Thermokarst lakes and ponds on Mars in the very recent (late Amazonian) past

Earth and Planetary Science Letters 272 (2008) 382–393

Contents lists available at ScienceDirect

Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Thermokarst lakes and ponds on Mars in the very recent (late Amazonian) past Richard J. Soare a,b,⁎, Gordon R. Osinski c, Charlotte L. Roehm d a

Department of Geography, Planning and Environment, Concordia University, 1455 de Maisonneuve W., Montreal, QC, Canada H3A 1M8 Department of Geography, Dawson College, 3040 Sherbrooke St. W., Montreal, QC, Canada H3Z 1A4 c Departments of Earth Sciences and Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 5B7 d Climate Impact Research Center & Department of Ecology and Environmental Science, Umeå University, SE-981 07, Abisko, Sweden b

A R T I C L E

I N F O

Article history: Received 13 November 2007 Received in revised form 1 May 2008 Accepted 5 May 2008 Available online 20 May 2008 Editor: T. Spohn Keywords: Mars periglacial geomorphology climate-change

A B S T R A C T The history of water is fundamental to understanding the geological evolution of Mars and to questions concerning the possible development of life on the Red Planet. Today, Mars is cold and dry; its regolith is permanently frozen and except under highly localised and transient conditions, liquid water is unstable at the surface. Intriguingly, we have identified geological features that could be markers of very late-Amazonian “wet” or ice-rich periglacial processes in Utopia and western Elysium Planitiae: 1. rimless, flat-floored and lobate, sometimes scalloped, depressions that are suggestive of terrestrial alases (evaporated/drained thermokarst lakes); 2. small-sized polygonal patterned-ground (perhaps formed by thermal-contraction cracking and possibly underlain by ice wedges); and, 3. circular/near-circular raised-rim depressions (consistent in morphology and scale with pingo-scars) that are nested in rimless depressions. In terrestrial cold-climate, non-glacial environments, landscape assemblages of this type occur only in the presence of icerich permafrost. Commenting upon the origin of the putative periglacial features on Mars, most workers have suggested that sublimation and not evaporation has been the dominant process. By contrast, we propose that two key characteristics of the rimless depressions – inner terraces and orthogonally-oriented polygons – are markers of stable, ponded water and its slow loss by evaporation or drainage. If the raised-rim landforms are pingo scars, then this also points to boundary conditions that are supportive of stable liquid water. With regard to the relative age of the features described above, previous work identified some lobate depressions superposed on crater-rim gullies in the region (Soare et al., 2007). Gullies could be amongst the youngest geological features on Mars; superposed depressions point to an origin that is more youthful than the gullies. In turn, as some raised-rim landforms are superposed on rimless depressions, this is indicative of an origin that is even more recent than that of the depressions. Together with the geological evidence showing that the rimless depressions could have been formed by ponded water, the stratigraphy of the putative periglacial-landscape in this region suggests that the very late Amazonian period could have been warmer and wetter than had been thought hitherto. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The history of water is fundamental to understanding the geological evolution of Mars and the potential for the existence of primitive life on the Red Planet. Mars today is a cold, dry planet. Except under highly localised and transient conditions, liquid water is unstable at the Martian surface (Haberle et al., 2001; Hecht, 2002; Mellon and Jakosky 1995). However, in the middle-latitudes of the northern hemisphere some landscapes display characteristics that are consistent with the very recent (late Amazonian) formation and flow of mountain-flank glaciers: fan-shaped and lobate deposits (sometimes ridged concentrically); parallel, converging and chevron-like ⁎ Corresponding author. Tel.: +1 514 848 2050; fax: +1 514 848 2032 E-mail address: [email protected] (R.J. Soare). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.05.010

lineations in potentially ice-rich deposits; and, lineated valley fill (Head et al., 2005; Forget et al., 2006; Head et al., 2006; Levrard et al., 2004; Madeleine et al., 2007; Shean et al., 2007). Climate models have shown that near-equatorial glaciation could be induced episodically as Mars reaches high obliquities (Forget et al., 2006; Head et al., 2003; Head et al., 2006; Madeleine et al., 2007). Interestingly, recent work also has identified landscape assemblages whose morphology, scale and characteristics are consistent with periglacial activity that is icy or “wet”, youthful (perhaps very late Amazonian in origin) and possibly a product of obliquity-driven temperature rises. The assemblages occur in Utopia and western Elysium Planitiae (Costard and Kargel, 1995, Costard et al., 2008; Morgenstern et al., 2007; Mustard et al., 2001; Seibert and Kargel, 2001; Soare et al., 2005a,b, 2007) as well as in the Cerberus Plains and surrounding lowlands (Burr et al., 2005; Page and Murray, 2006; Page 2007). The

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

383

constituent features include: 1. rimless, flat-floored and lobate, sometimes scalloped depressions (Fig. 1a) that are suggestive of terrestrial alases (evaporated/drained thermokarst lakes) (Fig. 1b) (Costard and Kargel, 1995; Costard et al., 2008; Morgenstern et al., 2007; Page and Murray, 2006; Page, 2007; Seibert and Kargel, 2001; Soare et al., 2005b, 2007); 2. small-sized polygonal patterned-ground (perhaps formed by thermal-contraction cracking and possibly underlain by ice wedges) (Mellon 1997; Page and Murray, 2006; Page, 2007; Seibert and Kargel, 2001; Soare et al., 2007); and, 3. circular/near-circular raised-rim depressions (consistent in morphology and scale with terrestrial pingoscars) that are nested in rimless depressions (Dundas et al., 2008; Osinski and Soare, 2007; Page and Murray, 2006; Page, 2007). In periglacial environments such as the Mackenzie River Delta and Tuktoyaktuk Coastlands (MRDTC) of northern Canada (Soare et al., 2008) and the Laptev Sea region of Russia (Romanovskii et al., 2000), assemblages of this type are found only in the presence of ice-rich permafrost. Commenting upon the origin of the putative periglacial landscapes on Mars, particularly the rimless depressions, most workers have suggested that sublimation and not evaporation has been the dominant process. By contrast, we propose that two key characteristics of the rimless depressions – inner terraces and orthogonally-oriented polygons – are markers of stable, ponded water and its slow loss by evaporation or drainage. If the raised-rim landforms are pingo scars, then this also points to boundary conditions that are supportive of stable liquid water. With regard to the relative age of the features described above, previous work identified some lobate depressions superposed on crater-rim gullies in the region (Soare et al., 2007). Gullies could be amongst the youngest geological features on Mars; superposed depressions point to an origin that is more youthful than the gullies. In turn, as some raised-rim landforms are superposed on rimless depressions, this indicates an origin that is even more recent than that of the depressions. Together with the geological evidence showing that the rimless depressions could have been formed by ponded water, the stratigraphy of the putative periglacial landscape in this region suggests that the very late Amazonian period could have been warmer and wetter than had been thought hitherto. 2. Thermokarst lakes and periglacial processes on Earth 2.1. Defining characteristics of thermokarst lakes, ponds and alases Terrestrial thermokarst-lakes are roughly circular to elongate (Figs.1b, c and 2a,b); they have relatively flat floors and lack raised rims (French, 2007; Mackay, 1998; Washburn, 1973). The absence of raised rims facilitates the ability of workers to differentiate possible thermokarst depressions on Mars from eroded or softened impact craters. Lake diameters range from meters to kilometers. Most lakes are no deeper than a few meters, although depths of ∼40 m have been reported in Siberia. Thermokarst lakes (Figs. 1b,c) occur in flat or low gradient topography that is dominated by ice-rich or “wet” permafrost (i.e., permafrost in which the volumetric occurrence of frozen water exceeds the sedimentary pore-space available to it in a column of soil) (French, 2007; Mackay, 1998; Washburn, 1973). When ice-rich permafrost thaws, losing its thermal equilibrium, the active layer deepens. In turn, this generates excess meltwater that was previously frozen within the permafrost column and was not an active part of the near-surface hydrological system. The excess water either drains away immediately, producing a thermokarst depression in its wake at the thaw site, or pools, forming a thermokarst lake or pond centred around the thaw site. An alas is a thermokarst-induced depression, pond or lake that has lost its water by evaporation (Fig. 2a) or drainage (Fig. 2b) (French, 2007; Washburn, 1973). The loss of thermal equilibrium could be produced by disparate variables such as the removal of a protective snow cover, thermal and/ or mechanical incision of running water or sustained regional-rises of mean temperature (French, 2007).

Fig. 1. (a) Rimless depressions in Utopia Planitia, Mars (45.0 °N, 275.6 °W, MOC r0301203, MSSS/JPL). (b) Frozen thermokarst-lakes on Baffin Island, Canadian Arctic (66.28°N, 72.45°W). Air photo 15358-58, National Air Photo Library, Canada. (c) Typical thermokarst-lake landscape, Tuktoyaktuk Coastlands, NWT, Canada. All scale bars are 500 m across.

2.2. Thermokarst-lake drainage and the formation of hydrostatic pingos Pingos are ice-cored hills that are long-lived (perennial), enduring hundreds to thousand of years, and relatively stable, in the absence of sustained rises of regional or global temperatures (Mackay, 1962). They tend to be conical to elongate in shape and range in height from a few metres to ~70 m; their diameter stretches from several metres to ~600 m (Fig. 3a,b) (Washburn, 1973). There are two types of pingos: hydraulic (open) and hydrostatic (closed).

384

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

Fig. 2. (a) Alas formation by evaporation (Tuktoyaktuk Coastlands, NWT, Canada). Lake is approximately 200 m across. (b) Alas formation by drainage (Tuktoyaktuk Coastlands, NWT, Canada). Lake is approximately 750 m across and drains to the left (west) of the image. Note the recessional terraces (pale region surrounding the lake) in a and b. (c) Ground view of recessional terraces showing antecedent lake-margins surrounding a thermokarst lake (Tuktoyaktuk Coastlands, NWT, Canada).

Most hydraulic pingos are found on lower valley-side slopes, valley bottom alluvium, alluvial fans, braided channels and outwash material (Cruickshank and Calhoun, 1965; French, 1993; O'Brien, 1971). Their origin and evolution is dependent on three related variables: 1. the occurrence of sub-permafrost water (a) upslope from the site of supra-permafrost (surface) water discharge and pingo formation or (b) at depth below the site in a confined aquifer; 2. the downslope and sub-permafrost flow of water, which creates a hydraulic head, or the upward migration of water through, under artesian pressure; and, 3. the existence of thin or discontinuous permafrost and of coarse sediments or fractured rock beneath the formation site, which permits the intra-permafrost migration of water from its sub-permafrost source to the surface (Cruickshank and Calhoun, 1965; French, 1993; Lasca, 1969; Müller, 1962; Mackay, 1998; O'Brien, 1971). The terrestrial regions where hydrostatic pingos grow and persist, such as the MRDTC, are characterised by the following features: icerich sediments; deep permafrost; glacio-fluvial sands (that facilitate the formation of injection ice) overlain by clays or other fine-grained sediments (that facilitate the formation of a pressure resistant cap or overburden); and, flat or low-lying topography (that facilitates the localised pooling of water into lakes and ponds) (Mackay 1962, 1972, 1998). The greatest concentration of hydrostatic pingos in the world (~ 1350) – growing, mature or collapsed – is located in the MRDTC (Mackay, 1998). Almost all of these pingos occur in alases, which are commonplace and ubiquitous in the region (Mackay, 1962; Washburn, 1973) (Fig. 3a). The relationship between pingo formation and alases is an important one to understand. When a thermokarst lake or pond loses its water by evaporation or drainage, wet sediments beneath the lake or pond floor are exposed to freezing temperatures (French, 2007; Mackay, 1966). A freezing front forms and permafrost begins to aggrade from the exposed area of the lake/pond-floor downwardly and from the margins inwardly. Pore

water becomes trapped in an increasingly small space and this, in turn, causes a rise of hydrostatic pressure (Mackay, 1966, 1979). As the centre of the lake or pond basin tends to be the last site to lose water, due to its depth and relative warmth, the permafrost column will be thinner here than in the surrounding area (Mackay, 1972). Thin permafrost also forms where residual water has pooled in highly localised pockets of negative topography (Mackay, 1998). It is precisely in these places that the lake or pond floors are most susceptible to deformation by hydrostatic pressure and where doming (incipient pingo-formation) begins (Mackay, 1998). Ongoing exposure to permafrost aggradation and to seasonally-induced freezing temperatures leads the expelled pore-water (now injection ice) to freeze into a solid mass of ice (Mackay, 1998). As a pingo grows, the overburden (formerly, the lake floor) must either stretch in response to the increase in surface area or fail in tension (Mackay, 1998). If the overburden thins too much, it becomes incapable of protecting the pingo's ice core from seasonally warm (thaw) temperatures; the loss of thermal stability generates meltwater which, in turn, could induce ponding, slumping and collapse (Mackay, 1998). Failure in the tensional strength of the overburden induces the formation of cracks, albeit by mechanical not thermal means, that radiate from the pingo summit (Fig. 3a). The formation of tensional cracks facilitates the penetration/propagation of thaw temperatures to the ice core and, much like the thinning of the overburden, leads to slumping and possible collapse conditions (Mackay, 1998). Collapsed pingos form shallow, usually round or ovoid, flat-floored depressions surrounded by raised rims or ramparts (Fig. 3c) (Mackay, 1998; Washburn, 1973). 2.3. Inward-oriented terraces in thermokarst lakes, ponds and alases When the loss of thermokarst-lake or-pond water by drainage or evaporation is intermittent or episodic, the basin may display an

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

385

other markers of ground ice such as polygonal patterned-ground (underlain by networks of ice-wedge polygons) and perennial icecored mounds (pingos). 2.4. Polygonal patterned ground and its orthogonal orientation in thermokarst lakes, ponds and alases Thermal contraction cracks begin to form when a severe and rapid decline of sub-zero temperatures occurs in frozen soil, either at the permafrost table or in the frozen active layer (Lachenbruch, 1962; French, 2007). The severe and rapid decline of temperature induces the tensile stress of the frozen ground to exceed its tensile strength, creating a small ground-crack (Lachenbruch, 1962). In wet periglacial environments, crack infill materials comprise hoarfrost or active-layer melt-water (Lachenbruch, 1962; Washburn 1973). In extremely cold and dry environments such as the Antarctic and eastern Greenland, the infiltrate may be sand (Bennike, 1998; Lachenbruch, 1962; Marchant et al., 2002; Sletten et al., 2003; Washburn, 1973). As seasonal ground temperatures fall below freezing, the crack infill freezes. Thermal contraction polygons of the Tuktoyaktuk peninsula, most of which are or have been underlain by ice, are ∼10–20 m in diameter. Ice-wedge polygons, ∼ 30–60 m in diameter, have been identified in the Fosheim Peninsula, Ellesmere Island, Canada (Lewkowicz and Duguay, 1999). Ice-wedge polygons that are ~ 60–80 m in diameter occur in the flood plains of the Yamal peninsula in Siberia, only to be surpassed in size by sea-terrace polygons in the region that are ∼100 m in diameter (Kuzmin et al., 2002). Thermal contraction polygons found on alas floors often show an “orthogonally-oriented” pattern (Fig. 4) (Lachenbruch, 1962) (i.e., the orientation of the polygons is normal (perpendicular) to and radial from the floor of the former thermokarst lake or pond). Lachenbruch (1962) relates the origin of the polygon orthogonality to “stress anisotropy”: horizontal disparities in stress (cracking conditions) induced by large horizontal temperature-differences. This occurs when previously unexposed and wet sediments are exposed to freezing temperatures and frost-cracking by the slow evaporation or drainage of lake or pond water (cf., Black, 1952; Everett, 1979; Kerfoot, 1972). By contrast, random and non-oriented polygons form when the loss of water is rapid (Lachenbruch, 1962). Orthogonal polygons could also be formed by two other means: in response to systematic topographic relief, i.e. gravity effects (Black, 1952; Lachenbruch, 1962) or to horizontal tensile stress as in steeply-dipping slates (Lachenbruch, 1962). 3. Possible thermokarst lakes and periglacial processes in Utopia and western Elysium Planitiae 3.1. Relict thermokarst features? Fig. 3. (a) Aerial view of pingo-thermokarst lake assemblage ∼6 km sw of Tuktoyaktuk. Split (centre-bottom of image) and Ibyuk (right-side of image). These pingos are ∼300 m across at their base. (b) View of Ibyuk from Split Pingo. (c) Collapsed pingo, ∼150 m across at its base (Tuktoyaktuk Coastlands, NWT, Canada). From Mackay (1986).

inwardly stepped or terraced appearance (Fig. 2a–c). These steps are geological markers. Each step represents a steady (albeit temporary) state of equilibrium as the lake or pond base-levels recede. Sometimes, thermokarst ponds or lakes grow and adjacent basins coalesce (Fig. 1c). This occurs in an aggrading environment where the high specific heat capacity – 4.187 (MJ/m3 K) – and thermal conductivity – 0.605 (W/m K) – of still water (Harris, 2002) promote the melting of peripheral ground ice and induce bank retreat through thermal and mechanical erosion (French, 2007; Washburn, 1973). When bank retreat occurs a gentle slope prevails; no tiers or terraces are produced. Often, as is the case with the MRDTC or the Laptev Sea region, the ice-rich environment in which thermokarst lakes form encompasses

Lobate, sometimes scalloped and rimless depressions dot the landscape in Utopia and western Elysium Planitiae (e.g. Fig. 1a) (Costard and Kargel, 1995; Lefort et al., 2007; Morgenstern et al., 2007; Seibert and Kargel, 2001; Soare et al., 2007). The depressions have relatively flat floors, lack raised rims and range in size from tens to hundreds of metres in diameter (Costard and Kargel, 1995; Morgenstern et al., 2007) (Figs. 1c and 5a–c). Their shape varies from round to roughly ovoid (Fig. 5a–c). The depressions are found on the walls, floors and ejecta structures of numerous impact craters in the region (Fig. 6); they are widespread in the surrounding plains (Fig. 6). Often, the depressions occur in assemblages that show a consistent loss of elevation on a scale of tens of meters (Morgenstern et al., 2007) along a north-south axis (Costard and Kargel, 1995; Morgenstern et al., 2007; Soare et al., 2007). Many of the depressions show inwardly-oriented terraces, tiers or benches (Fig. 5a–d), as well as superposed small-sized polygons that are oriented orthogonally (radially) (Fig. 5c–e).

386

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

Fig. 4. (a) Aerial view of thermal-contraction polygons oriented radially around a partially-drained thermokarst-lake (Tuktoyaktuk Coastlands, NWT, Canada). Image is approximately 150 m across. (b) Thermal-contraction polygons within a fully drained thermokarst-lake (i.e., an alas) (Tuktoyaktuk Coastlands, NWT, Canada). Image is approximately 80 m across. (c) An ice-and snow-covered alas showing radially-oriented thermal-contraction polygons (Tuktoyaktuk Coastlands, NWT, Canada). Image is approximately 250 m across.

Using all available Mars Orbiter Camera (MOC) narrow-angle images, we have carried out a systematic survey identifying the occurrence and distribution of the depressions within this region (240–285°W, 15–60°N, Fig. 7). Our data indicate that the depressions are ubiquitous, covering an area of ~ 2 × 106 km2 between longitudes 240–280°W and latitudes of 40–55°N (Fig. 7). This coincides roughly with the distribution of several late geological units, the youngest of which is the Astapus Colles unit “a” (cf., Morgenstern et al., 2007; Tanaka et al., 2005). The “a” unit overlies the Vastitas Borealis Formation. The latter generally shows few superposed impact craters and could be early Amazonian in age (Morgenstern et al., 2007; Tanaka et al., 2005). The former comprises a ~ 30–40 m thick ice-rich mantle that is thought to be late Amazonian in age and a product of obliquity-driven changes in global climate (Tanaka et al., 2005). Consistent with this chronological estimate is the fact that most depression margins are sharp and continuous. They show little or no sign of weathering, erosion or slumping. Moreover, high resolution HiRISE images of the depressions show no superposed impact craters greater than a few 10's of meters in size. Based on our earlier work, which identified alas-like depressions superposed on some crater-wall gullies in Utopia and western Elysium Planitiae (Fig. 8) (Soare et al., 2007), we suggest that at least some of the depressions in the region formed very recently. Gullies are amongst the youngest of Martian landforms identified so far (e.g., Christensen, 2003; Heldmann et al., 2005; Malin and Edgett, 2000; Malin et al., 2006; Mellon and Phillips, 2001; Reiss et al., 2004). For example, Reiss et al. (2004) found a number of gully aprons that were superposed on aeolian dunes in Nirgal Vallis. Using dune-crater counts as the means to estimate the recentness of dune activity, they

found that the dune activity occurred between 1.4 Mya and 0.3 Mya (Reiss et al., 2004). Superposition of the gully aprons on the dunes indicates that the former are younger than the latter. Recently, Malin et al. (2006) have documented a small set of gully-like flow-features on impact crater-walls that seem to have formed some time over the past seven years, during the lifetime of the Mars Global Surveyor, Mars Orbital Camera. We do not hypothesise that the depressions superposing the gullies in Utopia and western Elysium Planitiae were formed as recently as the flow features identified by Malin et al. (2006). However, we do suggest that these depressions, and possibly others in the region, are youthful, having originated in the very late Amazonian period and that their origin postdates the formation of some gullies. 3.2. Raised-rim (possibly collapsed-pingo) landforms Free-standing roughly circular-mounds that are crossed by smallsized polygonal patterned-ground also are ubiquitous in the region and are thought to be pingos (Fig. 9) (Allen and Kanner, 2007; De Pablo and Komatsu, 2007; Dundas et al., 2008; Osinski and Soare, 2007). Within our study area (particularly ~ 39–42 °N, 262–280 °W), numerous circular to irregularly-shaped, raised-rimmed landforms also occur (Osinski and Soare, 2007). The landforms are meso-scale in size (ranging from ~ 100–600 m in diameter); often, they are flatfloored (Fig. 9b). In some instances the landforms appear to have coalesced; sometimes they are clustered (Fig. 9b,c). Often, small-sized polygonal patterned-ground encompasses the landforms; on occasion the landforms are nested within the rimless depressions of the type discussed above. These depressions may be mistaken for secondary

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

387

Fig. 5. (a) Rimless depressions similar to terrestrial alases in Utopia Planitia, Mars (HiRISE TRA 000856-2265, 46°N, 268°W, NASA/JPL/University of Arizona). Note the coalescence of the depressions and the inwardly-oriented terraces possibly indicating antecedent high water marks (cf., Fig. 2a,b,c). (b and c) Enlargements of a, showing well-developed inwardlyoriented terraces. (d) Portion of HiRISE image PSP 001331_2260 showing small-sized polygons oriented radially or orthogonally within alas-like depression in Utopia Planitia (45.62°N, 266.4°W, NASA/JPL//University of Arizona). (e) Enlargement of a. Note the decrease in polygon size within the alas-like depression (bottom). The latter are also oriented radially or orthogonally with respect to the depression perimeter.

impact crater fields; however, ejecta structures, indicative of an impact origin, are absent from the feature margins (Fig. 9c). In addition, these raised-rimmed landforms are concentrated in particular units (e.g., bright regions in Fig. 9b) and sometimes occur within the rimless depressions. Such a correlation is inconsistent with a secondary impact crater origin. Under current conditions of low atmospheric pressure, temperature and humidity at latitudes above ∼ ±40° but below ~ ± 60°, nearsurface ground ice (at depths b1–2 m) is unstable and subject to

sublimation (Costard and Kargel, 1995; Fanale et al., 1986; Mellon and Jakosky, 1995; Mellon et al., 1997); at higher obliquities, this near-surface ground ice would have been stable (Fanale et al., 1986; Mellon and Jakosky, 1995; Mellon et al., 1997). At depths N2 m, perhaps under the protection of a sublimed dessication-lag (Head et al., 2003), relict ground ice to tens of metres of depth could still be in place (Costard and Kargel, 1995; Mellon and Jakosky, 1995). Importantly, the spatial distribution of the rimless depressions mapped by us (Fig. 7) and of the raised-rim landforms coincides with

388

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

Fig. 6. Alas-like depressions on (a) a crater floor, (b) a crater wall, (c) an ejecta structure and (d) in the plains of Utopia Planitia (44.93°N, 274.91°W, THEMIS image V05265217; image width ~ 3.2 km; NASA/JPL/ASU).

the latitudinal band where buried ground-ice dozens of metres deep could be present and stable (Fanale et al., 1986; Mellon and Jakosky, 1995; Mellon et al., 1997). 4. Landscape evolution and thermokarst processes in the northern plains 4.1. The origin of an ice-rich mantle in Utopia and western Elysium Planitiae The origin and emplacement of an ice-rich mantle in Utopia and western Elysium Planitiae, creating conditions that are necessary for and antecedent to thermokarst processes, could have occurred in the very late-Amazonian period as the result of six sequentially-related and obliquity-driven events (cf., Soare et al., 2007): 1 atmospheric transportation and subsequent deposition of water ice-dust mantle

from the north pole to the middle northern-latitudes (cf., Costard and Kargel, 1995; Head et al., 2003; Head et al., 2005; Head et al., 2006; Kargel, 2004; Mustard et al., 2001). A recent complementary hypothesis proposes that a latitudinally-based transfer of water ice also could have occurred between the Tharsis/Olympus volcanoes and the northern plains (Costard et al., 2008; Forget et al., 2006; Madeleine et al., 2007); 2. formation of an ice-rich mantle (Costard et al., 2008; Costard and Kargel, 1995; Head et al., 2003; Head et al., 2005; Head et al., 2006; Kargel, 2004; Madeleine et al., 2007; Mustard et al., 2001) and its subsequent thaw (Soare et al., 2007); 3. meltwater saturation of the near-surface regolith to meters, possibly tens of meters of depth; 4. freezing of the saturated regolith in situ; 5. localized thaw of the ice-rich regolith; and, 6. possible pooling of meltwater in a pond or a lake followed by its loss through evaporation or drainage, forming an alas. Support of the idea that the rimless depressions are the product of thermokarst processes and of sublimation in a recently emplaced and regional ice-rich mantle, as opposed to aeolian, cratering, tectonic or volcanic processes, seems to be widespread within the planetary science community (e.g., Bentham and Howard, 2008; Costard and Kargel, 1995; Seibert and Kargel, 2001; Costard et al., 2008; Morgenstern et al., 2007; Soare et al., 2007). A few workers have raised the possibility that the lobate depressions are formed by ponding and, subsequently, by episodic evaporation or drainage (Costard and Kargel, 1995; Soare et al., 2007). Here, we discuss two geological features of the depressions – 1. inward tiers or terraces; and, 2. orthogonally-oriented polygons – and show that their occurrence is consistent with this possibility. We also suggest that if the raised-rim depressions are collapsed pingos or pingo scars, then this offers further support to a landform-evolution hypothesis based on the presence of stable ponded-water and its subsequent loss by episodic evaporation or drainage. 4.2. Terracing and ponding in the rimless depressions As noted above (Section 2.3), the inwardly-oriented terraces in terrestrial alases represent individual equilibrium points, distinct and easily identifiable stages of recession (or of high water) in the

Fig. 7. Distribution of alas-like depressions within Utopia and western Elysium Planitia, as mapped during this study, superposed on a hillshade image created from the Mars Global Surveyor MOLA topographic 16 ppd digital elevation model and a portion of the Geologic Map of the Northern Plains of Mars, Map 2888 (Tanaka et al., 2005). One degree of latitude on Mars equals ~ 59 km. ABa = Astapas Colles unit; ABvi = Vastistas Borealis interior unit; ABvm = Vastistas Borealis interior unit; AEta = Tinjar Valles a unit; AEtb = Tinjar Valles a unit; AHc = Crater unit; AHEe = Elysium rise unit; HBu1 = Utopia Planitia 1 unit; HBu2 = Utopia Planitia 2 unit; HNn = Libya Montes unit.

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

389

Fig. 8. Impact crater in Utopia Planitia with well-defined gullies along the crater walls and alas-like depressions in the crater interior. Note that many gully aprons are cross-cut by the alas-like depressions (arrows) (MOC image S04-00681, 50.5°N, 276.°W, MSSS/JPL). Crater is ~ 7.5 km across.

transition from a basin full of ponded water to an empty one (Fig. 2a–c). Given appropriate boundary conditions above the triple point of water (under the influence of high obliquity), the tiers or terraces within the Martian depressions could mark points of relative sequentialequilibria, as ponded water is lost by episodic or intermittent evaporation or drainage. Sometimes, the terraces are neither radial nor symmetrical; they show sharply-tiered losses of elevation polewardly and graduallyuntiered losses of elevation towards the equator. We concur with Morgenstern et al. (2007), who suggested that the north-south differences in symmetry are due to insolation. As Mars moves away from high obliquity, the southward-facing slopes increasingly receive more solar energy; being warmer, these slopes would be more susceptible to the loss of near-surface ground ice by means of slumping and mass wasting than the northward slopes (Morgenstern et al., 2007). Were the rimless depressions formed by sublimation, one would expect the newly-dessicated regolith to be dotted with small surficial pockmarks and pits. This appears to be the case in isolated regions of Arcadia Planitia (Fig. 10). This difference may be due to regional variations in the present-day and/or paleoclimate during the last phase of high obliquity. A further possible explanation may lie in the physical properties of the regolith in the two regions. Frozen soils, by definition, have a higher cohesive-strength than unfrozen soils (Guymon et al., 1980). However, this strength can decrease rapidly under confining stress (Chamberlain, 1985) or as a function of thermal disequilibrium. More porous materials with high water content (e.g., clays) will exhibit strong ice-crystal structure and cohesive strength, resulting in a higher resistance to pressure melting and/or sublimation. This suggests that the pockmarks and pits could be the result of differential ice-cohesion and binding. Indeed, it is hypothesized that regolith of the Martian mantle has a mean uncompacted porosity of 54% and a compacted porosity of 44% (Allen et al., 1998), indicating that unconsolidated materials dominate. This can, therefore, support lower thermal conductivity, unless there is a high degree of cementation by either salt or ice. In highly compacted regoliths with high cohesivestrength, substantial sublimation should be limited to small regions due to the adsorption of water molecules to smectite clays which can block the molecules until adsorption equilibrium is reached; this depends highly on the thickness and homogeneity of the regolith (Ostrowski et al., 2004). Consequently, localized pockets of sublimation that are metres in scale should be produced rather than entire depressional-basins and would delineate the area where ground-ice loss occurs. Even if a column of ice-saturated regolith were exposed to multiple episodes of sublimation, this would produce pockmarks and pits characterized by greater depth or amplitude such as in Arcadia Planitia (Fig. 10), not inwardly-stepped terraces. The Martian terraces could be retrogressive thaw-slumps and active-layer detachments, which occur in ice-rich landscapes on Earth (Fig. 10). Retrogressive thaw-slumps are a type of mass periglacialmovement. They are initiated by the rapid exposure and thaw of icerich ground (Burn, 2000; French, 2007; Lantuit and Pollard, 2007; Summerfield, 1991; Washburn, 1973). Two commonplace triggers are basal undercutting by running river-bank water (Summerfield, 1991) or by lacustrine/marine wave action (Lantuit and Pollard, 2007).

Thaw slumps form by headwall or retrogressive ablation, which continues until either the exposed or affected ground ice has thawed, the headwall is insulated by slumped material or the foot slope rises to the surrounding terrain (Burn, 2000). The headwalls, comprised of

Fig. 9. (a) Pingo-like mounds in a region of small-sized polygonal patterned ground. Portion of MOC image e500488 (41.7°N, 277.2°W). (b) Sub-circular raised-rimmed depressions interpreted as collapsed pingos or pingo scars. Note the close association with particular substrates (e.g., bright areas) and alas-like depressions. Portion of THEMIS image V12342003 (42.5°N, 277.9°W, NASA/JPL/ASU). (c) Two impact craters displayed ejecta blankets with a cluster of raised-rimmed depressions to the left. Portion of THEMIS image V12342003. Scale bars are 1 km. Modified from Osinski and Soare (2007).

390

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

Fig. 10. Pitted, possible sublimation-landscape in Arcadia Planitia (HiRISE image PSP 004097-2185, 38.1°N, 169.8°W, NASA/JPL/University of Arizona).

(non-slumped) frozen ground, are steep and may reach ~ 15–20 m in height (Wolfe et al., 2001). By contrast, the foot slopes and slide planes underlying them are of a low gradient (Burn, 2000). Sometimes, headwall/foot-slope assemblages have the appearance of arcuate embayments (Fig. 11; Summerfield, 1991). On occasion, the thaw and exposure of ice-rich ground is prompted by active layer detachments (Burn, 2000; Lewkowicz, 2007). Activelayer detachments are a type of shallow landslide that may be triggered by the rise of regional mean-temperatures (Lantuit and Pollard, 2007; Lewkowicz, 2007). These temperature rises lead to a deepening of the active layer, the localised mobilisation of meltwater previously frozen in the upper part of the permafrost column and an increase in porewater pressure (Summerfield, 1991). The increase in pore-water pressure, accompanied by a decrease in effective strength at the base of the thaw mass, make the thaw mass highly susceptible to sliding or slumping (Lewkowicz, 2007). Although the morphology and scale of the Martian terraces is consistent with that of landscapes revised by retrogressive thawslumping on Earth, analogous processes on Mars would require undercutting or erosional work to be done by running or standing water alongside an ice-rich mass. Our explanation of terrace formation differs only in that we hypothesise an ice-rich recessional-landscape in which water is being lost episodically by evaporation or drainage, rather than an ice-rich erosional-landscape where slumping and headwall formation are being induced.

Similarly, active-layer detachments in periglacial environments on Earth require the substantial presence of high pore-water pressure and of basal meltwater beneath a thawed mass. This is consistent with the assumption that the alas-like features identified by us occur in an ice-rich unit that has been modified and thawed by recent obliquities. However, we point to the orthogonal draping of orthogonally-oriented and small-sized polygons on the Martian terraces as being a geological marker of ponding and of a highly localised lacustrine system in recession. On Earth, the orthogonal polygons are neither associated with retrogressive thaw-slumping nor with active-layer detachments. 4.3. The orthogonal orientation of small-sized (possible ice-wedge) polygons in Utopia and western Elysium Planitiae Recent HiRISE images show that small-sized polygons are ubiquitous within and superpose the alas-like depressions of Utopia and western Elysium Planitiae and in the surrounding plains (e.g., Fig. 5). Their size – ~ 5 m across for the inner polygons and 50–100 m diameter for the outer plains polygons (cf., Lefort et al., 2007) – lies well within the mean of terrestrial thermal contraction polygons underlain by ice wedges (Lachenbruch, 1962). Furthermore, as the polygons superpose the depressions, their origin and evolution must have superseded that of the depressions themselves. The radial or orthogonal orientation of some inner polygons is also widespread within the depressions (e.g., Fig. 5d,e). On the basis of terrestrial analogues such as those found in the MRDTC, we hypothesise that the polygons could have been underlain by ice wedges and that the orthogonal orientation of some polygons is the result of ponded water having been lost episodically by evaporation or drainage. Unequivocal evidence of the Martian polygons having been underlain by ice-wedges is lacking. However, three considerations are consistent with the possible occurrence of ice wedges beneath the polygons. First, the polygons occur at a latitude – poleward of 40° – where near-surface ground ice is thought to have been emplaced (Costard et al., 2008; Costard and Kargel, 1995; Head et al., 2003; Head et al., 2005; Head et al., 2006; Kargel, 2004; Madeleine et al., 2007; Mustard et al., 2001) in a stable state (Fanale et al., 1986; Mellon and Jakosky, 1995; Mellon et al., 1997; Mustard et al., 2001), perhaps under the protective blanket of a dusty sublimation-lag, in the very late

Fig. 11. Retrogressive thaw slump ∼5 km southwest of Tuktoyaktuk (early July 2007); long axis of land mass, ∼1 km.

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

Amazonian period. Second, Mustard et al. (2001) identify characteristics associated with viscous creep at these latitudes and suggest that this type of movement rheologically is consistent with an icesaturated regolith. Moreover, this geological interpretation is consistent with our belief that the rimless depressions, raised-rim landforms and small-sized polygons are the work of periglacial processes and of freeze-thaw cycles in a regional ice-rich landscape. As noted above (Section 2.3), other than the episodic loss of pooled water from a lake or pond basin there are two alternative explanations of polygon orthogonality: (a) systematic topographic relief (gravity effects) or (b) horizontal tensile stress in steeply-dipping slates. Close observation of the rimless depressions in our study region shows that there is a loss of elevation associated with the inward-looking terraces or tiers nested on the depression embankments. Orthogonal polygons seem to drape these terraces and, hypothetically, could have been formed by slope-induced stress. On the other hand, if the plains of Utopia and western Elysium are blanketed by a metres-thick mantle of ice (Costard and Kargel, 1995; Head et al., 2003; Head et al., 2005; Head et al., 2006; Kargel, 2004; Mustard et al., 2001) upon which the rimless depressions rest, then a periglacial intepretation of polygon orientation is at least as plausible as a topographic one. However, when the periglacial intepretation is buttressed by a landscape assemblage whose constituents show characteristics consistent with freeze-thaw processes and ponded water, its plausibility increases substantially. For polygon orientation to have occurred on steeply-dipped beds of shale, two antecedent conditions would have been necessary: lowgrade regional metamorphism, transforming a shale-type rock into slate, and faulting. Observations in favour of these conditions have not been reported in the literature. 4.4. Possible collapsed pingos in Utopia and western Elysium Planitiae Circular and near-circular raised-rim depressions are a late-stage characteristic of pingo evolution or, in a sense, degradation, on Earth. Evaluating whether the raised-rim mesoscale depressions identified by us in Utopia and western Elysium Planitia are degraded or collapsed pingos requires asking whether the polygonally-crossed sometimes fractured mounds thought to be pingos in the region possibly are precursor (periglacial) landforms. Towards this end, mound formation hypotheses not associated with periglacial processes have to be considered. Salt (Dundas et al., 2008) or mud diapirism (Costard and Kargel, 1995; Dundas et al., 2008) are two possible “uplift” processes associated with the formation of the pingo-like mounds. Salt diapirism, however, seems to be an unlikely candidate. The hypothesised scale of Martian salt diapirs is an order of magnitude larger (Beyer et al., 2000; Dundas et al., 2008) than the mounds found in Utopia and western Elysium Planitiae. Mud diapirism is equally improbable, as the key characteristics associated with mud diapirs putatively identified in southern Utopia Planitia do not match those of the pingo-like mounds in our study region (Dundas et al., 2008). For example, some of the hypothesised mud-volcanoes are composed of thin, overlapping lobes and often are located close to or on top of wrinkle ridges or large arches (Skinner and Tanaka, 2007). Moreover, these landforms also are an order of magnitude wider and higher than the mounds of Utopia and western Elysium Planitiae, and are thought to be the product of complex, overlapping geological processes that incorporate the mobilisation of soft-sediments (Skinner and Tanaka, 2007). Possible origins of the raised-rim landforms in Utopia and western Elysium Planitiae are impact cratering, lava-groundwater or magma/ ice interaction or, once again, soft-sediment mobilisation by mudvolcanism. As there are no impact or ejecta structures associated with the raised-rim features, an impact origin is unlikely. Debate concerning whether raised-rim features in Athabasca Valles are “rootless cones”, the product of multiple magmatic and aqueous flooding events remains unresolved (Lanagan et al., 2001; Page and Murray,

391

2006). However, the raised-rim landforms of Utopia and western Elysium Planitiae occur in a recent geological unit comprising an rich ice-dust mantle, not in units of volcanic and/or aqueous provenance; therefore, a “rootless” origin is unlikely. A soft-sedimentary origin seems just as unlikely. Pitted possibly sedimentary-cones in southern Utopia Planitia are almost an order of magnitude larger and higher than the raised-rim features to the north (in our study region) (Skinner and Tanaka, 2007). Also, the cones have broad-sloping flanks, and are associated with lobate materials, neither of which are characteristics of the northerly landforms (Skinner and Tanaka, 2007). 5. Age constraints on periglacial landforms and possible ponding We believe that the formation of the raised-rim landforms in Utopia and western Elysium Planitiae is consistent with the occurrence of ice-rich regolith, periglacial processes and a very recent origin. First, in morphology and scale, the landforms are similar to terrestrial collapsed pingos or pingo scars. Second, the spatial coincidence of the raised-rim landforms, the rimless depressions and small-sized polygonal patterned-ground, across a broad latitudinalband in our study region, is striking. This co-incidence is consistent with and possibly a geological marker of two things: 1. near-surface (extant or past) ground ice; and, 2. the occurrence of freeze-thaw processes in an environment where surface water could have ponded and been stable. Third, if the rimless depressions are youthful lateAmazonian landforms (see Section 3.1), then the raised-rim depressions that superpose the lobate depressions in some instances must be even younger. In turn, if the raised-rim depressions are pingo scars, especially if the pingos were formed by hydrostatic processes, then this points to triple-point conditions of temperature and pressure that are tolerant of and consistent with the occurrence of ponded water. 6. Concluding remarks There is general agreement amongst workers that the lobate depressions of Utopia and western Elysium Planitiae are the result of thermokarst processes; however, opinion concerning the boundary conditions associated with the evolution of these and other spatiallyassociated features has tended to favour sublimation. By contrast, we show that the tiered and orthogonally-oriented polygons draped on the depressions could be geological markers of ponded water and its episodic loss by evaporation or drainage. The stratigraphy of the rimless depressions and raised-rim landforms also suggests a very youthful origin of these and associated landscape features. We suggest that boundary conditions in the region during the very late Amazonian could have been slightly warmer and wetter than most workers have thought possible. Acknowledgements Fieldwork in Inuvik and Tuktoyaktuk, was supported by grants from the Canadian Space Agency, the Aurora Research Institute and the Northern Scientific Training Programme (Department of Indian and Northern Affairs, Government of Canada). Permits acquired from the Aurora Research Institute, Parks Canada (Western Arctic office), the Inuvialuit Land Administration, the Hunters and Trappers Associations of Inuvik and Tuktoyaktuk, N.W.T., the Environmental Impact Screening Committee, the Gwich'in Land Administration and the Gwich'in Renewable Resource Board facilitated access to the field sites. Special thanks must be given to James Pokiak and John Roland for their assistance in the field. Many thanks to Maude Ouellet, Bimal Patel and Geoff Pearce who were involved in the detailed examination of the MOC database in search of periglacial landforms. Finally, we thank the MOC, THEMIS, and HiRISE teams for the excellent imagery without which this study would not have been possible.

392

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393

References Allen, C.C., Morris, R.V., Jager, K.M., Golden, D.C., Lindstrom, D.J., Lindstrom, M.M., 1998. Martian regolith simulant JSC Mars-1. Lunar and Planetary Science Conference 29, Abstract 1690. Allen, C.C., Kanner, L.C., 2007. Exposures of water ice in the northern mid-latitudes of Mars. 7th International Mars Conference, Abstract 3065. Bennike, O., 1998. Pingos at Nioghalvfjerdsfjorden, eastern North Greenland. Geol. Greenl. Surv. Bull. 180, 159–162. Bentham, A.J., Howard, A.D., 2008. Elevated flat-floored depressions in the mid-latitude region of Mars. Lunar and planetary Science Conference 39, Abstract 2333. Beyer, R.A., Melosh, H.J., McEwen, A.S., Lorenz, R.D., 2000. Salt diapirs in Candor Chasma, Mars? Lunar and Planetary Science Conference 31, Abstract 2022. Black, R.F., 1952. Polygonal patterns and ground conditions from aerial photographs. Photogramm. Eng. 18 (1), 123–134. Burn, C.R., 2000. The thermal regime of a retrogressive thaw slump near Mayo, Yukon Territory. Can. J. Earth Sci. 37, 967–981. Burr, D.M., Soare, R.J., Wan Bun Tseung, J.M., Emery, J.P., 2005. Young (late Amazonian), near-surface, ground ice features near the equator, Athabasca Valles, Mars. Icarus 178, 56–73. Chamberlain, E.J., 1985. Shear strength anisotropy in frozen saline and freshwater soils, The Fourth International Symposium on Ground Freezing: Sapporo, Japan, p. 6. Christensen, P.R., 2003. Formation of recent Martian gullies through melting of extensive water-rich snow deposits. Nature 422, 45–48. Costard, F.M., Kargel, J.S., 1995. Outwash plains and thermokarst on Mars. Icarus 114, 93–112. Costard, F., Forget, F., Madeleine, J.B., Kargel, J., 2008. The origin and formation of scalloped terrain in Utopia Planitia: insight from a general circulation model. Lunar and Planetary Science Conference 39, Abstract 1274. Cruickshank, J.G., Calhoun, E.A., 1965. Observations on pingos and other landforms in Schuchertdal, northeast Greenland. Geogr. Ann. 471, 224–236. de Pablo, M.A., Komatsu, G., 2007. Pingo fields in the Utopia basin, Mars: 200 Geological and climatic implications. Lunar and Planetary Science Conference 38, Abstract 1278. Dundas, C.M., Mellon, M.T., McEwen, A.S., Lefort, A., Keszthelyi, L.P., Thomas, N., 2008. HiRISE observations of fractured mounds: possible Martian pingos. J. Geophys. Res. 35, L04201. doi:10.1029/2007GL031798. Everett, K.R., 1979. Evolution of the soil landscape in the sand region of the Arctic coastal plains as exemplified at Aktasook, Alaska. Arctic 32 (3), 207–233. Fanale, F.P., Salvail, J.R., Zent, A.P., Postawko, S.E., 1986. Global distribution and migration of subsurface ice on Mars. Icarus 67, 1–18. Forget, F., Haberle, R.M., Montmessin, F., Levrard, B., Head, J.W., 2006. Formation of glaciers on Mars by atmospheric precipitation at high obliquity. Science 311 (5759), 368–371. French, H.M., 1993. Cold-climate processes and landforms. In: French, H.M., Slaymaker, O. (Eds.), Canada's Cold Environments. McGill Queens Univ. Press, Montreal, pp. 271–290. French, H.M., 2007. The periglacial environment, 3rd edition. John Wiley & Sons. 458 pp. Guymon, G., Berg, R., Hromadka, T., 1980. A one-dimensional frost heave model based upon simulation of simultaneous heat and water flux. Cold Reg. Sci. Technol. 3, 253–263. Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., 2001. On the possibility of liquid water on present-day Mars. J. Geophys. Res. 106 (E10), 23,317–23,326. Harris, S.A., 2002. Causes and consequences of rapid thermokarst development in permafrost or glacial terrain. Permafr. Periglac. Process. 13, 237–242. Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent ice ages on Mars. Nature 426, 797–802. Head, J.W., et al., the HRSC Co-investigator Team, 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature 434, 346–350. Head, J.W., Marchant, D.R., Agnew, M.C., Fassett, C.I., Kreslavsky, M.A., 2006. Extensive valley glacier deposits in the northern mid-latitudes of Mars: evidence for Late Amazonian obliquity-driven climate change. Earth Planet. Sci. Lett. 241, 663–671. Hecht, M.H., 2002. Metastability of liquid water on Mars. Icarus 156, 373–382. Heldmann, J.L., Toon, O.B., Pollard, W.H., Mellon, M.T., Pitlick, J., McKay, C.P., Anderson, D.T., 2005. Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions. J. Geophys. Res. 110, E05004. doi:10.1029/2004JE002261. Kargel, J.S., 2004. Mars: a warmer, wetter planet. Springer-Praxis, New York. 557 pp. Kerfoot, D.E., 1972. Thermal contraction cracks in an arctic tundra environment. Arctic 25, 142–155. Kuzmin, R.O., Ershow, E.D., Komarow, L.A., Kozlov, A.H., Isaev, V.S., 2002. The comparative morphometric analysis of polygonal terrain on Mars and the Earth high latitude areas. Lunar and Planet Science 33th. Abstract 2030. Lachenbruch, A.H., 1962. Mechanics of thermal contraction cracks and ice-wedge polygons in permafrost. Geol. Soc. Amer., Special Paper 70 69 pp. Lanagan, P.D., McEwen, A.S., Keszthelyi, L.P., Thordarson, T., 2001. Rootless cones on Mars indicating the presence of shallow equatorial ground ice in recent times. Geophys. Res. Lett. 28, 2365–2368. Lantuit, H., Pollard, W.H., 2007. Fifty years of coastal erosion and retrogressive thaw slump activity on Herschel Island, southern Beaufort Sea, Yukon Territory, Canada. Geomorphology 95 (1–2), 84–102. Lasca, N.P., 1969. The surficial geology of Skeldal, Mesters Vig, northeast Greenland. Medd. Gronl. 176 (3), 2–54.

Lefort, A., et al., the HiRISE Team, 2007. Scalloped terrains in Utopia Planitia, insight from HiRISE. Lunar and Planetary Science Conference 38th, Abstract 1796. Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2004. Recent ice-rich deposits formed at high latitudes on Mars by sublimation of unstable equatorial ice during low obliquity. Nature 431 (7012), 1072–1075. Lewkowicz, A.G., Duguay, C.R., 1999. Detection of permafrost features using SPOT Panchromatic Imagery, Fosheim Peninsula, Ellesmere Island, N.W.T. Can. J. Remote Sens. 25, 34–44. Lewkowicz, A.G., 2007. Dynamics of active-layer detachment failures, Fosheim Peninsula, Ellesmere Island, Nunavut, Canada. Permafr. Periglac. Process. 18, 89–103. Mackay, J.R., 1962. The pingos of the Pleistocene Mackenzie Delta area. Geogr. Bull. 18, 21–63. Mackay, J.R., 1966. Pingos in Canada. Permafrost International Conference, Proceedings. National Research Council, vol. 1287. National Academy of Science, Ottawa, Canada, pp. 71–76. Publication. Mackay, J.R., 1972. Some observations on the growth of pingos, Mackenzie Delata Area Monograph, 22nd International Geographical Congress, Ed. D.E.Kerfoot, Brock University, Ontario. Mackay, J.R., 1979. Pingos of the Tuktoyaktuk Peninsula area, Northwest Territories. Geogr. Phys. Quat. 33 (1), 3–61. Mackay, J.R., 1986. The first seven years of ice wedge growth (1978–1985), Illisarvik experimental drained lake site, western arctic coast. Can. J. Earth Sci. 23, 1782–1795. Mackay, J.R., 1998. Pingo growth and collapse, Tuktoyaktuk Peninsula area, western Arctic coast, Canada: a long-term field study. Geogr. Phys. Quat. 52 (3), 271–323. Madeleine, B., Forget, F., Head, J.W.I., Levrard, B., Montmessin, F., 2007. Mars: a proposed climatic scenario for northern mid-latitude glaciation. Lunar and Planetary Science Conference 38, Abstract 1778. Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335. Malin, M.C., Edgett, K.S., Posiolova, L.V., McColley, S.M., Dobrea, E.Z.N., 2006. Presentday impact cratering rate and contemporary gully activity on Mars. Science 314, 1573–1577. Marchant, D.R., Lewis, A.R., Phillips, W.M., Moore, E.J., Souchez, R.A., Denton, G.H., Sugden, D.E., Potter Jr., N., Landis, G.P., 2002. Formation of patterned ground and sublimation till over Miocene glacial ice in Beacon Valley, southern Victoria Land, Antarctica. Geol. Soc. Amer. Bull. 114 (6), 718–730. Mellon, M.T., 1997. Small-scale polygonal features on Mars: seasonal thermal contraction cracks in permafrost. J. Geophys. Res. 102, 25617–25628. Mellon, M.T., Jakosky, B.M., Postawko, S.E., 1997. The persistence of equatorial ground ice on Mars. J. Geophys. Res. 102 (E8), 19,357–19,369. Mellon, M.T., Phillips, R.J., 2001. Recent gullies on Mars and the source of liquid water. J. Geophys. Res. 106, 23165–23180. Mellon, M.T., Jakosky, B.M., 1995. The distribution and behaviour of martian ground ice during past and present epochs. J. Geophys. Res. 100, 11781–11799. Morgenstern, A., Hauber, E., Reiss, D., van Gasselt, S., Grosse, G., Schirrmeister, L., 2007. Deposition and degradation of a volatile-rich layer in Utopia Planitia, and implications for climate history on Mars. J. Geophys. Res. 112, E06010. doi:10.1029/ 2006JE002869. Müller, F., 1962. Observations on pingos. Medd. Gronl. 153 (3), 1–117 Translated by D.A. Sinclair, National Research Council of Canada. Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice. Nature 412, 411–414. O'Brien, R., 1971. Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland. Medd. Gronl. 195 (1), 1–19. Osinski, G.R., Soare, R.J., 2007. Circular structures in Utopia Planitia, Mars: impact v. periglacial origin and implications for assessing ground-ice content. Lunar and Planetary Science Conference 38th Abstract 1609. Ostrowski, D.R., Chevries, V., Chastain, B.K., Sears, D.W.G., 2004. Experimental study of the water vapor interaction with clay regolith during ice sublimation on Mars. Lunar and Planetary Science Conference 38, Abstract 2097. Page, D.P., Murray, J.B., 2006. Stratigraphical and morphological evidence for pingo genesis in the Cerberus plains. Icarus 183, 46–54. Page, D.P., 2007. Recent low-latitude freeze-thaw on Mars. Icarus 189, 83–117. Reiss, D., van Gasselt, S., Neukum, G., Jaumann, R., 2004. Absolute dune ages and implications for the time of formation of gullies in Nirgal Vallis, Mars. J. Geophys. Res. 109, E06007. doi:10.1029/2004JE002251. Romanovskii, N.N., Hubberten, H.W., Gavrilov, A.V., Tumskoy, V.E., Tipenko, G.S., Grioriev, M.N., Siegert, C.H., 2000. Thermokarst and Land-Ocean interactions, Laptev Sea Region, Russia. Permafr. Periglac. Process. 11, 137–152. Seibert, N.M., Kargel, J.S., 2001. Small-scale Martian polygonal terrain: implications for liquid surface water. Geophys. Res. Lett. 28, 899–902. Shean, D.E., Head, J.W., Fastook, J.L., Marchant, D.R., 2007. Recent glaciation at high elevations on Arsia Mons, Mars: implications for the formation and evolution of large tropical mountain glaciers. J. Geophys. Res. 112, E03004. doi:10.1029/ 2006JE002761. Skinner, J.A., Tanaka, K.L., 2007. Evidence for and implications of sedimentary diapirism and mud volcanism in the southern Utopia highland-lowland boundary plain, Mars. Icarus 186, 41–59. Sletten, R.S., Hallett, B., Fletcher, R.C., 2003. Resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground. J. Geophys. Res. 108 (E4), 8044. Soare, R.J., Burr, D.M., Wan Bun Tseung, J.M., 2005a. Possible pingos and a periglacial landscape in northwest Utopia Planitia. Icarus 174, 373–382. Soare, R.J., Wan Bun Tseung, J.M., Peloquin, C., 2005b. Possible thermokarst and alas formation in Utopia Planitia, Mars. Lunar and Planetary Science Conference 36, Abstract 1103.

R.J. Soare et al. / Earth and Planetary Science Letters 272 (2008) 382–393 Soare, R.J., Kargel, J.S., Osinski, G.R., Costard, F., 2007. Thermokarst processes and the origin of crater-rim gullies in Utopia and western Elysium Planitia. Icarus 191, 95–112. Soare, R.J., Osinski, G.R., Costard, F., 2008. Recent, late Amazonian pingos, ice-rich landscapes and periglacial ponding in Utopia and western Elysium Planitia, Mars. Lunar and Planetary Science Conference 39, Abstract 1315. Summerfield, M.A., 1991. Global Geomorphology. Pearson Education Ltd, Harlow, England.

393

Tanaka, K.L., Skinner, J.A.J., Hare, T.M., 2005. United States Geological Survey, Atlas of Mars: Northern Plains Region, 1:15,000,000, Map 2888. Washburn, A.L., 1973. Periglacial processes and environments. St. Martin's Press, New York. 320 pp. Wolfe, S.A., Kotler, E., Dallimore, S.R., 2001. Surficial characteristics and the distribution of thaw landforms (1970 to 1999), Shingle Point to Kay Point, Yukon Territory. Open File 4115. Geol. Surv. Can. CD-ROM.