THE IMPACT OF SHADOWS IN A MESOSCALE CLIMATE MODEL ON
THE ATMOSPHERE-SURFACE INTERACTIONS RELATING TO ICERICH LANDFORMS
L.-A. Foley, School of Physical Sciences, Open University, UK (lori-ann.foley@open.ac.uk), S.R. Lewis, M.R.
Balme, J. Holmes, School of Physical Sciences, Open University, UK.

Introduction:
The amount of shadow cast on a landscape varies
with, inter alia, latitude and topography (e.g. 1–3),
and when shadow is included in a climate model it
can influence atmospheric variables such as surface
temperature, wind speed and wind direction. This
can in turn influence how the deposition and evolution over time of water, ice or snow are represented
in models of martian surface-atmosphere interaction.
Surface features that appear to be heavily influenced by the atmospheric conditions and the location’s topography include viscous flow features
(VFFs). Interpreted as debris-covered glaciers4, they
undergo a slow, plastic deformation due to gravity,
resulting in the movement of ice and debris down the
steep slopes on which they are preferentially found5–
7
. Their evolution is distinct from other nearby icerich landforms that exist in different topographic
settings. Characteristics of VFFs include evidence of
flow around or over obstacles7, surface lineations5,
compressional ridges5, extensional troughs7, and a
lobate plan-view morphology5. The causes of their
distinctive morphology and location are linked to the
atmospheric conditions they experience.
This study focuses on Lyot crater, Mars, a
∼215‐ km diameter impact crater centered at
50.5°N, 29.3°E (Figure 1). The crater floor is ∼3000
m below the surrounding landscape and ∼7000 m
below Mars Orbiter Laser Altimeter (MOLA) datum,
making it the lowest point in the northern hemisphere8. It has a relatively young age, and is thought
to have formed between the early Amazonian (1.6
Gyr 9) and the late Hesperian (3.4 Gyr ago10). Lyot
crater contains many geomorphological indicators of
surface ice and ice melt, both past and present8.
Lobate features with convex-outward ridges and
convex-upward profiles, identified as VFFs, can be
seen predominantly in the south of the crater along
the steep slopes of the crater rim and inner peak ring
(Figure 2)8,11. VFF formation is believed to be temperature dependent7 and such slopes are largely
shadowed, protecting the ice-rich landforms found
there from sublimation due to solar insolation.
Ridges found on the main body of the VFFs are interpreted as compressional ridges, indicating the ductile flow of material5,7. The aim of this study is to use
a high resolution climate model to determine when
and how VFFs formed, and why they form in these
specific locations, as opposed to elsewhere in the

crater.

Figure 1: MOLA topographic map showing Lyot
crater with the approximate locations of the outer
ejecta extent, inner ejecta extent, crater rim, inner
peak-ring and central peak marked by black lines.
From 11.

Methodology:
Simulations were run using the Laboratoire de
Météorologie Dynamique (LMD) Mars Mesoscale
Model (MMM)12, varying obliquity (the tilt of the
planet’s axis), eccentricity (how elliptical a planet’s
orbit around the sun is) and perihelion (the time of
year when the planet is closest to the sun). Obliquities used ranged from 5° to 55°, in 10° intervals. Eccentricity was set to be either the current value
(0.093) or zero. Perihelion was run at aerocentric
longitudes (Ls) of 251° (the current value, close to
northern hemisphere winter solstice of 270°) or 90°
(northern hemisphere summer solstice). The ice
source was set to be:
 at both poles at obliquities of 5° and 15°;
 at the north pole only when obliquity was
25° or 35° and perihelion was at Ls = 251°;
 at the south pole only when obliquity was
25° or 35° and perihelion was at Ls = 90°;
 in the Tharsis region when obliquity was 45°
or 55°.
The MMM simulations were performed for a duration of 10 days at three times of year: mid-summer
(Ls = 110°), late autumn (Ls = 230°) and early spring
(Ls = 350°). Boundary conditions were supplied at

hourly intervals from a simulation run using the
Open University version of the LMD Global Climate
Model (GCM; for a full description of the model and
its dust and water packages see 13–17). The GCM simulations were spun-up for 12 years until the simulation was running with an equilibrated water cycle.
The data passed from the GCM to the MMM was on
a common horizontal grid, where the GCM had a
resolution of approximately 2.50° x 2.50° latitude
and longitude, respectively, and the MMM used 6
km x 6 km grid boxes. The atmosphere was divided
into 41 vertical levels, giving data up to 80 km. Both

dust and clouds were radiatively inactive. The GCM
was forced using data reflecting a year when there
was no global dust storm.
Results:
In this study, the evolution of VFFs in Lyot crater
has been examined both over one present-day Mars
year (Figure 2) and using varying orbital parameters
to constrain how past climate conditions affected
them.
In late autumn, shadow falls to the south of higher ground, causing cooler stable surface temperatures

Mid-summer: Ls = 110°

Late autumn: Ls = 230°

Early spring: Ls = 350°

Figure 2: Results from the simulation carried out under current climatic conditions. Column 1 shows the
maximum amount of shadow over one martian sol. Column 2 shows the change in ice depth over the period.
Column 3 shows the surface temperature at 16:00. In columns 2 and 3, the pink features are the viscous flow
features, as mapped by 11. Note the colorbar range varies in columns 2 and 3.

throughout the day and leading to greater water ice
deposition, especially as obliquity increases. There is
also shadow on the east and west sides of the crater
at the start and end of the day, respectively, leading
to larger variation of surface temperature throughout
the day. Hence, deposition of water ice is heaviest in
the south, especially on the shadowed crater rim.
Generally, the surface temperatures are cooler where
VFFs are found, surrounded by unshadowed warmer
areas.
In the early spring, shadow predominates in the
south/south-west of the crater, with water ice deposition due to the cooler stable surface temperatures
throughout day. The start and end of the day again
experience shadow on the east and west sides of the
crater, respectively, with larger changes of surface
temperature there throughout the day. While surface
water ice is still deposited at all obliquities, there is
some sublimation as well. At lower obliquities (15°
and 25°), water ice deposition is specific to east
and/or west VFF areas, with only light sublimation in
west VFF areas. Patches of cooler surface temperature match VFF locations and are again surrounded
by areas with higher temperatures. The higher temperatures mean more sublimation occurs in unshadowed areas, while the cool patches at VFF locations
allow for more deposition, less sublimation, and perhaps even enhanced ice flow. At higher obliquities
(35°, 45°, 55°) widespread water ice deposition occurs, with the thickest ice forming in the south and
east VFF areas where shadow is most common.
There is some sublimation in the south and west VFF
areas. A moderate surface temperature occurs
throughout the day in all VFF locations, with cooler
patches in the south and east VFF areas, which again
promotes water ice deposition.
In the mid-summer, shadow occurs on the east
and west sides of the crater, which also experience a
greater range of surface temperatures throughout the
day. The southern part of the crater receives more
direct solar insolation so it is generally warmer but
has a more stable range of surface temperature
throughout the day. Sublimation occurs at all obliquities, with less in VFF-hosting areas due to shadowing
and cooler surface temperatures. The sublimation is
greatest at the north crater rim at low to moderate
obliquities (15° to 35°), when there are cooler surface temperatures in the VFF areas; sublimation is
greatest at the south crater rim at higher obliquities
(45° and 55°).
Discussion and conclusions:
Surface water ice mantles much of Lyot crater8,11
but water ice deposited on the heavily shadowed
steep slopes evolved differently over time compared
to the mantle in flatter or less shadowed locations.
Steeper terrains, especially in the south of the crater,
are shadowed more consistently than other areas in
the crater at all times of year and for all orbital pa-

rameters examined in this study. This has an impact
on the surface temperature in these areas, with the
average temperature in the most highly shadowed
regions being up to 10 K lower than in nonshadowed regions at the same latitude and elevation.
This in turn creates conditions which are more conducive for water ice accumulation when it is cold;
when it is warmer the icy landforms are relatively
protected from sublimation and may experience enhanced ice flow. These conditions lead to the formation of VFFs7. So not only are these the areas in
Lyot crater where we would expect VFFs to form,
they are the areas where VFFs have been mapped
today8,11. VFF-rich sites predominate at the midlatitudes4,7, at locations like Lyot crater, with its
steep slopes creating shadowed areas which affect
surface temperature and ice evolution, allowing
VFFs to form.
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