THE EFFECT OF GROUND ICE MIGRATION ON THE MARTIAN PALEOCARBON DIOXIDE BUDGET
E. David, O. Aharonson*, E. Vos, Weizmann Institute of Science, Rehovot, Israel (elad.david@weizmann.ac.il),
*also at: Planetary Science Institute, Tucson, Arizona, F. Forget, Laboratoire de Météorologie Dynamique, Jussieu, Paris, France.

Introduction:
The mass and distribution of the CO2 and H2O
ground ice (GI) reservoirs on Mars evolve in response
to the evolving orbit of Mars, which is characterized
by oscillations in eccentricity (e), longitude of perihelion (Lp) and obliquity (ε) [1]. The seasonal condensation/sublimation CO2 cycle intensifies with rising
obliquity, as more CO2 mass exchanges seasonally
between surface and atmosphere; it considerably subdues as obliquity decreases, at low enough obliquity
reaching atmospheric collapse and the formation of
massive CO2 deposits [2,3]. Similarly, ground ice extends equatorward at high obliquity, as mid-latitudes
become more humid and receive less insolation, and
recedes back poleward with decreasing obliquity
[4,5].
In addition to the individual evolution of the two
reservoirs, they also interact with each other: due to
the high thermal conductivity of water ice, ground ice
on Mars acts as a heat sink during the spring/summer
months and as a heat source during autumn/winter,
thereby inhibiting the accumulation of CO2 and increasing atmospheric pressure [6]. In this work, we
seek to provide a systematic analysis of the coupling
of these two phenomena, i.e., the effect of the orbitally-forced redistribution of ground ice on the evolution of the CO2 budget.

Methods:
We use the Global Climate Model developed by
the Laboratoire de M'et'eorologie Dynamique, CNRS,
Paris (LMD-GCM) [7] in order to simulate the paleoclimate with different orbital parameters and ground
ice distributions of choice. The model operates on a
three-dimensional 64x64x29 grid (resolution of
2.8125º latitude and 5.625º longitude) and calculates
the temporal evolution of variables such as temperature, atmospheric pressure, and various tracers. We
run the GCM for an array of orbital parameters within
the range of the past 1.5 Ma (0 ≤ e ≤ 0.12, 0º ≤ Lp ≤
360º, 15º ≤ ε ≤ 35º) [1] for two cases (Figure 1): (1) a
study case where ground ice distribution changes according to the orbital configuration (Equilibrium GI
scenario, or EqGI) and (2) a control case where
ground ice distribution is constant regardless of orbital configuration (Static GI scenario, or StGI). Comparing the two scenarios underlines the contribution
of ground ice redistribution to the CO2 evolution.

Figure 1: Schematic illustration of the equilibrium
ground ice (EqGI; left) and the static ground ice (StGI;
right) scenarios

To calculate the equilibrium ground ice distribution at each orbital configuration for the EqGI scenario, we use the Mars Subsurface Ice Model
(MSIM), a one-dimensional equilibrium ground ice
model, developed by Schorghofer & Aharonson [8].
The model calculates the depth of an ice table at icevapor equilibrium with the atmosphere under a dry
regolith layer. Mean annual daytime water vapor pressure, a critical parameter for MSIM, is derived from
GCM simulations (after a spin-up time of 15 Marsyears). MSIM ice table depths are interpolated to the
GCM subsurface grid consisting of 18 grid points of
exponentially growing depth from 0.14 mm to 18 m.
GCM ground ice is represented in terms of thermal
inertia (𝐼) with the relation between pore fill fraction
and thermal inertia following previous formulation
[8]. Pore space (Φ = 0.4) is completely filled with ice
under the ice table, as expected in a state of full icevapor equilibrium. At this stage of the study, we calculate the zonal mean ground ice distribution and expand it for all longitudes. For the StGI scenario, we
use a ground ice distribution based on MONS observations [9] (this distribution has been previously incorporated in the LMD-GCM and has been calibrated
to reproduce Viking seasonal surface pressure [10]).
Results:
Effect of ground ice equilibrium distribution at
low and high obliquities: Figure 2 shows the seasonal
evolution of CO2 ice accumulation at EqGI and StGI.
In EqGI, the extent of the equilibrium ground ice
roughly matches the extent of the seasonal CO2 cap.
At obliquity 20º the seasonal cap is mainly restricted

Figure 2: Seasonal evolution of zonal mean CO2 accumulation at Mars year 7, e = 0, ε = 20º (top) and ε = 35º (bottom);
dashed/solid CO2 mass correspond to StGI/EqGI scenarios; dashed/dotted ground ice profiles (black) correspond to static ground
ice at longitude -151º/135º.

to latitudes >60º at both poles, and the effect of
ground ice migration on accumulation is small. The
small decrease in accumulation in EqGI can be attributed to shallower burial depth of the equilibrium
ground ice relative to the static case. At obliquity 35º,
the seasonal cap extends toward the equator as a
greater range of latitudes experiences sub-CO2 frost
point temperatures. Ground ice now not only extends
to latitudes where it was previously absent, but also
becomes shallower at higher latitudes. It therefore has
a more substantial effect on the CO2 accumulation at
all latitudes.

effects of ground ice migration discussed above can
be seen from the perspective of volatile budget evolution: decrease in the seasonally accumulating mass
and attenuation of the orbitally-forced intensive oscillations in volatile budget. Finally, the bottom plot of
Figure 4 shows the evolution of seasonal surface pressure variations (global mean), with ground ice migration considerably decreasing the seasonal amplitude

Seasonal CO2 accumulation over the orbital parameters space: Total seasonal accumulation from all
simulations is summarized in Figure 3. This perspective allows us to look not only at the exchangeable
mass at individual orbital configurations (which is
lower for EqGI, as expected), but also the rate of
change of exchangeable CO2 mass with orbital parameters, which decreases due to ground ice migration.
The difference between scenarios increases with
obliquity, as exchangeable CO2 mass grows without
the inhibiting effect of ground ice. Exchangeable
mass is larger when Lp coincides with winter (aphelion winters are longer), yet interestingly, ground ice
has a particularly larger effect at the same Lp (perihelion summer is warmer, leading to higher humidity
and more extensive ground ice). Rise/decline in eccentricity increases/decreases the magnitude of the Lp
effects mentioned above (not shown in the figure).
Interpolation of seasonal CO2 accumulation for
the past 1.5 Ma: Exchangeable mass values are interpolated for Laskar et al. historical orbital parameters
values for the last 1.5 Ma (Figure 4, top and middle
plots; obliquity 15º is extrapolated, modeled exchangeable mass is expected to be much lower around
this obliquity, due to atmospheric collapse). The same

Figure 3: Total seasonal CO2 exchangeable mass, e = 0.12,
at northern (top) and southern (bottom) hemispheres;
dashed/solid contours correspond to StGI/EqGI scenarios,
respectively. Results for ε =15º are not included here because these simulations did not reach steady state.

at epochs of high obliquity. At its maximal effect,
ground ice migration decreases the seasonal surface
pressure variations by 42% relative to the non-migrating ground ice scenario.

Figure 4: Historic evolution of seasonal exchangeable
CO2 mass in the northern (top) and southern (middle) hemispheres); historic evolution of seasonal surface pressure
variations (bottom; in purple, left vertical axis) overlayed
with obliquity (in black, right vertical axis); dashed/solid
lines correspond to StGI/EqGI scenarios, respectively.

Discussion:
Ground ice is redistributed due to orbital evolution, and much like the seasonal CO2 cap it extends
towards the equator at high obliquity and recedes at
low obliquity. It acts as an adapting heat reservoir that
inhibits the seasonal accumulation of CO2 and attenuates the more extreme orbitally-forced oscillations in
the secular evolution of the seasonally-exchangeable
CO2 budget. Our work emphasizes the notion that
ground ice redistribution is non-negligible and has potentially significant implications on the reconstruction
of the Martian paleoclimate. Particularly, surface
pressure variations and CO2 ice accumulation are relevant to questions of liquid water stability [11] and
the interpretation of the enigmatic polar layered deposits [12], the massive south polar CO2 deposits [13]
and other climate records.
Further work will advance the current analysis of
secular CO2 evolution (e.g., including the steady state
seasonal CO2 accumulation at low obliquity) and improve our understanding of the effect of ground ice
distribution on the seasonal cycle. Finally, we must
consider more complex scenarios, such as non-equilibrium ground ice, ground ice in the presence of an
equatorial humidity source, and others.
Acknowledgments:
The authors wish to thank the Helen Kimmel Center for Planetary Sciences for support of this work and

thank Norbert Schorghofer and Paul Hayne for helpful discussions.
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