Water ice cliffs on Mars Some locations on Mars are known to have water ice just below the surface, but how much has remained unclear. Dundas et al. used data from two orbiting spacecraft to examine eight locations where erosion has occurred. This revealed cliffs composed mostly of water ice, which is slowly sublimating as it is exposed to the atmosphere. The ice sheets extend from just below the surface to a depth of 100 meters or more and appear to contain distinct layers, which could preserve a record of Mars' past climate. They might even be a useful source of water for future human exploration of the red planet. Science, this issue p. 199 
 
 Abstract 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 shallow ground ice. This ice is a critical target for science and exploration: it affects modern geomorphology, is expected to preserve 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 volume %) 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 suggested 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 identify 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 surface has the same origin and age as the deeper ice because the upper ice is most readily modified and best coupled to the recent climate. 
 
 We describe observations of the vertical structure of ground ice using the Mars Reconnaissance Orbiter (MRO). The observations target eight locations that have steep, pole-facing scarps created by erosion (Figs. 1 and 2 and figs. S1 to S3) in images from the High-Resolution Imaging Science Experiment (HiRISE); seven are located in the southern hemisphere, and the eighth location is a cluster of scarps in Milankovič Crater in the north (table S1). Each of the scarps is relatively blue (compared with surrounding terrain) in enhanced-color HiRISE images, and three locations have water-ice signatures in mid-summer spectral data (Fig. 3 and figs. S4 and S5) taken by MRO’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) (23). CRISM spectra show an H 2 O ice absorption feature at 1-μm wavelength (fig. S6), which can be masked by as little as 1% soil (14). Tens of micrometers of dust are needed to make an opaque cover (24), so the dust content is low and/or wind removes dust. 
 
 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. 
 
 Fig. 2 Enhanced-color transverse sections of icy scarps in late spring/early summer. (A to F) Arrows indicate locations where relatively blue material is particularly close to the surface. Downhill is to the bottom in (A) to (E) (Scarps 1 to 5, respectively) and to the top in (F) (Milankovič Crater). Banding or layering is visible in several scarps, and (E) shows a distinct change in color as well as multiple fractures cutting the ice. 
 
 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 H 2 O ice absorption features. The spike near 1.65 μm 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. 
 
 Several lines of evidence indicate that the scarps are exposures of subsurface ice rather than persistent seasonal frost. First, they remain distinct from the surrounding terrain in color in all HiRISE images, including many observations acquired long after seasonal frost has sublimed from steep pole-facing slopes at higher latitudes. Second, Mars Odyssey Thermal Emission Imaging System (THEMIS) observations indicate late-afternoon scarp temperatures above the likely atmospheric frost point (23). Last, nearby pits lack relatively blue material that would be expected for topographically controlled frost (fig. S7). 
 
 The units hosting the scarps drape the terrain, with some meter-scale boulders on the surface and few impact craters. The surface textures show some fracturing (fig. S8), probably due to thermal contraction. Ice is expected to be stable near 10 cm depth under level ground at these locations (3). The scarps are sharply defined and nearly straight, up to ~6 km long, and face slightly east of poleward. Each scarp constitutes the equatorward side of a few-kilometer-scale pit with a rugged and sometimes boulder-rich floor, except in Milankovič Crater, where they are part of the ragged edge of a smooth plateau (fig. S2). In several cases, ridge structures parallel the scarp within the pit and merge with cusps along the margin. The base of the exposed ice appears to be covered by colluvium (Fig. 2). In several cases, surface expressions of long fractures with surrounding depressions occur on the terrain above the scarp (fig. S8). 
 
 Topographic data (figs. S9 and S10) for Scarp 1 (numbering is provided in table S1) indicates that the ice exposure has a slope of ~45° (up to 55° in some locations), transitioning to a shallower slope for the regolith-covered lower scarp. We estimate the scarp-hosting material to be at least 130 m thick after correcting for the regional slope, although the exposed ice has ≤100 m of relief because the lower slope is buried. The former value provides a better estimate of the ice-rich thickness if all of the relief is due to ice loss. 
 
 These scarps reveal the vertical structure of the ice. At several locations, ice reaches close to the top of the scarp (Fig. 2), indicating that massive ice begins at shallow depths (≤1 m). Banded patterns or variations in color or slope at some sites suggest subunits within the ice. At Scarps 6 and 7, the banded patterns show unconformities where layers cut across each other (fig. S3). The uppermost section of the scarp at each site appears steeper and less blue than the remainder. Like the Martian polar deposits (25), the color and brightness of the surface may be partially controlled by a thin coating of dust (likely a sublimation lag) and thus not representative of the bulk properties. Because lags and contaminants will reduce the visibility of ice, it is likely that clean ice near the surface is more extensive than the limited locations where it is exposed (Fig. 2). 
 
 At Scarp 2, a lens-shaped section of the exposure lacks relatively blue coloration (Fig. 4A) and contains a concentration of boulders. Repeat images taken 3 Mars years apart show that meter-scale blocks fell from within or below this lens-shaped section (Fig. 4, B and C), providing evidence of active slope retreat. Because these blocks did not disappear after they fell [unlike ice blocks excavated by craters (10)] and originated as protrusions from the icy layer, they must be rocks that were embedded within the deposit, not chunks of ice broken off the scarp. Other sites have not shown similar blockfalls, but mottled shifts in tone and texture are common, and dark markings a few meters across appeared and disappeared at the base of some scarps (fig. S11). 
 
 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). 
 
 The Shallow Radar (SHARAD) on MRO has detected radar contacts (reflectors) that have been interpreted as the base of ice layers in the northern plains of Mars (15, 19–22). Similar features occur near several of the scarps (fig. S12) but are difficult to distinguish from radar clutter caused by local rugged topography. Candidate reflectors in the three SHARAD tracks closest to Scarp 1 do not appear in clutter simulations by using topography derived from either Mars Orbiter Laser Altimeter (MOLA) or High-Resolution Stereo Camera (HRSC) data (23), suggesting that they could be due to the bottom of the massive ice layer. However, candidate reflectors near Scarp 5 have possible counterparts in the HRSC-based clutter simulations, suggesting that they are not subsurface features. Only these two sites have topographic data available at suitable resolution, and given the rough terrain, we cannot currently rule out unresolved clutter as the cause of candidate reflectors. Nondetections may be due to surface roughness or stratigraphic contacts that are gradual or otherwise do not produce a sharp dielectric contrast on the scale of the radar footprint. 
 
 We interpret these scarps as exposures of subsurface ice originating as dusty snow or frost (26–29) and subsequently compacted and recrystallized. This interpretation is consistent with the high ice content and the mantling appearance of the host unit. Alternative explanations, such as growth of ice lenses (30) or enhanced vapor diffusion (31), are expected to be slow and operate at shallow depths. They should also produce layering parallel to the ground surface, unlike the layers at Scarps 6 and 7 (fig. S3). The latest ice deposition 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 material, but this veneer cannot be reliably distinguished 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 uppermost 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 simple 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). Furthermore, the observations demonstrate that the massive ice in many cases occurs at depths much shallower than that upper bound. The stratigraphy 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 covers of ice-free and ice-cemented soil because of reduced overall stability and more frequent unstable 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. 
 
 Supplementary Materials www.sciencemag.org/content/359/6372/199/suppl/DC1 Materials and Methods Supplementary Text Figs. S1 to S17 Tables S1 to S5 References (34–59) 
 
 http://www.sciencemag.org/about/science-licenses-journal-article-reuse This is an article distributed under the terms of the Science Journals Default License. ||||| Breaking News Emails Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings. 
 
 / Updated By Alex Johnson 
 
 Huge ice sheets more than 300 feet deep have been discovered on Mars, making it possible that human astronauts could have access to almost limitless water, scientists said Thursday. 
 
 "Astronauts could essentially just go there with a bucket and a shovel and get all the water they need," said one of the report's authors, Shane Byrne of the Lunar and Planetary Laboratory at the University of Arizona. 
 
 Snow and ice covering dunes on Mars' Northern hemisphere in a photo taken by NASA's Mars Reconnaissance Orbiter in May 2017. NASA / AFP - Getty Images file 
 
 It has been known for some time that some locations on Mars have water ice just below the surface — but until now, there has been no accurate way to know just how much. 
 
 The new data, collected in high-resolution three-dimensional images from two U.S. satellites, reveal at least eight locations where massive shelves of water ice are deposited from just below the Martian surface as far deep as 100 meters, or roughly 330 feet, according to the report, which was published in the journal Science. 
 
 Related: NASA details its plans for sending humans to Mars 
 
 "The discovery reported today gives us surprising windows where we can see right into these thick underground sheets of ice," Byrne said. "It's like having one of those ant farms where you can see through the glass on the side to learn about what's usually hidden beneath the ground." 
 
 The ice shelves contain distinct layers, which could preserve a record of Mars' past climate, the scientists said. Of probably wider public interest, however, is that the ice shelves "might even be a useful source of water for future human exploration of the red planet," they said. 
 
 👉Future astronauts: @NASA identified eight icy places on #Mars where you can get all the water you need. BYO bucket & shovel. https://t.co/mqgmOndneu pic.twitter.com/LE272hZunG — Curiosity Rover (@MarsCuriosity) January 11, 2018 
 
 The ice probably started as snowfall that compacted into massive fractured layers. They're at relatively easy-to-reach locations with less hostile conditions than at Mars' polar ice caps, according to NASA's Jet Propulsion Laboratory — at latitudes roughly equivalent on Earth to Scotland or the tip of South America. 
 
 "Astronauts could essentially just go there with a bucket and a shovel and get all the water they need." 
 
 The discovery could add fuel to NASA's efforts to send humans to Mars, one of the objectives President Donald Trump cited last month when he directed the space agency to focus its efforts on returning to the moon. 
 
 Related: Debate heats up over the search for life on Mars 
 
 Elon Musk, chief executive of SpaceX, has also discussed plans for private manned missions to Mars. 
 
 The research, which was funded by NASA, was conducted by the Lunar and Planetary Laboratory along with the U.S. Geological Survey, the nonprofit Planetary Science Institute, Georgia Tech, Johns Hopkins University and the University of Texas. ||||| A cross-section of a thick sheet of underground ice is exposed at the steep slope (or scarp) that appears bright blue in this enhanced-color view from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. The view covers an area about 550 yards (500 meters) wide. Figure 1 includes a 100-meter (109-yard) scale bar. North is toward the top. The upper third of the image shows level ground that is about 140 yards (130 meters) higher in elevation than the ground in the bottom third. In between, the scarp descends sharply, exposing about 260 vertical feet (80 vertical meters) of water ice. Color is exaggerated to make differences in surface materials easier to see. The presence of exposed water ice at this site was confirmed by observation with the same orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Credit: NASA/JPL-Caltech/UA/USGS Researchers using NASA's Mars Reconnaissance Orbiter (MRO) have found eight sites where thick deposits of ice beneath Mars' surface are exposed in faces of eroding slopes. 
 
 These eight scarps, with slopes as steep as 55 degrees, reveal new information about the internal layered structure of previously detected underground ice sheets in Mars' middle latitudes. 
 
 The ice was likely deposited as snow long ago. The deposits are exposed in cross section as relatively pure water ice, capped by a layer one to two yards (or meters) thick of ice-cemented rock and dust. They hold clues about Mars' climate history. They also may make frozen water more accessible than previously thought to future robotic or human exploration missions. 
 
 Researchers who located and studied the scarp sites with the High Resolution Imaging Science Experiment (HiRISE) camera on MRO reported the findings today in the journal Science. The sites are in both northern and southern hemispheres of Mars, at latitudes from about 55 to 58 degrees, equivalent on Earth to Scotland or the tip of South America. 
 
 "There is shallow ground ice under roughly a third of the Martian surface, which records the recent history of Mars," said the study's lead author, Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona. "What we've seen here are cross-sections through the ice that give us a 3-D view with more detail than ever before." 
 
 Windows into underground ice 
 
 The scarps directly expose bright glimpses into vast underground ice previously detected with spectrometers on NASA's Mars Odyssey (MRO) orbiter, with ground-penetrating radar instruments on MRO and on the European Space Agency's Mars Express orbiter, and with observations of fresh impact craters that uncover subsurface ice. NASA sent the Phoenix lander to Mars in response to the Odyssey findings; in 2008, the Phoenix mission confirmed and analyzed the buried water ice at 68 degrees north latitude, about one-third of the way to the pole from the northernmost of the eight scarp sites. 
 
 At this wedge-shaped pit on Mars, the steep slope (or scarp) at the northern edge (toward the top of the image) exposes a cross-section of a thick sheet of underground water ice. The image is from the High Resolution Imaging Stereo Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. The gray-scale portions on left and right are from a single waveband in the red-light portion of the visible spectrum. The middle section, in enhanced color, combines data from red, near-infrared and blue-green wavebands. The scene covers an area about three miles (five kilometers) wide. Figure 1 includes a one-kilometer scale bar. One kilometer is about six-tenths of a mile. The ice-exposing scarp at the northern edge of the pit has a steepness of about 45 to 55 degrees, plunging from the relatively level ground outside the pit. Credit: NASA/JPL-Caltech/UA/USGS 
 
 The discovery reported today gives us surprising windows where we can see right into these thick underground sheets of ice," said Shane Byrne of the University of Arizona Lunar and Planetary Laboratory, Tucson, a co-author on today's report. "It's like having one of those ant farms where you can see through the glass on the side to learn about what's usually hidden beneath the ground." 
 
 Scientists have not determined how these particular scarps initially form. However, once the buried ice becomes exposed to Mars' atmosphere, a scarp likely grows wider and taller as it "retreats," due to sublimation of the ice directly from solid form into water vapor. At some of them, the exposed deposit of water ice is more than 100 yards, or meter, thick. Examination of some of the scarps with MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) confirmed that the bright material is frozen water. A check of the surface temperature using Odyssey's Thermal Emission Imaging System (THEMIS) camera helped researchers determine they're not seeing just thin frost covering the ground. 
 
 Researchers previously used MRO's Shallow Radar (SHARAD) to map extensive underground water-ice sheets in middle latitudes of Mars and estimate that the top of the ice is less than about 10 yards beneath the ground surface. How much less? The radar method did not have sufficient resolution to say. The new ice-scarp studies confirm indications from fresh-crater and neutron-spectrometer observations that a layer rich in water ice begins within just one or two yards of the surface in some areas. 
 
 Astronauts' access to Martian water 
 
 The new study not only suggests that underground water ice lies under a thin covering over wide areas, it also identifies eight sites where ice is directly accessible, at latitudes with less hostile conditions than at Mars' polar ice caps. "Astronauts could essentially just go there with a bucket and a shovel and get all the water they need," Byrne said. 
 
 The exposed ice has scientific value apart from its potential resource value because it preserves evidence about long-term patterns in Mars' climate. The tilt of Mars' axis of rotation varies much more than Earth's, over rhythms of millions of years. Today the two planets' tilts are about the same. When Mars tilts more, climate conditions may favor buildup of middle-latitude ice. Dundas and co-authors say that banding and color variations apparent in some of the scarps suggest layers "possibly deposited with changes in the proportion of ice and dust under varying climate conditions." 
 
 This research benefited from coordinated use of multiple instruments on Mars orbiters, plus the longevities at Mars now exceeding 11 years for MRO and 16 years for Odyssey. Orbital observations will continue, but future missions to the surface could seek additional information. 
 
 "If you had a mission at one of these sites, sampling the layers going down the scarp, you could get a detailed climate history of Mars," suggested MRO Deputy Project Scientist Leslie Tamppari of NASA's Jet Propulsion Laboratory, Pasadena, California. "It's part of the whole story of what happens to water on Mars over time: Where does it go? When does ice accumulate? When does it recede?" 
 
 Explore further: Scientists discover evidence of recent water flows on Mars 
 
 More information: C.M. Dundas at U.S. Geological Survey in Flagstaff, AZ el al., "Exposed subsurface ice sheets in the Martian mid-latitudes," Science (2018). science.sciencemag.org/cgi/doi … 1126/science.aao1619 
 
 ||||| For the first time, high-resolution images show the three-dimensional structure of massive ice deposits on Mars. This photo by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter shows a detailed subsection of an icy scarp on the Red Planet in enhanced color. 
 
 Sizable deposits of water ice lurk just beneath the surface in some regions of Mars, a new study reports. 
 
 The newfound sheets appear to contain distinct layers, suggesting that studying them could shed considerable light on the Red Planet's climate history, researchers said. And the ice is buried by just a few feet of Martian dirt in places, meaning it might be accessible to future crewed missions. 
 
 "I'm not familiar with resource-extraction technology, but this may be information that's useful to people who are," study lead author Colin Dundas, of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona, told Space.com. [Photos: The Search for Water on Mars] 
 
 Dundas and his colleagues analyzed photos captured over the years by the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA's Mars Reconnaissance Orbiter (MRO). They identified eight locations where erosion had exposed apparent glaciers, some of which extend 330 feet (100 meters) or more into the Red Planet's subsurface. 
 
 These sites are steep, pole-facing slopes in Mars' midlatitudes, between about 55 and 60 degrees both north and south of the equator. The ice-harboring areas sport few craters, suggesting they're quite young, geologically speaking, the researchers said. 
 
 Interestingly, scientists think that Mars' obliquity — the tilt of the planet's axis relative to the plane of its orbit — has shifted a fair bit over the past few million years, varying between about 15 and 35 degrees, Dundas said. (The Red Planet's obliquity is currently about 25 degrees; Earth's is 23.5 degrees.) 
 
 This high-resolution HiRISE image shows an icy scarp on Mars in the context of a broader area. Credit: NASA/JPL/University of Arizona/USGS 
 
 "There've been suggestions that, when there's high obliquity, the poles get heated a lot — they're tilted over and pointed more at the sun, and so that redistributes ice toward the midlatitudes," Dundas said. "So, what we may be seeing is evidence of that having happened in the past." 
 
 Researchers already knew that Mars harbors subsurface water ice, and lots of it. For example, MRO's ground-penetrating Shallow Radar instrument recently found a buried ice layer that covers more ground than the state of New Mexico. (NASA's Phoenix lander also dug up some ice near the Martian north pole in 2008, but it's unclear if that stuff is part of a big sheet.) 
 
 But the newly analyzed HiRISE data give researchers more detailed looks at such deposits, Dundas said. 
 
 "The take-home message is, these are nice exposures that teach us about the 3D structure of the ice, including that the ice sheets begin shallowly, and also that there are fine layers," he said. 
 
 The new study was published online today (Jan. 11) in the journal Science. 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
– Scientists already knew that water ice lurked below Mars' surface. But a new study published in the journal Science sheds new light on what Space.com calls "apparent glaciers," seen anew thanks to high-res imagery from the HiRISE camera on NASA's Mars Reconnaissance Orbiter. Using this data, researchers found eight spots on the Red Planet that had been eroded, with steep slopes of ice sheets, or scarps, reaching from just below the surface (meaning it won't be hard to get to if humans or robots ever get there) to more than 325 feet deep. The images, which render the ice in 3D, show discrete layers within the ice, which could offer clues on how climate change has affected Mars. The scientists believe the ice was once snow that fell when Mars' axis was tilted more prominently toward the sun; that snow has since evolved into "massive, fractured, and layered ice." The ice sheets are located at latitudes that would be the equivalent of Scotland or the tip of South America on Earth, per NBC News. "It's like having one of those ant farms where you can see through the glass on the side to learn about what's usually hidden beneath the ground," study co-author Shane Byrne says of the detailed imagery, per Phys.org. Although scientists haven't yet figured out how these ice scarps form, they do know that once they peek out into the atmosphere, they turn from ice into water vapor, a process called sublimation, and "retreat" as they get taller and wider. The ice chunks could even prove handy as water sources if and when humans ever touch down to explore the planet, the study notes. "Astronauts could essentially just go there with a bucket and a shovel and get all the water they need," Byrne says. (What's up with those Mars "scratch marks"?)