SEASONAL VARIATION OF THE COLD AND BRIGHT ANAMOLIES ON
THE NORTH POLAR LAYED DEPOSITS
P. J. Acharya1 (pruthvi1@yorku.ca), I. B. Smith1 (ibsmith@yorku.ca), 1York University, Toronto, Ontario
Introduction: The north polar region of Mars is
a very active region, from dust storms on the scale of
5000 km to polar cliff avalanches on the scale of a
few meters. In the northern winter and summer, during the formation and the sublimation of the seasonal
frost cap, this region is responsible for a significant
change in the atmospheric pressure [1]. Due to this,
the seasonal variation of this region strongly affects
variation on the entire planet. A strong understanding of this region will help us predict the future climate for Martian exploration and understand the
climate in the past. There are many features and processes that are poorly classified. We focused on the
Cold and Bright Anomalies (CABA), first observed
by thermal observations [2,3].
The CABA are cold and bright regions that appear in visual and thermal data annually during early
summer [2]. There are so named because they respond differently from their surroundings that appear
to have otherwise similar properties. Equally anomalous, during late summer, the CABA rapidly undergo
a reduction in albedo, over just a few hours - becoming the darkest locations on the polar cap [2,3]. We
used observations from Mars Color Imager
(MARCI), Thermal Emission Imaging System
(THEMIS), and Mars Orbiter Laser Altimeter
(MOLA) to build a timeline and interpret CABA.
CABA MARCI Timeline: Before Ls 85,
the albedo of the CABA and their surroundings increases approximately linearly with time (Fig 1A)
and no albedo difference is observed between the
two regions (Fig 1B). At Ls 85/86, the CABA’s albedo begins to diverge from neighboring regions
(Fig 1B). At the same time, low albedo dust streaks
related with katabatic wind activity appear near spiral troughs [4] The streaks are less prominent or not
observed at the CABA locations (Fig 2A).
From Ls 97 to Ls 106/107, the CABA continue brightening, whereas the surrounding region’s
albedo is decreasing (Fig 1), reaching peak albedo
contrast at Ls 106/107 (Fig 1,2B). Then, until Ls 130
the albedo contrast begins the decrease due to capwide refrosting (Figs 1B,2C). During the refrosting
period, the CABA’s albedo continues to increase at a
linear rate however, the albedo of their surroundings
increases at a faster rate (Fig 1) [5,6].

Figure 1: Relative albedo measurement of the
CABA and surroundings. Data collected by taking
the mean pixel value of the yellow region seen in
Figure 2 and 4A. A) Relative albedo from Ls 70 to Ls
170. B) Albedo difference between the CABA and a
nearby normal region. Max albedo contrast at Ls
105, green arrow, major darkening events seen at Ls
150 and Ls 161, red arrows. At Ls 160, the entire yellow region has darkened.
Around Ls 138/140, widespread dust storms are
seen over the cap (Fig 2D), and some of the CABA
darken quickly in a matter of hours on the scale of a
few kilometers (Fig 1). The mean pixel value, which
is proportional to the albedo and the reflected light,
drops by a factor of 31% while surrounding remains
the same (Fig 1). Other darkening events have been
observed at Ls 150/151 and Ls 161/162 (Figs 1,
2E,2F). Each darkening event is more widespread,
and the dust storms are more intense. From some
observations, especially from MY 31 and MY 32,
the darkened regions cover half the residual cap (Fig
2F).
CABA THEMIS Timeline: Before Ls
83/84, there is no temperature difference observed
between the CABA and their surroundings, both
regions warm linearly (Fig 3B). At Ls 83/84, the
CABA begins to diverge from their surroundings and
warm slower than their surroundings (Fig 3). This
continues until Ls 104/105, when the temperature
difference between the two region is at a maximum,
the CABA are about ~20 K cooler than their surroundings (Fig 3). After this, both regions begin to
cool down due to refrosting, however the CABA
cool more slowly (Fig 3) [5,6]. Around Ls 140, at the
time of the first darkening observed by MARCI
(Figs 1, 2D), the CABA are about ~3-5 K warmer
than their surroundings (Fig 3).
From THEMIS visual observations, we find that
all the darkened CABA have a bright halo surroundings them (Fig 4A).

Figure 2: MARCI MY 32 observations. A) Ls 86.4, least amount of dust streaks seen at the CABA locations. B)
Ls 106.8, peak albedo contrast. C) Ls 131.0, albedo contrast has decreased due to refrosting. D) Ls 137.8, first
darkening event which dust storms seen around the cap. E) Ls 150.3, second darkening event with more intense
dust storm. F) Ls 161.8, half the residual cap is darkened with more intense dust storm. Red outlines the most
common locations for the CABA, black boxes outline the location for the strongest dust streaks and the yellow
box outlines the region used for temperature and albedo observations.
higher than the surrounding terrain. In the visible
spectrum, a halo is formed on either side of this rise
(Fig 4B).

Figure 3: THEMIS temperature measurements. A)
Temperature Ls 60 to Ls 163. B) Temperature difference between the CABA and a nearby normal region.
Peak temperature at Ls 104 (green) the darkened
CABA is warmer at Ls 140 (red). Measurements taken from regions outlined in Figure 2 and 4A.
Comparison with MOLA: The darkened
CABA form on a topographic rise, ~30-50 meters

Figure 2: CABA outlined in yellow in Figure 2. A)
THEMIS Visual observation (V55595018). B) MOLA
topography. Halos seen surrounding the CABA, orange arrow, and found on either side of the darkened
CABA.
CABA Hypothesis/Interpretation: During
springtime, between Ls 70 and Ls 90 and before the

CABA are observed by MARCI or THEMIS, there
are fast surface winds called katabatic winds. These
winds peak at Ls 83 [7,8] and due to their proximity
to the surface, they remove surface ice by two
means: shear and enhanced sublimation caused by
forced convection that removes water vapor [6,9].
From THEMIS observations of clouds associated
with katabatic winds and from detailed Mars
Mesoscale Simulations [7,9], we can see that CABA
location have the slowest katabatic winds and the
least amount of katabatic storm clouds (Fig 5).

Figure 3: A) MARCI MY 32 Ls 86.4. B) THEMIS
katabatic cloud observations from Ls 30 to Ls 120 C)
Detailed Mars Mesoscale Simulation of the surface
wind speed at Ls 50 [7], yellow region outlines the
region seen in Figure 4A. Black arrows show the
locations of the most intense katabatic clouds and
dust streaks. Red boxes outline the most common
CABA locations.
We interpret this to mean that the CABA locations are relatively calm during katabatic wind season, reducing the removal of fresh small-grained ice.
Neighboring regions, that exhibit dust streaks that
lower surface albedo and provide evidence for the
removal of small-grained ice (Fig 6), trap more radiative flux from the Sun. For the CABA locations that
retained fine-grained surface ice, the surface albedo
continues to increase, which decreases the sublimation rate and provides a means by which they remain
colder during summertime (Fig 3).

Figure 4: Heuristic model to explain the change in
albedo. Greater number of arrows indicates faster
winds (purple) and enhances removal of smallgrained ice (red). A) Faster wind speeds lead to less
surface ice and higher shear/sublimation rates. B)
Lower wind speeds lead to more surface ice and
lower shear/sublimation rates
During refrosting, which is a well documented

period [5,6], the higher reflectance CABA act as
cold traps for the sublimated water ice (Fig 7), creating a positive feedback loop and increasing the albedo and temperature contrast (Figs 1,3, 2A, B).

Figure 5: Heuristic model to explain the cold trapping process. Sublimated water ice from surroundings regions (red) gets condensed from the atmosphere as small-grained ice (blue). This will reflect
more incoming solar flux (yellow) and create a feedback loop.
After peak mid-summer temperatures, when
the cap is cooling, the CABA would cool down
slower because of Newton’s Law of Cooling that
states the rate of cooling is proportional to the differences in the final and starting temperatures. The
starting temperatures are ~200 K and ~220 K for the
CABA and their surroundings respectively (Fig 3).
Additionally, the albedo contrast will decrease because the CABA are already covered in smallgrained reflective ice and the neighboring regions are
refrosting (Fig 1, 2C).
Darkening Hypothesis: During late summer,
when refrosting has stopped, we hypothesize that
surface winds in part driven by large scale eddies
[10] rapidly strip small-grained ice and transport
them into the atmosphere, lowering the albedo by
exposing dustier, larger grained ice. This happens
extremely rapidly, within hours (Fig 8). This layer
would be warmer than the surrounding because the
heat trapped during summertime (Fig 7). Additionally, as the small-grained ice is lifted, there would be
regions on either side of the topographical rise where
the ice remains on the surface (Fig 8). This would be
the halos seen by THEMIS visual images (Fig 4A).

Figure 6: Heuristic model to explain the darkening
process. Winds (purple) traveling over the refrosted
topographical rise speed up and increase the shear
rate. This removes surface ice (green) and leaving
halos on either side, outlined in red.
The darkened CABA location we studied, seen
in Figure 2, has areas of 136 km2, 6,030 km2 and
156,551 km2 at Ls 140, Ls 151 and Ls 162 respectively. Assuming the ice layer is between 1 and 5 mm,
the average grain size is 10 μm [6] and a density of
0.01 kg m-3 [11], we calculate the amount of ice lifted is ~4,080 kg, ~176,820 kg and ~4,519,710 kg at

Ls 140, Ls 151 and Ls 162 respectively. The mass
estimation assumes once the region has darkened no
more ice is removed from these locations and the
areas are only calculated for the darkened areas seen
by MARCI, the true number is most likely within an
order of magnitude.
Future Work: In order to further test these
hypotheses, we plan to study mesoscale, near-surface
winds with Le Laboratoire de Météorologie Dynamique (LMD) Mars Mesoscale Model over the
CABA and neighboring locations. Simulations will
have resolution better than 15 km from Ls 83 to Ls
170. No high-resolution simulations have been published for each Ls date after Ls 140, leaving a void in
the study of CABA and surface processes in general.
We plan to use these simulations to determine wind
speed, wind direction, and shear stress for removing
fine grained particles. We also plan to increase visual
and thermal observations of the CABA locations
after Ls 140. There are few thermal and visual observations for us to use, and more observations will lead
to a better understanding of factors governing the
CABA.
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