MILANKOVITCH FORCING OF EQUILIBRIUM GROUND-ICE ON MARS.
O. Aharonson1 , E. Vos, Dept. of Earth & Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel
(Oded.Aharonson@weizmann.ac.il), 1 Also at: Planetary Science Institute, Tucson, Arizona, USA., N. Schorghofer,
Planetary Science Institute, Tucson, Arizona, USA., F. Forget, Laboratoire de Meteorologie Dynamique/IPSL, Sorbonne Universite, ENS, PSL Research University, Ecole Polytechnique, CNRS, Paris, France.

Summary
Exchangeable ice deposits are present today on Mars
in polar caps and as shallow subsurface mid-latitude
ground-ice. Geologic evidence indicates these reservoirs waxed and waned in the past in concert with the
emplacement and loss of equatorial glaciers. Here we
use a climate model [1], to determine the distribution of
the mid-latitude ground ice deposits assumed to be in diffusive vapor equilibrium with the atmosphere, at present
and under past orbital configurations. The ground-ice
and its distribution have profound consequences for various aspects of Mars climate, not only playing a central
role in the water cycle, but also determining the seasonal
duration of CO2 stability at the surface. The model predictions for the extent of the equilibrium ice-table in
the past are successful in matching the latitudinal distribution of terrain softening geologic features previously
mapped [2, 3, 4] and exposures of sub-surface ice in
recently formed craters [5].

Introduction
Water vapor exchange between the Martian atmosphere
and ice in the shallow subsurface governs the presentday distribution of ground-ice ice on Mars. Ice sheets
that form on the surface retreat vertically until covered
by an ice-free layer of a thickness that allows for vapor
equilibrium between the atmosphere and the ice table.
Or, if no ice is present initially but the atmosphere is sufficiently humid and the ground porous, ice is sequestered
through thermal pumping of vapor [e.g. 6, 7]. Comparisons of models with remote mapping using neutron and
gamma-ray emissions and in-situ observations by the
Phoenix lander show that ground-ice within the top few
decimeters is in approximate diffusive equilibrium with
the mean annual atmospheric humidity at present, a state
also expected from lab measurements of the diffusive
properties of regolith [e.g. 8].
Water ice has also been detected equatorward of the
latitude boundary for equilibrium by recent impacts and
exposures at scarps [9, 10, 11, 5]. Depending on the
thickness of the overlying layer and the lithic content
of the ice (which produces a sublimation lag), massive
pure ice deposits may be an out-of-equilibrium relic of
a recent more humid period.
Calculations of the equilibrium ice table for a past

climate period [12, 13] require inputs from a model for
the vapor content of the atmosphere. The evolution of
the subsurface ice volume depends on the atmospheric
humidity and, vice versa, the ice content changes the
thermal properties significantly, and therefore influences
the state of the atmosphere (such as through the amount
of seasonal CO2 frost). Here, we present calculations
of the paleoclimate equilibrium ice table using a coupling between a full 3D Global Climate Model for the
atmosphere and an equilibrium ice table model. The
model results are compared to the geographic and depthdistribution of recent ice deposits.

Methods
We use the LMD-GCM [1] to simulate the climate of
Mars at various orbital conditions and surface ice distributions. This model has been extensively tested and calibrated with present-day observations, implements the
full water cycle, and includes treatment of surface ice,
atmospheric vapor, and ice clouds. The model used to
find the depth of the equilibrium ice table relies on the
condition that the saturation vapor density at the mean
annual temperature (as a function of depth) equals the
mean annual vapor density in the atmosphere. Once we
identify the depth below which subsurface ice is stable, we assign the thermal inertia of all layers below
this depth a value of 1,700 J m−2 K−1 s−1/2 , representing
ice-cemented soil. The GCM is then restarted, and the
process is repeated until the ice table variations from
year to year are smaller than one resolution layer. Initialization of the subsurface temperatures is performed
with a custom algorithm and we ensure convergence is
achieved (∆T < 1 K).

Results
Figure 1 shows the predicted equilibrium ice table for
present-day conditions, displayed as a geographic map
(c) and the zonal average (a). As found in past models
[6, 14, 15], ground ice is stable polewards of ∼ 58◦ latitude in both hemispheres. The Figure also shows similar maps, but for past orbital conditions. Specifically,
we compare obliquities 15◦ , 25.1◦ (as today), 35◦ , and
45◦ , with the remaining orbital elements fixed at their
present-day value. Several noteworthy results emerge.

Figure 1: Equilibrium ice-table depth for various obliquities experienced in the past ∼10 Myr. Panels b–e correspond to obliquities
of 15◦ , 25.1◦ , 35◦ , and 45◦ . Black points indicate locations where dissected mantle terrain has been previously mapped [3, 4], and
red circles where recent crater events expose subsurface ice deposits [5].

At low obliquity of 15◦ , the distribution of ice retreats
polewards, with a boundary reaching 70◦ latitude. Conversely, at high obliquity, the distribution of stable ground
ice extends equatorward, with regions of stability appearing even at equatorial latitudes, in the Western Tharsis and Arabia Terra regions. The stability in this region
results from a combination of favorable thermal properties and geographic variations in atmospheric humidity,
and thus a full 3D GCM is required to capture the effect.
At obliquity of 35◦ , near the highest value attained in
the last few Myr, the regions of stability grow in extent,
covering up to a third of the low latitudes. The significance of the result that extensive regions of ground-ice
stability are present at low latitudes is twofold. It not
only suggests the possible diffusive emplacement of ice
in the regolith pore space but also, promotes the stability
of ice emplaced by other mechanisms.

In Figure 1 (b-e) we also plot locations where dissected mantle terrain has been mapped [3, 4], and interpreted to indicate atmospheric deposition and subsequent dissection of ice. Latitude-dependent terrain softening features with viscous flow morphology have also
been proposed to indicate regions rich in ground ice [2].
The two geomorphic classes share some similar characteristics, appearing over similar latitude bands, mantling
pre-existing topography, and showing some evidence of
internal layering. A compilation of recent craters found
to expose subsurface ice [5] is also shown in the figure.
The latitudinal extent of the dissected mantle terrain (and
of the smaller number of recently exposed ice deposits)
shows remarkable agreement with the maximum extent
of the region of ground-ice stability predicted by our
model at past obliquities.

REFERENCES

References

[8]

F. Forget et al. “Improved general circulation models
of the Martian atmosphere from the surface to above
80 km”. In: J. Geophys. Res. Planets 104.E10 (1999),
pp. 24155–24175.

T. L. Hudson et al. “Water vapor diffusion in Mars subsurface environments”. In: J. Geophys. Res. 112.E5,
E05016 (2007), E05016.

[9]

S. Byrne et al. “Distribution of Mid-Latitude Ground
Ice on Mars from New Impact Craters”. In: Science 325
(2009), pp. 1674–1676.

[2]

S. W. Squyres and M. H. Carr. “Geomorphic evidence
for the distribution of ground ice on Mars”. In: Science
231.4735 (1986), pp. 249–252.

[10]

[3]

J. F. Mustard, C. D. Cooper, and M. K. Rifkin. “Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice”. In: Nature 412.6845 (2001), pp. 411–414.

C. M. Dundas et al. “HiRISE observations of new impact craters exposing Martian ground ice”. In: J. Geophys. Res. 119.1 (2014), pp. 109–127.

[11]

C. M. Dundas, A. M. Bramson, L. Ojha, et al. “Exposed
subsurface ice sheets in the Martian mid-latitudes”. In:
Science 359.6372 (2018), pp. 199–201.

[12]

M. T. Mellon and B. M. Jakosky. “The distribution and
behavior of Martian ground ice during past and present
epochs”. In: J. Geophys. Res. 100.E6 (1995), pp. 11,781–
11,799.

[1]

[4]

R. E. Milliken, J. F. Mustard, and D. L. Goldsby. “Viscous flow features on the surface of Mars: Observations
from high-resolution Mars Orbiter Camera (MOC) images”. In: J. Geophys. Res. Planets 108.E6 (2003).

[5]

C. M. Dundas, M. T. Mellon, S. J. Conway, et al. “Widespread [13]
Exposures of Extensive Clean Shallow Ice in the Midlatitudes of Mars”. In: J. Geophys. Res. 126.3, e2020JE006617
(2021).
[14]
M. T. Mellon and B. M. Jakosky. “Geographic variations in the thermal and diffusive stability of ground ice
on Mars”. In: J. Geophys. Res. 98.E2 (1993), pp. 3345–
[15]
3364.

[6]

[7]

N. Schorghofer and O. Aharonson. “Stability and exchange of subsurface ice on Mars”. In: J. Geophys. Res.
110.E5, E05003 (2005), E05003.

N. Schorghofer and F. Forget. “History and anatomy
of subsurface ice on Mars”. In: Icarus 220.2 (2012),
pp. 1112–1120.
N. Schorghofer and O. Aharonson. “Stability and exchange of subsurface ice on Mars”. In: J. Geophys. Res.
110.E5 (2005).
O. Aharonson and N. Schorghofer. “Subsurface ice on
Mars with rough topography”. In: J. Geophys. Res. 111.E11,
E11007 (2006).