SEASONAL AND INTERANNUAL VARIABILITY IN CO2 SNOWFALL
WITHIN THE MARTIAN NORTH POLAR VORTEX
N. R. Alsaeed, P. O. Hayne, V. Concepcion, Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, USA (noora.alsaeed@colorado.edu)
Introduction: The polar regions on Mars play an
important role in the thermal regulation and circulation of the atmosphere. In the winter, strong westerly
jets isolate the cold air above the poles from lower latitudes. This aptly named polar vortex inhibits
transport of dust and other condensates into the poles
[1,2]. Temperatures within the polar vortex boundary
drop to the freezing point of carbon dioxide, allowing
it to condense out both directly onto the surface and
as snowfall [3]. As it is the main constituent of the
Martian atmosphere, the surface-atmosphere exchange of CO2 is one of the fundamental processes in
both the present and past climate on Mars, mainly due
to its ability to significantly alter the atmospheric
pressure and heat balance of the planet on seasonal
and orbital timescales [4,5]. It is therefore important
to understand what forces affect the dynamics of the
polar vortex and subsequently the atmospheric processes such as snowfall that occur within it.
In this work we use atmospheric retrievals of temperature and CO2 ice cloud opacity from the Mars Climate Sounder (MCS) on board NASA’s Mars Reconnaissance Orbiter (MRO) to analyze patterns in the
winter polar vortex and CO2 ice clouds over the north
pole for Mars years (MY) 29 to 34. We also couple
the MCS data with a simple snowfall model to determine precipitation rates of CO2 snow and analyze patterns in the amounts and distribution of snowfall.
Data and Methods: We use data from MCS, an
infrared radiometer that measures the thermal emission of the atmosphere to retrieve vertical profiles of
temperature, dust, water vapor and other condensates
[6]. In this work we use retrievals of the pressure, temperature, and dust opacities [7] which are converted
to CO2 ice opacities as described in [8]. We limit the
data to the northern polar winter between LS = 180°
and 360° with latitudes above 65 N for MY 29 to 34.
For each LS in each year, we convert the CO2 opacities
to CO2 density profiles, and use different binning procedures in latitude and longitude to parse out patterns
in the data.
In our analysis of the polar vortex, we bin the temperature between 10 and 20 km above the surface into
5-degree latitude and longitude bins for each LS and
define the vortex boundary at T = 170 K. We do this
for the entirety of northern winter for all Mars years
between MY 29 and 34 and observe changes in the
area covered by the polar vortex over seasonal and annual timescales. Results are shown in figures 1 and 2.
For the snowfall analysis, we input the CO2

profiles into a snowfall model to track deposition onto
the surface. The snowfall model we use is a cloud settling model described by a Fokker-Planck-Smoluchowski diffusion equation which incorporates both
diffusion and gravitational settling of the ice particles
[9]. We aggregate the snowfall amounts over the entire winter season for each Mars year.
Results: In our analysis, four main patterns
emerge: 1) There is strong coupling between dust activity in the southern hemisphere and polar vortex dynamics in the northern polar winter. 2) The stable polar vortex tends towards an elliptical wave 2 shape. 3)
Snowfall rates vary with dust activity from both localized and global dust storms. 4) There is a persistent
longitudinal stationary wave in CO2 ice cloud densities within the polar vortex.

Figure 1: Northern polar vortex area (black) and mean temperature (red) for MY 29 to 34. The light purple shaded areas correspond to weaker dust events where the dust opacity
(scaled to 610 Pa) was larger than 0.25, and darker purple
areas correspond to stronger dust events where the dust
opacity was larger than 0.375. The polar vortex boundary is
defined to be at T = 170 K. The area is given as a fraction
of the total area encompassed above 50 N. Dust information
was sourced from the Mars Climate Database [10].

Discussion:
Polar vortex coupling with dust activity. Unlike
the polar vortex on Earth, which grows during the
winter season, the northern polar vortex on Mars
shrinks in size multiple times throughout the winter
[2]. As described in previous work [11, 12], this
shrinking is due to the presence of a dust cycle on
Mars. Figure 1 shows the strong coupling between
dust events in the southern hemisphere and the shrinking of the polar vortex in the northern pole. We observe that the reduction in size of the polar vortex is
also coupled with warmer temperatures within the
vortex, and enhanced snowfall as shown in figure 4.

Snowfall variability with dust activity. Our results
show active snowfall throughout the entirety of the
northern winter season, with slight year to year variation in the total amounts of snowfall as shown in table
1 and figure 3. We find that there is significant and
measurable interannual variability in the snowfall,
with most snowfall occurring in MY 29 followed by
a gradual decline. This is of note as it follows the
planet-encircling dust storm of MY 28.
Table 1: Total CO2 ice deposition due to snowfall at
the end of northern winter.
Mars
Year

CO2 ice mass deposited (kg)

CO2 ice equivalent
thickness (cm)

29

1.23e15

13

30

1.18e15

13

31

1.25e15

13

32

1.03e15

11

33

1.05e15

11

34

1.02e15

11

Figure 2: Shape of the north polar vortex given by the
boundary lines between MY 29 to 33. The boundaries were
found by taking the temperature contour at T = 170 K for
each LS, over-plotted with a normalized color bar.

Elliptical wave 2 shape is most stable. The north
polar vortex on Mars displays a unique characteristic
wave 2 shape. This shape has been observed and modeled before and is likely driven by topography in the
polar region [1,2]. It is characterized by an ellipticity
of roughly 0.5, with the semi-major axis pointing in
the direction of 150 E. We find that this elliptical
shape is disrupted during strong dust storms when the
vortex shrinks in size. We believe this disruption may
be a driving factor in the variation of snowfall discussed in the next section.

Figure 3: Total CO2 snowfall deposited onto the northern
polar surface at the end of winter as a function of latitude.

Figure 4: CO2 snowfall deposition per latitude onto the
northern polar surface as a function of LS. Plotted values are
the median deposition for MY 29 to 33.

Within each winter season, figure 4 shows clear
enhancement in the amount of snowfall around LS
220° and at LS 320°, timeframes that align with the
categorized B and C dust storms that kick off in the
south [13]. This may indicate that smaller dust storms
enhance snowfall by allowing more dust to be injected
into the polar vortex area at its boundary which serve
as condensation nuclei for ice to precipitate out. However, when parsing it out by year, not every dust storm
enhances snowfall. The more likely scenario is that
the dust storms introduce instabilities within the polar
vortex which can create localized pockets of
cooler/warmer air that could affect CO2 condensation
either way.

Figure 5: Winter CO2 ice cloud column densities as a function of longitude. The column densities span 10 to 60 km
above the polar surface and are averaged over 80° – 90° N
for the entire winter season LS 180° – 360° of each year.

Longitudinal stationary wave. Within the polar
vortex region, a clear longitudinal wave pattern
emerges in the density of the CO2 clouds, as seen in
figure 5. This pattern is persistent from one Mars year
to the next and enhances during the global dust storm
in MY 34. The stationary planetary wave is likely a
forced oscillation driven by topography [14], which
possibly affects the ellipticity of the vortex as well [2].
Acknowledgements: Part of this work was supported
by NASA through the MRO project and by the United
Arab Emirates Mohammed bin Rashid Space Center
through support for N. R. Alsaeed.
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