covering the troposphere, the
mesosphere and the thermosphere of Mars. In parallel, the strong H2O band at 2.66-2.67 μm is also
observed for the first time up to 120 km.
In the paper, we discuss our scheme of the temperature and CO2, H2O density retrievals [4, 6] validating with atmospheric models and with simultaneous measurements by ACS-NIR below 100 km [2].
The climatology is provided by the seasonal and
latitude coverage of observations that includes about
600 vertical profiles spreading over 1.5 Martian
Years (MY), from the middle of MY 34 to the end of
MY 35. The dataset allows observing seasonal and
latitudinal variations of the thermal structure at the
dawn and dusk terminators. Temperature and pressure conditions for H2O and CO2 saturation are analyzed as well.
Results of observations:
Our measurements cover several Martian seasons
containing the second half of MY 34 and the entire
MY 35, which correspond to ACS MIR observations
from May 2018 to January 2021. The selected data

set comprises 308 occultation sessions in the Northern Hemisphere and 301 sessions in the Southern
Hemisphere, encompassing seasonal periods from LS
= 180° to LS = 356° in MY 35. We highlight several
key points of our observations:
 Seasonal variability of the Martian temperature and CO2 density is observed at altitudes from 20
to 180 km [6].
 The mesopause altitude rises from 70-90
km in the high latitudes of winter to 130-150 km in
the summer season for both hemispheres [6].
 The homopause altitude varies from 80 km
at aphelion to 110 km at perihelion in the MY 34 and
35, and it depends on dust activity [6].
 For the first time, water relative abundances
are reported in a previously unexplored altitude
range: from 100 to 120 km [4].
 Both the global dust storm in MY 34 and
the two perihelion seasons (MY34, 35) reveal 10–50
parts of H2O per million by volume (ppmv) at 100–
120 km. At the same time, it does not exceed 2 ppmv
during aphelion of MY 35 [4].
 High altitude H2O super saturation conditions (>10) occur at 80-110 km every perihelion
season. For CO2, the condensation episodically occurs at aphelion Polar Regions in troposphere altitudes.
Acknowledgements:
The ACS experiment is led by the Space Research Institute (IKI) in Moscow, assisted by
LATMOS in France. Retrievals and analysis of temperature and water distributions, performed by the
authors at IKI, are funded by grant #20-42-09035 of
the Russian Science Foundation.
Bibliography:
[1] Bougher et al. (2017). Chapter 14: Upper
Atmosphere and Ionosphere, in The Atmosphere and
Climate of Mars, ed. B. Haberle, M. Smith, T. Clancy, F. Forget, R. Zurek, Cambridge University Press,
https://doi.org/10.1017/9781107016187.
[2] Fedorova et al. (2020). Stormy water on
Mars: The distribution and saturation of atmospheric
water during the dusty season. Science, 367(6475),
297–300. https://doi.org/10.1126/science.aay9522.
[3] Fedorova et al. (2021). Multi-annual monitor-

ing of the water vapor vertical distribution on Mars
by SPICAM on Mars Express. Journal of Geophysical Research: Planets, 126, 2020JE006616.
doi:10.1029/2020JE006616.
[4] Belyaev et al. (2021). Revealing a high water
abundance in the upper mesosphere of Mars with
ACS onboard TGO. Geophysical Research Letters,
48,
e2021GL093411.
https://doi.org/10.1029/2021GL093411.
[5] Korablev et al. (2018). The Atmospheric
Chemistry Suite (ACS) of three spectrometers for the
ExoMars 2016 trace gas orbiter. Space Science Reviews, 214(1). https://doi.org/10.1007/s11214-0170437-6.
[6] Belyaev D. et al. (2022). Thermal Structure of
the Middle and Upper Atmosphere of Mars from
ACS/TGO CO2 Spectroscopy. Submitted to Journal
of Geophysical Research: Planets. Under Review.