ON THE CONNECTION BETWEEN MARTIAN GRAVITY WAVES, DUST
STORMS, AND ATMOSPHERIC ESCAPE.
E. Yiğit, George Mason University, Fairfax, VA, USA (eyigit@gmu.edu), A. S. Medvedev, Max Planck Institute
for Slar System Research, Göttingen, Germany, M. Benna, NASA Goddard Space Flight Center, Greenbelt, MD,
USA,.

Introduction
Gravity waves play an important role in the dynamics
and maintaining the thermal structure of all planetary
atmospheres [Yiğit and Medvedev, 2019]. On Mars, they
are generated in the lower atmosphere and propagate upward to the mesosphere and thermosphere, where they
saturate and deposit their energy and momentum to the
mean flow. Here, we present recent progress in the observation of upper atmospheric effects of gravity waves,
focusing on observations provided by the NGIMS instrument onboard NASA’s MAVEN spacecraft.

Thermospheric gravity waves during dust storms
Gravity waves continuosly propagate from the lower atmosphere to the thermosphere. Since global dust storms
significantly change the large-scale dynamical and thermal structure, especially, of the lower atmosphere, it
is expected that the generation, propagation, and dissipation of gravity waves are influenced by dust storms.
Retrievals of gravity wave activity between 20-30 km
from the Mars Climate Sounder (MCS) on board Mars
Reconnaissance Orbiter (MRO) suggested that the gravity wave activity decreases during dust storms [Heavens
et al., 2020]. High-resolution Martian general circulation modeling showed that gravity wave activity actually increases in the upper mesosphere by up to a factor
of two [Kuroda et al., 2020]. More recently, retrievals
of gravity wave activity from density measurements by
NGIMS/MAVEN showed that the GW activity increased
by at least a factor of two in the Martian thermosphere as
shown in Figure 1 (as adapted from the work of Yiğit et al.
[2021]). The relative density fluctuations produced by
gravity waves increased from 14-16% before the onset
of the storm on 1 June 2018 up to 40% during the peak
phase of the storm.

Figure 1: Variation of thermospheric gravity waves activity
during the 2018 planet-encircling dust storm.

Thermospheric gravity waves at solar minimum
MAVEN has been observing the upper atmosphere of
Mars since late-2014 to present. The latter part of the
mission covers the most recent solar minimum, which
provides an unprecedented opportunity to characterize
the Martian gravity wave activity for the first time during low solar irradiation. Previous studies on Earth had
suggested that GW activity is stronger during solar minimum than solar maximum [Yiğit and Medvedev, 2010].
On Mars, Yiğit et al. [2021] have conducted an extensive analysis of gravity wave activity during the last

Gravity waves

(b) Solar Minimum (Solar Cycle 24)
Monthly Mean
Sunspot Number
13-month Running
Mean Sunspot Number

14
12

Sunspot number

minimum. It is seen that the 13-month running mean

Chosen MAVEN solar
minimum observations
used in this study

10
8
6
4
2

20

19
Ma
r
Ma
y
Ju
l
Se
p
No
v
20
20
Ma
r
Ma
y
Ju
l
Se
p

0

Earth Date
Figure 2: Variation of sunspots during the minimum of the
Solar Cycle 24. The temporal range of the chosen MAVEN
data
set to analyze
is shown.
[Yiğit
Figure
4. Altitude,
latitude,thermospheric
local time, and GW
solar activity
zenith angle
variations
of the monthly mean gravity-wave activity in terms of relative density ﬂuctuations from 2019
spring
and summer
May et
to al.,
20202021,
February,
corresponding
to Ls = 18°. 6–158°. 8, representative of northern
Figure
4: Solar
zenithseasons.
angle variation of thermospheric gravFigure
1].
ity wave activity during the solar minimum from May 2019 to
averages
of Distribution
relative density
3.2. 2020.
GlobalMontly
and Local
Time
of fluctuaGravity-wave
shown in Figure 6. Within 2019 December 5, MAVEN has February
tions
due
to
gravity
wave
variations
are
shown.
See
the
legend
Activity
completed
6
orbits,
which
are
represented
by
different
colors
solar minimum. Figure 2 shows the sunspot number
corresponding to different UTs. Except for the longitude in Figure 3 [Yiğit et al., 2021, Figure 4a]

Figure 8 shows the altitude variations between 150 and 230
km of the relative density ﬂuctuations during northern spring
(upper panels) and summer (lower panels), binned as a function
of latitude (left panels) and local
(right& panels).
Yiğit,time
Medvedev,
Hartogh Each
seasonal result is based on a ﬁve-month average, considering
all NGIMS data from 2019 August to 2019 September
(Ls = 18°. 6–86°. 7) and from 2019 October to 2020 February
(Ls = 86°. 8–158°. 8) as representative of northern hemisphere
spring and summer, respectively. MAVEN orbital coverage is
similar globally for the chosen seasons, as shown above in
Figure 2, with a good local time and latitude (75°S–75°N)
coverage.
During northern spring season, GW activity maximizes with
∼26% in the southern hemisphere high latitudes around
160–170 km. Generally, GW-induced ﬂuctuations of density
are much larger in the southern hemisphere, but this difference
should be interpreted with caution since the northern hemisphere mid-latitudes are poorly sampled by MAVEN during

variations in 2019-2020 period and the temporal range
variations, all other geophysical parameters vary to a minor
of the
datasetOverall,
used, which
includes
the solarand
degree
fromMAVEN
orbit to orbit.
the latitudes
of 5–55°N
local times of 20.5–22 hr with solar zenith angles of 100°–120°
The Astrophysical Journal, 920:69 (15pp), 2021 October 20
are observed. The peak density of ∼1.5 × 109 cm−3 is found at
1126 UT at periapsis around −20° longitude and 30°N latitude.
The altitude variations of the GW activity during the six
orbits on December 5 are plotted in Figure 7, presented by
subdividing each orbit into their inbound and outbound passes
shown in panels (a) and (b), respectively. The instantaneous
values of GW activity vary signiﬁcantly (up to ±50%),
occasionally jumping up to 100%. Orbit-to-orbit variations
are noticeable, which points out a longitudinal variability of
GW activity. There are major differences between the inbound
and outbound passes for a given orbit as well, which is
indicative of latitudinal variability since the inbound and
outbound passes correspond to somewhat different latitudes, in
this case varying between 5–30°N and 30–55°N, respectively.

6

Figure 3: Variation of thermospheric gravity wave activity
during the solar minimum from May 2019 to February 2020.
Montly averages of relative density fluctuations due to gravity
wave variations are shown [Yiğit et al., 2021, Figure 4a]

Figure 5: Variation of ing the solar minimum from May 2019
to February 2020. Montly averages of relative density fluctuations due to gravity wave variations are shown [Yiğit et al.,
2021, Figure 4a]

Gravity waves

(a)

Altitude [km]

200
180

180

160

160

140

140

120

120

100

100

80

80

60

60
120

140

160

Altitude [km]

180

Temperature [K]

10

160

160

140

140

120

120

100

100

80

80

60

60
10

10

9

15

Wave amplitude, u 0

20

7

10

10

5

(d)

200
180

5

11

Dissipation and growth rate [m 1]

180

0

Day ( mol)
Night ( mol)
Day ( non)
Night ( non)
Day (1/H)
Night (1/H)

200

(c)

200

(b)

200

25

u0w00 = 10 6 m2s
= 100 km
ci = 120 ms 1

0

5

10
0

2

15

/ [%]

20

Figure 6: Gravity wave activity at night and day as simulated by a column model based on the whole atmosphere gravity wave
parameterization of Yiğit et al. [2008] [Yiğit et al., 2021, Figure 11]

sunspot number reaches a minimum in December 2019.
They have studied the montly mean GW activity during
the solar minimum as a function of altitude, latitude,
local time, and solar zenith angle . Figure 3 presents
the altitude variation of the gravity wave-induced relative density fluctuations as retrieved from the NGIMS
instrument on board MAVEN. Gravity wave activity typically peaks around 160-190 km but varies from month to
month between 5-25%. Figure 4 shows the solar zenith
angle variations of the monthly mean gravity wave activity. Increasing gravity wave activity with increasing
solar zenith angle suggests that the nighttime gravity
wave activity is greater than the daytime one. Figure

5 shows gravity wave induced density fluctuations for
six consecutive MAVEN orbits on 5 December 2019.
Instantaneously GW activity can reach up to 100% with
significant degree of orbit-to-orbit variations, depending
on the altitude. During 5 December 2019, mainly the
longitude varies while the other orbital parameters, such
as latitude and local time do not vary much between
the different orbits. Hence, these variations indicate
the presence of longitudinal variability in thermospheric
GW activity. Overall, these results highlight the variable nature of gravity wave propagation and dissipation
processes in the thermosphere.
The day-night difference in gravity wave activity

INSIGHTS | PERSPECTIVES

Getting water off of Mars

mosphere to the thermosphere (15), where
they can potentially control Jeans escape of
hydrogen to space through wave-induced
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the thermosphere
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higher altitudes only during the perihelion
propagation conditions from the lower at10.1126/science.abg5893

Atmospheric coupling processes that play a major role in the direct transport of water to the thermosphere are
shown. Atmospheric gravity (buoyancy) waves may have a key role in strengthening the meridional circulation
responsible for the upward water transport and in enhancing hydrogen escape to space.

Altitude (kilometers)

modulate the upward transport of water into the thermosphere, to the regions where it can be dissociated into
its constituents, hydrogen and hydroxl. Recent general
circulation modeling studies have provided evidence for
this mechanism [
, 2022]. Hydrogen
can then easily escape to space via Jeans’ escape mechanism.

10 DECEMBER 2021 • VOL 374 ISSUE 6573

Gravity waves and atmospheric escape
Influence of gravity waves on atmospheric escape is a
long-range multi-step process. So far there is a significant degree of evidence not only for the dynamical
importance of gravity wave in the thermosphere, but
also for a potential role of gravity waves in atmospheric
escape on Mars. Figure 7 illustrates how lower atmospheric gravity waves can modulate loss of water on
Mars, as adapted from the work by Yiğit [2021]. When
gravity waves dissipate in the upper atmosphere, they
can alter the mean meridional circulation, which can

GRAPHIC: V. ALTOUNIAN/SCIENCE

shown in Figure 4 is worth revisiting. In order to study
how the gravity wave activity varies as a local time,
[2021] conducted column model simulations
“…hydrogen escape
using the whole atmosphere gravity wave parameterizacannot be fully
tion of
[2008].
Figure
6 presents the altitude
understood
without
variations of temperature,
gravity
considering
lowerwave dissipation rate,
atmospheric
processes.”
amplitude, and relative
density
fluctuations for representative daytime and nighttime. It is seen that that
the nighttime wave amplitude growth rate exceeds the
daytime growth rate in the thermosphere, while the dissipation at nighttime due to molecular viscosity is greater
than the dissipation during daytime, however, the growth
rates exceed the dissipation rates up to 170 km. The ultimate
effect of these differences between the growth
1324
and dissipation during day and night lead to stronger
nighttime gravity wave activity.

Downloaded from https://www.science.org at George Mason University on December 09, 2021

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