MIGRATING THERMAL TIDES IN THE MARTIAN ATMOSPHERE
DURING APHELION SEASON OBSERVED BY EMM/EMIRS
Siteng Fan, François Forget, Sandrine Guerlet, Antoine Bierjon, Jiandong Liu, Ehouarn Millour,
LMD/IPSL, Sorbonne Université, PSL Research Université, École Normale Supérieure, École Polytechnique,
CNRS, Paris, France (sfan@lmd.ipsl.fr), Michael D. Smith, NASA Goddard Space Flight Center, Greenbelt,
MD, USA, Khalid M. Badri, Hessa R. Al Matroushi, Mohammed Bin Rashid Space Center, Emirates Institute
for Advanced Science and Technology, Al Khawaneej Area, Dubai, UAE, Samuel A. Atwood, Justin Deighan,
Laboratory for Atmospheric and Space Physics, Boulder, CO, USA, Roland M. B. Young, Department of Physics & National Space Science and Technology Center, United Arab Emirates University, Al Ain, UAE, Christopher S. Edwards, Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ, USA,
Philip R. Christensen, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA.

Overview:
Temperature profiles retrieved using the first set
of data obtained during the science phase of the
Emirates Mars Mission (EMM, Almatroushi et al.
2021) by the Emirates Mars InfraRed Spectrometer
(EMIRS, Edwards et al. 2021) are used for the analysis of thermal tides in the Martian atmosphere. The
selected data cover a solar longitude (LS) range of
60°-90°of Martian Year (MY) 36. The novel orbit
design of the Hope Probe leads to a good local time
coverage that significantly improves thermal tides
analysis. Wave mode decomposition suggests dominant diurnal tide and important semi-diurnal tide
with maximal amplitudes of 6K and 2K, respectively,
as well as the existence of ~0.5K ter-diurnal tide.
The results agree well with predictions by the LMD
Mars General Circulation Model (GCM, Forget et al.
1999), but the observed diurnal tide has earlier phase
(3h), and the semi-diurnal tide has much larger
wavelength (~200km).

Introduction:
Thermal tides are planetary-scale harmonic responses driven by diurnal solar forcing and influenced by planetary topography. Excited by solar
heating absorbed by the atmosphere and energy exchange with the surface, thermal tides grow in Martian atmosphere. These tides usually have large amplitudes due to the low atmospheric heat capacity,
and dominate its diurnal variations. To investigate
such global and diurnal/sub-diurnal mechanisms,
data with planetary-scale spatial coverage that sample all local times within a short range of season is
necessary. Observations obtained by EMM/EMIRS
can meet such a requirement due to the novel design
of spacecraft orbit, which is the subject of this work.

Observations:
Analysis in this work is based on the first set of
data obtained by EMIRS (Edwards et al. 2021), a
Fourier transform infrared spectrometer onboard the
Hope Probe. The selected data are during LS= 60°-

90°of MY 36, which contains ~7.0×104 temperature
profiles, and have a full coverage in geography and
local time (Figure 1). This is a dust-clear season in
the late northern spring when Mars is near aphelion.

Figure 1. Data coverage of EMIRS observations
during LS= 60°-90°of MY 36.

Results:
Individual temperature profiles are first binned in
longitude, latitude, and local time with grid sizes of
5°, 10°, and 1h, respectively (Figure 1). Daily temperature anomalies are then obtained across most of
the latitudes by subtracting the zonal and diurnal
means (Figure 2). This is the first time that such variations are observed on a global scale without any
significant gaps in local time or sampling bias in

Figure 2. Zonal mean daily temperature anomaly
between ±5°in latitude. The magenta dashed and
solid lines denote the local time of daily temperature maximum, and the linear fit result of its
downward phase progression, respectively.

season. Near the equator, the daily temperature
anomaly shows signatures of dominant downward
phase propagating of diurnal tide with an amplitude
of ~6K at <10Pa to ~2K at >100Pa. The daily temperature maximum propagates approximately from
23h at 5Pa to 19h at 500Pa.
Through wave mode decomposition, contributions of the first three migrating thermal tides are
derived. In the equatorial region, the diurnal tide
dominates the diurnal temperature variation with an
amplitude of ~2-6K, with a similar vertical wavelength compared to model predictions, but an earlier
phase of ~3h in local time. The semi-diurnal tide
shows an amplitude of ~1.5-2K across all pressure
levels with a wavelength of ~200km, far larger than
that in the model prediction. As a new finding, the
observations show the existence of the ter-diurnal
tide with an amplitude of ~0.3-0.5K.
Latitudinal and vertical distributions of amplitudes and phases of the migrating tides are obtained
by repeatedly applying the wave mode decomposition to each latitude bin at each pressure level (Figure 3). The diurnal tide has a maximal amplitude of
~6K near the equator at ~5Pa, and also large values
north of 30°N (Figure 3a). Its downward phase progression is well constrained in the equatorial region
between ±30°, but the phase is close to ~±π at midlatitudes (Figure 3b). Similar to the example shown
above for the equatorial bin, the semi-diurnal tide
between ±20° in latitude have similar amplitudes
(Figure 3c), but slightly different downward propagating phases (Figure 3d). The phase in the northern
hemisphere is earlier than that in the south. The ter-

diurnal tide has a maximal amplitude of ~0.5K at
~20Pa (Figure 3e), and also downward phase progression at most of the latitudes (Figure 3f). Its phase
distribution seems to have a symmetric patter by
20°N.

Discussion:
The sampling advantage of EMM/EMIRS observations that cover all geographic locations and local
times within 10 days (~5°in LS) largely address the
sampling problem in analyzing observations obtained by previous missions, and enables detailed
tide investigations with good constraints on their
amplitudes and phases. The EMM/EMIRS data obtained during Martian aphelion season shows dominant diurnal tide and important semi-diurnal tide, as
well as the existence of ter-diurnal tide. Compared to
the LMD Mars GCM, the observed diurnal tide
shows similar wavelength but an earlier phase, while
the wavelength of the semi-diurnal tide is much larger than predicted.
Discrepancies of the migrating tide phases suggest improvements of GCMs. Preliminary attempts
show that the phases of tides in the GCMs are not
sensitive to any currently included parameters. Upgrades of the GCMs with more mechanisms are anticipated. Moreover, the large wavelength of the
semi-diurnal tide suggests improvements on the
physical properties of the modeled Martian atmosphere, and the asymmetric and symmetric distributions of semi-diurnal and ter-diurnal tides provide
further indication, which is possibly important for

Figure 3. Amplitudes and phases of migrating thermal tides.

further understanding of the Martian atmosphere on
a daily basis.

References:
Almatroushi, H., et al. (2021), Space Sci. Rev.,
217, 89.
Edwards, C. S., et al. (2021), Space Sci. Rev.,
217, 77.
Forget, F., et al. (1999), JGR, 104, 24155.