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Johansson, A. Johnsson, M. Olvmo, E. Carlsson, H.A.B. Johansson and S. McDanielE. Hauber, D. Reiss, M. Ulrich, F. Preusker, F. Trauthan, M. Zanetti, H. Hiesinger, R. Jaumann, L. landforms on MarsPeriglacial landscapes on Svalbard: Terrestrial analogs for cold-climate

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177

The Geological Society of AmericaSpecial Paper 483

2011

Periglacial landscapes on Svalbard: Terrestrial analogs for cold-climate landforms on Mars

E. Hauber1,†, D. Reiss2, M. Ulrich3, F. Preusker1, F. Trauthan1, M. Zanetti2, H. Hiesinger2, R. Jaumann1, L. Johansson4, A. Johnsson4, M. Olvmo4, E. Carlsson5, H.A.B. Johansson6, and S. McDaniel7

1Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany2Institut für Planetologie, Westfälische Wilhelms-Universität, 48149 Münster, Germany3Alfred-Wegener-Institut, 14473 Potsdam, Germany4Department of Earth Sciences, University of Gothenburg, Box 460, SE-405 30 Göteborg, Sweden5Swedish Institute of Space Physics, Box 812, SE-981 28 Kiruna, Sweden6Department of Earth Sciences, Stockholm University, S-10691 Stockholm, Sweden7Reactive Surfaces, Ltd., 300 West Avenue, Austin, Texas 78701, USA

ABSTRACTWe present landforms on Svalbard (Norway) as terrestrial analogs for possible Martian periglacial surface features. While there are closer climatic analogs for Mars, e.g., the Antarctic Dry Valleys, Svalbard has unique advantages that make it a very useful study area. Svalbard is easily accessible and offers a periglacial landscape where many differ-ent landforms can be encountered in close spatial proximity. These landforms include thermal contraction cracks, slope stripes, rock glaciers, protalus ramparts, and pingos, all of which have close morphological analogs on Mars. The combination of remote-sensing data, in particular images and digital elevation models, with fi eld work is a promising approach in analog studies and facilitates acquisition of fi rst-hand experi-ence with permafrost environments. Based on the morphological ambiguity of certain landforms such as pingos, we recommend that Martian cold-climate landforms should not be investigated in isolation, but as part of a landscape system in a geological context.

Hauber, E., Reiss, D., Ulrich, M., Preusker, F., Trauthan, F., Zanetti, M., Hiesinger, H., Jaumann, R., Johansson, L., Johnsson, A., Olvmo, M., Carlsson, E., Johansson, H.A.B., and McDaniel, S., 2011, Periglacial landscapes on Svalbard: Terrestrial analogs for cold-climate landforms on Mars, in Garry, W.B., and Bleacher, J.E., eds., Analogs for Planetary Exploration: Geological Society of America Special Paper 483, p. 177–201, doi:10.1130/2011.2483(12). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

†[email protected].

INTRODUCTION

The surface of Mars shows many landforms that resemble cold-climate features on Earth (e.g., Lucchitta, 1981; Rossbacher and Judson, 1981). The potential use of these landforms as

indicators of the past and present Martian environment makes them prime targets for paleoclimatic investigations (Clifford et al., 2000; MEPAG Special Regions–Science Analysis Group, 2006; NRC, 2007). Since permafrost on Earth is known to host rich habitats containing cold-adapted microbial communities



178 Hauber et al.

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(e.g., Friedmann, 1994; Gilichinsky and Wagener, 1995; Rivkina et al., 2004; Gilichinsky, 2002; Steven et al., 2007), its exploration is also important for exobiologic studies. Life might even have originated in cold environments (e.g., see the studies of Trinks et al. [2005], which were inspired by fi eld experiments in Sval-bard), and cryophilic (i.e., cold-tolerating) organisms may be ana-logs for possible psychrophiles (extremophilic organisms capable of growing and reproducing in cold temperatures, ranging from −15 °C to +10 °C) that exist on, or deep inside, the surface of Mars (Seckbach, 2003; see also Buford Price, 2010, and references therein). Here, we introduce landforms on Svalbard (Norway) as useful terrestrial analogs for a number of surface morphologies on Mars that have been interpreted to be the result of permafrost or periglacial processes.

The use of terrestrial analogs has a long tradition among scientists who study planetary landscapes (Sharp, 1988). The basic premise is that a planetary feature looks similar to a terrestrial feature, the properties and origin of which are known. The known causes of the terrestrial analog might allow us to infer the causes of the planetary feature under study. This method of analogical reasoning was probably fi rst formally described for the fi eld of geoscience by Gilbert (1886), and the reader is referred to Baker (2008) for a more in-depth discussion of analogical reasoning in planetary geomorphology. It has to be emphasized, however, that analogs do not prove any causal relationships. Instead, they can help to fi nd lines for further reasoning (e.g., multiple working hypotheses) (see also Baker, 1996).

The most successful terrestrial analogs in planetary science are based not on terrestrial fi eld observations alone, but also on additional remote-sensing data that have a quality and scale comparable to that of planetary data. A similar scale is particularly important, since only certain geomorphic forms are scale-invariant. A classic case of self-similarity in geomorphology (Burrough, 1981; review by Xu et al., 1993) is the shape of a shoreline (Mandelbrot, 1967). Other typical examples are the fractal geometry of the drainage pattern of river basins (Rodriguez-Iturbe and Rinaldo, 1997) and the size distribution of a fault population that is characterized by a power law (e.g., Marrett and Allmendinger, 1992; however, note that not all systems with power-law properties are frac-tal; Paola et al., 2009). In general, fractal geometries have mostly been observed on erosional landscapes (Paola et al., 2009). Among permafrost features, the regular forms of pat-terned ground might have the highest likelihood to be fractal, and, e.g., polygons on Mars have been described to display a fractal-like geometry (Mangold, 2005). Scale-invariant spatial form does not imply, however, that the responsible process also operates at all scales, and indeed processes such as chemical weathering, frost action, or soil creep operate only at microscale levels (Klinkenberg, 2004). Geomorphic systems are commonly allometric, i.e., the components of the systems do not change in constant proportions (Church and Mark, 1980). One consequence of this is that many properties of natural surfaces and landscapes are nonfractal, at least at

certain scales. Therefore, the question of how to transfer results from one scale of investigation to another is one of the most fundamental challenges in geomorphology (Slaymaker, 2006, 2009). This problem is overcome, at least partly, if the scales of observations are similar for the planetary study objects and their terrestrial analogs. Here, we report on selected periglacial landforms on Svalbard and contrast them with Martian landforms that have been interpreted to be the result of permafrost processes. The scope of this paper is not to prove or disprove these interpretations, or to put forward new hypotheses, but to offer easily accessible examples of periglacial landscapes that are (1) morphologically strik-ingly similar to Martian landforms and (2) well described in the literature. As such, they should be useful analogs for planetary scientists who seek to understand the climatic his-tory of Mars. We begin with a short review of permafrost on Mars and Earth, including important terrestrial analogs that have been frequently cited in past studies. The next section provides information on the image data used to illustrate the Martian landforms. It also explains the High-Resolution Ste-reo Camera (HRSC-AX) instrument and the fl ight campaign that provided the aerial images of Svalbard shown here, and it gives an overview of the fi eld sites visited in 2008 and 2009. We then briefl y describe the climate, geology, soils, and vegetation of the study areas. We continue with the com-parison of permafrost landforms on Mars and Earth, and then discuss possible implications.

PERMAFROST AND PERIGLACIAL FEATURES ON MARS AND SVALBARD

Permafrost is defi ned as “ground (soil or rock […]) that remains at or below 0 °C for at least two consecutive years, regardless of the water content” (van Everdingen, 2005; French, 2007). Permafrost is not necessarily frozen, since the presence of mineral salts or pressure can depress the freezing point of water below 0 °C. Ground ice is ice in freezing or fro-zen ground, and it is one of the most important attributes of the terrain in permafrost regions. Ground ice occurs mainly as structure-forming ice, bonding the enclosing sediments, and as large bodies of more or less pure ice (e.g., Burgess and Smith, 2000; Heginbottom, 2000). The varieties of structure-forming ice include segregated ice, injection ice, reticulate vein ice, ice crystals, and icy coatings on soil particles (for a more compre-hensive list of ground ice varieties, see Shumskii, 1959; for a classifi cation of ground ice, see Pihlainen and Johnston, 1963). Large bodies of pure ground ice exist mainly in the upper part of the ground. They form pingo cores, massive icy beds, and ice wedges. The presence of ground ice infl uences topography, geomorphology, vegetation, and the response of the landscape to environmental changes (Burgess and Smith, 2000; Heginbot-tom, 2000).

Both permafrost and ground ice belong to the realm of the periglacial zone (Troll, 1944; Büdel, 1944). The term



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“periglacial” was fi rst introduced by the Polish geologist Walery von Łozinski (Łozinski, 1909), who established the concept of a “periglacial zone” at the XI International Geological Congress in Stockholm to describe climatic and geomorphic phenomena occurring peripheral to the Pleisto-cene ice sheets (e.g., Huggett, 2003). In fact, the periglacial concept came to be widely accepted during the postcongress fi eld excursion to Spitsbergen (French, 2003, 2008), and shortly thereafter Meinardus (1912) and Blanck (1919) described cold-climate patterned ground on Svalbard. Later refi nements of the defi nition of the term periglacial, which was often used for both regions and processes, led to the modern usage of the term as a “range of conditions, processes and landforms associated with cold, non-glacial environments” (Dylik, 1964; Washburn, 1973, 1979; van Everdingen, 2005; French, 2007). The two diagnostic criteria of periglacial environments are (1) ground freezing and thawing (Tricart, 1968), and (2) the presence of permafrost (Péwé, 1969), and the reader is referred to French (2007, 2008) and Thorn (1992) for a further discussion of the history of the periglacial concept. Permafrost and ground ice produce a wide variety of periglacial landforms on Earth, and a wide body of literature covers their morphology and origins (e.g., Jahn, 1975; Embleton and King, 1975; Washburn, 1979; French, 2007; Yershov, 1998).

Following the defi nition of permafrost cited here, Mars may well be regarded as a permafrost planet. Over most of its geological history, the shallow subsurface probably expe-rienced temperatures that were continuously below 0 °C (e.g., Shuster and Weiss, 2005). In the current Martian climate, ground ice is thought to be stable only at higher latitudes (e.g., Leighton and Murray, 1966; Smoluchowski, 1968; Clifford and Hillel, 1983; Fanale et al., 1986). This view has been vindicated by the instruments of the gamma-ray spec-trometer (GRS) suite on Mars Odyssey (Boynton et al., 2002; Feldman et al., 2002, 2004; Mitrofanov et al., 2002), which have detected near-surface water-equivalent hydrogen (inter-preted as ice) at latitudes higher than ~60°. The stability of near-surface ground ice on Mars depends on the obliquity of its rotational axis; at an obliquity of 32° (today: ~25°), it becomes globally stable (Mellon and Jakosky, 1995). Since the average value of the obliquity in the Martian past was probably higher than today (Laskar et al., 2004), ground ice can be expected to have been a signifi cant factor in Martian landscape evolu-tion. This view was recently confi rmed by the detection of relatively pure ice in the shallow subsurface in polar (Bibring et al., 2004), subpolar (Smith et al., 2009), and midlatitude regions (Byrne et al., 2009). Interestingly, relatively pure water ice has been found at some locations in midlatitudes (Holt et al., 2008; Byrne et al., 2009), where it should not be in thermodynamic equilibrium today (Mellon et al., 1997). The depth of the ground ice table on Mars is variable with respect to the geographic distribution (e.g., Kuzmin et al., 1988) and geologic time (Reiss et al., 2006). Permafrost- and ground ice–related landforms on Mars were fi rst reported from the

study of Mariner 9 and Viking Orbiter images, with resolu-tions not better than tens of meters per pixel. These early reports emphasize the morphological similarity of many Mar-tian landforms to terrestrial analogs of the periglacial zone (e.g., Carr and Schaber, 1977; Squyres, 1978, 1979; Ros-sbacher and Judson, 1981; Lucchitta, 1981, 1984; Jankowski and Squyres, 1993; Costard and Kargel, 1995). More recently, higher-resolution data have confi rmed these early notions and added new morphological evidence that permafrost processes might have played an important role in shaping the Martian landscape (see reviews by Clifford et al., 2000; Masson et al., 2001; Baker, 2001, 2006; Head et al., 2003; Helbert et al., 2007). While most of these studies discuss Amazonian-aged periglacial surfaces, new data provide compelling evidence that equatorial regions on Mars were affected by permafrost processes even much earlier, during the Hesperian (Warner et al., 2010). A comprehensive overview of Martian cold-climate landforms in particular, including comparisons with Earth, is given by van Gasselt (2007).

In lieu of direct observations of permafrost processes on Mars, comparisons with terrestrial analogs can constrain mod-els of Martian cold-climate environments and their potential as habitats. Since present-day Mars is cold and dry, surface processes acting in terrestrial cold deserts should be consid-ered as useful analogs. Probably the closest climatic analogs to Mars on Earth are the Antarctic Dry Valleys (Anderson et al., 1972; Marchant and Head, 2007), a polar desert environment with exceptionally cold and dry conditions (Doran et al., 2002; Bockheim et al., 2007). Many authors have studied this unique environment to obtain insights into the climate, landforms, and possible biologic activity on Mars (e.g., Gibson et al., 1983; Priscu et al., 1998; Wynn-Williams and Edwards, 2000; Wentworth et al., 2005; Levy et al., 2008a, 2008b). Nevertheless, access to the Dry Valleys is logistically complicated and costly, and some features of the morphologic inventory of permafrost terrain (e.g., pingos) are missing. Therefore, other polar areas (and perhaps high-altitude arid mountain regions) should also be considered as terrestrial cold-climate analogs for Martian landscapes. The circum-Arctic realm provides numerous peri-glacial landforms that have already been compared to Mars. Ice-wedge polygons in permafrost-dominated coastal plains of North America are similar to Martian polygonal ground (e.g., Seibert and Kargel, 2001; Mangold, 2005). Periglacial gullies and debris fans on Greenland might have been formed similar to young Martian gullies (Costard et al., 2002). Dundas et al. (2010) compared pingos, e.g., on the Tuktoyaktuk peninsula in Canada, to fractured mounds on crater fl oors on Mars. Retrogressive thaw slumps on Herschel and Ellesmere Islands (Lantuit and Pollard, 2008; Grom, 2008) have been used as terrestrial analogs for morphologically similar niches near the Cerberus Fossae in Elysium Planitia, Mars (Balme and Gallagher, 2009). Also situated in Elysium Planitia, some polygonal patterns bear a superfi cial similarity to sorted stone circles, e.g., on Ellesmere Island in Canada (Balme et al., 2009).



180 Hauber et al.

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While these terrestrial analogs are scattered throughout the huge area of the arctic zone of North America, the archipelago of Svalbard and its largest island, Spitsbergen (Fig. 1A), offer a diverse inventory of periglacial landforms in close spatial proximity. Terrain phenomena such as pingos, ice wedges, and rock glaciers are ubiquitous, especially in the dry central regions of Spitsbergen. Periglacial features such as solifl uction lobes occur primarily in the more humid western regions. Vari-ous forms of patterned ground, such as stone circles and stripes, are widespread and well developed (for a review of perigla-cial forms of Svalbard, see Åkerman, 1987). Svalbard is easily accessible, and examples of periglacial landforms are closely located to the settlements of Longyearbyen and Ny Ålesund on the main island, Spitsbergen, making it a very useful morpho-logical analog to Martian cold-climate landforms, which is so far underrepresented in the literature.

DATA

Mars Images

The Mars Reconnaissance Orbiter (MRO) mission (Zurek and Smrekar, 2007) carries two camera instruments that commonly act in concert, i.e., they operate simultaneously and cover the same ground area at different resolutions. The Context Camera (CTX) (Malin et al., 2007) is a pushbroom or line-scanning instrument with a 5000-element Charge-Coupled Device (CCD) detector. From a near-polar orbit with an altitude of ~290 km above the ground and a fi eld of view of 5.7°, it acquires ~30-km-wide images with a spatial ground resolution of ~6 m/pixel. The second camera, High- Resolution Imaging Science Experiment (HiRISE), provides images up to 20,000 pixels wide with a typical ground resolution of

Figure 1. Context maps of study areas. (A) Map of the Arctic, with location of Svalbard highlighted. (B) Map of Svalbard with study areas highlighted. (C) Study area in Adventdalen (cf. Fig. 3A). White outline shows coverage by High-Resolution Stereo Camera (HRSC-AX) (base map: hillshaded version of Advanced Spaceborne Thermal Emission and Refl ection Radiometer (ASTER) digital elevation model [DEM]). (D) Brøgger peninsula with white outline showing the coverage by HRSC-AX (cf. Fig. 3B) (base map: hillshaded version of ASTER DEM). The study area, Kvadehuksletta, is located at the northwestern tip of the peninsula.




spe 483-01 page 181

~30 cm/pixel (McEwen et al., 2007). The swath width of HiRISE is ~6 km, so that the higher-resolution images of HiRISE are nested within the footprint of the CTX images, which provide the geologic context over a wider area. All images displaying Martian landforms in this paper were obtained by CTX and HiRISE.

High-Resolution Stereo Camera (HRSC-AX)

HRSC (Fig. 2) is a multisensor pushbroom instrument with 9 CCD line sensors mounted in parallel that has been in orbit around Mars since January 2004 on Mars Express (Jaumann et al., 2007). It simultaneously obtains high-resolution stereo, multicolor, and multiphase images. Based on fi ve stereo channels, which provide fi ve different views of the ground, digital photo-grammetric techniques are applied to reconstruct the topography.

The four color channels (blue, green, red, and near-infrared) are used to make true orthophotos in color and false color. As of June 2011, high-quality digital elevation models (DEMs) and corresponding ortho-images were available for more than 36% of the Martian surface. The particular value of HRSC is the ste-reo capability, which allows the systematic production of high-resolution DEMs with grid sizes between 50 and 100 m (Wewel et al., 2000; Scholten and Gwinner, 2004; Scholten et al., 2005; Gwinner et al., 2005, 2009, 2010). An airborne version of the HRSC was used for the acquisition of stereo and color images in Svalbard. Since 1997, different airborne versions of HRSC have been developed. The principles of HRSC-AX data pro-cessing are described by Gwinner et al. (2006). The orientation data of the camera are reconstructed from a global positioning system inertial navigation system (GPS INS). HRSC-AX has been applied in diverse technical and scientifi c applications (e.g., Gwinner et al., 1999, 2000; Hauber et al., 2001; Otto et al., 2007) and has also been successfully used to investigate rock glacier activity (Roer and Nyenhuis, 2007). The fl ight campaign in July–August 2008 covered a total of seven regions in Sval-bard: (1) Longyearbyen and the surroundings of Adventfjorden, (2) large parts of Adventdalen, (3) large parts of the Brøgger-halvøya (halvøya = peninsula) in western Spitsbergen, (4) the Bockfjorden area in northern Spitsbergen, (5) the northeastern shore of the Palanderbukta and the margin of the adjacent ice cap in Nordaustlandet, (6) an area on Prins Karls Forland, and (7) the area of the abandoned Russian mining settlement of Pyramiden together with the nearby Ebbedalen. The landforms discussed in this study are located on the Brøgger peninsula and in Adventdalen and its vicinity (Fig. 3). Examples of true-color and false-color HRSC-AX images of permafrost landforms in Svalbard are shown in Figure 4.

Field Work

In two fi eld campaigns conducted during the summers of 2008 and 2009, selected landforms were investigated in situ. One of the objectives was the acquisition of ground truth data for HRSC-AX measurements, e.g., by determining with laser range meters the dimensions of landforms that had been observed with HRSC-AX. More important, however, we collected sam-ples (e.g., soil samples of the active layer) and took in situ mea-surements of soil properties such as thermal conductivity. Field photographs helped to increase the range of scales for which textural information was available from airborne images. Three fi eld camps were established. In July and early August 2008, the base camp was situated near Hiorthhamm, at the northeast-ern shore of Adventfjorden, opposite to Longyearbyen (central Spitsbergen; Fig. 3A). The main purpose was to study gullies and fans resembling the young gullies on Mars that had been detected in MOC (Mars Orbiter Camera) images (Malin and Edgett, 2000). First results of these investigations are reported elsewhere (Hauber et al., 2009; Reiss et al., this volume). The second study area was Kvadehuksletta, a strandfl at at the

Figure 2. Operating principle of High-Resolution Stereo Camera (HRSC) on Mars Express, and viewing geometry of the individual Charge-Coupled Device (CCD) sensors. The airborne version of HRSC (HRSC-AX), which was used for the Svalbard campaign, fol-lows the same principle. ND—nadir channel; S1, S2—stereo 1 and stereo 2 (±18.9° viewing direction, measured from nadir orientation); P1 and P2—photometry 1 and photometry 2 (±12.8°); IR—near-infra-red channel (+15.9°); GR—green channel (+3.3°); BL—blue channel (–3.3°); RE—red channel (–15.9°). All nine line sensors have a cross-track fi eld of view of ±6°.



182 Hauber et al.

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northwesternmost part of Brøggerhalvøya in western Spitzbergen (Fig. 3B). The site is renowned for its well-developed stone circles and was visited in July 2009. Another objective at this site was the exploration of protalus ramparts, a class of rock glaciers, which are prominent landforms at a nearby location called Stuphallet. A third study area (July–August 2009) was the Adventdalen in central Spitsbergen (Fig. 3A). It offered a variety of periglacial landforms in close spatial proximity, such as polygons and other types of pat-terned ground, rock glaciers, and pingos.

SVALBARD: CHARACTERIZATION OF STUDY AREAS

Climate

The present climate of Svalbard is arctic. The mean annual air temperature ranges between about –6 °C at sea level and –15 °C in the high mountains. In Longyearbyen (78°13�0�N, 15°38�0�E), which is located near the study area in Advent-dalen (Figs. 1 and 3A), the coldest (February) and warmest (July) months have mean temperatures of –15.2 °C and 6.2 °C, respectively (Table 1). The mean annual air temperature is –5.8 °C (average 1975–2000), but it can get as low as -15 °C in mountain areas. Precipitation is low and reaches only ~180 mm in central Spitsbergen (Table 1). At the coasts of Svalbard, the precipitation is ~400–600 mm. The central part of Spitsbergen can therefore be considered to be a polar (semi)desert, which is

defi ned as an area with annual precipitation less than 250 mm and a mean temperature during the warmest month of less than 10 °C (Walker, 1997). Interannual differences in mean precipitation and temperatures can be very high. Heavy snow-falls can occur in December and January in some years, and snow is the dominant type of precipitation. Snow avalanches are frequent, especially on downwind slopes. Svalbard lies at the border zone between cold arctic air in the north and mild maritime air in the south. This border zone can be meteorologi-cally very active, with cyclones generating unstable and often stormy weather. Strong winds can occur in winter and redistrib-ute the snowpack, so that wind-exposed sites, particularly in the more arid regions of central Spitsbergen, can be more or less snow-free even in high winter, enhancing heat loss from the ground (Humlum et al., 2003). At the more maritime western coast, the thicker snowpack in winter acts as an effective insu-lator (Winther et al., 2003). About 60% of Svalbard is covered by glaciers and ice caps. The remaining part (~25,000 km2) is characterized by continuous permafrost (Brown et al., 1997). Permafrost thickness is 10–40 m in coastal regions and ~100 m in the major valleys, but it can increase to more than 450 m in the highlands (Liestøl, 1976; Isaksen et al., 2001; Sollid et al., 2000). The age of the permafrost on Svalbard is Weichselian in the mountains and late Holocene in the coastal areas and in the valleys (Humlum et al., 2003). Permafrost temperature is between –2.3 °C and –5.6 °C, with a trend toward warming (Christiansen et al., 2010).

Figure 3. Locations of fi eld sites and fi gures shown in this paper. (A) High-Resolution Stereo Camera (HRSC-AX) image mosaic (approximately true color) of Longyearbyen and large parts of Adventdalen. (B) HRSC-AX image mosaic (approximately true color) of Brøggerhalvøja (western Spitsbergen). See Figure 1 for context.




spe 483-01 page 183

Geology

The geology of Svalbard is extremely diverse, given the limited size of the archipelago. A comprehensive description is available from Harland (1997). For the purpose of this study, a brief description of the overall geography and geology of the study sites is suffi cient.

At the largest scale, the topography of central Spitsbergen (Nordenskjøld Land) is dominated by mountain massifs that are separated from each other by valleys, which are in some cases interconnected. The largest of these valleys are Sassendalen, Rein-dalen, Colesdalen, and Adventdalen. Adventdalen, the site where most of the terrestrial features discussed in this study are located, has been ice free since ~10,000 yr ago (Mangerud et al., 1992). It is ~40 km long and hosts many periglacial landforms. Geologi-cally, the bedrock of the massifs bordering Adventdalen consists of Jurassic and Cretaceous sediments (Dallmann et al., 2002). Most of the bedrock in the study area belongs to the Helvetiafjellet and Carolinefjellet Formations (Dallmann et al., 2001). Their lithol-ogy is characterized by sandstones, siltstones, shales, and some thin coal seams (Parker, 1967; Major and Nagy 1972). The rocks are thinly layered (centimeters to tens of centimeters), and the lay-ering is generally subhorizontal. The rocks are heavily fractured by frost-shattering and—particularly near the coast—salt weath-ering. Valley marginal terraces in lower Adventdalen are thought to be proximal loess deposits that were likely derived by defl ation and local deposition of fl uvial sediments (Bryant, 1982). Most of the periglacial landforms in Adventdalen (e.g., pingos, ice-wedge polygons) were formed in the late Holocene and are only ~3000 yr old (Svensson, 1971; Jeppesen, 2001; Humlum, 2005), but some ice wedges at high elevations might have survived the Weichse-lian ice age under cold-based ice (Sørbel and Tolgensbakk, 2002). A useful overview of glacial and permafrost features in Norden-skjøld Land is given by Meier and Thannheiser (2009).

The second study site is located on the outermost part of the Brøggerhalvøya. The lithology of the mountains in the hinterland is predominantly dolomitic (Challinor, 1967). The study area at Kvadehuksletta is a strandfl at with well-developed raised beach ridges up to an elevation of 80 m above sea level (asl). Forman and Miller (1984) subdivided the beach ridges with respect to their elevation into three classes, at the intervals 0–44 m asl, 44–55 m asl, and 55–80 m asl, and assigned the associated depos-its ages of <12 ka, 60–160 ka, and 130–290 ka, respectively. The sediments covering the strandfl at have a calcitic and dolomitic composition (Dallmann et al., 2002). The reader is referred to Svendsen et al. (2002) for an in-depth description of the physical environment of the Kongsfjord region.

Soil

The surfaces of the northernmost portions of ice-free land are primarily mantled with polar desert soils (Tedrow, 1966). Such soils are commonly characterized by low temperatures, relatively dry soil conditions, a desert pavement, mildly acid to alkaline reac-tion, and salt effl orescences (Tedrow, 1966). For a map of soil types on Svalbard, see Låg (1993). Portions of Svalbard, in particu-lar at higher elevations, lie within the polar desert soil area (Char-lier, 1969; Tedrow, 1977), which is characterized by mineral soil without identifi able horizons, and with little humus content. Polar desert soil is also found at the study site on Brøggerhalvøya (Mann et al., 1986; Ugolini and Sletten, 1988). The recently published Soil Atlas of the Arctic (Jones et al., 2010) classifi es the soils of Sval-bard as leptic cryosols, i.e., shallow permafrost soils developed over a rocky substrate. A detailed description of the micromor-phology of cryosols, including a discussion of cryoturbation and solifl uction processes, is given by Van Vliet-Lanoë et al. (2004). Since the respective map in the Soil Atlas of the Arctic has a very small scale, however, it does not differentiate between mountains and valley fl oors, the latter of which have greater soil thicknesses.

TABLE 1. CLIMATE AT SVALBARD AIRPORT 1912 TO 1993 Season Observed temperature

(°C) Modeled temperature

(°C) Correlation†

Mean SD Minimum Maximum Mean SD Minimum Maximum Year –6.3 1.7 –12.2 –3.1 –6.4 1.0 –8.9 –4.0 0.61 Winter (DJF) –14.0 3.6 –23.2 –7.6 –14.1 2.4 –19.1 –9.1 0.62 Spring (MAM) –10.8 2.4 –19.3 –6.7 –10.8 1.7 –15.2 –7.5 0.58 Summer (JJA) 4.3 0.7 2.5 6.1 4.2 0.5 3.2 5.4 0.54 Autumn (SON) –4.8 2.0 –11.3 –1.3 –4.9 1.5 –8.7 –1.8 0.66

Season Observed precipitation (mm)

Modeled precipitation (mm)

Correlation†

Mean SD Minimum Maximum Mean SD Minimum Maximum Year 180.7 49.8 86.4 317.0 178.7 33.5 93.5 286.6 0.54 Winter (DJF) 53.4 24.3 16.8 140.0 52.8 11.5 24.5 86.8 0.40 Spring (MAM) 35.6 10.4 6.4 125.9 34.3 13.6 10.6 65.5 0.60 Summer (JJA) 43.7 21.2 3.0 114.0 43.7 18.7 8.3 100.8 0.57 Autumn (SON) 48.1 17.0 18.4 109.0 47.9 13.1 21.5 79.1 0.54

Note: For the series of observed and modeled annual and seasonal precipitation sums from 1912 to 1993, the following values are given: mean, standard deviation (SD), absolute minimum, and absolute maximum. For comparison, the mean annual temperature at the floor of the Dry Valleys in Antarctica ranges from –14.8 °C to –30 °C, and the mean annual precipitation is <<100 mm, but can be as low as 13 mm (Doran et al., 2002; Campbell and Claridge, 2004).

†Correlation coefficient between observed and modeled precipitation series (data from Hanssen-Bauer and Førland, 1998).



184 Hauber et al.

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In Adventdalen, loess (Bryant, 1982) and arctic meadow soil are present (Van Vliet-Lanoë, 1998). The relatively high organic content of the soils in Adventdalen is responsible for the limited depth of the active layer (<1 m; Christiansen and Humlum, 2008), since the insulating effect of the thick vege-tation cover prevents the wave of enhanced summer tempera-tures from penetrating deeply into the subsurface. In contrast, the active layer in sandy and gravelly soils has a larger depth. The effect of a vegetation cover has been found to be a more

important control on the variations of active-layer depth than regional geographic differences (Stäblein, 1970).

Vegetation

For a general description of circum-Arctic vegetation zones, see Bliss and Matveyeva (1992). With respect to plant ecology, three climatic zones are present on Svalbard: The mid-arctic tundra zone, the northern arctic tundra zone, and the polar

Figure 4. Examples of High-Resolution Stereo Camera (HRSC-AX) coverage of permafrost features in Svalbard. (A) Unsorted nets and stripes on the western slopes of Hiorthfjellet. The fl at top of a SE-NW–trending ridge is characterized by unsorted nets (“N”). The low troughs be-tween the centers of the net are accentuated by vegetation (shown in red tones in this HRSC-AX false-color view, which was produced via IHS (Intensity-Hue-Saturation) transformation from the near-infrared, red, and green color channels to a red-green-blue representation, respectively). Where the slope gets steeper, the nets transform into stripes (“S”; see, e.g., Kessler and Werner, 2003; 78.258°N, 15.722°E). (B) Debris-covered glacier on Operafjellet. HRSC-AX false-color image (78.227°N, 16.0°E). (C) Polygons on fl uvial terrace in central Adventdalen (east of the mouth of Helvetiadalen, upper part of image). A ground view of the distinct troughs between the polygon centers is shown in Figure 6B. A small pingo (labeled “P” and named “Riverbed Pingo” by Yoshikawa, 1993) is visible in the braided river bed. HRSC-AX false-color image (78.195°N, 16.451°E). (D) “Lagoon pingo” (Yoshikawa, 1993) in lower Adventdalen at the distal end of a debris-fl ow fan (“DFF”). This pingo group (“P”) is very young and was only initiated when the area was raised above sea level. Note the irregular form of the mounds (maximum elevation: 4.5 m) and craters (the youngest of which formed in 1993; Yoshikawa and Harada, 1995). Small lakes could be thaw lakes resulting from the degrada-tion of ice cores. HRSC-AX false-color image (78.230°N, 15.755°E). (E) Rock glacier (“RG”) on Hiorthfjellet (Isaksen et al., 2000). HRSC-AX false-color image (78.256°N, 15.786°E). (F) Thaw slumps or active-layer detachment slides near Advent City, an abandoned mining site near the mouth of Hannaskogdalen. These slumps (white arrows point to slump heads) resemble active-layer detachment failures in morphology and dimension (e.g., see fi g. 6 in Lewkowicz, 2007). HRSC-AX true-color image (78.283°N, 15.614°E). North is up in all images.




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desert zone (Elvebakk 2005; Alsos et al., 2007). The mean air temperature during the warmest month in these three zones in Svalbard are 4–6 °C, 2.5–4 °C, and 1–2.5 °C, respectively (Elvebakk, 2005). Mann et al. (1986) mentioned polar desert vegetation on Brøggerhalvøya, but Elvebakk (2005) assigned the strandfl at of Kvadehuksletta to the northern arctic tundra zone. Adventdalen is more densely vegetated than the Brøg-ger peninsula. The main plant species are Salix polaris and the mosses Polytrichum hyperboreum and Saninonia uncinata (Rozema et al., 2006).

MORPHOLOGIC COMPARISONS BETWEEN MARS AND SVALBARD

In this chapter, we compare examples of presumed periglacial features on Mars with terrestrial analogs on Spitsbergen (Table 2). The selection is restricted to features that have been covered by HRSC-AX data, so that a comparison among Mars, HRSC-AX, and fi eld photographs is enabled.

Wherever possible, we tried to present features that have similar scales on Mars and on Spitsbergen. The use of genetic, not descriptive, terms for the section headings applies for the features on Svalbard only and does not imply that the Martian landforms have the same origin.

Patterned Ground

The term patterned ground was introduced by Washburn (1956, p. 824) to denote all sorts of “more or less symmetrical forms, such as circles, polygons, nets, steps, and stripes, […].” This defi nition includes sorts of patterned ground that are not restricted to cold-climate environments (e.g., desiccation poly-gons), but generally—and in the following—the term is used to refer to cold-climate patterned ground. Patterned ground consists of sorted and nonsorted varieties. The sorted classes of patterned ground commonly exhibit a marginal zone of stones that surround a central area of fi ner material. The mechanisms thought to form patterned ground include particle sorting,

TABLE 2. OVERVIEW OF PERIGLACIAL LANDFORMS noitinifeDmrofdnaL

(van Everdingen, 2005) Selected references

(Earth; general) Images

(this study)Selected

references (Mars)

Selected references(Svalbard)

Ice-wedge polygons

A polygon outlined by ice wedges underlying its boundaries

Dostovalov and Popov (1966); Lachenbruch (1962, 1966); Black

(1976); Washburn (1979); Mackay and Matthews

(1983)

Figure 5 Figure 6

Mangold (2005); Levy et al.

(2009a, 2010)

Christiansen (2005)

Sorted and unsorted stripes

Sorted stripes form patterned ground with a striped and sorted appearance, due to parallel strips of stones and intervening strips of finer material, oriented down the steepest available slope

Nonsorted stripes form patterned ground with a striped and nonsorted appearance, due to parallel strips of vegetation-covered ground and intervening strips of relatively bare ground, oriented down the steepest available slope

Washburn (1956) Figure 7 Mangold (2005) Van Vliet-Lanoë (1991)

Sorted circles A sorted circle is a patterned ground form that is equidimensional in several directions, with a dominantly circular outline, and a sorted appearance commonly due to a border of stones surrounding a central area of finer material

Washburn (1956) Figure 8 Balme et al. (2009)

Hallet and Prestrud (1986); Hallet et al. (1988); Etzelmüller and Sollid (1991)

Rock glaciers A mass of rock fragments and finer material, on a slope, that contains either interstitial ice or an ice core and shows evidence of past or present movement

Capps (1910); Washburn (1979); Barsch (1996)

Figures 9 and 10

Squyres (1978); Degenhardt and Giardino (2003)

Sollid and Sørbel (1992); Humlum et al. (2007); Isaksen

et al. (2000) Pingos A perennial frost mound consisting of a core of

massive ice, produced primarily by injection of water, and covered with soil and vegetation

Porsild (1938); Mackay (1973, 1979); Washburn

(1979)

Figure 11 Dundas et al. (2008); Dundas

and McEwen (2010)

Liestøl (1976); Yoshikawa (1993);

Yoshikawa and Harada (1995);

Ross et al. (2007) Retrogressive thaw slumps or active-layer detachments

A slope failure resulting from thawing of ice-rich permafrost

Mackay (1966); McRoberts and

Morgenstern (1974); Washburn (1979); Lantuit

and Pollard (2008); Lewkowicz (2007); Lacelle et al. (2010)

Figure 4F Balme and Gallagher (2009)

Larsson (1982)

Note: Overview of periglacial landforms discussed in this study. Definitions are from the “English Language Glossary of Permafrost and Related Ground-Ice Terms,” provided by the Frozen Ground Data Center of the National Snow and Ice Data Center (NSIDC) (van Everdingen, 2005). Where possible, those references on Svalbard’s periglacial landforms were selected that cover the study sites described in this work.



186 Hauber et al.

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freeze-and-thaw cycles, the deformation of frozen soil, and soil creep (see Kessler and Werner, 2003, and references therein), but there is still no consensus whether the range of forms can be explained by a single model (e.g., Kessler and Werner, 2003).

PolygonsPolygonal ground in cold climates on Earth is widespread. It

is caused by thermal contraction cracking (Lachenbruch, 1962, 1966), but depending on the temperatures and amount of avail-able liquid water, different formation mechanisms are known. They all involve cooling and cracking of the ground in the win-ter season and subsequent fi lling of the cracks by water, sand, or soil. The most common type of thermal contraction poly-gons are ice-wedge polygons (Leffi ngwell, 1915; Black, 1976), which form by refreezing of the molten liquid water that fi lled the cracks in spring and summer. If there is a lack of liquid water in hyperarid polar deserts (e.g., the ice-free areas of Antarctica or the polar deserts of the Canadian High Arctic), the cracks may be fi lled by loess or sand (sand-wedge polygons; Péwé, 1959; Sletten et al., 2003), or soil (Black, 1976; French, 2007). Repeated cracking in subsequent years leads to lateral expan-sion of the ice or sand wedge. A further type of cold- climate polygonal ground has been described by Marchant et al. (2002) from the Antarctic Dry Valleys, where ice buried beneath sedi-ment is cracking by thermal contraction. Fine-grained sediment collects in the cracks, and coarser-grained material (>2 cm) is left at the surface near the cracks, enhancing sublimation due to the relatively higher porosity and permeability. These so-called sublimation polygons can be considered to be a special type of sand-wedge polygons (Marchant and Head, 2007).

Polygonal ground is widespread on Mars and has been recog-nized at different scales, with polygon diameters ranging from meters to tens of kilometers (e.g., Lucchitta, 1981; Lucchitta et al., 1986; Mellon, 1997; Malin and Edgett, 2001; Seibert and Kargel, 2001; van Gasselt et al., 2005; Pondrelli et al., 2008). While the giant polygons in the northern lowlands might have a tectonic origin (Pechmann, 1980; Hiesinger and Head, 2000), and some very small-scale polygonal patterns might be the result of rock jointing, several classes of polygons with diameters of meters and tens of meters bear a striking resemblance to terrestrial polygons that formed by freeze-thaw processes in cold-climate regions (e.g., Figures 5A and 5B). There seems to be a clear geographic control of their distribution on Mars (Kuzmin and Zabalueva, 2003; Seibert and Kargel, 2001; Mangold, 2005; Levy et al., 2009a). This distribution indicates a possible control by climatic factors, and many workers have inferred that these polygons formed as thermal contraction cracks anal-ogous to terrestrial ice-wedge or sand-wedge polygons (e.g., Seibert and Kargel, 2001; Mangold, 2005; Levy et al., 2009a). Mechanical modeling shows that the maximum size of poly-gons formed by thermal contraction cracking on Mars is limited, however, and that a formation of many polygons on crater fl oors by desiccation might be a viable alternative (El Maarry et al., 2010). Recently, detailed comparisons among polygons on Mars, particularly those found at the Phoenix landing site,

and terrestrial polygons in the Antarctic Dry Valleys have led Levy et al. (2008b, 2009b) to conclude that high-latitude Mar-tian polygons are more likely to be sand wedge or sublimation polygons than ice-wedge polygons.

Polygonal ground occurs throughout Adventdalen in many locations on terraces of the valley’s river, Adventelva, and on the slopes of the adjacent massifs (Tolgensbakk et al., 2000) (Figs. 5C–5F). Polygons are known to exist on slopes with an inclination of up to 25° (Sørbel and Tolgensbakk, 2002). Typi-cally, these polygons have diameters of 10–80 m, separated by 1–6-m-wide troughs (Sørbel et al., 2001). According to most researchers, they are active ice-wedge polygons (e.g., Sørbel and Tolgensbakk, 2002; Christiansen, 2005, and references therein). With respect to their morphology, however, different varieties can be distinguished. Low-centered polygons appear to be more common in the fl at parts near Adventelva in lower Adventdalen (Figs. 5E and 5F). On the other hand, high- centered polygons prevail on many fl at and inclined surfaces in central and upper Adventdalen (Figs. 5C and 5D). They have diameters of 10–20 m and are separated by well-developed wide and deep troughs (Figs. 6A and 6B). A direct correlation could be established between the widths of the ice wedge and the overlying trough, and it was found that the oldest, most distinct troughs occur on the highest terrace (Malmström et al., 1973; cited in Christiansen, 2005). This notion is supported by HRSC-AX image inspection. Field observations show that the shoulders of the troughs are often disrupted by trough-parallel fractures (Fig. 6B). HRSC-AX data have a suffi cient spatial and vertical resolution to enable the three-dimensional analy-sis of polygons and troughs (Fig. 6F). Troughs are commonly ~50 cm deep (Fig. 6F), but they can reach depths of up to 1 m (Fig. 6A).

Of the many morphologically different polygons on Mars (Mangold, 2005; Levy et al., 2009a), some polygons, for example, those situated near gully alcoves, share the characteristics of the high-centered polygons of the upper Adventdalen. A direct comparison reveals this similarity (Figs. 6D and 6E). In both cases, distinct troughs separate polygon centers with fl at surfaces. The troughs display an orthogonal pattern in both cases, but on Mars, it is oriented-orthogonal, whereas on Svalbard, it appears random-orthogonal (for an overview on the various geometries of polygonal ground, see French, 2007). The diameters of the polygons are 5–10 m and 10–20 m on Mars and Svalbard, respectively.

Based on morphology alone, however, it would appear to be problematic to interpret the Martian polygons shown in Figure 6D as ice-wedge, sand-wedge, or sublimation polygons. The morphology of different thermal contraction cracks has recently been discussed by Levy et al. (2010, their Fig. 2). For example, polygons on the northern margin of Victoria Valley (Antarctic Dry Valleys) appear morphologically very similar to the examples in Svalbard (with the exception of the trough shoul-ders, which are slightly elevated with respect to the polygon cen-ters in Victoria Valley), yet they are more likely to be sand-wedge polygons (cf. Fig. 6 in Marchant and Head, 2007). On the other




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Figure 5. Polygonal structures on Mars and in Svalbard. (A) Polygons of different sizes on the fl oor of an impact crater on Mars (High-Resolution Imaging Science Experiment [HiRISE] image PSP_007372_2475 at 67.2°N, 47.8°E). (B) Enlarged detail of A. Note how a population of smaller polygons (diam-eters of a few meters to 10 m) is nested within a population of larger polygons (˜50–100 m). North is up. (C) Polygons of different sizes on the northern side of central Adventdalen (Spitsbergen). Labels “x” and “y” denote two polygons highlighted in panel D. High- Resolution Stereo Camera (HRSC-AX) false-color image; north is up (78.196°N, 16.584°E). (D) Field photograph of area shown in C. Both small and large polygons are high-centered. The ratio between the diameters of small and large polygons is similar on both examples shown here from Mars and Svalbard. (E) Well-developed ice-wedge polygons on the northern side of lower Advent-dalen, near Longyearbyen (HRSC-AX approximately true-color image; north is up; 78.206°N, 15.888°E). (F) Oblique aerial photograph of scene shown in E. Ponding water indicates low-center polygons.

Figure 6. Polygons on Mars and Svalbard. (A) Large trough-bounding high-center polygons in central Adventdalen (Spitsbergen). Person for scale. (B) Trough between high-center polygons in central Adventdalen. Note the fractured and degraded appearance of the trough shoulders. Spade for scale. (C) Ice-wedge polygons in lower Adventdalen, near Isdammen. Person for scale. (D) Oriented-orthogonal polygons pattern on a ridge between two gullies on the northern wall of Hale crater, Mars (see Fig. 10A for location). The polygons have high centers and diameters between ~5 and ~10 m (High-Resolution Imaging Science Experiment [HiRISE] image PSP_004072_1845; near 34.6°S, 323.1°E). (E) High-center orthogonal polygons in central Adventdalen. The polygons have high centers and diameters between ~10 and ~20 m. A typical trough between these polygons is shown in panel B (78.196°N, 16.545°E). (F) Hillshaded digital elevation model derived from High-Resolution Stereo Camera (HRSC-AX) stereo images of a subscene of panel E (in lower left part). White line a–a� marks location of profi le shown below. The profi le shows that the troughs are typically ~50 cm deep (cf. panel B).



188 Hauber et al.

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hand, sublimation polygons, e.g., in Beacon Valley (Antarctica), can also display an almost identical morphologic expression with high-centered, fl at-topped, 10–20-m-wide polygons (cf. fi g. 4 in Marchant et al., 2002). A further complication is introduced by the possibility that some varieties of polygonal ground on Mars might be desiccation cracks, resulting from the drying out of ancient crater lakes (El Maarry et al., 2010). Hence, we conclude that an unambiguous interpretation of polygonal ground based on remotely sensed data alone is diffi cult even on Earth, let alone on Mars. This is even more true because the reconstruction of paleoenvironmental conditions from current-day (ice-wedge) polygons (which are the basis of analog studies) appears to be problematic (e.g., Harry and Godzik, 1988) due to the limited understanding of the constraints on cracking within modern permafrost environments (see Christiansen, 2005). A promis-ing approach is the detailed study of not only polygons, but the entire suite of associated landforms, as was shown by Levy et al. (2009b) in their study of the Phoenix landing site.

Nets and StripesSorted patterned ground (polygons, circles, stripes) is a class

of patterned ground that forms as a result of differential frost-heaving (e.g., Washburn, 1956; Goldthwait, 1976; Van Vliet-Lanoë, 1991). Unsorted stripes and nets resemble their sorted relatives, but according to Washburn (1956, p. 837), the striping is due to the presence of vegetated and nonvegetated areas.

Slope stripes on Mars have been reported by Malin and Edgett (2001) and Mangold (2005). From the inspection of MOC images, Mangold (2005) found them to be concentrated in two latitudinal belts (±55° to ±75°), with a higher frequency in the southern hemisphere. Stripes occur with and without association to polygons. Higher-resolved images acquired by the HiRISE (High-Resolution Imaging Science Experiment) onboard the Mars Reconnaissance Orbiter (MRO) add new evidence for slope stripes (Fig. 7). The examples shown in Figures 7C and 7D are situated on slopes of the inner wall of an impact crater (Fig. 7A) in the immediate vicinity of gully alcoves (Fig. 7B). These stripes display an alternating pattern of bright and dark albedo. Their width typically ranges from ~50 cm to 1.5 m, and their orientation is con-sistently downslope, although it cannot be excluded that it some-times slightly deviates from the steepest topographic gradient. It is unlikely that the stripes are the result of wind-sculpting, since the pattern is clearly controlled by local slope azimuth (Fig. 7C). These Martian stripes are almost identical in appearance to sorted stripes that can be observed on slopes of the valley sides in Adventdalen (Bibus et al., 1976; Figs. 7E and 7F). The width of the stripes in Adventdalen is commonly between 50 and 100 cm, and the orientation is downslope. The bright stripes correspond to vegetation-free areas of rock particles, and the dark parts corre-spond to vegetated areas. In many cases, the stripes are connected to sorted nets on fl at-lying terrain, from which they develop downslope (Fig. 4A). This spatial association between nets on

Figure 7. Comparison between alternating bright and dark stripes on Mars and sorted stripes on Svalbard. (A) Context for Martian examples; the location of B is marked by white box (center of crater at 39°S, 195.9°E; Context Camera [CTX] images P14_006497_1406 and B11_013894_1412). (B) Gullies on the pole-facing inner wall of the crater; locations of C and D are marked by white boxes (High-Resolution Imaging Science Experiment [HiRISE] PSP_001684_1410). (C) Stripes on the crater wall near the gullies. Three different expressions of the stripes (I–III) are controlled by local slope orientation (HiRISE image PSP_001684_1410; near 38.9°S, 196.0°E). (D) Alternating dark and bright stripes near gullies on the inner wall of a Martian impact crater (HiRISE image PSP_001684_1410; near 38.9°S, 196.0°E). The orien-tation of the stripes is approximately downslope. (E) Sorted stripes on the western slopes of the Hiorthfjellet massif (east of Adventfjorden, Spitsbergen; 78.255°N, 15.701°E). Note the striking similarity in scale between D and E. (F) Sorted stripes in Adventdalen (Spitsbergen). Coarser and slightly elevated unvegetated stripes alternate with fi ner-grained and vegetated stripes (fi eld photography, person for scale).




spe 483-01 page 189

fl at terrain and stripes on adjacent slopes is well known from observations (Washburn, 1956), and it could also be reproduced by numerical modeling (Kessler and Werner, 2003).

The presence of slope stripes on Mars might indicate the action of cryoturbation processes. If so, the difference between the bright and dark stripes could perhaps be associated to frost heave and particle sorting (in contrast to vegetated and nonvegetated bands in unsorted stripes on Earth; see Fig. 7C). An alternative model could be the preferential deposition of relatively brighter dust or frost in long and narrow, parallel depressions that are oriented downslope. Even if this were the case, the existence of such depressions requires an explanation.

Stone CirclesStone circles are a class of sorted ground. The exact pro-

cess of formation is still not fully understood (see discussion in Balme et al., 2009), but there is a consensus that some form of churning of the ground by freeze-thaw cycles in ice-rich sedi-ment is required. Some researchers hold that free convection in thawed fi ne-grained soils might be involved (Hallet and Pre-strud, 1986), but this view was challenged by Van Vliet-Lanoë (1991), who questioned the assumption that conditions in the active layer meet the requirements for convection. The signifi -cance of weathering and accumulation of fi nes was emphasized by Etzelmüller and Sollid (1991), who also pointed out in their study of the stone circles at Kvadehuksletta that dissolution of dolomitic bedrock and, therefore, bedrock petrography play an important role.

Polygonal structures in Elysium Planitia, Mars, with zones of coarse (up to 1 m) particles surrounding interior areas of homogeneous texture have been interpreted by Balme et al. (2009) as possible sorted stone circles or nets (Fig. 8D). These authors infer that ground ice in equatorial regions on Mars may have been subject to freeze-thaw cycles and recent cryoturba-tion, with implications for a warmer-than-thought climate in the geologically young history of Mars. According to Balme et al. (2009), given the known ratio of 3–4 between the diameter of sorted circles and the depth of the active layer, respectively (Ballantyne and Harris, 1994), the uppermost regolith in Elysium Planitia would have contained ice to depths of a few meters.

Sorted stone circles and nets are particularly well devel-oped at Kvadehuksletta on Brøggerhalvøya (Figs. 8A–8C). They are commonly found in shallow depressions dammed by beach ridges (Tolgensbakk and Sollid, 1987). Typical diameters are a few meters (Figs. 8E and 8F), and the raised rims, consist-ing of stones with diameters of a few centimeters, reach heights of up to 50 cm (Figs. 8A and 8B; Etzelmüller and Sollid, 1991). The planform shape and dimensions are signifi cantly different among the polygonal patterns in Elysium Planitia and on Kva-dehuksletta. The size of particles in the stone rings, in particular, is much larger on Mars than in Svalbard, but the diameter of the circles is also larger. The mesoscale topography (i.e., the loca-tion of the sorted circles in areas behind the beach ridges, where water supply is high) appears also to be different. Nevertheless, larger sorted circles than those on Svalbard are known from the colder and drier Canadian Arctic (e.g., Bjorne Peninsula, Ellesmere Island; see also Goldthwait, 1976) and match in scale

Figure 8. Stone circles on Svalbard and polygonal structures on Mars. (A) Well-developed closed and circular stone circles on Svalbard. (B) Network of irregular stone circles (sorted circles or sorted nets) on Svalbard. Both A and B are located at Kvadehuksletta (Brøgger peninsula, western Spitsbergen). Black squares on scale bar (arrows) are 5 × 5 cm. (C) Close-range aerial photograph of stone circles near Ny Ålesund (Brøgger peninsula, western Spitsbergen; diameter of individual circles is ~2 m). (D) Polygonal structures in Elysium Planitia, Mars. A network of coarse particles out-lines low-center polygons with fi ner-grained interiors (High-Resolution Imaging Science Experiment [HiRISE] image PSP_004072_1845, near 4.5°N, 156.0°E). (E–F) Examples of stone circles as seen by the High-Resolution Stereo Cam-era (HRSC-AX), which has a similar spatial resolution (~20 cm/pixel) as the HiRISE camera (25–32 cm/pixel). Both images are from Kvadehuksletta (Brøgger peninsula, western Spitsbergen, see Fig. 1D for location).



190 Hauber et al.

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the purported Martian examples of Balme et al. (2009). If the polygonal patterns observed by Balme et al. (2009) are indeed sorted circles, this would have interesting implications for the recent Martian climate at equatorial regions. The observed differences between the features in Elysium Planitia and on Sval-bard, however, with respect to their morphology (cf. Figs. 8D, 8E, and 8F), geomorphic setting, and clast lithology (i.e., dolomitic at Kvadehuksletta and basaltic on Mars [e.g., Diez et al., 2009], with implications for dissolution processes) require some caution in the interpretation, and careful investigations should identify if some mantling process could preferentially deposit fi nes (e.g., airborne dust) in the low-lying interior of the polygons, creating a homogeneous texture.

Rock Glaciers

Rock glaciers are common phenomena on Earth, and they occur both as active and relict (fossil) features in a variety of mountainous environments. They are sensitive to environmental changes and, therefore, are important landforms in climatic stud-ies (e.g., Humlum, 1998). Many defi nitions of rock glaciers exist in the literature (for overviews, see Barsch, 1996; van Gasselt, 2007), and here we refer to that of Barsch (1988, p. 72), who defi nes active rock glaciers as “lobate or tongue-shaped bodies of frozen debris with interstitial ice and ice lenses, which move downslope or downvalley by deformation of the ice contained within them.” It is still a matter of debate (e.g., Humlum et al., 2007) whether rock glaciers are strictly periglacial landforms (a view promoted by Barsch, 1996), or if they are part of a con-tinuum, with rockfall-derived talus on one end and true glaciers on the other end (Giardino and Vitek, 1988). This debate shall not further be considered here. The defi nition does not address the source of the ice and is, therefore, applicable to both models.

Rock glacier–like landforms on Mars have been reported since the acquisition of the Viking Orbiter images. The descriptive term “lobate debris apron” was assigned to distinctive geomorphic landforms showing evidence for the creep and deformation of ice-rich debris in Martian midlatitudes (e.g., Carr and Schaber, 1977; Squyres, 1978, 1979; Lucchitta, 1984; Degenhardt and Giardino, 2003). They were fi rst described in detail by Squyres (1978, 1979), who ascribed them to downslope transport of erosional debris mixed with ice, analogous to ter-restrial rock glaciers (e.g., Barsch, 1996). The global distribu-tion of lobate debris aprons was found to be concentrated in two latitudinal bands with a width of 25°, centered at 40°N and 45°S (Squyres, 1979; Squyres and Carr, 1986; Hauber et al., 2008), implying a climatic infl uence on their formation. A con-centration of small-scale viscous fl ow features in the same latitudinal belts was later observed by Milliken et al. (2003), who compared them with terrestrial rock glaciers. On Svalbard, more than 500 active and inactive rock glaciers have been iden-tifi ed in aerial images (Kristiansen and Sollid, 1986). Many of them are located at coastal areas in western and northern Spitsbergen (see fi g. 2 in Sollid and Sørbel, 1992), but some of the best explored examples are also situated in Adventdalen.

Based on their geometrical or morphological expression (Wah-rhaftig and Cox, 1959), rock glaciers have been subdivided into different classes (e.g., Humlum, 1982; Martin and Whalley, 1987). It should be emphasized that there exists some ambigu-ity in the classifi cation of rock glacier landforms. The inter-ested reader is referred to Hamilton and Whalley (1995) for a discussion of rock glacier nomenclature. Here, we focus on two frequently mentioned classes of rock glaciers and related land-forms that will be compared among Mars and Svalbard: tongue-shaped rock glaciers and protalus lobes or ramparts.

An example of a tongue-shaped fl ow feature on Mars is shown in Figures 9A and 9C. It is located on the inner, south-facing wall of an impact crater in Promethei Terra and has a size of ~2600 m × 600 m. The shape in plan view resembles that of the tongue-shaped rock glaciers in Adventdalen (Figs. 4E and 9B), which occur on the south-facing slopes of the mountain massifs north of the valley fl oor (Isaksen et al., 2000). Their movement has been measured in several studies, and a rock glacier on the Hiorthfjellet massif moves at a rate of ~10 cm yr–1 (Isaksen et al., 2000; Ødegård et al., 2003), while another tongue-shaped rock glacier on Birkafjellet has about half this velocity. A third rock glacier of this type is perched on the southern slopes of Oper-afjellet between two bedrock spurs (Figs. 9B and 9C). Despite the similarity of the planform, however, some differences are appar-ent between the lobate feature in the Martian crater (Figs. 9A and 9C) and the tongue-shaped rock glaciers in Adventdalen. First, the former is ~10 times larger. Second, the interior of the Martian feature seems to be bounded by high-standing sharp ridges, but there are no such ridges at the Adventdalen rock glaciers. Third, the Adventdalen rock glaciers have well-developed, relatively steep fl ow fronts. Fourth, there is a pattern of contour-parallel bands or stripes on the Martian fl ow, whereas there is little vis-ible texture (at this scale of observation) on the surfaces of the Adventdalen rock glaciers (see also Ødegård et al., 2003). Prob-ably the most important differences are the low-lying interior, the marginal high-standing ridges, and the lack of a conspicuous fl ow front of the Martian feature. A possible explanation is that the fl ow features on Mars were initially ice-rich (perhaps more like glaciers or debris-covered glaciers) and have since experi-enced a volume loss by sublimation, leaving behind a “defl ated” interior and lateral moraines. This would agree with the notion of Head et al. (2008) and Dickson and Head (2009), who theorized that the current morphologies in midlatitude craters refl ect a late-stage phase in the most recent ice age on Mars.

The second class of viscous fl ow features that we compare to possible Martian counterparts consists of protalus lobes and protalus ramparts (for a review, see Shakesby, 1997). Protalus lobes are different from tongue-shaped rock glaciers in their geomorphic setting (along the side of a valley, in contrast to tongue-shaped rock glaciers, which occur usually in cirque-like depressions) and in their aspect ratio (width > length). Some researchers, e.g., Hamilton and Whalley (1995), suggest that protalus lobes should be distinguished from rock glaciers, but others do not make this distinction (for a discussion, see Whalley and Azizi, 2003). Fine examples of protalus lobes occur




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Figure 9. Comparison between possible rock glaciers on Mars and tongue-shaped rock glacier on Svalbard. (A) Tongue-shaped structure (white arrows) at inner wall of an unnamed crater on Mars (at 35.5°S, 111.9°E; image; Context Camera [CTX] image P03_002386_1444). The shape in plan view suggests the viscous fl ow of material. (B) Tongue-shaped rock glacier (white arrows) on northern margin of central Adventdalen (Spitsbergen). Part of High-Resolution Stereo Camera (HRSC-AX) image mosaic. (C) Close-up of detail in A. The central part of the feature seems to be lowered with respect to its margin, possible indicating a degraded (or “defl ated”) rock glacier, having experienced a loss of its ice content. (D) Close-up of detail in B. Note the morphological (planform) similarity between C and D. (E) Hillshaded version of a HRSC-AX digital elevation model (DEM) of the rock glacier shown in B. The steep distal scarp is clearly visible. Artifacts are caused by poor texture in snow-covered areas (cf. the snow-covered areas in Fig. 9B). Inset shows slope map derived from HRSC-AX DEM. The surface slope of the rock glacier becomes gradually more gentle from the prox-imal parts (>25°) toward the distal edge (<10°), and it steepens again along the fl ow front (35°–40°). These values are in excellent agreement with independent measurements of tongue-shaped rock gla-ciers in Adventdalen (see Table 1 in Isaksen et al., 2000). North is up in all images.

at Fuglehuken, on the northernmost tip of Prins Karls Forland, an island off the western coast of Spitsbergen (Berthling et al., 1998). Other protalus lobes are observed at Nordenskjøldkysten in western Spitsbergen (Kääb et al., 2002; Farbrot et al., 2005). Protalus ramparts are similar to protalus lobes. A defi nition of a protalus rampart that considers form and process has been given by Ballantyne and Benn (1994, p. 146): “A protalus rampart is a ridge or ramp of predominantly coarse detritus, usually located at or near the foot of a talus slope, that has formed through the accumulation of debris along the downslope margins of a perennial fi rn fi eld following supranival gravitational transport.”

The Martian surface displays landforms that are closely analogous to protalus lobes (Figs. 10A, 10C, and 10E). They

occur at the foot of talus-producing slopes, commonly at inner or outer crater walls. They exhibit all macroscopic morphological characteristics of protalus forms, i.e., concave-upward topo-graphic profi les, a width that is much larger than their length, and steep distal scarps. The examples shown in Figures 10A and 10C bear a remarkable similarity to the protalus forms on Prins Karls Forland (cf. fi g. 3 in Berthling et al., 1998), but protalus forms at the study site on Brøggerhalvøya are also close morphological analogs (Figs. 10B, 10D, and 10E). Depressions between the protalus ramparts and the talus-producing scarp in the hinterland (Fig. 10D, middle background) are thought to have hosted peren-nial snowfi elds or small glaciers (cf. fi g. 4.9 in Barsch, 1988). A similar interpretation of such proximal depressions as former



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locations of ice in association with Martian lobate fl ow features was put forward by Milkovich et al. (2006). An inactive phase in the development of Martian rock glacier–like landforms is also indicated by the dissection and superposition by gullies and debris-fl ow cones, respectively (Fig. 10C). Similar spatial relation-ships at Kapp Mitra in western Spitsbergen have been interpreted to refl ect a fossilization of protalus forms (or “talus-foot rock gla-ciers”; cf. fi g. 7 in André, 1995).

These comparisons demonstrate that some landforms on Mars, i.e., the protalus-like forms shown in Figs. 10A,

10C, and 10E, are indeed closely analogous to rock glaciers on Earth. For other surface features, however, the case is less obvious, and interpretations as glacier-derived moraines or degraded debris-covered glaciers (Milkovich et al., 2006) seem to be plausible alternatives (Figs. 9A and 9C). Regardless of this, the degradation of the Martian features (depressed interior [Fig. 9A]; superposition by debris-fl ow cones [Fig. 10C]) as compared with active forms on Sval-bard might indicate that the possible rock glaciers on Mars are not currently active.

Figure 10. Possible protalus forms on Mars (left) and Svalbard (right). (A) Protalus lobe-like structures at the base of a large scarp on the northern wall of Hale Crater (Context Camera [CTX] image P15_006756_1454; near 34.6°S, 323.1°E) (Reiss et al., 2009). (B) Oblique aerial photograph of protalus forms at Stuphallet on the Brøgger peninsula (western Spitsbergen; viewing direction toward west; image from Sollid and Sørbel, 1992; photograph by Johan Ludwig Sollid). Note the similarity between A and B, both of which display steep talus-producing cliffs in the hinter-land, a concave-upward slope at the base of the cliff, and a steep distal scarp. (C) Possible protalus lobes on the eastern wall of Hale crater. The steep distal scarp is partly dissected by fans, which are fed by gullies carved into the cliff in the hinterland. Similar morphologic assemblages near Kapp Mitra (Svalbard) have been interpreted as protalus ramparts (or “talus-foot rock glaciers”) that have been fossilized and dissected by debris-fl ow deposits (André, 1995) (CTX image P14_006545_1445; image center at 35.26°S, 324.7°E). (D) Field photograph of the same area as shown in B, viewing direction toward northeast. The snow-covered lower slopes highlight the concave-upward topography of the deposits. Pro-talus lobes are in the foreground, and protalus ramparts are in the background. Kongsfjord is in the far background. (E) Possible protalus rampart at the outer-wall slope of an unnamed crater on Mars (near 36°S, 112.3°E; CTX image mosaic of P04_002742_1439 and P05_003164_1446). (F) Close-up image of protalus rampart shown in A and C. Note the sharp transition from the steep scarp toward the fl at foreland (see person for scale). North is up in all Mars images.




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Pingos

Pingos are perennial, ice-cored mounds that are only known from permafrost environments (e.g., Mackay, 1987). Two types of pingos can be distinguished with respect to their hydrology, which both need a pressure gradient for their formation. Closed- or hydrostatic-system–type pingos form by pore-water expulsion, typically from a formerly unfrozen body of water (i.e., a talik) that is exposed to freezing by the drainage of an overlying and protective thaw lake (e.g., Mackay, 1998). Spectacular examples of this type are observed in Arctic low-lands, e.g., on the Tuktoyaktuk Peninsula area in the Northwest Territories (Canada) (Mackay, 1979). The second type is the open- or hydraulic-system–type pingo, which is supplied by pressurized (artesian) water from a hydraulic head. This type of pingo is found in areas of considerable topographic relief, e.g., in valleys of East Greenland (Müller, 1959; Worsley and Gurney, 1996; Mackay, 1998). The basal diameters of both types of pingos commonly do not exceed a few hundred meters, and their heights are a few meters to a few tens of meters (Gurney, 1998).

Small mounds at many sites on Mars have been tentatively interpreted as pingos (see Dundas and McEwen, 2010, and references therein). Some of the earlier observations were based on relatively low-resolution Viking Orbiter images (commonly 60–100 m/pixel). One problem with the identifi cation of pingos in remotely sensed data is that other landforms of completely different origin may be morphologically very similar. Indeed, higher-resolution images suggest that alternative interpretations (e.g., pseudo-craters, volcanic cones, eroded craters, etc.) seem to be more plausible explanations for several of the purported Martian “pingos” (Burr et al., 2009a; Dundas and McEwen, 2010). It seems that some previous interpretations of small mounds on Mars as pingos and the subsequent paleoenvi-ronmental inferences were premature and did not take into account the geologic and hydrologic context. An assessment of different pingo-like features on Mars based on the very high-resolution HiRISE images concluded that fractured mounds on the fl oors of midlatitude craters (Figs. 11A and 11B) are mor-phologically most consistent with terrestrial pingos (Dundas and McEwen, 2010).

Figure 11. Comparison of possible ice-cored mounds on Mars and pingos on Svalbard. (A) Low mound with radial fractures on top (Mars, southern hemi-sphere at 33.6°S, 124°E; detail of High-Resolution Imaging Science Experiment [HiRISE] image PSP_002135_1460; see Dundas et al., 2008). (B) Frac-tured mound on fl oor of crater in southern hemisphere (detail of HiRISE image PSP_007533_1420; near 37.9°S, 347.2°E; see Dundas and McEwen, 2010). (C) Pingo in upper Eskerdalen, central Spitsbergen. Note the pattern of radial fractures on top of the eastern, more elevated part of the pingo, which closely resembles the fractures seen in A. High-Resolution Stereo Camera (HRSC-AX) image, acquired in July 2008. (D) Synthetic perspective view of the pingo shown in C, generated from HRSC-AX stereo images. (E) Field photograph of shallow pingo in DeGeerdalen, central Spitsbergen (described by Liestøl, 1976). Note the morphologic similarity to the shallow fractured mound shown in A. See group of sitting persons on summit for scale (circle), the diameter of the pin-go is ~150 m. North is up in panels A–D and toward the left in panel E.



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Pingos are numerous in Svalbard and occur mainly on the fl oors of the large valleys of Nordenskjøld Land in central Spits-bergen (Tolgensbakk et al., 2000). Most, if not all, of them are thought to be of the open-system type (Liestøl, 1976; Yoshikawa, 1993; Ross et al., 2007), but the exact mechanism by which they form is still poorly understood. One of these questions is: How can hydraulic pingos, which are thought to require artesian pressure, develop in river beds (Fig. 4C)? (For a discussion, see Gurney [1998].) Several pingos are located in Adventdalen and Esker-dalen and are covered by HRSC-AX data. They are described in detail by Yoshikawa and Harada (1995). A large and well-developed example is located in upper Eskerdalen (central Spits-bergen) (Figs. 11C and 11D). It has an area of ~440 m × 144 m, with the long axis oriented parallel to the valley. The height is variable, with the easternmost (and probably youngest) part reaching a height of ~35 m above the valley fl oor. The dimen-sions of this pingo are remarkably large (for comparison, the Ibyuk pingo on the Tuktoyaktuk Peninsula has a basal diameter of ~300 m and a height of 49 m, and it was considered to be “[…] one of the largest pingos in the world” by Mackay [1986, p. 68]). The fl ank slopes can be quite steep, in particular, at the fl anks paralleling the valley, and they exceed 45° on the southern fl ank of the highest, eastern part of the pingo. Erosion by the river is probably responsible for this steepness, which is unusual for pin-gos (Gurney, 1998). The top of the easternmost mound displays a radial pattern of up to 5-m-deep fractures, similar to those seen at some Martian fractured mounds (cf. Figs. 11A and 11B). Inter-estingly, the morphological expression of the pingos on Svalbard is quite diverse, and Liestøl (1976) noted that the “classical” regular cone shape seems to be the exception rather than the rule. Instead, a spatial shift in activity might have caused a change of the active part of a pingo, resulting in a cluster of cones and older, more degraded craters (Liestøl, 1976). If such clustering of pingos and irregularity in shape (e.g., Fig. 4D) are also true for Martian pingos, the implication is that an identifi cation based on morphology alone is even more complicated than suggested previously. It would seem that independent evidence (e.g., other periglacial landforms in close association) is required before fi rm conclusions on paleoclimatic implications may be drawn.

Thermokarst

The term thermokarst is used for “the process by which characteristic landforms result from the thawing of ice-rich permafrost or the melting of massive ice” (van Everdingen, 2005). Thermokarst is a characteristic process in the northern lowlands of Alaska, Canada, and Siberia (e.g., Grosse et al., 2007). The probably best-known examples of landforms result-ing from thermokarst are thaw lakes (e.g., Hinkel et al., 2003). Other thermokarst features are retrogressive thaw slumps and active-layer detachments (Lewkowicz, 2007; Lantuit and Pollard, 2008; Lacelle et al., 2010).

Thermokarst possibly affected the surface at different areas on Mars (e.g., Costard and Kargel, 1995; Soare et al., 2008; see

also Morgenstern et al., 2007, and references therein). A landscape exhibiting evidence of thaw slumping was recently discovered by Balme et al. (2009). Perhaps the best example for morpholo-gies indicative of thermokarst, including evidence for melting, is reported by Warner et al. (2010), who showed depressions in the Ares Vallis area that are strongly reminiscent of thermokarst depressions on Earth. Some of these depressions are connected by small channels, which is very similar to thaw lakes in Alaska that are drained by outlet channels (e.g., fi g. 2 in Brewer et al., 1993; see also Jones et al., 2009). Svalbard is (so far) not heavily affected by thermokarst processes. Some small lakes (e.g., associated with the pingo cluster shown in Fig. 4D) might have been formed by the degradation of a former ice core and could be interpreted as thaw lakes. Active-layer detachments have been described by Larsson (1982), and HRSC-AX images reveal several small slumps on a fl uvial terrace south of the mouth of Hannaskogdalen (Fig. 4F). They are isolated features, however, and local factors (e.g., fl uvial erosion of the river terrace; Lacelle et al., 2010) were probably responsible for their formation. It is not obvious that their investi-gation as terrestrial analogs would reveal any signifi cant properties of the Martian environment.

DISCUSSION AND CONCLUSIONS

The surface of Mars exhibits a variety of landforms that resemble terrestrial periglacial structures (Table 2). The un ambiguous interpretation of Martian surface features as periglacial or permafrost landforms is not without problems, however, since a particular morphologic form can be the result of different processes. The fact that different initial states can evolve to indistinguishable fi nal states (convergence or equifi nality: e.g., King, 1953; Chorley, 1962; Pitty, 1982; Haines-Young and Petch, 1983; Beven, 1996; Harrison, 2009) is especially impor-tant for planetary geomorphologists, who lack the possibility to acquire ground truth data by fi eld work. For example, rows of pitted cones on Mars, e.g., in Isidis Planitia (“thumbprint terrain”), have alternatively been interpreted as pseudo-craters, cinder cones, pingos, and moraines (see Hiesinger et al., 2009, and references therein). The morphology of landforms can be ambiguous (for a discussion of the problems in distinguishing pingos from morphologically similar landforms, e.g., see Burr et al., 2009b), and inferences on climatic conditions from morph-ologic interpretations seem to be tenuous. This is even more true because it is not clear whether periglacial landforms really develop in periglacial climates (see André [2003], who empha-sized the importance of chemical reactions and microbiologi-cal activity in landscape evolution). In fact, it is suggested that many periglacial landscapes bear a heritage of either glacial or non–cold-climate environments (French, 2007). It seems man-datory, therefore, that any study using morphological features to infer climatic conditions should investigate not a single class of landforms, but a suite of landforms (a landscape) in their geo-logical context. Such approaches have recently been realized in a number of studies (e.g., Soare et al., 2005; Balme et al., 2009;




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Levy et al., 2009b; Mellon et al., 2009). Interestingly, even the additional information available from landed missions does not always lead to consensus. While Mellon et al. (2009) suggested that cryoturbation has affected the soils at the Phoe-nix landing site, Levy et al. (2009b) held that the environment is more stable, and churning of the subsurface did not occur.

The investigation of Martian polar environments has benefi ted from the in situ data acquired by the Phoenix mis-sion, since these data revealed information that was not acces-sible from orbit. For instance, “puzzle” rocks (for a terrestrial example, see fi g. 10 in Marchant and Head, 2007) cannot be identifi ed even in the very high-resolution images of the HiRISE instrument (~30 cm/pixel) and are only visible in images taken by the lander camera (Levy et al., 2009b). In a similar manner, the use of terrestrial analogs in planetary sci-ence benefi ts from both the use of remote-sensing data and fi eld work. The former provide an integrated view of the large-scale relationships (Sharp, 1988; for the use of remote-sensing data in Arctic periglacial research, see Grosse et al., 2005, and references therein). They are ideal for creating base maps for permafrost mapping (Heginbottom, 2002), and the additional availability of a DEM enables morphometric measurements over large areas (e.g., slope maps [Fig. 9E]), which are hard to obtain by fi eld work. Field work is the ideal complement for analyses based on remote-sensing data. It provides ground truth, increases the spatial resolution of the observations (e.g., particle sizes), and allows us the subsurface to be sampled. The combination of remote-sensing data, which provide both (multispectral) imagery and topographic information (e.g., HRSC-AX), with fi eld work seems to be a promising trend in the study of terrestrial analogs to increase our knowledge of Martian landforms.

ACKNOWLEDGMENTS

This study would not have been possible without the logistical support of the German-French research station AWIPEV and the kind hospitality of their staff, in particular, Marcus Schumacher and Damien Isambert. The generous help from University Centre in Svalbard and the Norwegian Polar Institute with transport and safety equipment for the fi eld campaigns is highly appreciated. We thank the High-Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) teams for making their data publicly available. Detailed and insightful reviews by Caleb Fassett and Nick Warner helped to improve the manuscript and are warmly acknowledged. We also thank Brent Garry and Jacob Bleacher for their editorial support. This research has been partly supported by the Helmholtz Association through the research alliance “Plan-etary Evolution and Life.”

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