Institutes of Health (P50GM085764), and the National Institute of
Allergy and Infectious Diseases (NIAID; R01AI103058 and
R01AI50234). Whole-genome sequencing was performed at the
Institute for Genomic Medicine Genomic Center, UCSD. We thank
the Penn State Metabolomics Core Facility (University Park, PA)
for critical analytical expertise. A.C. received support from the
UCSD Division of Infectious Diseases institutional NIAID training
grant (T32AI007036). E.L.F. received support from a NIAID
National Research Service Award fellowship (F32AI102567). V.C.
received support from a UCSD Genetics Training Program through
an institutional training grant from the National Institute of General
Medical Sciences (T32GM008666). G.M.L. is supported by an
A.P. Giannini Post-Doctoral Fellowship. The authors declare no
competing financial interests. The red blood cells used in this work
were sourced ethically, and their research use was in accord with
the terms of the informed consents. All relevant sequence data
that were not previously published have been deposited in the
National Center for Biotechnology Information (NCBI) Sequence
Read Archive (SRA) database with accession code SRP096299.
Previously published sequences are deposited in the NCBI SRA
with the accession codes listed in table S3. This work is licensed
under a Creative Commons Attribution 4.0 International (CC BY
4.0) license, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly
cited.To view a copy of this license, visit http://creativecommons.
org/licenses/by/4.0/. This license does not apply to figures/
photos/artwork or other content included in the article that
is credited to a third party; obtain authorization from the rights
holder before using such material.
Materials and Methods
Figs. S1 to S10
Tables S1 to S16
15 May 2017; accepted 2 November 2017
Exposed subsurface ice sheets in the
Colin M. Dundas,1 Ali M. Bramson,2 Lujendra Ojha,3 James J. Wray,4
Michael T. Mellon,5 Shane Byrne,2 Alfred S. McEwen,2 Nathaniel E. Putzig,6
Donna Viola,2 Sarah Sutton,2 Erin Clark,2 John W. Holt7
Thick deposits cover broad regions of the Martian mid-latitudes with a smooth mantle; erosion
in these regions creates scarps that expose the internal structure of the mantle. We
investigated eight of these locations and found that they expose deposits of water ice that
can be >100 meters thick, extending downward from depths as shallow as 1 to 2 meters below
the surface. The scarps are actively retreating because of sublimation of the exposed water
ice. The ice deposits likely originated as snowfall during Mars’ high-obliquity periods and have
now compacted into massive, fractured, and layered ice. We expect the vertical structure of
Martian ice-rich deposits to preserve a record of ice deposition and past climate.
One-third of the Martian surface contains hallow ground ice. This ice is a critical target for science and exploration: it affects modern geomorphology, is expected to pre- serve a record of climate history, influences
the planet’s habitability, and may be a potential
resource for future exploration. The extent of
Martian ground ice and the depth to the ice table
have been predicted in theory (1–3) and have
been tested both in situ (4) and from orbital observations (5–11). However, the vertical structure
of subsurface ice remains poorly known, including its layering, thickness, and purity, which
record its emplacement and subsequent modification processes. Information about the structure,
depth, and purity of shallow ice is also required to
plan possible in situ resource utilization (ISRU) on
future missions (12).
Early theoretical predictions suggested that
Martian subsurface ice would be ice-cemented
ground (2). Orbital neutron-spectrometer data
have revealed ice contents greater than the likely
pore space volume in the upper few centimeters
of the ice table in many locations (5–7). Shallow
ice (<1 to 2 m) exposed by fresh impacts remains
distinct for months or years, also indicating a low
rock or dust content, although possibly modified
by the impact process (9, 10, 13). The Phoenix
lander on the northern plains uncovered both ice-
cemented regolith and deposits of pure (~99 vol-
ume %) ice (4, 14) at a few centimeters depth.
The ice at that site likely extends to 9 to 66 m
depth on the basis of shallow radar reflections
(15), but geomorphic interpretations suggest that
ice-cemented ground is dominant there (16).
However, ice-loss landforms indicate that several
regions on Mars have high ice volume fractions
extending through substantial subsurface depths
(17, 18), and radar echo power data have sug-
gested high ice contents beneath areas of the
northern plains (8). Subsurface radar reflections
indicate the presence of debris-covered glaciers
(19, 20) as well as buried regional ice sheets in
the Utopia and Arcadia Planitiae regions that are
up to 170 m thick and nearly pure ice (21, 22).
Because the radar did not resolve the top of the
ice, it is likely to be within ~20 m of the surface,
the limit of the radar instrument’s ability to iden-
tify shallow signals (15). The smaller-scale structure
of the ice sheets and rocky cover are unresolvable
with current radar data. It remains unknown
whether the ice within a few meters of the sur-
face has the same origin and age as the deeper
ice because the upper ice is most readily modi-
fied and best coupled to the recent climate.
1Astrogeology Science Center, U.S. Geological Survey, 2255 N.
Gemini Drive, Flagstaff, AZ 86001, USA. 2Lunar and Planetary
Laboratory, University of Arizona, Tucson, AZ, USA.
3Department of Earth and Planetary Sciences, The Johns
Hopkins University, Baltimore, MD, USA. 4School of Earth and
Atmospheric Sciences, Georgia Institute of Technology, Atlanta,
GA, USA. 5The Johns Hopkins University/Applied Physics
Laboratory, Laurel, MD, USA. 6Planetary Science Institute, 1546
Cole Boulevard, Suite 120, Lakewood, CO 80401, USA. 7Institute
for Geophysics, Jackson School of Geosciences, University of
Texas at Austin, Austin, TX 78758, USA.
*Corresponding author. Email: firstname.lastname@example.org
Fig. 1. Pits with scarps exposing ice. (A and B) Scarps 1 and 2. Both (A) and (B) show HiRISE
red-filter data merged with the center color strip (23) in early-summer observations. Parallel ridges
indicate retreat of scarps (fig. S1). North is up and light is from the left in all figures.