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Planetary and Space Science 57 (2009) 541–555

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Planetary and Space Science

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Pingos on Earth and Mars

Devon M. Burr a,b,Ã, Kenneth L. Tanaka c, Kenji Yoshikawa d a Earth and Planetary Sciences Department, University of Tennessee Knoxville, Earth and Planetary Sciences Department Building 412, 1412 Circle Dr., Knoxville, TN 37996, USA b Carl Sagan Center, SETI Institute, 515 N Whisman Rd, Mountain View, CA 94043, USA c USGS Astrogeology Team, 2255 N. Gemini Dr., Flagstaff, AZ 86001, USA d Institute of Northern Engineering, University of Alaska, Fairbanks, AL 99775, USA article info abstract

Article history: Pingos are massive ice-cored mounds that develop through pressurized groundwater flow mechanisms. Received 25 March 2008 Pingos and their collapsed forms are found in periglacial and paleoperiglacial terrains on Earth, and have Received in revised form been hypothesized for a wide variety of locations on Mars. This literature review of pingos on Earth and 9 November 2008 Mars first summarizes the morphology of terrestrial pingos and their geologic contexts. That Accepted 12 November 2008 information is then used to asses hypothesized pingos on Mars. Pingo-like forms (PLFs) in Utopia Available online 27 November 2008 Planitia are the most viable candidates for pingos or collapsed pingos. Other PLFs hypothesized in the Keywords: literature to be pingos may be better explained with other mechanisms than those associated with Mars terrestrial-style pingos. Surface & 2008 Elsevier Ltd. All rights reserved. Ground ice Periglacial Terrestrial analogues

1. Introduction shape. As discussed in Cabrol et al. (2000) and Burr et al. (2005), pingo size is not a function of gravity. Pingos are meso-scale (order 100-m-diameter) ice-cored Given pingos’ utility for understanding Martian climate, mounds (Figs. 1–5) that form in periglacial terrain as a result of hydrology, and potential astrobiology niches, a review of Mars’ pressurized groundwater flow and progressive freezing. These numerous pingo-like forms (PLFs) provides a perspective on these specific formation conditions make them helpful climatic and issues. In the last decade, a series of visible-wavelength cameras hydrologic markers. The potential of the ice core to host on Mars spacecraft have provided high-resolution images of psychrophilic (i.e., extremophilic cold-loving) microorganisms various hypothesized periglacial features, including fields of makes them attractive as potential astrobiologic targets. These inferred pingos. This work surveys the published literature considerations make identifying pingos on Mars useful for piecing describing these PLFs, in order to assess the plausibility of their together Mars’ climatic and hydrologic history and for providing origin as ice-cored mounds and map out the distribution of likely potential biotic or pre-biotic targets. On Mars, pingos have been pingos on Mars. First are reviewed the two terrestrial pingo types hypothesized in the literature as the explanation for a variety of and their associated water pressurization mechanisms, as well as mound or ring forms at a range of latitudes in the northern other ice-mound features. This review is followed by an historical hemisphere, with much more limited examples at some similar summary of the terrestrial pingo literature by geography location latitudes in the southern hemisphere. These hypotheses are based and context. Then terrestrial pingo morphology is discussed, largely on their similarity to terrestrial pingos in morphology and focusing on features that would be discernable in Mars spacecraft size. Their geologic contexts on Mars are not always similar to images. In the discussion of Martian PLFs the information about those on Earth. However, the local or regional Martian geology terrestrial pingos is used in conjunction with available Mars data does provide indications of water and/or periglacial processes. to assess the likelihood that features hypothesized in the Thus, pingos are commonly although not universally identified on literature to be Martian pingos are pingos in the terrestrial sense. Mars based on the axiom that if their formation mechanisms are Whereas a wide variety of mound or ring features are found on similar, terrestrial and Martian pingos should have a similar Mars, we focus here only on those features hypothesized in the literature to be pingos. The Mars data include images and compositional spectral information from the Thermal Emis- Ã Corresponding author at: Earth and Planetary Sciences Department, University sion Imaging Spectrometer (THEMIS), the Thermal Emission of Tennessee Knoxville, Earth and Planetary Sciences Department, Building 412, Spectrometer (TES), the Compact Reconnaissance Imaging Spec- 1412 Circle Dr., Knoxville, TN 37996, USA. E-mail addresses: [email protected] (D.M. Burr), [email protected] (K.L. Tanaka), trometer for Mars (CRISM), the Mars Orbiter Camera (MOC), the [email protected] (K. Yoshikawa). High-Resolution Imaging Science Experiment (HiRISE), and the

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Fig. 2. (a) An oblique aerial photo showing an open-system pingo (arrow) at the toe of a fan (to left), Hulahula River, Brooks, Range, Alaska. (b) A generalized schematic showing the inferred formation mechanism for the pingo in (a). Groundwater flow occurs from higher elevation to lower elevation through an unfrozen zone (‘talik’) within the permafrost. The hydraulic head is provided by elevation and confinement within the permafrost.

Fig. 1. (a) An oblique aerial photograph showing three open-system pingos in a mountain river valley, Spitsbergen, Norway. The two pingos on the left are collapsed and have central ponds. ‘Ny’ pingo on the right shows both radial cracks from dilation and diagonal cracks trending from upper left to lower right. A glacier is located in the center background (flow direction is out of the photo), with two moraines in front of it. (b) A generalized schematic of the hydrological and glaciological situation in the photo in (a). Subglacial meltwater percolates through an unfrozen zone in the discontinuous permafrost to form a pingo.

High-Resolution Stereo Camera (HRSC), as well as topographic information from the Mars Orbiter Laser Altimeter (MOLA).

2. Terrestrial pingos

2.1. Pingo types: water pressurization mechanisms

Terrestrial pingos are canonically divided into two types, depending on the mechanism for pressurizing the groundwater. Open-system or hydraulic pingos form through artesian pressure (Mu¨ ller, 1959; Holmes et al., 1968; French, 1996). Water is supplied under hydraulic pressure develop surrounding areas and flows beneath a low-permeability layer, which is commonly provided by permafrost. Permafrost is defined as a terrain whose temperature is less than 0 1C for two years (Associate Committee on Geotechnical Research, 1988). In areas of continuous perma- frost, permeability to groundwater is low because of the freezing conditions, although localized taliks—sites of unfrozen ground—- Fig. 3. (a) A ground photograph of a pingo located adjacent to South Kunlun Fault, may occur beneath shallow lakes. In comparison, the discontin- Kunlun Mountain, China (4700 m a.s.l.). Artesian pressure of sub-permafrost water uous permafrost zone is characterized by ‘through taliks’ or is 22 m higher than the ground surface. Polygonal terrain due to ice wedging is unfrozen ground that connects the unfrozen ground below the visible in the foreground. (b) A generalized schematic of the situation inferred for permafrost and the active layer that undergoes seasonal thawing the photo in Fig. 3a. Pingos develop along minor faults associated with major fault systems as a result of flow along the minor faults. above the permafrost. ‘Through taliks’ increase the permeability of discontinuous permafrost regions over continuous permafrost regions. Most open-system pingos are found in the discontinuous discontinuous permafrost provides the permeability for ground- permafrost zone, in or near areas of significant relief, where water upwelling. Artesian groundwater environments cause the relief supplies the hydraulic gradient (potential) and the repeated injection of water via taliks to the surface, followed by Author's personal copy ARTICLE IN PRESS

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river bottoms (Fig. 2), in highly fractured or faulted uplands (Fig. 3), and on rebounding marine terraces (Fig. 4; Yoshikawa and Harada, 1994). In each of these settings, surrounding elevated terrain imparts artesian pressure to the groundwater, and discontinuous permafrost permits its upwelling at lower elevations. In contrast to open system or hydraulic pingos, closed system or hydrostatic pingos (Fig. 5) are found in regions of continuous permafrost (as reviewed in Mackay, 1979, 1998), which creates an impermeable layer at depth. Most closed-system pingos form in drained thaw lakes, in which the saturated lake basin sediments are exposed to the atmosphere. This exposure causes the saturated sediments to freeze, forming new permafrost. However, residual ponds within the lakes retard permafrost growth, resulting in locally thinner permafrost and lesser resistance to deformation. As the permafrost aggrade downward, the pore water is expelled ahead of the freezing front. Blockage of this downward pore water flow by the continuous permafrost at depth redirects the flow inward from the basin edges. Injection of expelled water beneath the overlying permafrost, followed by freezing of the injected water, causes progressive doming of the permafrost overburden. This doming, commonly sited beneath the Fig. 4. (a) An aerial photo showing a swath of pingos (arrows) about 1 km in total length along a formerly submerged coastline on the river delta of Adventdalen, least permafrost aggradation, is a result of the massive ice forming Spitsbergen. The overburden of pingo ice is composed mainly of fine silt and mud, the core of the pingo. but has a thin peat layer. The largest pingo is approximately 4.5 m high, 500 m For both pingo types, the 9% expansion due to phase change long, and 140 m wide, and is situated approximately 25 km from the former shore from water to ice is insufficient to account for the size of pingos; line. (See Yoshikawa and Harada, 1995, for details.) (b) A generalized schematic of groundwater flow is required to supply the volume of a pingo ice the hypothesized situation in the photo in Fig. 4a. The marine terrace is rebounding from glacial isostasy, and developing new permafrost. Deglaciation core. Both open- and closed-system pingos may undergo vertical permits upwelling of artesian groundwater through unfrozen zones (‘taliks’) in the pulsing (periods of uplift) as a result of injection of groundwater newly forming permafrost. New permafrost thickness is 30 m. to a sub-pingo water ice lens (Mackay, 1998; Yoshikawa, 2008).

2.2. Other ice-cored mounds

Closed-system (hydrostatic) pingos are often associated with frost mounds (Mackay, 1998). Frost mounds are smaller, seasonal, ice-cored features that may precede pingo formation or develop on the sides of pingos after they form. Both pingos and frost mounds form through similar processes, namely, the pressuriza- tion of ground water during freezing of the ground (Pollard and van Everdingen, 1992). Their morphology is likewise similar in being mounded often with cracks. However, terrestrial pingos and frost mounds differ in size by roughly one to two orders of magnitude. That is, pingos may be hundreds of meters in diameter (Figs. 1–5), whereas frost mounds are typically meters in diameter. Pingos and frost mounds also differ in the length of time they exist. Dating of closed-system pingos by the willows growing on their surfaces indicates ages on the order of a thousand years (Mackay, 1973, 1986), whereas frost mounds form over months and do not last more than one season (Pollard and van Everdingen, 1992). Other large ice-mound features include palsas and lithalsas. Whereas pingos contain a massive ice core, palsas and lithalsas are comprised largely of segregated ice. Segregated ice is a result of cryosuction, the process by which ice exerts a suction effect on Fig. 5. (a) A black-and-white aerial photograph showing two closed-system pingos on the Tuktoyaktuk Peninsula, NWT, Canada. ‘Split’ is located to the lower left and the surrounding pore water by capillary action (Davis, 2001). This ‘Ibyuk’ is approximately at image center. Dilational cracking is visible on both suction continues until the thickness of the water-poor layer pingos, and thermal contraction polygons are visible on the surrounding terrain. beneath the ice layer is too great for further cryosuction. Because ‘Ibyuk’ sits within a drained thaw lake, and ‘Split’ has been inundated by the Arctic of the limit this thickness imposes on ice layer development, Ocean (just off the image to the left). ‘Ibyuk’ is 300 m in diameter for scale. (b) A maximum palsa and lithalsa heights are of order 10 m on Earth. generalized schematic depicting formation of a closed-system pingo (after Mackay, 1998, Fig. 4). Arrows show the direction of permafrost aggradation. Pore water If the climate is sufficiently cold, subsequent layers of ice may expulsion ahead of the aggrading permafrost causes subsurface pooling of the pore form below the water-poor layer. The term palsa refers to water and upward (hydrostatic) pressure. Freezing of this pore water creates the segregation ice mounds associated with peat bogs, which pingo ice (blue). constitute most segregated ice mounds on Earth; the term lithalsa freezing. This process leads to the development of the massive ice (or ‘mineral palsas’) refers to segregation ice mounds that form in forming the core of the pingo. Thus, this pingo type is commonly sedimentary material and are considerably more rare (Gurney, found in glacial valleys (Fig. 1), at the toes of alluvial fans near 2001; Pissart, 2002). Author's personal copy ARTICLE IN PRESS

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Palsas and lithalsas have a very different genesis in comparison residual ponds which retard permafrost growth and allow for with hydrostatic or even hydraulic pingos, but basic similarities pingo development. The controls on open-system pingo size are and differences may be suggested between these two types of ice- less well understood. Pingo diameters may reach up to 2 km, but cored mounds. Palsas and lithalsas are composed of segregation are commonly reported to be of order of 100 m. The surfaces of ice formed generally through cryosuction (Davis, 2001), whereas incipient pingos are generally smooth (Fig. 6a), unless the terrain pingos are composed mainly of massive ice formed through has already been cracked by thermal contraction caused by groundwater pressurization. As with open-system or hydraulic sudden drops of sub-freezing temperature (see, e.g., French, 1996 pingos, palsas and lithalsas require an influx of (water) volume, or Davis, 2001 for descriptions of thermal contraction cracking). along with a less significant contribution from phase-change- During their growth, the flow of groundwater continues to cause induced volume increase, to induce doming of the overlying accretion of ice onto the pingo ice core (Mackay, 1998). Actively surface. Like pingos, terrestrial palsas and lithalsas are associated growing pingos have a convex, domical or mound shape (Fig. 6b). with regions of permafrost. However, pingos are surrounded by Fully developed pingos may reach up to several tens of meters in (continuous or discontinuous) permafrost which is required to height, with the largest known examples being about 50 m tall. focus groundwater flow during formation (see, e.g., Jorgenson et During this growth stage, the pingo commonly develops dilational al., 2008), whereas palsas and lithalsas are themselves loci of cracks (Fig. 6c), as the ice core expands beyond the overburden permanently frozen ground, thus constituting the permafrost. tensile strength (see, e.g., Mackay, 1998, his Fig. 9). These Pingo and lithalsa morphologies are very similar (Gurney, 2001), dilational cracks are generally oriented radially to the pingo including ‘ramparted depressions’ as relict forms (e.g., Matthews center (Fig. 5a), but may exploit any pre-existing weaknesses, et al., 1997). However, the lower height attained by lithalsas such as thermal contraction cracks. Semi-concentric dilational should produce less pronounced relict forms, or scars. As expected cracks may (rarely) develop as normal faults, with the upthrown from the limitations of the cryosuction formation mechanism, side towards the pingo center (Mackay, 1998). Ice core growth terrestrial palsas and lithalsas and their relict forms reported in may steepen the pingo sides beyond the angle of repose, causing the literature are generally smaller and shorter than pingos (e.g., slumping of material from the pingo. Exposure through cracking Worsley et al., 1995; Matthews et al., 1997), although systematic of the central ice core causes melting and/or sublimation of the measurements are not available to quantify this characterization. ice, collapse of the pingo summit, and formation of a summit crater (Fig. 6d). This summit crater may have marked topography 2.3. Morphology and the perennial water that results from the ice core melting may create a pond within the crater (Fig. 7), which enhances the Pingo morphology varies with growth stage (Fig. 6; see also permafrost degradation. In addition to melting, vapor diffusion Mackay, 1998, his Fig. 4 for diagrams illustrating closed-system through the sedimentary substrate may also contribute to ice core pingo genesis and growth, and Burr et al., 2005, their Fig. 5 for collapse. In its final stage, collapsed pingos form relict features or photographs of pingos in various stages of growth). Incipient ‘pingo scars’ (cf., Flemal, 1976)(Fig. 6e). These raise-rimmed pingos are low mounds (Fig. 6a). The diameter of the closed- depressions are a result of the transport of sediment by slumping system pingos is largely set at this stage, based on the size of the down the sides of the pingo, and so may show slight centrifugal (outward) displacement of mass from the original pingo circum- ference. Because of this mass displacement, the floor of the pingo may be slightly lower than the original or surrounding land surface. Pingos of different developmental morphologies may be co- located. Of the approximately 1350 closed-system pingos on the Tuktoyaktuk Peninsula, only about 50 are actively growing (Mackay, 1973). The remaining pingos are in various morpholo- gical stages of collapse.

Fig. 6. Cartoon showing profile view of pingo morphology during growth and collapse: (a) incipient pingo, (b) growing pingo, (c) growing or collapsing pingo with dilational cracking and slumping (dark areas) on the sides, (d) pingo in advanced stage of collapse, (e) collapsed or relict pingo or pingo scar. In all cases, Fig. 7. Ground photo of the summit of Ibyuk Pingo (see Fig. 5 for aerial view), stippled pattern connotes subsurface ice. Aspect ratio is exaggerated for ease of showing the topography created by dilational cracking and a frozen pond (outlined viewing. in black arrows) created by ice core melting. Persons at left suggest scale. Author's personal copy ARTICLE IN PRESS

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Features associated with the surrounding periglacial terrain, North American arctic coast regions are commonly closed system. such as polygonal cracking, are commonly observed on the pingos. The first published studies date back to the early 20th century. Seasonal frost mounds may develop on the sides of pingos, and These early workers explored multiple working hypotheses, narrow circumferential annuli of lower terrain (‘moats’) have been including pingo formation as (1) the erosional remnants of pre- documented surrounding a few pingo bases (e.g., Mackay, 1979). existing mounds of uncertain origin, (2) as glacial deposits of Sediment slumps are commonly visible on the sides of pingos, and combined sediments and ice, and (3) by pressurization of icings may be observed at the pingo base due to discharge from a groundwater by mechanisms similar to those that give rise to water lens beneath the ice core (Mackay, 1998). hydraulic or open-system pingos (e.g., Liffingwell, 1919). Con- temporary observations show that closed-system pingos (e.g., Fig. 2.4. Substrate 5) predominate on the North Slope and Seward Peninsula, Alaska, and the Mackenzie Delta in Canada (Mackay, 1979, 1998). These Pingos are almost always found in unconsolidated sedimentary areas are dominated by continuous permafrost and heavily substrates, which allow both groundwater flow and deformation populated with thaw-lake basins, in which 98% of these coastal during pingo growth. Closed-system pingos in the North Slope of pingos formed. Alaska are reported only in regions where finer-grained sediments In contrast, inland pingos are commonly open-system types. overlie coarser-grained sediments; no pingos are reported in About 760 pingos are found in Central Alaska and the Yukon adjacent lake basins that are fine-grained throughout (Carter and Territory, Canada, of which only a few are closed-system pingos Galloway, 1979). On the Tuktoyaktuk Peninsula, 98% of the pingos (Holmes, 1967; Hughes, 1969; Hamilton and Obi, 1982). Both of form in drained thaw lakes, where fine-grained lacustrine these inland areas are dominated by discontinuous permafrost. material commonly overlies glaciofluvial sands. As an example Many of these pingos formed at the toes of alluvial fans (e.g., Fig. of this stratigraphy, the overburden of Ibyuk Pingo (Figs. 5 and 7) 1), where sub-permafrost water was likely to attain a maximum is comprised of 4 m of lake silts, 2 m of diamicton with glacial hydraulic head and where localization of artesian flow would boulders, and 9 m of medium glaciofluvial sands (Mackay, 1998). perhaps occur. However, theoretical calculations and limited This stratigraphy of fine-grained over coarse-grained sediments groundwater data from Alaska (Holmes et al., 1968) suggest that allows pore water movement (inflow) through the coarser, hydraulic pressure alone is insufficient to overcome the tensile subsurface sediments, while limiting water transport (outflow) strength of the permafrost layer and the static load of the through the overlying finer-grained sediments. This limitation of overburden. Holmes et al. (1968) suggested that freezing water outflow through the fine-grained capping layer creates pressures probably are also required to lift open-system pingos. upward pressure. At the same time, the atmospheric conditions in periglacial regions cause freezing of this water. As discussed 2.5.2. European studies and Greenland above, the result of continued ground water inflow, and to a lesser Open-system pingos in East Greenland have been described extent the expansion due to the phase change of the water to ice, since the late 1800s and researched since the 1950s (Mu¨ ller, 1959; is uplift of the sedimentary overburden over a massive ice core. Cruickshank and Colhoun, 1965; Washburn, 1969; O’Brien, 1971). Open-system pingos can develop with a wider variety of Early calculations of the total up-doming pressure required to substrates. Open-system pingos have been reported in clay counteract the cohesion and weight of the overlying permafrost (Yoshikawa and Harada, 1995), in sand (Yoshikawa, 1991), and in material suggested that 600–2200 kPa of pressure was needed to silt (Yoshikawa et al., 2003). For open-system pingo formation, form pingos. Subsequent work focused on pingo hydrology. The water temperature should be near freezing in order to form a origins of open-system pingos in the Svalbard archipelago, in the massive ice core. In addition, discharge should be moderate in Arctic Ocean north of Norway, were described as resulting from order to prevent breakthrough of the permafrost layer and the groundwater accumulation beneath a glacier’s terminal moraine formation of spring flow instead of formation of a pingo and flow toward the coast through the unfrozen zone beneath the (Yoshikawa, 1993). However, pingos have also been observed in permafrost (Liestøl, 1977, 1996). This mechanism is consistent shale (Matsuoka et al., 2004) and in sandstone (Mu¨ ller, 1959). with the Svalbard glaciers, which exhibit surging behavior with Shale and sandstone may have extensive fractures, especially in excess water at their base. Of the approximately 80 pingos in Svalbard (Yoshikawa, 2003) and Greenland (Mu¨ ller, 1959), which Svalbard (Yoshikawa and Harada, 1995), almost all are of the may allow artesian groundwater to heave up the overburden, open-system type. Pingos are situated in river beds in central developing a pingo. However, the extreme scarcity of such Spitsbergen (the largest of the Svalbard islands) and about 20–30% observations suggests this is very unusual condition. of these have artesian flow caused by the flow of subglacial groundwater. Pingos in Svalbard also occur in near-shore uplift 2.5. Historical, geographic, and contextual overview areas and at fault zones (Yoshikawa and Harada, 1995), showing the diversity of locations that may be conducive to groundwater This section describing pingos on Earth concludes by present- flow and pingo formation. ing an overview of historical research on terrestrial pingos by No active pingos are known in the Scandinavian Peninsula or in geographic region. We focus on features that have been more the European mainland, although many palsas are reported in easily accessible to past investigators (and presumably to future Scandinavian countries. Relict pingos in Europe, i.e. in Ireland researchers as well). Regional and local effects, such as proximity (Mitchell, 1973), Great Britain (Pissart, 1963; Watson, 1971), to bodies of water and geohydrologic conditions, account for these Belgium (Pissart, 1965), and Germany(Weigand, 1965), have been geographic associations. Such factors are poorly understood in the subject of research for the evidence they provide of Martian cases, which are thus more conservatively grouped and paleopermafrost and paleoclimate conditions. presented by latitudinal region (see Section 3 below). 2.5.3. Eurasian studies 2.5.1. North American studies Pingos in Siberia have been studied since the mid-20th century Porsild (1938) was the first to suggest the word pingo (Inuit for (see Tikhomirov, 1948). They are concentrated in the Yakutia ‘small hill’) as a generic term for ice cored hills in permafrost region of north-central Siberia, but can be found from the White regions, based on his observations in North America. Pingos in the Sea in the west to the Bering Strait in the east. These pingos are Author's personal copy ARTICLE IN PRESS

546 D.M. Burr et al. / Planetary and Space Science 57 (2009) 541–555 located at the toes of alluvial fans or in valley bottoms, which are The central pits, being difficult to produce through uniform quintessential locations for open-system pingos in Alaska. Pingos sedimentary accumulation, are interpreted as indicative of pingo in the Boguchansk region of central Siberia are open-system formation (Sakimoto, 2005a). The asymmetry is held to be pingos (Serov and Leshchikov, 1978), located on permafrost with indicative of a volatile-driven process, namely loss through temperature profiles similar to those in central Alaska. solar-driven sublimation (Sakimoto, 2005a). Numerical modeling In China, pingos are found in two major regions. The first shows that sufficient flow volume could be generated by hydraulic region is the northeast part of China, continuous from Siberia and pressure from an hypothesized regional aquifer to produce the Mongolia. The second region is the Tibetan Plateau, which may observed shapes (Sakimoto, 2005b; Bacastow and Sakimoto, present useful analog conditions to Mars. The Tibetan Plateau has 2006). The mounds’ hypothesized groundwater source is a a widespread area of permanently frozen terrain above 4000 m regional aquifer, which would supply water through impact elevation, producing cold, dry permafrost. In addition, initial fracturing of the crust. The large size of the PLFs relative to studies show that many of the groundwater systems have briny terrestrial pingos is proposed to be a result of the large subsurface water (Yoshikawa et al., 2006a, b). Pingos have been reported and water supply and a sustained cold climate in Mars’ polar regions. sampled near Kunlun Pass, northern Tibet (An, 1980), but to our Korolev crater (Fig. 9), a large (83-km diameter) and well- knowledge, very few other pingo studies have been carried out on preserved north polar crater (731N, 1631E), provides an example of the Tibetan Plateau. Related permafrost and periglacial features in a polar crater with ‘pingo-like fill’ (Bacastow and Sakimoto, 2006). Tibet were investigated along the Golmud–Lasha Road (e.g., Kuhle, Previous work on Korolev crater using data from TES, THEMIS, and 1985; Wang, 1982). CRISM indicates that the surface of the mound in Korolev crater is composed of water ice (Armstrong et al., 2005; Brown et al., 2007). These data were all collected during the northern summer. 3. Martian PLFs In growing terrestrial pingos, the ice core is buried beneath an active layer (at least 1 m of overburden) (Mackay, 1987, his Knowledge of terrestrial pingos, in conjunction with knowl- Fig. 10). Thus, an entirely water ice surface would be inconsistent edge of Martian conditions, allows assessment of PLFs on Mars. with formation analogous to that of terrestrial pingos and would Martian PLFs discussed in the literature may be grouped by instead be more analogous to a terrestrial icing mound. Icing latitude. We discuss Martian PLFs moving from the polar regions, mounds are a common site in Arctic regions of groundwater through the mid-latitudes, to the equator. Fig. 8 shows the generic upwelling, where they occur during winter as a result of artesian locations of the various study areas presented in the literature. pressure (Yoshikawa et al., 2007) and are visible on some open- Detailed locations are provided in the analysis below. A summary system pingos. Frost cover over regolith could provide the of information on PFLs by location is provided in Table 1. observed water ice signature, but modeling of the thermal inertia indicates the water ice was several meters thick (Armstrong et al., 2005). Alternatively, extensive cracking in the overburden could 3.1. Polar PLFs: large impact craters expose the water ice to detection, consistent with formation as a terrestrial pingo. Thus, one test of the pingo hypothesis is whether Pingos have been hypothesized by Sakimoto and colleagues for the ice detection derives from cracks in the mound. We examined nearly circular mounds in circum-polar impact craters poleward available THEMIS mosaics, MOC images, and one CTX image of 601; all but one are in the northern latitudes (Sakimoto, (P01_001592_2530_XI_73N195W) of Korolev crater and the other 2005a, b; Bacastow and Sakimoto, 2006). These impact craters are PLFs given in Bacastow and Sakimoto (2006, their Fig. 1), in order 48 km in diameter, and the inset mounds occupy a large portion to look for large cracks that may have exposed interior ice to of the crater. These mounds are surrounded by partial or complete detection in spectral data. For terrestrial-style pingos (i.e., that moats, and MOLA topography shows that they usually have a formed through water ice accumulation beneath a thin regolith shallower equator-ward exposure than poleward exposure. They cover), we would also expect to see frost mounds and polygonal occasionally show central pits. terrain, as are commonly associated with terrestrial pingos.

Fig. 8. Generic locations of Martian PLFs as proposed in the literature and summarized in Table 1, plotted on MOLA shaded relief topography. The plot stretches from 901N to 901S and 1801Wto1801E. Author's personal copy

Table 1 Summary of information and assessment regarding hypothesized pingos on Mars..

Location Pingo Context Data Primary Morphologies Inferred terrain Tectonic or Potential Potential Potential Pingo likelihood references type impact periglacial lacustrine volcanic mechanism(s) structures features features features proposed

Polar regions Circular; some Circum-polar Less likely Large MOLA Sakimoto Large impact Occurs within Hydraulic, a,b , have central pits regions impact (2005) craters, minimum impact craters sourced from Bacastow and craters and moats; some size 8 km tens of km in global aquifer Sakimoto (2006) may underlie diameter layered deposits

Mid-latitudes Small mounds, Crater floor Polygonal Hydrostatic NW Utopia More likely Impact MOC, Osinski and Soare Astapus Colles a,b Soare et some with terrain Planitia crater THEMIS (2007) and Vastitas al. (2005) summit (40–300 m 541–555 (2009) 57 Science Space and Planetary / al. et Burr D.M. visible Borealis interior a and diameter) depressions; units circular to superposed crater irregular rimmed

depressions, PRESS IN ARTICLE some with interior layered mounds Utopia basin More likely Basin floor MOC de Pablo and Domes, cones, Various units of Wrinkle ridges Polygonal Megalahar Magmatically 33–431 N Komatsu (2008) rings; most have possible fluvial troughs deposits, driven melting central pits and volcanic several km in possible lava of permafrost origin, especially diameter flows proximal possible to Elysium

megalahara deposits 30–471N, More likely Mid- Scattered Some in areas None for HiRISE, Dundas et al. Circular/oval/ Various northern 80–120 1E latitudes in wrinkle ridges of polygonal/ pingos; flat MOC (2008) irregular; some a,b plains and fractured Utopia domical, some topped highland units terrains Basin flat-topped, morphologies profiles; some have concentric and radial grabens

Equatorial regions c, North of Thira Less likely Impact Viking Cabrol et al. Circular/elongate/ Small wrinkle Associated Hydrostatic interpretedUnit AHbm1 as crater on crater (2000) composite/ ridges with possible (closed- fluvio-lacustrine floor of collapsed fluvio- system) sediments Gusev mounds; some lacustrine crater have central pits; deposits dense cratering Cerberus Polygonal Polygonal Hydrostatic Athabasca Less likely Outflow MOC Burr et al. (2005) , Mounds, pitted Cererusa Fossae 3 , interpreted Fossae terrain terrain, lava Valles channel Page and Murray cones, possible unit (closed- as lava flows with (2006) , Jaeger et dilation cracks, flows common system) or minor fluvial al. (2007, 2008) , irregular rimmed hydraulic deposits Page (2008) forms (open-system) Cerberus Palus Less likely Basin, along MOC Nussbaumer et al. Pitted cones; low Mixture of MFF Inferred northern (2003, 2004) , population and lava rootless cones margin of Nussbaumer density MFF (2005) 547 a Tanaka et al. (2005) . b Scott et al. (1986–87) . c Kuzmin et al. (2000) . Author's personal copy ARTICLE IN PRESS

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Fig. 9. Korolev crater (near 731N, 1631E) as seen in image mosaics. (a) The MOC wide angle images show the bright frost deposit. (b) The THEMIS daytime IR images show the relatively high thermal inertia compared to the surrounding terrain. The 50-km scale bar on (a) applies to both images.

Neither cracks on the scale of the spectral data nor other may have been caused by initial enrichment and subsequent associated features were apparent in these images. Thus, we removal of volatiles in the brecciated crust within the impact surmise that the water ice inferred from spectral data reflects the crater floor adjacent to the volcanic mound (Rodriguez and overall surface composition of the mounds, and is not due to Tanaka, 2006). In this scenario, a volcanic, non-impact (or exposure through cracking of an interior ice core. This conclusion enhanced impact) central peak is formed much later than the is consistent with the interpretation of Armstrong et al. (2005) for impact event, perhaps during the Hesperian as suggested by the spectral data from Korolev crater. Ghatan and Head (2002). Other southern, circum-polar impact Moreover, the mound displays local fine, even layering mostly craters contain layered and dune-capped mounds that preferen- overlain by relatively dark, pitted deposits (e.g., MOC image R20- tially infill the southern parts of the crater floors and in some 00307). The finely layered deposits are similar to late-stage, ice- cases merge with layered deposits of the south polar plateau rich materials making up upper parts of the north polar plateau (Planum Australe). These features have been mapped as outliers of that are locally overlain by dunes, sedimentary veneers, and volatile-rich, south polar layered deposits (Tanaka and Scott, residual ice (Rodriguez et al., 2007; Seelos et al., 2007; Tanaka et 1987), whose form and preferred poleward occurrence appear to al., 2008); no PLFs have been identified thus far in these other be related to climate-controlled energy mass-balance (Russell et stratigraphically similar materials. On the basis of this layering, al., 2004). None of these proposed processes for southern, high- these northern PLFs have been hypothesized to be outliers of latitude mounds is similar to the formation of pingos by ground- north polar layered terrain locally capped by residual water ice water flow and freezing. (Tanaka and Scott, 1987; Tanaka et al., 2005). The absence of such In summary, the various mechanisms proposed for the forma- mounds in all polar craters is held to be counterindicative of this tion of mounds in large polar impact craters are not similar to the origin (Bacastow and Sakimoto, 2006). However, this may be a processes that form terrestrial pingos. The stratigraphy, form, and function of crater diameter, depth, elevation, and latitude. Korolev orientations of the polar PLFs suggests that these layered mounds is the largest crater north of 701N. The next largest, Dokka, is are outliers of ice-rich polar layered terrain (Tanaka and Scott, 52 km in diameter at 77.21N, 214.41E and also contains a mound 1987; Russell et al., 2004; Tanaka et al., 2005), whereas the flat- (Tanaka et al., 2005), as does an unnamed, 30-km-diameter crater topped Sisyphi Montes are likely some sort of volcanic feature at 77.21N, 89.21E. Somewhat smaller craters along the margin of (Ghatan and Head, 2002; Rodriguez and Tanaka, 2006). In these and atop Planum Borum also have mounds of layered deposits. In cases, the features commonly associated with pressurized growth contrast, Lomonosov (110 km diameter but at 651N) does not of ice beneath a layer of regolith are lacking in available images. have a mound. Some smaller craters also include mounds formed Thus, our analysis of available data does not support the hypothesis by dune deposits, showing that sediment collection in polar that these PLFs in large polar impact craters are perennial ice-cored craters is complex (e.g., three craters 16–21 km in diameter near mounds analogous to terrestrial open-system pingos. 751N, 3451E). In the southern hemisphere, a regional suite of large mounds, called the Sisyphi Montes, occur at the centers of degraded impact 3.2. Mid-latitude PLFs: Utopia Planitia craters. Ghatan and Head (2002) interpreted these features to be subglacial volcanoes beneath a 41.4-km-thick ice sheet, based on Pingos have been hypothesized to explain variously-shaped, their flat tops, conical shapes, summit craters, and associated meso-scale features in the mid-latitudes. Although Dundas et al. channels. Rodriguez and Tanaka (2006) further observed that the (2008) note one PLF in the southern hemisphere, to date mapping features (1) occur at the centers of highly rimmed and rimless of PLFs in the mid-latitudes has focused on Utopia Planitia. circular depressions interpreted to be degraded impact craters, (2) Centered near 501N1201E, Utopia Planitia is an ancient include adjacent moats o200 m deep within the crater floors in multiring impact basin about 3300 km in diameter (McGill, about half the cases, and (3) appear to be organized along possible 1989). As part of Mars’ northern lowlands, it may have been traces of Hellas basin rings. Their alternative explanation for the inundated by a northern ocean (Baker et al., 1991; Parker et al., southern mounds is that of a regional phase of magmatism 1993; Head et al., 1999). The lowlands are filled with the Vastitas involving magma ascent along a combination of Hellas basin Borealis units, interpreted to be Early Amazonian periglacially structures and brecciated zones beneath larger impact craters reworked sediments possibly deposited in an ancient ocean, as resulting in eruptions at crater centers; the surrounding moats well as Hesperian marginal-plains deposits emplaced largely by Author's personal copy ARTICLE IN PRESS

D.M. Burr et al. / Planetary and Space Science 57 (2009) 541–555 549 mass wasting (Tanaka et al., 2005; Skinner and Tanaka, 2007). contraction polygons and meltwater flow features, respectively Interstitial water within these sediments may have frozen (Chap- (Soare et al., 2005). Thus, the higher resolution data are consistent man, 1994, 2003), producing an ice-rich permafrost zone over- with the earlier interpretation of the mounds as pingos. The water lying wet sediments. Widespread polygons and possible source for the pingos was hypothesized to have been atmospheric, thermokarst provide supporting evidence for this scenario tied to obliquity variations (Soare et al., 2005). (Costard and Kargel, 1995). Channels on the western Elysium Subsequent classification and interpretation of some sub- Mons slopes may also have contributed wet sediments to the circular to circular landforms as pingos has been made throughout basin through magmatically induced debris flows and lahars Utopia basin (Osinski and Soare, 2007; de Pablo and Komatsu, (Christiansen, 1989; Tanaka et al., 1992; Russell and Head, 2003). 2007, 2008; Dundas et al., 2008). These features show a range of Local fractured rises, elliptical mounds, pitted cones, and rounded morphologies. Using MOC and THEMIS images, Osinski and Soare depressions indicative of sedimentary diapirism and mud volcan- (2007) identify five morphologic types, namely: (1) small sub- ism involving water-saturated sediments and ice-rich permafrost circular mounds, (2) shallow depressions surrounded by con- are prevalent in Hesperian materials in the marginal plains centric and radial cracks, (3) sub-circular to irregularly shaped between the Vastitas Borealis plains materials and the Utopia rimmed depressions, (4) sub-circular rimmed features containing basin rim materials. These features may be influenced by buried layered mounds, and (5) circular rimmed-depressions surrounded impact-basin structure and topography (Skinner and Tanaka, by ejecta. Whereas types 2 and 5 are suggested to be impact 2007). Thus, the necessary elements for terrestrial-style pingo craters, types 1, 3, and possibly 4 are held to resemble pingo formation—namely, a sedimentary substrate, subsurface ground- morphologies at different stages of growth or collapse. ‘Small sub- water, and freezing conditions, as well as structural anomalie- circular mounds’, including some with radial cracks (Fig. 11a), are s—have all been inferred in Utopia Planitia. morphologically similar to incipient or growing pingos, and are Early identification of PLFs in northwestern Utopia focused on inferred by Osinski and Soare (2007) to be closed-system the floor of a crater near 651N, 671E, showing a set of roughly (hydrostatic) pingos. As shown in Mackay (1998), radial (dilation) circular mounds 25–50 m in diameter (Soare et al., 2005). These cracks on pingos may connect with polygonal (thermal contrac- mounds were hypothesized to be pingos on the basis of their size tion) cracks on the surrounding terrain. ‘Sub-circular to irregularly and morphology, which included radial cracks and summit shaped rimmed depressions’ are similar in size and morphology to depressions visible in MOC image E03-00299. As discussed under collapsed pingos or pingo scars (Fig. 11b). As noted in Section 2 Section 2 above, radial cracking is observed on terrestrial pingos above, terrestrial pingos in various morphological stages of as dilation cracking and summit depressions on terrestrial pingos development are commonly co-located, so the multiple adjacent result from melting or sublimation of the ice core and subsequent morphologies is consistent with interpretation of Osinski and collapse. Associated features inferred to be periglacial in origin, Soare (2006) that these features are collapsed pingos. In some namely polygonal cracking and a curvilinear (channel-like) cases the depth of the surface inside these rimmed depressions feature, led to interpretation of the crater floor as supporting an appears noticeably greater than the terrain surface outside the assemblage of features most consistent with formation by rim. As noted in Section 2 above, transport of material away from aqueous periglacial processes. The proffered terrestrial analog is the pingo center by slumping may cause a collapsed pingo center the assemblage of features found in the Tuktoyaktuk Peninsula, to be slightly lower than the surrounding terrain, although the Northwest Territories, Canada, which includes one of the largest amount of this effect is not known. Examples of a third concentrations of closed-system (hydrostatic) pingos on Earth morphology, ‘sub-circular rimmed features containing layered (Mackay, 1998 and references therein). Images from HiRISE mounds’ (Fig. 11c), are similar to features located on the floors of (e.g., Fig. 10a) show the same suite of features inferred from the inferred thermokarst depressions and previously interpreted by lower resolution MOC image, namely, mounds with radial cracks Morgenstern et al. (2006) to be impact craters partially filled with that in some cases extends onto the surrounding terrain (Fig. 10b) layered material. Osinski and Soare (2007) note that not all such and/or summit depressions (Fig. 10c). In addition, polygonal features are located within thermokarst basins and suggest that cracking of the surrounding crater floor and bright curvilinear they could be due to pingo regrowth within a slumped basin. features are visible, previously hypothesized to be thermal Horizontal layering is not characteristic of terrestrial pingos,

Fig. 10. (a) Subset of HiIRSE image PSP_008492_2450 (centered near 65.41N, 67.31E), showing the floor of an impact crater in Utopia Planitia with features hypothesized by Soare et al. (2005) to be pingos, thermal contraction polygons, and drainage features. White boxes show the locations of Fig. 10b and c. (b) and (c): Subsets of HiRISE image PSP_008492_2450, showing hypothesized pingos with (b) with radial cracking (white arrows), and (c) a summit depression. The 50-m scale bar on (b) applies to both images (b) and (c). Author's personal copy ARTICLE IN PRESS

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Fig. 11. High-resolution images showing examples of PLFs in Utopia Planitia as classified by Osinski and Soare (2007). In all images, north is up and illumination is from the left. (a) Subset of HiRISE image PSP_007951_2220 (centered near 41.61N, 82.91E), showing small sub-circular mounds with radial cracks (black arrows denote the most prominent crack). (b) Subset of HiRISE image PSP_007951_2220, showing sub-circular to irregularly shaped rimmed depressions. (c) Subset of HiRISE image PSP_008162_2220 (centered near 41.61N, 82.91E), showing sub-circular rimmed features containing layered mounds. Both scale bars are 50 m; scale bar on (b) also applies to (c). however, and the observed layering in the mounds is not by uplift of the surrounding terrain, and thus to be most explained for an ice-growth mechanism consistent with forma- consistent with formation as pingos. They found a concentration tion of a closed-system pingo. On the basis of other inferred of these features between 351N and 451N, similar to that of de ground-ice features in the area, Osinski and Soare (2007) Pablo and Komatsu (2007). Alternative mechanisms besides uplift speculate that the water emplacement was recent, although a are discounted as being inconsistent with the observed character- specific process is not proposed. istics of the surface. Conversely, the latitudinal control of the Using both MOC and HRSC images, de Pablo and Komatsu (2007, mounds is held to be best explained by ground ice processes 2008) provide a simpler, tripartite classification of features—domes, (Dundas et al., 2008). Pingos are presented as the most similar cones, and rings—although each of these classes includes a range of ground ice landform on Earth, although the ‘trapezoidal profile’ of individual examples. Domes and cones tend to occur in groups of ten the Utopia Planitia mounds is noted as being atypical of terrestrial or more individual features, whereas rings tend to occur in smaller pingos. Flat-topped pingos have been noted on Earth; a well- groups. A high-resolution image of Utopia Planitia shows examples of documented example from the Tuktoyaktuk Peninsula is Pingo 14 these features (Fig. 12a). Domes are defined by de Pablo and Komatsu (Geyser Pingo) of Mackay (1998). This large closed-system pingo (2007, 2008) as circular, domical features with diameters ranging (42400 m in its longest dimension) is located in a drained thaw from 20 to 200 m (average 90 m) and heights of less than 45 m lake, along with two other pingos which are more mound shaped. (Fig. 12b). Most domes are flat without cracks or fissures; a few show Its location in a thaw lake supports its identification as a pingo, as shallow summit depressions. Cones are defined as conical features does its large sub-pingo water lens, determined by yields of with summit depressions and having diameters ranging from 11 to artesian water from repeated drilling. Pingo 14 gained and lost 890 m (average 85 m) (Fig. 12c). Rings are defined as annular features elevation over 18 years through water influx and egress (in part characterized by smooth, thin rims, which may form complex due to drilling), so that its final elevation was significantly below structures. They range in diameter from 30 to 850 m (average its highest elevation. Such elevation gain and loss is not unusual, 150 m) (Fig. 12d). Most of these PLFs are located between 331Nand however, and also occurs in mounded pingos. Pingo 14 is unique 431N(de Pablo and Komatsu, 2008, their Fig. 1), although a few in the morphology of its dilational cracks, which originate from outliers are located outside of this concentration. Because the three the pingo summit, but then bifurcate before reaching the pingo types of features are closely related in space, a common origin was flanks. This crack morphology is not understood, nor is the cause sought for all three types. They are hypothesized to be pingos based for the unusually flat top of the Utopian features. As suggested by on their morphology, which is similar to pingo morphology at Dundas et al. (2008), the flat-topped mounds could be morpho- different stages of development, and their location within geologic logically similar to lithalsas. As discussed in Section 2 above, units that were inferred to be water-rich (i.e., megalahars). Rootless lithalsas tend to have flat upper surfaces and so may be a viable cones are presented as another possible hypothesis, but de Pablo and analogue for the Utopia mounds. Komatsu (2007, 2008) did not find clear evidence of volcanic In summary, research in the Utopia Planitia shows that the materials co-located with the PLFs. The water for the PLFs may have regional context is favorable to terrestrial-style pingo formation derived from melting of ground ice, hydrothermal water flows, or and that the observed features span a range of forms that are both mechanisms together. The heat source for these mechanisms is similar to the range of terrestrial pingos undergoing growth and speculated to be a recent, deep magma chamber, allowing pingo collapse. However, some questions remain about these Utopia formation under current climatic conditions (de Pablo and Komatsu, Planitia features, including the water source for the hypothesized 2007, 2008). pingos, the timing of their formation, and the flat-topped Using HiRISE images, Dundas et al. (2008) focus on a subset of morphology observed in some cases. the features proposed as pingos by Osinski and Soare (2007) and de Pablo and Komatsu (2007), namely, near-circular isolated mounds with steep sides and flat summits (Fig. 13). They note 3.3. Equatorial PLFs: a range of contexts several similarities to terrestrial pingos, including scale, size, and radial and concentric fractures. The similarity of the surrounding In Mars’ equatorial regions, PLFs are found in a plethora of terrain with the surface of the PLFs is held to indicate formation locations. Author's personal copy ARTICLE IN PRESS

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Fig. 12. High-resolution images showing examples of PLFs in Utopia Planitia as classified by de Pablo and Komatsu (2008). In all images, north is up and illumination is from the left. (a) Subset of HiRISE image PSP_002095_2130 (centered near 32.71N, 127.61E), providing context and location of sub-images, which show examples of (b) domes, (c) cones, and (d) rings. The 50-m scale bar on (b) applies to all three sub-images.

300 to 1000 m, and heights were measured from shadow measurements as being between 35 and 240 m. These mounds have circular, elongated or composite planforms, and some show interior depressions which may be interpreted as evidence of collapse. They were hypothesized to be pingos on the basis of their plan view morphology and morphometry (ratio of length to width), which are shown to be roughly similar to published data (Pissart and French, 1976) on terrestrial pingos. An inferred periglacial origin for other features in Gusev crater appeared to provide a supporting geologic context to the pingo interpretation (Cabrol et al., 2000). More recent data, however, do not support these inferences regarding the material in Gusev crater. Data from the Spirit Mars Exploration Rover show that much of Gusev crater is filled with lava (McSween et al., 2006 and references therein), but the materials in the vicinity of the proposed pingos are less well characterized. THEMIS (Fig. 14b) and MOC images (e.g., S15- 00196) show that these PLFs are highly degraded by impact Fig. 13. Subset of HiRISE image PSP_004180_2165 (centered near 36.21N 84.01E) gardening and embayed by a thin sheet deposit of uncertain showing an example of a flat-topped PLFs in Utopia Planitia as identified by origin. Although the size and irregular shape are similar to some Dundas et al. (2008). North is up and illumination is from the lower left. hydraulic pingos on Earth, their density appears high for hydraulic pingo formation and their spatial distribution is inconsistent with that of terrestrial pingos (Bruno et al., 2006). Based on this lack of 3.3.1. Gusev crater a sedimentary substrate and anomalous spatial distribution, we Rounded mounds commonly with raised margins in Gusev discount the hypothesis that these PLFs in Gusev crater are crater (Fig. 14) were hypothesized to be pingos by Cabrol et al. terrestrial-style pingos. Instead, they may relate to deflation and (2000). As derived from Viking images, mound lengths were deformation of a debris-flow deposit or other wet sedimentary or measured as ranging up to 2200 m, widths were measured as hydrovolcanic material. Similar processes but with more distinct ranging up to 900 m, diameters were measured as ranging from volcanic and tectonic associations have been proposed in the Author's personal copy ARTICLE IN PRESS

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Galaxias Fossae region of eastern Utopia, where volcanic flows 3.3.2. Cerberus region/Athabasca Valles and lahars led to deflated and deformed-appearing flows along Rings, cones, and mounds have been hypothesized to be pingos with linear grooves and cratered edifices (Skinner et al., 2007). at a variety of sites in the Cerberus region. In the Athabasca Valles outflow channel (Fig. 15a), a limited area of PLFs (Fig. 15b and c) in a proximal location in the channel was hypothesized to be a field of pingos based on their size and morphology (Burr et al., 2005). Small non-concentric inner cones, wide exterior moats, and a continuum of PLF morphologies with size were all interpreted as most consistent with pingo formation in comparison with other potential analogs. The size of the surrounding polygonal terrain was also interpreted as being consistent with periglacial pro- cesses, namely, thermal contraction cracking. The geologic context could have supported formation of either hydraulic or hydrostatic pingos. Similar reasoning was extended by other workers to PLFs found in both proximal and distal locations (Page and Murray, 2006). In an analysis of MOC images, inferred secondary impact craters (cf., McEwen et al., 2005) were observed to be crosscut by the PLFs. Such stratigraphic relationships were interpreted as showing that the formation of the PLFs postdated formation of the impact craters, implying a time lag between emplacement of the Fig. 14. PLFs in Gusev Crater. (a) MOLA shaded relief image of Gusev Crater. White substrate in the channel and formation on that substrate of asterisk shows location of (b). (b) A portion of THEMIS image V15085003 (centered the PLFs. Such timing would have been consistent with pingo near 14.31S 175.81E), showing PLFs of Cabrol et al. (2000), approximately numbers formation (which would lag substrate emplacement) and incon- 10–15 (the correspondence between features in the Viking image used in the original work and the THEMIS image is not exact). North is up, and illumination is sistent with formation of the PLFs as rootless cones (which would from the left. occur near-simultaneously with the substrate emplacement).

Fig. 15. Examples of PLFs in Athabasca Valles. (a) MOLA topography of Athabasca Valles (oriented northeast to southwest across the image), showing locations of (b) (designated by the letter ‘b’) and approximate locations (designated by x’s) of fields of PLFs as discussed in the multiple cited references. (One field in Page and Murray (2006) is off the image to the southwest.). (b) Portion of MOC image R0100745 (centered near 9.71N2041W/1561E), showing a portion of a field of PLFs and subaqueous dunes on the north side of a streamlined form. North is up and illumination is from the left. The black box outlines the area shown in (c). (c) Portion of MOC image R0100745 showing a close up of PLFs. The black box shows outlines the area shown in (d). (d) Portion of HiRISE image PSP_002661_1895 (centered near 9.61N 204.71W/156.31E) showing PLFs. Author's personal copy ARTICLE IN PRESS

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This and various other fields of PLFs in Athabasca Valles have been given a variety of interpretations. These interpretations include kettle holes (Gaidos and Marion, 2003), which form from deposition and subsequent melting or sublimation of large ice blocks, and basaltic ring structures (Jaeger et al., 2003, 2005), which form through hydrovolcanic interactions and subsequent extensive erosion. The first interpretation of conical landforms in this channel was as rootless cones, which also result from hydrovolcanic interactions but do not require subsequent erosion (Lanagan et al., 2001; Greeley and Fagents, 2001; Lanagan, 2004; Jaeger et al., 2007). From analysis of recent HiRISE images, this channel is interpreted as being draped in lava that once filled the channel and subsequently drained downslope into Cerberus Palus (Jaeger et al., 2007). In some locations, the moats around PLFs are rimmed with thin, high-relief morphology, interpreted as up- tilted slabs of lava that embays the channel (Fig. 15d). Other PLFs overlap to form chains along the direction of flow, indicating that the volatile source for the PLFs came from below the flow surface. Fig. 16. Portion of MOC image E1701551 (centered near 31N 142.31E/142.31W), These observations are interpreted as supporting the rootless cone showing PLFs at the edge of the western Medusae Fossae Formation (seen at upper left). North is up, and illumination is from the left. hypothesis for the PLFs over the various other hypotheses, including the pingo hypothesis (Jaeger et al., 2007). These observations and their interpretation remain controver- Planitia are the most likely candidates to be terrestrial-style sial. Important points of discussion include whether the PLFs and pingos or pingo scars. PLFs reported to date elsewhere on Mars are the surrounding polygonal terrain overlie or are overlain by less likely pingo candidates or are better explained by some other secondary impact craters (cf., McEwen et al., 2005). Some earlier mechanism. observations of the PLFs were not addressed in the analysis of That the landforms most likely to be pingos on Mars are HiRISE data (Jaeger et al., 2007), such as the observed interior clustered in Utopia basin may be due to general conditions in this cones, which are common in pingos (Burr et al., 2005). And a thin, northern mid-latitude basin. As discussed above (Section 3.2), high-relief rim morphology may not be unique to lava, as a similar Utopia basin has been a spatially extensive site for sediment morphology can be seen in MOC images of the Utopia PLFs as well accumulation throughout the Amazonian Period. Except for a (e.g., de Pablo and Komatsu, 2008, their Fig. 5A and F), for which single reported example of a pingo forming in bedrock (Mu¨ ller, pingos are the preferred formation hypothesis, as discussed above. 1959), all terrestrial pingos form in unconsolidated sediments. So However, lack of comparable (e.g., stereo) HiRISE coverage in sediment accumulation appears generally required for pingo Utopia Planitia to that over Athabasca Valles makes this formation. In addition, Utopia basin also experienced repeated comparison uncertain at the present time. The reader is referred aqueous inflow during the Amazonian. Such surface aqueous flow to the original references, including a recent comment (Page, would provide near-surface groundwater, which is also generally 2008) and response (Jaeger et al., 2008), for further discussion on necessary for pingo formation. this topic. To date, candidate pingos have not been hypothesized in large, southern mid-latitude basins that experienced sedimentary in- 3.3.3. Cerberus Palus/western Medusae Fossae Formation filling and aqueous inflow. Landforms in Hellas and Argyre basins A series of abstracts has proposed the idea that cones have been hypothesized to be glacial (e.g., Kargel and Strom, apparently eroding out from beneath the western Medusae Fossae 1992). Thus, it would be reasonable for the basins to also host Formation (Fig. 16) are pingos (Nussbaumer, 2005; Nussbaumer et periglacial (‘near-glacial’) landforms, such as pingos. To date, al., 2003, 2004). This hypothesis is based on the cones’ similarity however, PLFs in these basins have not yet been identified. in morphology and size to terrestrial pingos. As other features in A limited number of PLFs have been identified in small (10-km the region are interpreted to be glacial, the inferred context is also diameter) craters in the southern mid-latitudes (Dundas et al., consistent with the hypothesis that the cones are pingos. 2008). Additional data analysis is required to determine the extent Although the features’ conical morphology is pingo-like, the of PLFs in the southern hemisphere. Such future research will help arguments in these abstracts are not well-developed. Potential to determine the veracity of this seeming north–south asymmetry water pressurization mechanisms for the inferred pingos are not in PLF occurrence and, if true, its climatologic, hydrologic, and explained, and the interpretation of other features (e.g., positive- sedimentologic implications. relief linear hills) as glacial (e.g., as glacial fluting) is not convincingly argued in view of previous and more detailed interpretations (e.g., as yardangs). Similar PLFs nearby were Acknowledgements hypothesized to be rootless cones on the basis of the PLF morphology and the inference that the underlying substrate was This work was supported by the Mars Fundamental Research lava (Lanagan and McEwen, 2003; Lanagan, 2004). In our view, Program. We thank Jim Head and Goro Komatsu for constructive the interpretation remains viable, but speculative, with better critiques of the original manuscript that led to significant improve- supported interpretations available. ments, and Windy Jaeger and Jim Skinner for helpful comments.

4. Summary References

A tabular summary of the data and our evaluations of PLFs on An, Z., 1980. Formation and evolution of permanent ice mound on Qing-Zang Plateau. J. Glaciology Cryopedology 2 (2), 25–30. Mars is provided in Table 1. Of the Martian landforms that have Armstrong, J.C., Titus, T.N., Kieffer, H.H., 2005. Evidence for subsurface water ice in been hypothesized in the literature to be pingos, those in Utopia Korolev crater, Mars. Icarus 174, 360–372. Author's personal copy ARTICLE IN PRESS

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