ELEVATION DEPENDENT METEOROLOGY IN GALE CRATER
S. D. Guzewich, NASA Goddard Space Flight Center, Greenbelt, MD, USA (scott.d.guzewich@nasa.gov), M.
T. Lemmon, Space Science Institute, College Station, TX, USA, G. Bischof, York University, Toronto, ON,
Canada, E. L. Mason, NASA GSFC/CRESST II/University of MD Baltimore County, Greenbelt, MD, USA, C.
E. Newman, M. I. Richardson, Aeolis Research, Pasadena, CA, USA, M. de la Torre Juárez, Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA, USA.

Introduction: Airflow over and around Mars’
extreme topography drives much of its sensible
weather. On global scales, the large Argyre and Hellas impact basins of the southern hemisphere drive
stationary waves [1-3]. In the northern hemisphere,
the smoother and lower terrain provides channels for
baroclinic and barotropic traveling waves [4-5].
Many of these massive terrain features also force
vertically-propagating gravity and inertia-gravity
waves [6-7]. On smaller scales, craters of all sizes
produce daily- and seasonally-varying patterns of
upslope and downslope winds [8-9] and hydrostatic
adjustments in air pressure [10], resulting in complex
aeolian activity and possibly dispersal of trace gases
from localized sources [11-12].
The Mars Science Laboratory Curiosity rover
landed on the floor of Gale Crater in 2012/Mars Year
(MY) 31 at an elevation of approximately -4500 m
below the martian datum. At the center of Gale,
nearly 6 km higher, lies Aeolis Mons/Mt. Sharp.
This extreme altitude variation over a short distance
produces fascinatingly complicated atmospheric
flows through the crater, which Curiosity has measured to some degree and has been modeled [e.g., 9].
In nearly 10 Earth years on Mars, Curiosity has
climbed approximately 500 m higher on Mt. Sharp
and will continue to ascend in the years ahead. This
change in altitude over 5 Mars years now allows us
to directly interrogate how aspects of the martian
atmosphere change with altitude, a perspective that
has not been previously available to Mars atmospheric science either on the ground or in orbit (due to
much coarser vertical resolution of remote sensing
instruments).
Specifically, we investigate two meteorological
factors that are expected to be functions of altitude:
the amount of dust in the atmosphere and the atmospheric pressure.
Methods: Dust opacity is measured by Curiosity
through direct solar imaging by the Mast Camera
(Mastcam) 880 nm neutral-density-5 extinction filter
[13]. These images are typically taken every 1-3
martian sols, although particular periods have higher
cadences (up to 3/sol), while periods of solar conjunction or rover faults have none for several weeks.
880 nm dust opacity (“tau” for short) throughout the
mission has typically ranged between 0.3 and 1.5
with a very repeatable seasonal cycle outside of sig-

nificant perturbations such as the MY34 global dust
storm [14] or the recent pre-equinox MY36 regional
dust storm.
To complement and extend the opacity record
from Curiosity, we also use Mars Climate Sounder
(MCS) retrievals of dust extinction. MCS retrieves
dust extinction in the thermal infrared with a nominal
vertical resolution of 5 km, but the retrievals are
oversampled to output dust extinction at ~1.5 km
intervals [15]. We use the two-dimensional MCS
retrieval version 5.2 for this work, which provides
increased sensitivity at low altitudes [15]. However,
MCS retrievals frequently do not extend below 20-30
km altitude.
Other meteorological data used in this study has
been collected by the Rover Environmental Monitoring Station (REMS). REMS includes sensors measuring air pressure, air and ground temperature, ultraviolet radiation, relative humidity, and wind [16].
One wind sensor was damaged during landing and
the remaining one was damaged approximately 2.5
Mars years into the mission [11]. REMS measurements are collected for the first 5 minutes of every
hour local mean solar time (LMST) and in rotating 13 hour extended observation blocks every sol [e.g.,
14].
Results: We discuss each result individually.
Dust Scale Height. As previously stated, the atmospheric opacity over Gale Crater is largely repeatable each year outside of large dust storms (Figure
1).

Figure 1. Smoothed 880 nm midday and afternoon
atmospheric opacity measured by Mastcam over the
duration of the MSL Curiosity mission for each Mars

year. The black line represents the multi-year average.
Opacity reaches a minimum each year near Ls = 120140° before increasing during the dusty season. Two
periods of higher opacity occur during the southern
hemisphere spring and summer seasons, corresponding to the “B” and “C” dust storm phases [17]. The
repeatability of the seasonal cycle in dust opacity,
particularly in the less dusty Ls = 340-180° period,
allows us to use Curiosity’s change in altitude over
time to probe how dust opacity decreases with
height. To best isolate dust’s contribution to atmospheric opacity, and reduce or eliminate the contribution from water ice clouds, we only use Mastcam tau
measurements taken after 10 am LMST. We also
remove the four most anomalous dust storms that
Curiosity has experienced: the MY34 global dust
storm, the MY34 large regional dust storm near Ls =
330°, the MY35 southern hemisphere autumn regional dust storm near Ls = 50°, and the recent
MY36 large regional dust storm near Ls = 150°.
Then using the rover’s position information and
combining the opacity measurements into 5° solar
longitude bins, we can determine how opacity has
changed as a function of altitude. In this binning,
MY31 and MY32 measurements are frequent outliers
to an otherwise robust linear trend of opacity as a
function of altitude. In Figure 2, it can be seen that
in the Ls=5-10° time period of MY33-36, opacity has
closely followed a linear trend and implies a dust
scale height of ~2 km.

Figure 2. MSL opacity over altitude in the Ls = 510° solar longitude bin. The dashed line represents
the linear fit to the opacity trend with altitude in
MY33-36, which has a chi-squared statistic of
0.0038 and results in a dust scale height of ~2 km.
This pattern is common to most 5° Ls periods during
the martian year outside of the dust storm season of
Ls = 180-330°. During the dust storm season, there
is frequently little or no trend in opacity over altitude, which is expected due to the variability in dust
activity during this season. To make our results

more robust, we only calculate dust scale height for
5° solar longitude bins that contain at least 3 Mars
years with opacity measurements that fall on a linear
trend line with a chi-squared statistic of less than
5*10-4.
Figure 3 presents the results for dust scale height
across the entire martian year. Dust is strongly vertically confined within Gale Crater for most of the
year, with dust scale heights near 2 km outside of the
dust storm season. The increase to 4-6 km in the Ls
= 90-120° period could either reflect a true increase
in dust scale height or may be due to contamination
by daytime aphelion cloud belt water ice clouds.

Figure 3. Dust scale height within Gale Crater determined by the change in Mastcam opacity as a
function of altitude over 5 Mars years. Each 5° solar
longitude bin’s value is indicated by a cross with the
black line representing a smoothed running mean.
Solar longitude bins without at least 3 Mars years of
values or an insufficiently low chi-squared statistic
are excluded.
The same process can be completed for MCS
retrievals of dust extinction (converted to 880 nm
opacity). Note that the lowest retrieved altitude is
typically 5-10 km above the martian datum, or 9-14
km above Curiosity’s location. We find dust scale
heights typically less than the pressure scale height of
approximately 11 km. More typical values for MCS
retrieved dust scale height are 5-8 km, but occasionally values as low as 2-4 km are seen in northern
spring and summer. By extrapolating the MSL dust
opacity trend line to the lowest retrieved MCS altitude, we can infer what the dust scale height would
be in order for the MSL and MCS observations to be
self-consistent. This follows work done in [18] to
infer the vertical dust profile over Gale Crater. Connecting these two observed regions of atmosphere
implies that while dust is vertically confined within
Gale Crater itself, above Gale Crater, dust is wellmixed (with dust scale heights of approximately 11
km) for 1-2 atmospheric pressure scale heights.

Hydrostatic Pressure Adjustment. Surface air
pressure on Mars varies widely over a given sol due
to atmospheric tides [8]. The exact cycle of pressure
at a given location is a combination of multiple different tides with periods that are integer fractions of
a solar day. The most prominent are the diurnal
(once per sol) and semidiurnal (twice per sol). Within topographic depressions such as craters, however,
there is an additional enhancement to the daily pressure cycle due to “hydrostatic adjustment” [8].
Richardson and Newman [2018] describe the physics
of this process in detail. Essentially, variations in
surface air temperature over the course of the day
drive lateral atmospheric flows to maintain hydrostatic balance across elevation. They estimate that as
much as 15 Pa variations around the daily mean
pressure are due to this process, which is additive to
the planetary-scale diurnal tide.
Figure 4 shows how the diurnal pressure tide
(representing the total diurnal period variation in
pressure, i.e., the planetary-scale tide plus any local
hydrostatic adjustment term) has varied each Mars
year in the Ls = 340-90° period. We choose this period for being largely free of any significant dust
storm activity around the planet which can alter the
amplitudes of the tides [19]. It can be seen that while
the pattern of variation over time in each Mars year
is quite similar, the amplitude of the tide does vary
year-over-year. This is compared to the semidiurnal
tide, which has very little year-to-year variation.
Presumably, some of this is directly due to the
change in mean surface pressure over ~500 m of
altitude, but there should also be a change in the hydrostatic adjustment forcing.

change in PDRSP reflects either a change in the
global-scale diurnal tide or in the changing hydrostatic adjustment term. By calculating this term in the
clearer southern late summer and fall, we ideally
minimize any interannual variability in the globalscale tide and assume any change in PDRSP over
altitude is due to the hydrostatic adjustment forcing
decreasing. In Figure 5, we plot PDRSP over altitude in two solar longitude bins as an example. The
change in PDRSP falls closely to a linear trend with
altitude over 5 Mars years. The slopes in these two
periods are typical of the period analyzed and have
slopes of ~1 %/km.
Figure 6 presents the results for each 5° solar
longitude period and shows a mean change in hydrostatic adjustment forcing of ~1% of PDRSP per km
of altitude. This can be directly compared to Figure
5a-b of Richardson and Newman [2018] which
shows ~1% change in PDRSP over 500 m of altitude
over the Curiosity traverse route.

Figure 5. The change in PDRSP over rover altitude
in the Ls = 5-10° and 10-15° periods for each Mars
year. The linear fit is included in a dashed line with
the slope (%/km) and chi-squared statistic indicated
on each panel.

Figure 4. Diurnal and semidiurnal pressure tide amplitude (Pa) as measured by REMS in the Ls = 34090° period of each Mars year.
We normalize the change in diurnal pressure tide
change by dividing by the mean surface pressure to
compute the percentage of daily range of surface
pressure (PDRSP) [8]. As it is reflects the hydrostatic change in surface pressure over altitude, any

Figure 6. The change in hydrostatic adjustment forc-

ing (%/km) to the diurnal pressure range over the
MSL mission in each 5° solar longitude bin from Ls
= 340-90°. The mean value is ~1 %/km.
Conclusions: Curiosity’s ascent up Mt. Sharp allows us to directly interrogate elevation-dependent
atmospheric phenomena. We have measured the
change in dust opacity and atmospheric pressure as a
function of elevation over the 5 Mars year mission
lifetime to-date.
We find that over much of the year, dust must be
strongly vertically confined within Gale Crater, with
a dust scale height of ~2 km. This is much lower
than the atmospheric pressure scale height. During
the dustier second half of the martian year, the dust
scale height is more variable and often matches or
exceeds the pressure scale height.
Second, using routine measurements of atmospheric pressure by REMS, we can isolate the change
in hydrostatic adjustment forcing to the daily pressure cycle in Gale Crater. We find this factor changes by ~1%/km of elevation gain.
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