could have been geologically recent because there
are few craters on the surface. The fractures and
steep slopes indicate that the ice is cohesive and
strong. The presence of banding and color variations suggest layers, possibly deposited with
changes in the proportion of ice and dust under
varying climate conditions, similar to the Martian
polar-layered deposits (25). The lens-like body of
lower ice content at Scarp 2 (Fig. 4A) may be a
buried moraine transported by ice flow because
subtle arcuate surface topography suggests glacial flow at this location (fig. S13). Alternatively, it
could be a former sublimation lag; that would
require some additional process to explain the
presence of boulders, but some are found on the
surface of the mantling deposit elsewhere (23),
so this cannot be ruled out.
It is likely that the scarps are currently retreating owing to sublimation. Slope retreat caused
by sublimation would explain rocks falling from
Scarp 2, although the proximate cause could be
seasonal frost processes or thermal cycles. Because the boulders that fell from the scarp are
meter-scale, with a few percent of the boulders
present having fallen over an interval of 3 Mars
years, the likely retreat rate is on the order of a few
millimeters each summer (unresolvable with remote
imagery). This estimated retreat rate indicates that
the pits may have formed over a time scale on an
order of 106 years; however, the retreat rate is
probably not constant. Ridges and bands paralleling
the scarps may indicate former positions (Fig. 1).
The vertical structure of the ice deposits in
these exposures appears simple (fig. S14). Ice
with low rock and dust content occurs at shallow
depths as small as ~1 to 2 m and extends to at
least many tens of meters in depth. The ice may
be capped by a cover of ice-cemented lithic ma-
terial, but this veneer cannot be reliably distin-
guished from massive ice with a superficial lag.
In some cases, the scarp color does not clearly
indicate massive ice at the shallowest depths, but
the textures are continuous across the ice unit.
Variations in the apparent depth to massive ice
could arise from both true lateral heterogeneity
in ice content and from debris covering the up-
permost ice, so the depth to the top of the ice
sheet may be variable. Debris falling from an ice-
free surface layer would be particularly effective
at covering the uppermost massive ice. This sim-
ple structure matches radar interpretations of
thick, regional-scale sheets of ice elsewhere on
Mars, covered by lags thinner than the radar-
constrained upper bound of 20 m (21, 22). Fur-
thermore, the observations demonstrate that the
massive ice in many cases occurs at depths much
shallower than that upper bound. The stratigra-
phy indicated by the scarps may be relevant to
other locations with ice-rich deposits, such as
Arcadia and Utopia Planitiae. We expect that the
structure at lower latitudes will be more complex
(for example, more layering) or have thicker co-
vers of ice-free and ice-cemented soil because of
reduced overall stability and more frequent un-
stable conditions (28, 29).
Erosional scarps on Mars reveal the vertical
structure of geologically young, ice-rich mantling
deposits at mid-latitudes. The ice exposed by the
scarps likely originated as snow that transformed
into massive ice sheets, now preserved beneath
less than 1 to 2 m of dry and ice-cemented dust or
regolith near ±55° latitude. These shallow depths
make the ice sheets potentially accessible to
future exploration, and the scarps present cross-
sections of these ices that record past episodes of
ice deposition on Mars.
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Observation planning was funded by the MRO project, and analysis
was funded by NASA grants NNH13AV85I (to C.M.D.) and
NNX16AP09H (to A.M. B.). L. O. was supported by the Blaustein
Postdoctoral Fellowship at Johns Hopkins University. We thank NASA/
Jet Propulsion Laboratory and the instrument teams for their efforts
collecting and processing data. The SHARAD instrument was provided
to NASA’s MRO mission by the Italian Space Agency (ASI). We thank
M. Christoffersen for help with the HRSC-based clutter simulations,
which are available at https://doi.org/10.6084/m9.figshare.5537893.
The authors declare no competing financial interests. All of the primary
spacecraft data used in this study are available via the Planetary
Data System. HiRISE and CTX data are at https://pds-imaging.jpl.nasa.
gov/volumes/ mro.html, SHARAD data are at http://pds-geosciences.
wustl.edu/missions/mro/sharad.htm, CRISM data are at http://pds-geosciences.wustl.edu/missions/mro/crism.htm, and THEMIS data
and derived products are at http://viewer.mars.asu.edu/viewer/
themis#T=0; observation ID numbers are listed in tables S2 to S5.
Derived thermal inertia data are available via https://se.psi.edu/~than/
inertia and dust cover index and albedo via www.mars.asu.edu/data.
Materials and Methods
Figs. S1 to S17
Tables S1 to S5
21 June 2017; accepted 4 December 2017
SCIENCE sciencemag.org 12 JANUARY 2018 • VOL 359 ISSUE 6372 201
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
Fig. 3. CRISM spectra of three icy scarps. (Top
to bottom lines) Scarps 2, 3, and 1 (vertically
offset for clarity) compared with a laboratory
water ice spectrum (32). Dashed lines indicate
the centers of H2O ice absorption features.
The spike near 1.65 mm is an instrument artifact;
differences in band shape are likely due to grain
size (33) or minor effects of the neutral ratio
region used in the CRISM spectra.
Fig. 4. Blocks falling from an ice-rich scarp. (A) Enhanced-color context with arrow pointing to
a lens that is redder than the rest of the scarp. (B and C) Before and after images, respectively,
with arrows indicating boulders that have fallen from the lens (B) to the boulder-rich pit floor
(C). (B) and (C) are separated by ~3 Mars years (table S2).