Co-evolution of polygonal and scalloped terrains, southwestern Utopia Planitia, Mars

Earth and Planetary Science Letters 387 (2014) 44–54

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Co-evolution of polygonal and scalloped terrains, southwestern Utopia Planitia, Mars T.W. Haltigin a,b,∗ , W.H. Pollard b , P. Dutilleul c , G.R. Osinski d , L. Koponen a,e a

Space Exploration, Canadian Space Agency, 6767 Rte. de l’Aéroport, St. Hubert, QC, J3Y 8Y9, Canada Department of Geography, McGill University, 805 Sherbrooke St. W., Montreal, QC, H3A 2K6, Canada c Department of Plant Science, McGill University, Macdonald Campus, 21111 Lakeshore Rd., Ste-Anne-de-Bellevue, QC, H9X 3V9, Canada d Departments of Earth Sciences/Physics and Astronomy, University of Western Ontario, 1151 Richmond St., London, ON, N6A 5B7, Canada e Department of Geography and Environmental Studies, Carleton University, Loeb Building, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canada 1 b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 20 September 2012 Received in revised form 15 September 2013 Accepted 5 November 2013 Available online 3 December 2013 Editor: C. Sotin

Thermal contraction crack polygons and scalloped depressions, two of the most common landforms found in Utopia Planitia, Mars, have previously been linked to the presence of ice-rich deposits in the subsurface. Although the formation and evolution of these features individually are relatively well understood, little to no effort has been directed towards elucidating possible interactions that occur between them during their development. Thus, the overarching goal of this research was to investigate if there is an evolutionary link between polygonal and scalloped terrains by correlating metrics representing polygon and scallop maturity. A variety of statistical analyses were performed using HiRISE and MOLA datasets to quantify interactions between four sets of polygonal and scalloped terrains. Our results demonstrate the existence of a negative relationship between polygonal subdivision and surface elevation, indicating that polygon networks become more ‘evolved’ as the surface subsides. These results suggest that the permafrost landscape in Utopia Planitia may once have been extremely ice-rich, and that multiple geomorphic processes may be responsible for its evolution. Ultimately, this work demonstrates that landscape reconstruction is more complete when a system approach is followed, quantifying interactions between landforms as opposed to examining an individual landform in isolation. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Keywords: permafrost geomorphology ground ice Mars polygonal terrain scalloped depressions Utopia Planitia

1. Introduction The Utopia Planitia region of Mars likely contains ice-rich sedimentary deposits that may have been emplaced in relatively modern times (Costard and Kargel, 1995; Soare et al., 2007). Along various lines of reasoning, some of the strongest evidence suggestive of significant ice reserves in the shallow subsurface is the presence of a number of landscape formations similar to those found in terrestrial periglacial environments (e.g. Head et al., 2003; Lefort et al., 2009; Levy et al., 2009; Marchant and Head, 2007; Morgenstern et al., 2007; Soare et al., 2008; Ulrich et al., 2010). In particular, two such features – polygonal terrain and scalloped depressions – are commonly used to support the presence of a mid-latitude ice-rich mantle (Levrard et al., 2004; Madeleine et al., 2009), and are amongst the most intensively studied per-

*

Corresponding author at: Space Exploration, Canadian Space Agency, 6767 Rte. de l’Aéroport, St. Hubert, QC, J3Y 8Y9, Canada (current address). Tel.: +1 450 926 5020; fax: +1 450 926 4449. E-mail addresses: [email protected] (T.W. Haltigin), [email protected] (W.H. Pollard), [email protected] (P. Dutilleul), [email protected] (G.R. Osinski), [email protected] (L. Koponen). 1 Current address. 0012-821X/$ – see front matter Crown Copyright http://dx.doi.org/10.1016/j.epsl.2013.11.005

©

mafrost landforms on the planet (e.g. Lefort et al., 2009; Levy et al., 2008b; Mangold, 2005; Mellon et al., 2008, 2009; Morgenstern et al., 2007; Séjourné et al., 2011; Ulrich et al., 2010, 2011; Zanetti et al., 2010). Polygonal terrain refers to a network of interconnected trough-like features in the ground that form as a result of thermal contraction cracking of ice-rich substrates (Lachenbruch, 1962). Scalloped terrain comprises a series of degradational landforms resulting from surface subsidence caused by the removal of underlying ground ice (Lefort et al., 2009, 2010; Morgenstern et al., 2007; Ulrich et al., 2010). Individually, each of these two types of landform has been reasonably well characterized with respect to its formational processes and basic morphological evolution (Lefort et al., 2009; Levy et al., 2010; Mellon and Jakosky, 1995; Séjourné et al., 2011). However, relatively little effort has been directed towards understanding the feedbacks that take place as they mature. Given that both polygons and scallops are believed to develop as a result of ongoing adjustments of ice-rich ground to local environmental conditions, it is reasonable to postulate that some level of coevolution should exist. For example, polygonal network evolution is related to increasingly subdivided trough segments (Mackay and Burn, 2002), so smaller polygons can be used as an indicator of more advanced stages of network maturity (Haltigin et al., 2012).

2013 Published by Elsevier B.V. All rights reserved.

T.W. Haltigin et al. / Earth and Planetary Science Letters 387 (2014) 44–54

Moreover, progressive scalloped terrain development is associated with greater levels of surface subsidence (Lefort et al., 2009; Séjourné et al., 2011). Assuming a relatively flat original terrain, a lowered surface elevation should thus represent more evolved depressions. To illustrate that co-evolution of polygonal and scalloped terrains is taking place, a variety of morphometric parameters representing average polygon size must be related to surface elevation. Therefore, the overarching goal of this work was to establish the existence of an evolutionary link between polygonal and scalloped terrains by demonstrating a quantifiable relationship between polygon network arrangement and the elevation of the surface on which the network is formed. With the supporting analyses, we hoped to demonstrate that landscapes in this region have evolved as a system, and thus cannot be satisfactorily reconstructed by tracking the evolution of individual features in isolation. 2. Landform evolution in Utopia Planitia 2.1. Origins of thermal contraction crack polygons Current understanding of Martian polygonal terrain formation has drawn extensively from analogy to similar features in terrestrial permafrost environments (Levy et al., 2008a, 2008b; Marchant and Head, 2007). On Earth, thermal contraction cracks develop in ice-cemented permafrost as a rheological response to climate induced stresses (Lachenbruch, 1963). In regions where the nearsubsurface remains predominantly dry, open cracks can become filled with fine-grained sediments (Black, 1974; Péwé, 1959). As the cracking and infilling processes continue over hundreds to thousands of years (Sletten et al., 2003), multiple cracks interconnect and form the boundaries of enclosed polygonal patterns, and the sediment deposits take on an increasingly inverted-triangular, or ‘wedge’, shape. These features are commonly referred to as sand-wedge polygons. As a subset of the sand-wedge polygons described above, “sublimation polygons” are found in particularly cold, arid regions characterized by the lack of a seasonally-thawed active layer and extremely ice-rich substrates (Levy et al., 2011). Although the two types of sand-wedge polygons differ moderately in characteristic evolution and morphology, both are ultimately the result of thermal contraction cracking of ice-rich ground and thus can be considered as two separate outcomes of the same basic process. There remains some debate as to whether the polygonal patterns observed on Mars are more similar to typical terrestrial wedge-type or sublimation polygons (Levy et al., 2010; Mellon et al., 2009). However, given the likelihood that sublimation is the dominant process acting upon ice-rich terrains in this region (Lefort et al., 2009; Ulrich et al., 2010), for the purposes of our study we work under the assumption that the observed polygonal networks behave in accordance with the geomorphic processes dictating the latter. 2.2. Polygonal terrain development Sublimation polygon formation is initiated when a thermal wave resulting from rapid decreases in air temperature propagates into ground where a thin veneer of sediment is underlain by materials with ice content well in excess of 30% by volume (Marchant and Head, 2007). The induced tensile stress causes the ground to crack when the stress exceeds the substrate’s tensile strength (Lachenbruch, 1962), with the open crack providing a pathway whereby underlying ice can sublimate directly into the atmosphere (Levy et al., 2006). When numerous cracks intersect, enclosed geometrical shapes tens to hundreds of meters across become visible at the surface (Marchant et al., 2002).

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Enhanced sublimation along the crack widens and deepens a trough-like depression following the crack’s trajectory, leading to a redistribution of more dense material towards the polygon trough via slumping of adjacent mineral particles (Marchant and Head, 2007). Eventually, the troughs become sufficiently filled with slumped sediment to provide a cap preventing indefinite ice sublimation and trough growth (Levy et al., 2006). Although these ‘capped’ troughs will no longer widen, individual enclosed polygons can become subdivided by newly forming thermal contraction cracks that effectively reduce the average polygon size within the network to tens of meters or meters across (Levy et al., 2008a). 2.3. Scalloped terrain development Scalloped depressions are single or aggregated curvilinear depressions in the ground, measuring tens of meters to kilometers across and meters to tens of meters deep (Zanetti et al., 2010), and are believed to form due to surface subsidence resulting from the loss of subsurface ice bodies. Although alternative mechanisms have been suggested (Soare et al., 2008), prevailing convention suggests that this loss of ice occurs through direct sublimation to the atmosphere (Lefort et al., 2009; Ulrich et al., 2011). It is believed that small, meter-scale topographic variations in ice-rich substrates are subjected to differential insolation intensities, leading to enhanced sublimation and subsidence on their equator-facing slopes (Lefort et al., 2009). As a result, circular to sub-circular depressions are formed, displaying relatively shallow equator-facing slopes and sharper pole-facing slopes (Séjourné et al., 2011). Over time, the sublimation process enhances the scallop morphology, deepening and widening the depression and lengthening the gentle equator-facing slope. The pole-facing scarp becomes increasingly steep during its retrogressive equatorward retreat into the surrounding upper plains, and after several retreat episodes leaves scarp-parallel ridges on the scallop floor (Séjourné et al., 2011). Eventually, individual scallops that have grown sufficiently can coalesce, resulting in complex degradational patterns (Morgenstern et al., 2007; Zanetti et al., 2010). 2.4. Polygon-scallop spatial relationships Inspection of images returned by the Mars Orbiter Camera (MOC) (Malin and Edgett, 2001) for our study region revealed that the plains in which the scallops form consistently display polygonal terrain (Haltigin et al., 2008; Lefort et al., 2009; Soare et al., 2007). At MOC resolution (∼1.5–12 m/pixel), the observed polygons within the networks are generally characterized by trough spacings of 50–100 m, trivalent intersections (three trough segments per intersection), central surfaces higher in elevation than the bounding troughs, troughs lacking raised substrate shoulders, and overall irregular network geometry (Haltigin et al., 2008; Lefort et al., 2009). Subsequent images from the HiRISE camera (McEwen et al., 2007) aboard Mars Reconnaissance Orbiter (Zurek and Smrekar, 2007) have shown the presence of polygonal patterns within the scalloped depressions, morphologically distinct from those on the surrounding upper plains (Fig. 1). On or at the base of the polefacing scarp face, the polygons display trough spacings <10 m across, raised trough shoulders in some cases, tetravalent intersections (four trough segments per intersection), and an overall orthogonally-oriented trough geometry (Lefort et al., 2009; Levy et al., 2009; Morgenstern et al., 2007; Soare et al., 2008). Further away from the scarp face along the scallop floor, intermediatesized polygons (∼20–30 m across) are predominant, displaying tri- and tetravalent trough intersections and lacking raised trough shoulders (Haltigin et al., 2009; Lefort et al., 2009).

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T.W. Haltigin et al. / Earth and Planetary Science Letters 387 (2014) 44–54

Fig. 1. Subset of HiRISE image PSP_007740_2250, showing the spatial coincidence between polygonal terrain and scalloped depressions. Insets highlight variations in polygon morphology. Bottom to top: the largest most irregular polygons are found in the upper plains; the smallest, most regular polygons are found immediately in the lee of the scarp face; and polygons of intermediate morphology are found in the scallop floor. North is towards the top of the image. HiRISE image courtesy NASA/JPL/University of Arizona.

3. Study sites 3.1. Regional geology of Utopia Planitia Utopia Planitia is a large topographic depression in the northern plains of Mars, interpreted as an ancient impact basin (McGill, 1989; Thomson and Head, 2001), historically thought to have contained large volumes of liquid or frozen water (Chapman, 1994; Scott et al., 1992) and possibly glacial ice sheets in the early Amazonian (Kargel et al., 1995; Osinski et al., 2012). Early geologic mapping of the area suggested that the majority of the surface is covered by late Hesperian- to early Amazonian-aged deposits (Greeley and Guest, 1987). Tanaka et al. (2005) later suggested that the area is almost entirely underlain by the Vastitas Borealis Formation (VBF), a unit of reworked fluvial and/or marine sediments deposited during the late Hesperian (Tanaka et al., 2003). In western Utopia Planitia, two surface units currently dominate the physical setting (Fig. 2). In the southwestern portion of the region lies the Astapus Colles (ABa) unit, a glacial–periglacial unit tens of meters thick that is thought to have been deposited in the late-Amazonian (Tanaka et al., 2005). The ABa materials appear to be draped upon the Vastitas Borealis interior unit (ABvi ), which dominates the eastern portion of the area. The ABvi unit is believed to consist of materials deposited during outflow events during the early Amazonian, and presently represents a sublimation lag from large bodies frozen water that have since disappeared (Kreslavsky and Head, 2002). More recent mapping efforts indicate that the stratigraphic relationships in this region may be even more complex. Soare and Osinski (2009) and Capitan et al. (2012) demonstrate that the region may contain a third, as-of-yet

unmapped and unnamed, ice-rich unit containing several of the features described above, while Skinner et al. (2012) describe a unit tens of meters thick dominated by an ice-rich, fine-grained, loess-like sedimentation, believed to have been deposited in the mid- to late-Amazonian. 3.2. Site selection Although polygons and scallops are virtually ubiquitous in southwestern Utopia Planitia, the selection of appropriate study sites was somewhat limited by the available data. Specifically, locations with both HiRISE data for high-resolution morphometric mapping and elevation data against which to compare the appearance of the polygonal networks were required. The most widely available form of elevation data on Mars is provided by the Mars Orbiter Laser Altimeter (MOLA) instrument (Zuber et al., 1992; Smith et al., 1999). Interpolated MOLA Digital Terrain Models (DTM) provide insufficient spatial resolution to perform reliable numerical assessments at our scale of interest (Som et al., 2008), and thus point measurements from individual MOLA orbits were needed instead. Therefore, the identification of candidate study sites was restricted to locations at which MOLA track fully traversed a HiRISE image containing scalloped and polygonal terrains. It is possible that High-Resolution Stereo Camera (HRSC) DTMs could also have been used (Jaumann et al., 2007), but gaps in coverage would have restricted the number of HiRISE images from which to choose. Three HiRISE images with the characteristics described above were selected for investigation, displaying a range of scalloped terrain development stages (Fig. 3). Site 1 has one well-developed

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Fig. 2. Location of the four study sites within southwestern Utopia Planitia, Mars, with geological units identified by Tanaka et al. (2005) superimposed upon a MOLA shaded relief map. Note that surface elevation decreases towards the northeast. Inset is a global MOC mosaic highlighting the region of interest. The locations of Figs. 8(a) and 8(b) are indicated for reference.

Fig. 3. HiRISE images of the three study sites, representing (a) low, (b) moderate, and (c) high modification by scalloped depression development. In all cases, north is up. Dots show locations of MOLA elevation measurements, and outlined boxes represent the Frames of Interest (FOI) described in Section 4.2. Note that polygonal terrain does not appear in (c) northwards of the final FOI. HiRISE images courtesy NASA/JPL/University of Arizona.

depression, and contains primarily polygons in the upper plains. Site 2 shows an increasing level of scallop activity, and contains all morphologies of polygons. Site 3 displays a suite of scallop development stages, from small and isolated shallow depressions within the upper plains in the south to more complex aggregations towards the northern section of the image. It is important to note that polygon morphology on Earth is dependent on local climatic and rheological conditions (Fortier and Allard, 2005; Mackay, 1990, 1992). In order to reduce the potential influence of these parameters on variations observed for the Martian network morphologies, attempts were made to minimize the spatial separation amongst sites. Each of the images was located within the ABa unit and was separated by less than one degree longitude and 0.2 degree latitude. Thermal inertia values for the three locations suggest that the surficial materials at the three sites are quite similar, with values ranging from 250–340 J m−2 K−1 s−1/2 corresponding to grain sizes of approximately 300–1100 μm (Fergason et al., 2006).

4. Polygon network development stage as a function of surface elevation 4.1. Polygon trough spacing vs. surface elevation: cross-sectional analysis Using a Geographic Information System (ArcMap 9.3.1), one MOLA elevation track was plotted for each image (Fig. 3), traversing relatively unmodified (Site 1), marginally modified (Site 2), and heavily modified (Site 3) terrains. Locations where the transect crossed a polygon trough were manually identified and digitized. Planform coordinates (x– y; units = meters) were extracted for the digitized points and used to calculate distances along the transect, while an elevation value (z; unit = meter) was derived for each point from linear interpolation along the MOLA track. Points were then plotted as cross-sections showing trough-intersection locations (Fig. 4).

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Fig. 4. Locations of thermal contraction cracks along pre-defined MOLA transects at each of the study sites. Dots indicate location of MOLA measurements. Note that the vertical extent of thermal contraction cracks are not to scale. Topographic profiles have been exaggerated vertically by 15 times to highlight variations in elevation.

Qualitative inspection of each site reveals an apparent association between trough-intersection spacing and surface elevation, the frequency of troughs tending to be greater in local topographic lows. This observation suggests the existence of a positive relationship between surface elevation and the distance between successive polygon troughs along the transect. We tested the relationship between polygon trough spacing and surface elevation statistically. The data were first aggregated into 300 m bins, each centered upon a MOLA measurement. Two separate metrics were defined for troughs within a bin: (i) the mean Nearest-Neighbor (NN) distance, or mean distance between adjacent troughs, and (ii) the mean spacing amongst troughs, or average distance between all troughs within the bin. Because of the 1-D spatial nature of the data and the clear presence of heterogeneity of the mean reflected by trends, the correlation analyses between the mean NN distance for troughs within a bin and MOLA elevation and between the mean spacing between all troughs of a bin and MOLA elevation were performed on raw data and on estimated trends, following Dutilleul (2011, Chaps. 6 and 7). The local drift estimation procedure L 1 , available in the method of coregionalization analysis with a drift, Phase 1 (Pelletier et al., 2009), was used for the estimation of trends because it takes spatial autocorrelation into account, when present. The probabilities of significance (p) associated with the correlation statistic values (r) are generally smaller than 0.01 and none of them is greater than 0.10 (Table 1). These results demonstrate that the distance between troughs decreases as the elevation decreases, showing that the polygon network becomes increasingly subdivided and thus increases in maturity as the ground surface subsides. 4.2. Polygon subdivision vs. surface elevation: planform analysis The results presented in the previous section demonstrate a cross-sectional trend in polygonal subdivision along transects. The next step was to test whether similar patterns could be identified using metrics representative of planform geometry, which would further support the notion that polygons at lower elevations are more evolved than those at higher elevations. Several “frames of interest” (FOI) were defined and centered as closely as possible on a MOLA measurement (Fig. 3). Within each

Table 1 Results of correlation analysis between the mean Nearest-Neighbor (NN) distance for troughs within a bin and MOLA elevation and between the mean spacing between all troughs of a bin and MOLA elevation, where r denotes Pearson’s correlation coefficient and p is the corresponding probability of significance. (A) on raw data; (B) on trends estimated using a moving window in the local estimation procedure L 1 of the method of coregionalization analysis with a drift, Phase 1 (Pelletier et al., 2009). Site

NN distance r

Spacing p

r

p

(A) 1 2 3

0.407 0.342 0.659

0.0834 0.0595 0.0001

0.403 0.516 0.721

0.0875 0.0030 <0.0001

(B) 1 2 3

0.956 0.714 0.860

<0.0001 0.0006 <0.0001

0.977 0.705 0.925

<0.0001 0.0007 <0.0001

FOI, all locations where polygon troughs intersected were manually identified and digitized, a technique estimated to capture ∼90% of all intersections within the FOI (Haltigin et al., 2010). The total number of intersection points (n) was used to calculate the density of trough intersections for each FOI (n/km2 ). The resulting density values were plotted against the corresponding MOLA elevation data (Fig. 5). For each of the three sites, the majority of FOIs analyzed contain between a few hundred and a few thousand intersections per square kilometer. In Sites 2 and 3, however, one FOI had density values nearly an order of magnitude greater than the prevailing trend. This subset of the data was removed from consideration here, and was interpreted separately (see Section 5.3). After removal of these atypical observations, more than 80% of the variation in intersection point density is explained solely by differences in elevation (as indicated by the R 2 value greater than 0.8), and a negative linear relationship between the two becomes very clear. Such a finding confirms that areas at higher elevations display significantly less polygon subdivision than areas at lower elevations, and thus strongly supports the assertion derived in the previous section that more evolved polygonal sub-networks are related to vertical landscape subsidence.

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Fig. 5. Scatterplots illustrating polygon trough intersection density vs. elevation for the study sites. When statistical outliers are removed, a strong linear relationship becomes apparent.

4.3. Independent confirmation of relationship between polygon subdivision and elevation A major limitation of using individual MOLA point measurements in statistical analyses is the uncertainty inherent to the spatial location of the measurement and the spatial averaging that the measurement represents; a single MOLA shot has a spot size of approximately 170 m (Smith et al., 1999). Thus, reliance on a single elevation data point to represent an area of several thousand square meters creates challenges in assessing accurately the error associated with landscape trends estimated statistically. Recent advances in topographic modeling can help overcome such a limitation. For example, DTMs created from HiRISE stereoimages have a spatial resolution of ∼1 m/pixel and are unambiguous with respect to the localization of each data point (Kirk et al., 2008). Thus, the average elevation of a given area can be more accurately derived using tens of thousands of data points available from the DTM. At the time of our study, only one publicly available DTM contained polygonal and scalloped terrains in varying stages of development. Fig. 6 shows a heavily modified landscape (Site 4), with polygons ranging from meters across to tens of meters across superimposed on an extremely complex aggregation of scallop formations. Unlike the three sites previously examined, Site 4 is situated within the ABvi unit, but comparable thermal inertia values imply a similar surface grain size distribution. Using HiRISE and DTM data, the same analysis as performed in the previous section was repeated by digitizing all polygon trough intersections within 18 FOIs located at various elevations. Elevation data were extracted from each pixel to generate one average value for each FOI, and trough intersection density values were regressed on those average elevation values. As with Sites 2 and 3, a subset of the data for Site 4 was removed and treated separately so as to reveal the dominant relationship (Fig. 7). Likewise with the three previous datasets, a strongly negative correlation between trough intersection density and surface elevation is evident thereafter for Site 4, confirming the relationship between the maturity of its polygon network and the surface subsidence. 5. Implications for landscape interpretation 5.1. Demonstrating co-evolution of landforms Many geomorphological studies tend to focus on understanding and explaining the processes responsible for the evolution of an

Fig. 6. HiRISE image and DTM of a fourth study site, demonstrating severe modification by scallop development. Outlined boxes represent FOIs within which all trough intersections were identified. HiRISE image courtesy NASA/JPL/University of Arizona.

individual landform. Given that both terrestrial and Martian permafrost landscapes can comprise many types of features, it is natural to expect some level of interaction between processes as they develop (Hauber et al., 2011). Until such interactions are identified and quantified, any description of landscape evolution remains incomplete. Concerning links between polygonal terrain and scalloped depressions, several qualitative morphological relationships have been reported (Lefort et al., 2009, 2010), but the more rigorous

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Fig. 7. Scatterplots illustrating polygon trough intersection density against elevation for all study sites, highlighting Site 4. As for the other three sites, when the statistical outliers are removed a strong linear relationship is apparent. Table 2 Results of the linear regression of trough intersection density on elevation, where R 2 denotes the coefficient of determination, and a and b are, respectively, the regression line intercept and slope, reported with their 95% confidence interval. Note that b increases in absolute value with sites at lower elevations located towards the northeast, i.e. towards the center of the Utopia Basin, with significant differences between Sites 1–2 and 4; the 95% confidence intervals do not overlap. Approximate altitudes above the Martian datum reported are the mean y-axis values from Fig. 7 for each respective site rounded to the nearest meter. Site

Approximate altitude (m)

R2

a

95% CI for a

b

95% CI for b

1 2 3 4

−3950 −4140 −4180 −4561

0.873 0.832 0.838 0.855

−14 3408 −15 4588 −19 9951 −32 7626

−178 005, −108 811 −207 181, −101 996 −255 484, −144 418 −409 356, −245 895

−36.53 −37.58 −48.09 −72.70

−45.27, −27.78 −50.27, −24.88 −61.36, −34.82 −90.62, −54.78

quantitative analyses performed have tended to focus on only one of these landforms at a time (e.g. Séjourné et al., 2011; Ulrich et al., 2010). However, our results demonstrate that the appearance of a given polygonal network and the relative stage of scallop development are most likely linked, and thus should be considered as elements of a single, interactive system. It follows that the results reported here provide a more holistic view into how a given landscape adjusts to local environmental conditions, and therefore may serve as a basis to interpret its history more completely.

two local environmental conditions: the surrounding climate and substrate composition (Lachenbruch, 1962). Increased tensile stress can be caused by more rapid or greater magnitude drops in air temperature (Fortier and Allard, 2005), while decreases in strength can be associated with greater ground ice content (Arenson et al., 2004). Therefore, it is possible that either temperature gradients are stronger towards the northeast or that variations in polygonal geometry are associated with greater ground ice contents towards the center of the Utopia Basin. A more detailed regional assessment of scallop and polygon occurrences might provide additional insight.

5.2. Regional trends in landscape development 5.3. Landscape vs. landform scales of investigation The identification of a strong linear relationship between trough intersection density and surface elevation supports the existence of a link between polygon maturity and landscape subsidence. While the R 2 values are relatively consistent among regressions – indicating that the relationship is equally robust at the four sites – some level of variation exists in the reported regression slopes (Table 2), which are standardized rates of change by definition. Such variability suggests slight differences in the efficiency with which geomorphic processes are shaping the landscape at each site. Locations with a steeper regression slope demonstrate greater polygonal subdivision for a given change in elevation, suggesting a relative ‘ease’ of polygon evolution. In comparing the four studied locations on the basis of their regression slopes Sites 1–4 being located from southwest to northeast and characterized by steadily increasing regression slopes (Table 2), it appears that thermal contraction cracking is enhanced at lower altitudes within the Utopia Basin. The rate and frequency of cracking within a given substrate are dependent on two factors: the tensile stress induced by the thermal wave and the tensile strength of the substrate itself (French, 1996). The tensile stress and strength are, in turn, dependent on

In addition to the regional comparison amongst sites, the results presented in Figs. 5 and 7 allow for more localized assessments of the interaction between polygons and scalloped depressions. Although our results are strongly suggestive of a mechanistic relationship between polygon maturity and surface subsidence, this relationship became evident only after the removal of a small number of atypical observations, or outliers. Therefore, it is important to reconcile the existence of such data points that are not reflective of the overall landscape tendency. Visual inspection of the HiRISE images reveals that each of the outliers is associated with an FOI located at the base of a scallop scarp face. Previous studies have noted that polygons in such locations are morphologically distinct from those found elsewhere with respect to scallops, being both high- and low-centered, and in some cases having raised rims along the trough edges (Haltigin et al., 2009; Lefort et al., 2009; Séjourné et al., 2011). Under the assumption that associations established statistically are representative of the action of a certain suite of geomorphic processes, such a clear distinction of features implies that the polygons at the scarp base are subjected to processes different from those in

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the surrounding plains and scallop floors. Accordingly, the relationship between polygon network maturity and elevation appears to be relevant at the landscape scale only. 5.4. Re-evaluating the sequence of ice-rich terrain evolution in Utopia Planitia If the relationships illustrated in Figs. 5 and 7 are reflective of geomorphic processes acting at the landscape scale, it is important to situate them within the current understanding of generalized scallop development. The basic premises of the evolutionary model proposed by Lefort et al. (2009) have become commonly accepted, and thus serve as a useful starting point for comparison. While our findings support many aspects of this model, certain elements require closer examination. A major challenge pertains to the notion that sublimationinduced subsidence of the ground surface brings it closer to an ice-rich layer at depth, which increases tensile stress and leads to enhanced thermal contraction cracking. Fig. 18 in Lefort et al. (2009) suggests that the surface must be lowered by several meters to reach the ice table, at which point polygon subdivision can increase dramatically. The implication of the model is that enhanced polygon development postdates terrain subsidence. However, sublimation of ice-rich ground produces an increasingly thick lag deposit of ice-free material that insulates ground ice from further evacuation (Marchant et al., 2002). It follows that greater surface subsidence creates thicker lag deposits, and inhibits further removal of underlying ice. Therefore, it is difficult to propose a mechanism whereby the surface could be lowered by several meters without producing a lag thick enough to preclude additional vapor transfer from the subsurface into the atmosphere. If we are to envision a scenario where the surface lowers by 5 m (a very conservative value based on the MOLA elevation data presented in Fig. 4) and is still subjected to sublimationinduced subsidence, this would have important implications for the ice content of the original mantle deposit. Marchant et al. (2002) suggest that ground-ice sublimation in the Antarctic Dry Valleys requires an overlying lag less than 1 m thick. Assuming similar values for Mars – where estimates of ice table depth under present-day conditions are <1 m for seasonally stable ice at 45◦ N and permanently stable ice poleward of 60◦ N (Schorghofer and Aharonson, 2005) – lowering the surface by 5 m while producing less than 1 m of lag implies an excess ice content of >80% in the original substrate. This estimate is consistent with values previously noted in the literature, which range from ice completely filling all pores spaces (40–70%) (Skinner et al., 2012) to an 80:20% ice-dust ratio (Levy et al., 2010), to nearly pure ice (99%) (Byrne et al., 2009). Even when considering that some amount of desiccated mantle material would be removed through aeolian activity (Zanetti et al., 2010), which would slightly reduce our estimated ice content, a paradox is created. If the original surface already contained ∼80% excess ice, it would not need to subside by 5 m to reach an ‘ice-rich’ layer; instead, it is more likely that enhanced thermal contraction cracking would have already commenced in such material if the relationship between ice content and tensile stress reported by Lefort et al. (2009) was valid. We therefore propose that increased polygonal development does not occur after the surface lowers, but rather is triggered slightly before surface subsidence and then coincides with it. 5.5. Landform scale evolution The initial stages of scallop development have been discussed by both Morgenstern et al. (2007) and Lefort et al. (2009), who propose that microtopographic variations cause the equator-facing

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slopes to receive more direct insolation, which in turn promotes greater rates of sublimation and enhanced ground subsidence. Because the ice within the pole-facing slope is somewhat shadowed and thus less prone to exchange with the atmosphere, the early pole-facing headwall can maintain a greater stability because the increased ice content acts as a cementing agent. Over time, subsidence along the equator-facing slope continues and the scarp face becomes more entrenched. While it is certainly reasonable to assert that minor topographic variations could be responsible for scallop initiation, the origin of these variations has yet to be explained. It is possible that localized heterogeneities in ice content could have existed within the original mantle deposits. Assuming that variation in ice content exists on the scale of meters to tens of meters, localized ice-rich deposits would likely be more readily cracked and undergo greater rates of sublimation than the immediately surrounding areas. Locations with greater ice contents would thus initiate enhanced polygon development earlier and subside more rapidly than their surroundings, introducing a small depression whose evolution could then be dictated by the differential insolation mechanism outlined above. Many models (Lefort et al., 2009, 2010; Séjourné et al., 2011; Ulrich et al., 2010; Zanetti et al., 2010) also propose that the equatorward retreat of scallop scarp faces is due to increased sublimation rates resulting from greater insolation during periods of high planetary obliquity (Laskar et al., 2004). However, if our data indicate that the form-process relationships describing polygon networks at the landscape scale are different from those at the scale of an individual depression, we must then conclude that an insolation-driven sublimation model may be insufficient to fully explain the evolution of landscapes in both directions, towards the pole and the equator. Specifically, if scarp retreat was purely a function of insolation, then all scarp walls should only exist facing directly towards the pole. Conversely, the presence of numerous scallops in the region illustrates that scarp faces can also be formed along the east–west axis (Fig. 8). Moreover, the stability of ground ice at mid-latitudes actually increases during periods of high obliquity (Mellon and Jakosky, 1995; Chamberlain and Boynton, 2007), suggesting that ground ice degradation may actually be more difficult during obliquity excursions. As a result, we must consider the possibilities that (i) conceptual models aimed at describing the retrogressive development of scalloped depressions via simple reversal of prevailing sun direction during periods of high obliquity do not fully explain the observed landform patterns, and that (ii) such processes may be occurring under present-day climate conditions. 5.6. Equatorward scarp face retreat under present-day conditions Within individual scallops, areas immediately poleward of the initial scarp face will be shadowed, meaning both that the ground ice will be more stable in this region (resulting in an increased near-surface ice content) and that microclimatic variation will cause the shadowed regions to be colder than the surrounding areas receiving more insolation. Combining increased ice contents with colder temperatures should theoretically lead to accelerated thermal contraction cracking, as the propagation of the thermal wave is dependent on the magnitude of the drop in surrounding air temperature (Mackay, 1974), and the thermal stress is proportional to near-surface ice content (Mellon, 1997). Fittingly, a drastic increase in polygonal subdivision is observed near the scarp bases compared to areas outside the scallop and on the depression floor. Assuming that sublimation is the dominant geomorphic process acting on the region, a greater number of thermal contraction cracks would provide more direct pathways for ground ice to escape from the subsurface to the atmosphere, which theoretically should augment sublimation rates. However, in

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Fig. 8. HiRISE images ESP_025831_2260 (45.8◦ N, 90.5◦ E) and PSP_009480_2265 (45.9◦ N, 91.1◦ E) demonstrating scalloped depressions characterized by scarp faces retreating along an east–west axis. HiRISE images courtesy NASA/JPL/University of Arizona.

the absence of direct insolation on the scarp faces, it is clear that another mechanism must be driving – or, at a minimum, contributing to – accelerated sublimation. In terrestrial retrogressive thaw slumps (RTS), an imperfect but conceptually appropriate geomorphic analogue to scalloped depressions (Ulrich et al., 2010), microclimatic variations within the slump related to variations in wind speed and direction are correlated to rates of headwall retreat (Grom and Pollard, 2008). On Mars, if we consider the scarp-face and scalloped depression floor as the equivalents of RTS headwalls and slump floors, a scenario is proposed below whereby local airflow effects may accelerate ice loss under present-day Martian obliquity. In Fig. 13 of Lefort et al. (2009), the authors suggest that as the scallop evolves, the scarp becomes both steeper and deeper, further enhancing the shadowing effect, and only after the headwall slope reaches sufficient steepness do the advanced stages of polygonal patterns appear. Following the climate model of Forget et al. (1999), Morgenstern et al. (2007) show that the prevailing winds in central Utopia Planitia during the period of expected maximum sublimation (L s = 100 ◦ –110 ◦ ) originate from the South–Southeast, roughly normal to the majority of observed scallop scarp faces. Assuming a logarithmic flow profile for near-surface air currents, the introduction of a slope or step at the ground surface would modify the velocity vectors increasingly until a critical slope is achieved whereby flow would detach from the surface (Kaiktsis et al., 1991). When such a flow separation occurs, the region immediately behind the step experiences a negative dynamic pressure that induces eddy formations and increases air turbulence (Kaltenbach and Janke, 2000). Given that ice sublimation is enhanced by turbulent airflow of the surrounding near-surface air column (Ivanov and Muhleman, 2000; Pathare and Paige, 2005), vortices resulting from flow separation at the scarp face may provide the necessary energy to hasten the removal of water vapor, thereby creating a positive feedback mechanism where increased contraction cracking leads to exposure of ground ice to the atmosphere, enhancing vertical and horizontal subsidence, and thus creating a steeper scarp face and strengthening the turbulent vortices.

We believe that this turbulence-driven sublimation model is consistent with the geomorphic observations of Lefort et al. (2009), Séjourné et al. (2011), and others, but represents an improvement on existing explanations for scarp face retreat because: (i) it eliminates the requirement of obliquity variations to explain the retrogressive behavior; (ii) it could account for non-pole-facing (i.e. east–west axis of retreat) scarp walls if local topographic forcing modified the direction of predominant local winds; and (iii) it provides a mechanism whereby retrogressive erosion due to ground ice ablation could take place under current climate conditions. Future efforts devoted to three-dimensional computational fluid dynamics simulation of Martian winds over a step could further investigate this phenomenon, and – when combined with HiRISE Digital Terrain Models as they become increasingly available – could potentially be used to determine what wind speeds and critical slope are required before the necessary flow separation occurs. 6. Summary and conclusions Analyses of HiRISE and MOLA data at four sites in Utopia Planitia, Mars have shown that thermal contraction crack polygons and scalloped depressions become more evolved as the ground surface increasingly subsides. Moreover, statistical comparisons of metrics associated with polygonal and scalloped terrain maturity provided evidence for links between these features with respect to their geomorphic development. Although a relationship between polygonal terrain and scalloped depression morphologies has been inferred previously, we believe that this is the first quantitative demonstration of such a co-evolution. Interpretation of the results produced several implications for the reconstruction of the area’s geomorphic history. At the regional scale, ground ice content within the mid-latitude mantle deposits may have been 80%, potentially increasing in proportion towards the center of the Utopia Basin. At the landscape scale, enhanced polygonal subdivision likely coincides with terrain subsidence, contrary to a prior assumption. Finally, at the landform scale, it is

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possible that obliquity-driven sublimation events do not fully explain the observed retrogressive retreat of scallop scarp faces. Ultimately, this work demonstrates that investigating an individual landform in isolation may not be sufficient to characterize its history, and instead shows that landscape features should be considered as part of an interactive system. By developing the ability to deconvolve landscape- from landform-scale processes, this work could represent a first step towards developing a more integrated approach to landscape reconstruction of ice-rich terrains on Mars. Acknowledgements Support for this work was provided by the Canadian Space Agency and Canada’s Natural Sciences and Engineering Research Council (NSERC). We thank Matt Balme and Ernst Hauber for thoughtful and insightful commentary on an earlier version of this manuscript. We also wish to thank the HiRISE and MOLA teams for providing the exceptional data products that facilitate research such as ours. References Arenson, L.U., Johansen, M.M., Springman, S.M., 2004. Effects of volumetric ice content and strain rate on shear strength under triaxial conditions for frozen soil samples. Permafr. Periglac. Process. 15 (3), 261–271. http://dx.doi.org/10.1002/ ppp.498. Black, R.F., 1974. Ice wedge polygons in northern Alaska. In: Coates, D. (Ed.), Glacial Geomorphology. State University of New York, Binghampton, pp. 247–275. Byrne, S., Dundas, C.M., Kennedy, M.R., Mellon, M.T., McEwen, A.S., Cull, S.C., Daubar, I.J., Shean, D.E., Seelos, K.D., Murchie, S.L., Cantor, B.A., Arvidson, R.E., Edgett, K.S., Reufer, A., Thomas, N., Harrison, T.N., Posiolova, L.V., Seelos, F.P., 2009. Distribution of mid-latitude ground ice on Mars from new impact craters. Science 325 (5948), 1674–1676. http://dx.doi.org/10.1126/science.1175307. Capitan, R.D., Osinski, G.R., van de Wiel, M.J., Kerrigan, M., Barry, N., Blain, S., 2012. Mapping Utopia Planitia: Morphometric and geomorphologic mapping at a regional scale. Lunar Planet. Sci. XLIII. Abstract 2237. Chamberlain, M.A., Boynton, W.V., 2007. Response of Martian ground ice to orbitinduced climate change. J. Geophys. Res. 112, E06009. http://dx.doi.org/10.1029/ 2006JE002801. Chapman, M.G., 1994. Evidence, age, and thickness of a frozen paleolake in Utopia Planitia, Mars. Icarus 109, 393–406. Costard, F.M., Kargel, J.S., 1995. Outwash plains and thermokarst on Mars. Icarus 114, 93–112. Dutilleul, P., 2011. Spatio-Temporal Heterogeneity: Concepts and Analyses. Cambridge University Press, Cambridge. Fergason, R.L., Christensen, P.R., Kieffer, H.H., 2006. High-resolution thermal inertia derived from the Thermal Emission Imaging System (THEMIS): Thermal model and applications. J. Geophys. Res. 111 (E12004). http://dx.doi.org/10.1029/ 2006JE002735. Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S.R., Read, P.L., Huot, J.P., 1999. Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. 104 (E10), 24155–24176. Fortier, D., Allard, M., 2005. Frost-cracking conditions, Bylot Island, eastern Canadian arctic archipelago. Permafr. Periglac. Process. 16, 145–161. French, H.M., 1996. The Periglacial Environment, 3rd ed. John Wiley and Sons, Chichester. Greeley, R., Guest, J.E., 1987. Geologic map of the eastern equatorial region of Mars, 1:15,000,000 scale. U.S. Geological Survey Map I-1802. Grom, J.D., Pollard, W.H., 2008. A study of high arctic retrogressive thaw slump dynamics, Eureka Sound Lowlands, Ellesmere Island. In: Kane, D.L., Hinkel, K.M. (Eds.), Proceedings of the Ninth International Conference on Permafrost. University of Alaska Fairbanks, June 29–July 3, pp. 545–550. Haltigin, T., Pollard, W., Dutilleul, P., 2010. Comparison of ground- and aerialbased approaches for quantifying polygonal terrain network geometry on Earth and Mars via spatial point pattern analysis. Planet. Space Sci. 58, 1636–1649. http://dx.doi.org/10.1016/j.pss.2010.08.008. Haltigin, T.W., Pollard, W.H., Dutilleul, P., Osinski, G.R., 2012. Geometric evolution of polygonal terrain networks in the Canadian High Arctic: Evidence of increasing regularity over time. Permafrost Periglac. 23, 178–186. http://dx.doi.org/ 10.1002/ppp.1741. Haltigin, T.W., Pollard, W.H., Osinski, G.R., Dutilleul, P., 2009. Polygon terrain morphology within scalloped depressions, Utopia Planitia, Mars. Lunar Planet. Sci. XL. Abstract 2566.

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