Icarus 201 (2009) Contents lists available at ScienceDirect. Icarus.

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1 Icarus 201 (2009) Contents lists available at ScienceDirect Icarus Geologically recent gully polygon relationships on Mars: Insights from the Antarctic Dry Valleys on the roles of permafrost, microclimates, and water sources for surface flow J.S. Levy a,,j.w.head a, D.R. Marchant b,j.l.dickson a, G.A. Morgan a a Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA b Department of Earth Science, Boston University, 675 Commonwealth Ave., Boston, MA 02215, USA article info abstract Article history: Received 29 May 2008 Revised 17 October 2008 Accepted 22 December 2008 Available online 21 January 2009 Keywords: Mars surface Earth Geological processes Ices Regoliths We describe the morphology and spatial relationships between composite-wedge polygons and Marslike gullies (consisting of alcoves, channels, and fans) in the hyper-arid Antarctic Dry Valleys (ADV), as a basis for understanding possible origins for martian gullies that also occur in association with polygonally patterned ground. Gullies in the ADV arise in part from the melting of atmosphericallyderived, wind-blown snow trapped in polygon troughs. Snowmelt that yields surface flow can occur during peak southern hemisphere summer daytime insolation conditions. Ice-cemented permafrost provides an impermeable substrate over which meltwater flows, but does not significantly contribute to meltwater generation. Relationships between contraction crack polygons and sedimentary fans at the distal ends of gullies show deposition of fan material in polygon troughs, and dissection of fans by expanding polygon troughs. These observations suggest the continuous presence of meters-thick icecemented permafrost beneath ADV gullies. We document strong morphological similarities between gullies and polygons on Mars and those observed in the ADV Inland Mixed microclimate zone. On the basis of this morphological comparison, we propose an analogous, top down melting model for the initiation and evolution of martian gullies that occur on polygonally-patterned, mantled surfaces Elsevier Inc. All rights reserved. 1. Introduction Gullies on Mars are a class of geologically young features, initially interpreted to have formed by surficial flow of released groundwater (Malin and Edgett, 2000, 2001; Mellon and Phillips, 2001), and which may still be active (Malin et al., 2006). Martian gullies are geomorphic features composed of a recessed alcove, one or more sinuous channels, and a depositional fan or apron (Malin and Edgett, 2000). Alternative hypotheses for the source of gully-carving fluids include obliquity-driven melting of near-surface ground ice (Costard et al., 2002), melting of dust-rich snow deposits (Christensen, 2003), and melting of atmospherically emplaced frost and/or snow (Hecht, 2002; Dickson et al., 2007a; Head et al., 2007; Dickson and Head, 2008; Williams et al., 2008). Complementing gully formation models, recent GCM results (Forget et al., 2007) predict the deposition and potential for melt of up to 25 mm/yr of water ice at martian northern midlatitudes ( N) during obliquity conditions modeled to have occurred within the past 10 My (and potentially within the past <1 My; Laskar et al., 2004). Other workers have proposed that * Corresponding author. Fax: address: joseph_levy@brown.edu (J.S. Levy). gullies can form by dry avalanche processes alone (Treiman, 2003; Pelletier et al., 2008). Concurrent with advances in understanding of gully processes on Mars, modeling and observational studies have documented the distribution and origin of various types of martian thermal contraction crack polygons (Mellon, 1997; Mangold, 2005; Levy et al., 2008a). Despite the observation of polygonally patterned ground in gullied terrains on Mars and Earth (Malin and Edgett, 2000, 2001; Bridges and Lackner, 2006), and an increasing awareness of the importance of polygonally patterned permafrost in the development of terrestrial polar fluvial systems (Fortier et al., 2007; Levy et al., 2007a; Levy et al., 2008b), there has been little analysis of the interactions between thermal contraction crack polygons and gullies on Mars. In this contribution, we explore interactions between gullies and polygons in the Mars-like Antarctic Dry Valleys (Marchant and Head, 2007), and then assess similarities and differences with features observed on Mars. We first summarize recent research on the spatial distribution, formation, and modification of gullies and polygons in selected regions of the Antarctic Dry Valleys (ADV). In the next section we show how gully development on polygonally patterned ADV surfaces affects gully morphology and enhances water-flow processes. Further, we show how the morphology of /$ see front matter 2009 Elsevier Inc. All rights reserved. doi: /j.icarus

2 114 J.S. Levy et al. / Icarus 201 (2009) Fig. 1. Perspective view of a portion of the South Fork study area in upper Wright Valley, Antarctica. Black arrows indicate channels on the southern wall of the valley and white arrows indicate large alcoves present in the dolerite bedrock, approximately 1000 m above the valley floor. The dark, tongue-shaped lobe of dolerite boulders at center is approximately 300 m wide. Inset. Boxed region showing a small concavity present in the colluvium slope. White arrow indicates the center of the depression. Channels enter into and emanate from the concavity. polygons is altered by proximity to developing gullies. We describe such reciprocal modification relationships as gully polygon systems. We then analyze HiRISE images that document the interplay between polygonally patterned ground, ice-cemented permafrost, and gullies on Mars. If strong morphological similarities exist between gullies and polygons observed on Mars and those documented in the ADV, then this evidence would suggest that, to a first order, some martian gullies formed and were modified by processes analogous to those occurring in ADV gully polygon systems. Such morphological comparisons can help constrain the physical and hydrological properties of gully flow. We address concerns over equifinality (similar morphologies produced by different processes) by focusing our analysis on morphological relationships that illustrate specific spatial and stratigraphic relationships. 2. The Antarctic Dry Valleys (ADV) The Antarctic Dry Valleys are a suitable laboratory for understanding the geomorphological effects of water moving through temperature-dependent phase transitions (freezing, melting, sublimation, evaporation). On the basis of summertime air temperature, relative humidity, soil temperature, and soil moisture conditions, the ADV region is divided into three microclimate zones. The three zones include a coastal thaw zone, an inland mixed zone, and a stable upland zone (Marchant and Denton, 1996; Marchant and Head, 2007). In the inland mixed zone, melting, evaporation, and sublimation occur, whereas in the stable upland zone, sublimation is the dominant phase transition (Ragotzkie and Likens, 1964; Marchant et al., 2002; Kowalewski et al., 2006; Marchant and Head, 2007). The stable upland zone is interpreted to be closely analogous to Mars under current, average climate conditions, whereas the inland mixed zone may be a good analog for more clement martian conditions produced by orbitally-driven climate change (Marchant and Head, 2007) or for short duration peak temperature and insolation conditions. Landforms that are produced in equilibrium with microclimate conditions in each zone are termed equilibrium landforms (Marchant and Head, 2007). Gullies and polygons are the two dominant equilibrium landforms on inland-mixed zone valley walls Gully polygon systems in the ADV In the inland mixed zone of the ADV, gullies are characterized by a recessed alcove, sinuous channels with seasonally moist hyporheic zones (McKnight et al., 1999; Gooseff et al., 2002; Levy et al., 2008b), and one or more distal fans (Figs. 1 and 2). The hyporheic zone is the area marginal to and beneath a stream that exchanges water with the stream channel. Within and adjacent to most gullies, dry, ice-free sediment overlies sediment that is cemented by pore ice. The lower depth of this pore ice is unknown, but its surface, called the ice-cement table, is fairly uniform and occurs on average at about cm depth (Bockheim et al., 2007; Levy et al., 2007b). Typically, the ice-cement table deepens with increasing distance from isolated snow banks and gully channels. In ADV areas with extensive pore ice, the ground commonly shows well-developed thermal contraction crack polygons (Berg and Black, 1966). All gullies save one observed in the Wright Valley study site are present on polygonally-patterned slopes (Levy et al., 2008b; Morgan et al., 2008). Across the ADV, active and recently active gullies are typically present in association with contractioncrack polygons; relict gullies in the coldest and driest portion of the ADV that have been inactive for up to 10 My (Lewis et al., 2007) typically lack polygons characteristic of the Wright Valley site. The most common polygons present in the South Fork area are composite-wedge polygons (Levy et al., 2008b). Composite-wedge polygons are those in which alternating layers of sand and ice fill thermal contraction cracks (Berg and Black, 1966). Importantly, areas in the Dry Valleys that lack pore ice within the upper 1 m of soils tend to lack all varieties of thermal contraction crack polygons (Marchant and Head, 2007) ADV gully water sources Soil-temperature measurements indicate that melting along the ice-cement table in the inland mixed zone is uncommon, and

3 Geologically recent gully polygon systems on Mars and Earth 115 Fig. 2. Summary of key observations from gully polygon systems in the South Fork of upper Wright Valley, Antarctica. (a) Polygon troughs accumulate wind-blown snowbanks that contribute meltwater to gully flow. (b) Trough excavated through dry colluvium across a downslope-oriented polygon trough. The ice-cement table is depressed along polygon troughs, channelizing flow over the ice-cement table. Snow-derived meltwater moves down-slope (from left to right) along the top of impermeable ice-cement table. (c) Stratigraphic relationships between an Antarctic gully and surrounding polygons. Black arrows indicate embayment of surrounding polygons by gully fan material. The fan is dissected by underlying polygons. A white arrow indicates a polygon trough that has been annexed by the channel. The fan is 100 m wide. is not a significant source for surface meltwater (Marchant and Head, 2007; Morgan et al., 2007b). Rather, surface water arises from insolation-driven snowmelt (Head et al., 2007; Morgan et al., 2007a; Levy et al., 2007b). Snowbanks accumulate in polygon troughs and in gully channels through seasonal capture and preservation of windblown snow (Dickson et al., 2007b; Head et al., 2007; Levy et al., 2007b; Morgan et al., 2007a). A comparable volume of snow to that stored in gully channels can be stored in polygon troughs adjacent to gully channels, and in the polygon troughs present in gully alcoves (Levy et al., 2008b). Large snowbanks in gully channels and polygon troughs endure for weeks despite high rates of summertime sublimation (see Kowalewski et al., 2006). Southern hemisphere peak-summer daytime insolation causes melting of snowbanks that produces ephemeral water flow capable of eroding and redistributing sediments (Dickson et al., 2007b; Head et al., 2007; Levy et al., 2007b; Morgan et al., 2007a). Walking surveys at the field site were conducted over 3 kmof polygonally-patterned valley wall, rising from the valley floor to elevations of 800 m, in order to ascertain the presence or absence of deep groundwater sources for gullies. Inspection of five gully polygon systems in the field resulted in no observations of overland flow associated with springs emanating from faulted bedrock exposed at the surface or of high-pressure scouring of sediment or bedrock due to catastrophic release from confined aquifers. In the South Fork study area of upper Wright Valley, gully polygon systems are overwhelmingly present on north-facing (equator-facing) valley slopes (Morgan et al., 2007b). This distribution reflects enhanced peak-summer warming and subsequent melting of wind-blown snow and perennial snowbanks on warm, equator-facing slopes; shadowed, pole-facing slopes accumulate less abundant snowbanks and produce minimal volumes of summer meltwater (Morgan et al., 2008). These observations of gully water sources in the ADV establish two important processes to gully hydrology in permafrost environments. First, networks of polygon troughs can accumulate meltwater derived from broadly distributed melt source (e.g., polygontrough snowbanks) into a concentrated, channelized flow. Without the presence of polygons, snowbanks would have fewer valley wall accumulation sites, and any snowbank meltwater would simply percolate downslope. Second, all the water present in the Antarctic gully systems analyzed is derived from sources lying above the impermeable permafrost ice-cement table there is no deep aquifer component to these cold desert gullies ADV gully polygon morphological relationships: Alcoves, channels, and fans Large alcoves in the Antarctic Dry Valleys commonly form in dolerite bedrock cliffs (Fig. 1, inset; Head et al., 2007; Morgan et al., 2007b). These alcoves have little to no polygonal patterning due to thin to non-existent sediment cover (Levy et al., 2007b). Large alcoves present in the ADV span m in width, and can be up to 400 m long, with aspect ratios of close to 1. Below large bedrock alcoves, small concavities are present on valley walls in the ADV (Fig. 1; Dickson et al., 2007b; Head et al., 2007; Levy et al., 2007b; Morgan et al., 2007a). Concavities may form by erosion of colluvium by braided channels or as typical nivation hollows. Concavities exhibit a dessicated, near-surface sediment layer over ice-cemented debris (Fig. 1). The concavity located in the study area is 290 m long, 150 m wide, and has an aspect ratio of 1.6. Channels cut the concavity from upslope and emanate from it. Bank erosion of braided channels is intense within and around the concavity. Composite-wedge polygons occur within concavities as well as in adjacent colluvium, and are commonly 16 m in diameter, spanning m in diameter, with a standard deviation of 2.8 m (Fig. 1; Levy et al., 2008b). Surface-generated meltwater follows local topography and may be captured in polygon troughs. Where flow is concentrated in polygon troughs, erosion of trough/wedge sediments is enhanced (Fig. 2c), a process termed trough annexation (e.g., Levy et al., 2008b). Annexed polygon troughs are generally wider and deeper than unaffected troughs, and typically have rounded trough intersections (in contrast to angular intersections between pristine polygon troughs; Levy et al., 2008b). Annexed polygon troughs commonly have sandy floors, composed of layers of bedded and cross-bedded sediment (Levy et al., 2008b). Water flow directly observed within annexed troughs is restricted to the ground surface and to the shallow subsurface (Fig. 2b); the impermeable boundary at the top of the ice-cement table ( cm depth) prevents deeper meltwater infiltration. Meltwater that drains to the ice-cement table may flow downslope along its surface for tens of meters before emerging as overland flow (Levy et al., 2008b).

4 116 J.S. Levy et al. / Icarus 201 (2009) Over some reaches of the gullies, meltwater derived from snowbank melting was observed to flow several meters over the surface, before infiltrating into the colluvium, only to emerge from the sediment and resume overland flow several meters downstream (usually at small breaks in slope; Levy et al., 2007b). At the base of ADV valley walls, fan deposits overprint and embay active composite-wedge polygons. Some fan material is deposited in topographic lows (troughs) between high polygon centers, suggesting that polygon troughs provided a topographic barrier to fan emplacement (Fig. 2). Other polygons are completely covered by fan material. These stratigraphic relationships (Fig. 2c) indicate that some fan material was deposited over existing polygons. Continued polygon development results from elevation of the ice-cement table through the fan, and enables contraction cracks to propagate upward, dissecting overlying fan material. This process is analogous to the formation of syngenetic contraction-crack polygons first described by MacKay (1990) and also by Levy et al. (2008b). In summary, these observations are interpreted to indicate that Wright Valley gully development is strongly influenced by the presence of polygonally patterned ground. The presence of polygonally patterned ground does not directly cause the formation of gullies, however, polygons enhance the accumulation of snow that feeds gully flow, concentrate and direct the flow of gully meltwater, and modify depositional fans. Stratigraphic relationships between polygons and gully fans indicate that the Wright Valley gullies studied formed on surfaces continuously underlain by metersthick, ice-cemented, impermeable permafrost, effectively removing the possibility of groundwater contributions to gully flow. 3. Distribution of gully polygon systems on Mars Prompted by the developmental relationship between ADV gullies and polygons, we undertook a comprehensive survey of MRO- HiRISE images in order to assess morphological and stratigraphic relationships between martian gullies and polygons. A survey of HiRISE primary science phase images of the martian surface (McEwen et al., 2007), spanning orbits PSP_ to PSP_007207, and ranging between north and south latitude (a region known for concentrations of gullies, e.g., Milliken et al., 2003; Balme et al., 2006; Dickson et al., 2007a) forms the basis for this analysis. After selecting images based on latitude, images spanning orbits were analyzed sequentially by orbit number ( 530 images). Next, a subset of images from orbits were selected on the basis of geographical location within the latitude bands in order to increase the density of analyzedimagesinlocationssparselysampledinearlyorbits.asense of the magnitude of the dataset, and the degree of morphological detail present in each image is achieved by considering that, a typical 1 GB HiRISE image contains approximately 200 times more information than a typical 5 MB Mars Orbiter Camera image, resulting from increased spatial resolution within a comparable surface footprint. Of the 722 images studied, 168 contain gullies, and 93 contain gullies present on clearly polygonally patterned surfaces (Figs. 3 and 4; Supplementary data). Features present in HiRISE images were classified as gullies if they were composed of at least two of the three gully structural elements defined by Malin and Edgett (2000), namely: a recessed alcove, a sinuous channel, and a distal fan or apron. Geographically, gullies are predominantly observed in HiRISE images in the southern hemisphere (125 southern hemisphere occurrences compared to 43 northern hemisphere occurrences), as are gully polygon systems (71 southern hemisphere occurrences compared to 22 northern hemisphere occurrences). We partially correct for targeting bias by dividing the number of occurrences of gullies and gully polygon systems by the number of survey images in the latitude bands in which the features are present. Gullies are present in 48% of survey images between S, and in 21% of survey images between N; gully polygon systems are present in 26% of survey images between S, and in 9% of survey images between N. These distributions (Fig. 4) are consistent with rougher topography in the southern highlands (e.g., Neumann et al., 2003) providing steep-sloped surfaces important for gully formation (Kreslavsky and Head, 2002; Dickson et al., 2007a; Kreslavsky, 2008). The distribution of gullies and polygons between latitude strongly correlates with the distribution of dissected and continuous latitude-dependent mantle terrain, a meters thick dust and ice-rich deposit interpreted to have been emplaced during recent ice ages caused by spin-axis obliquity excursions (Mustard et al., 2001; Head et al., 2003; Fig. 4). Polygonally patterned ground is present on both the continuous and dissected mantles, while gullies are concentrated in dissected mantle terrains. These distributions suggest that polygons form over a wide range of zonal climate conditions, with and without gullies. In contrast, within the limited latitude range of gullies, the abundance of images containing gullies interacting with polygons suggests that gullies may preferentially form on polygonally patterned surfaces. Using insight from the ADV, gullies and polygons may be landforms which can be used to interpret the range of cold-desert geomorphological processes that have modified latitude dependent mantles on Mars. Where morphological similarities exist between spatially associated gullies and polygons on Mars and ADV gully polygon systems, we suggest that these features may have formed by analogous processes. 4. Morphological relationships between gully polygon systems on Mars 4.1. Alcoves and polygons The largest gully alcoves observed in this survey, characterized by lengths of 1000 m and widths in excess of 500 m, are localized in a latitude band between S and are less common elsewhere. These alcoves (Fig. 5) are triangular in shape and have aspect ratios of <1 to 3, comparable to alcoves described in previous surveys (e.g., Malin and Edgett, 2000; Dickson et al., 2007a). These large alcoves uniformly lack polygons, particularly on steep slopes where mantle material has been eroded, exposing bedrock (Fig. 5). Alcoves with polygonally patterned surfaces are commonly elongate and have a rectangular shape (Fig. 6), comparable to the lengthened alcoves of Malin and Edgett (2000). Elongate alcoves mapped in this study average 820 m in length (n = 29, minimum = 250 m, maximum = 2200 m) and have high lengthto-width aspect ratios (mean = 6, minimum = 4, maximum = 12). Elongate alcoves form within a surficial mantling unit (Fig. 6), and do not generally expose underlying bedrock or crater-fractured material and boulders (e.g., Fig. 5). One or more channels are commonly present within these elongate alcoves (Fig. 6), and emanate out from the alcoves. Thermal contraction crack polygons commonly form in the icerich sediments of the martian latitude-dependent mantle (Mustard et al., 2001; Milliken et al., 2003; Mangold, 2005) or pastedon terrain within or adjacent to gully alcoves (Costard et al., 2002; Christensen, 2003; see Fig. 4: most HiRISE images in mantled terrains polewards of 40 feature polygons). Pasted-on terrain is a relatively smooth-surfaced unit typical of martian midlatitudes, that is commonly superposed on pole-facing surfaces and is thought to have formed by atmospheric deposition of ice and/or dust (Malin and Edgett, 2001; Mustard et al., 2001; Christensen, 2003). Boulders present on some pasted-on terrain

5 Geologically recent gully polygon systems on Mars and Earth 117 Fig. 3. Distribution of polygonally patterned ground, gullies, and gully polygon systems mapped using HiRISE images. Small black dots indicate HiRISE images which do not contain gullies or polygons. (Top) HiRISE images containing polygonally patterned ground (triangles). (Middle) HiRISE images featuring gullies (circles). (Bottom) HiRISE images with gully polygon systems (circles with black and white fill). Gully polygon systems tend to occur in the region between regions with gullies and regions which have polygonally patterned ground. outcrops have been interpreted to indicate a rock-glacier origin for pasted-on terrain (e.g., McEwen et al., 2007); however, the wasting of fractured crater-rim materials located upslope from pastedon terrain may also account for the presence of boulders atop pasted-on surfaces. Polygons present in pasted-on terrain and on mantle surfaces are commonly flat-topped, with elevated interiors and depressed troughs. This morphology is consistent with sand-wedge polygon or sublimation-polygon structures that form preferentially in fine-grained and ice-rich substrates (Lachenbruch, 1962; Washburn, 1973; Maloof et al., 2002; Marchant et al., 2002; Marchant and Head, 2007). Analysis of 136 alcove polygons on Mars, in 8 HiRISE images, indicates a mean martian alcove polygon diameter of 11 m, spanning 5 21 m, with a standard deviation of 3.4 m. Some martian alcove polygons are outlined by bright deposits that are present preferentially in polygon troughs (Fig. 7). Bright indicates pixel DN values in processed HiRISE images that are several times higher than proximal pixels sampled from polygon centers or gully channels. These deposits may be water-ice deposited seasonally as frost (for images taken during winter periods; Mangold, 2005), salt deposits (Burt and Knauth, 2007), dusty lag deposits (Williams et al., 2008), or some other form of high-albedo, particulate deposit, such as snow, that accumulates preferentially in shielded topographic lows (Head et al., 2008). In some images, bright material is distributed broadly over surfaces containing gully polygon systems (Figs. 7a 7b); in others bright material is present in polygon troughs within gully alcoves (e.g., Fig. 7c). Dust cover is not pronounced in the analyzed images (e.g., dust ripples are uncommon and boulders are clearly visible), and no salts have been spectroscopically detected in the examined HiRISE images (e.g., Osterloo et al., 2008). Rather, these deposits are seasonally present, and are commonly blue-toned in HiRISE color data: ob-

6 118 J.S. Levy et al. / Icarus 201 (2009) Fig. 4. Histogram of gully, polygon, and gully polygon system distribution by latitude. The number of feature-containing images in each latitude band has been normalized to the total number of HiRISE images in the latitude band in order to remove bias in spatial coverage (thus, a normalized value of 0.5 indicates that half the HiRISE images surveyed in a latitude band contain the feature plotted). Gullies and gully polygon systems are found primarily in latitudes where dissected mantle terrain is present (Mustard et al., 2001; Head et al., 2003), while polygonally patterned ground spans the continuous and dissected mantles. Fig. 5. Large, triangular alcoves. Layered outcrops interpreted to be exposed crater wall bedrock surfaces are visible within the alcoves (arrows). (a) Portion of PSP_002368_1275, located at 52 S, 247 E, on a crater wall. Ls = : southern winter. (b) Portion of PSP_002054_1325, located at 47 S, 177 E, on a crater wall. Ls = : southern winter. (c) Portion of PSP_001882_1410, located at 39 S, 194 E, on a crater wall. Ls = : southern winter. servations consistent with the seasonal deposition of water ice (Gulick et al., 2008). On the basis of these observations, we interpret bright, trough-filling material present in or around alcoves, that has been imaged during winter or early spring time periods, to be atmospherically-emplaced frost or, possibly, particulate ice Channels and polygons Channel-like features were observed in analyzed HiRISE images that are (1) continuous and sub-linear; (2) present in widened, curved, and down-slope-oriented polygon troughs; and (3) present

7 Geologically recent gully polygon systems on Mars and Earth 119 Fig. 6. Elongate alcoves with thermal contraction crack polygons. Elongate alcoves commonly have a length-to-width aspect ratio of 6 or greater. (a) Portion of PSP_001846_1415, located at 38 N, 97 E, on a crater wall. (b) Portion of PSP_001882_1410 located at 39 S, 194 E, on a crater wall. Ls = : southern winter. (c) Portion of PSP_001357_2200, located at 40 N, 105 E. Illumination is from the left in all images. nearby to typical gully channels (Fig. 8). These features are present individually and in braided groups on polygon-surfaced slopes, and can be distinguished from typical polygon troughs by variations in surface texture, relief, and continuity (Fig. 9). These channels are commonly m in length. Deposits in these linear features are light-toned in HiRISE red-filter images (using DNcomparison of unstretched images), and are distinguished from blue-toned bright deposits present in polygon troughs in alcoves by a difference in texture (trough-channel deposits are slightly rippled with possible small boulders present), a lower albedo, and different color. Given their morphological similarity to terrestrial gully channels that have formed through the annexation of preexisting polygon troughs (Levy et al., 2008b), we interpret these martian features to be remnants of polygon troughs annexed by incipient gully channels. We interpret in-trough deposits to be fluvially deposited sediments, similar to fan-forming sediments, deposited during periods of channelized gully flow Fans and polygons Several gully fans observed in HiRISE images embay polygonally patterned ground (Fig. 10), consistent with observations of fans overprinting polygons in MOC image analyses (e.g., Malin and Edgett, 2000; Heldmann et al., 2007). Some fans terminate abruptly at relatively deep polygon troughs. Other fans appear to wrap around elevated polygon centers and elevated outcrops of patterned ground (Fig. 10a). These relationships suggest that polygon topography provided a barrier to fan emplacement. Small gully fans on polygonally patterned slopes range in surface area from m 2. These fans have little topographic relief and appear to be thin, surficial deposits (Fig. 10). Polygon troughs are visible through small fan surfaces. These polygon troughs are typically continuous with, and are extensions of, surrounding trough networks (Fig. 10). These observations suggest that polygons have remained active through fan deposition, and have winnowed fan sediments into underlying polygon wedges. In contrast to small-scale fans, several large fans were observed in HiRISE images that show different morphological and stratigraphic relationships with polygons (Fig. 10c). Large fans have surface areas spanning m 2 (one to two orders of magnitude larger than small fans described above). Large fans generally have significant topographic relief, and rise convexly up from inter-fan slopes. Typically, these large fans lack modification by surface polygons, though some are characterized by an array of fine-scale fractures that superficially resemble small polygon troughs. These fine fractures are not continuous with troughs from adjacent polygon networks (Fig. 10c). These observations suggest that large fans have buried surrounding polygon networks. Light-toned material can be seen displaced from the fan in some HiRISE images, suggesting partial redistribution of the fine fraction of fan-forming sediments by subsequent aeolian processes (Figs. 10a and 10b). The continued presence of bright fan material in polygon centers, rather than preferential redistribution of fan material into polygon troughs, suggests that much of the polygon-interior fan material has been preserved in place, despite winnowing of fan sediments into polygon troughs and aeolian erosion of fines. Lastly, the majority of polygons present on gully fans, and on surfaces topographically lower than the fans, do not display morphologies characteristic of seasonal saturation of sediments

8 120 J.S. Levy et al. / Icarus 201 (2009) Fig. 7. Bright material present in polygon troughs (white arrow) in proximity to alcoves and channels (black arrows). Images were taken during winter/spring in all cases, suggesting bright material is frost, ice, or snow. Downslope is towards image bottom in all images. (a) Portion of PSP_003920_1095, located at 70 S, 2 E. Ls = : southern spring. Illumination from left. (b) Portion of PSP_3511_1115, located at 69 S, 1 E. Ls = : southern spring. Illumination from above. (c) Portion of PSP_002165_1270, located at 53 S, 28 E. Ls = : southern winter. Illumination from left. by gully flow (e.g., Lyons et al., 2005; Levy et al., 2008b), such as concentration of boulders at the surface (heaving), sorting of sediments through cryoturbation (which might be detectable as changes in surface brightness or texture), or the formation of icewedge polygons with upturned shoulders. These observations suggest that water involved in the transport and deposition of fan sediments rapidly froze-on to the ice-cement table within the fan and/or sublimated until local equilibrium conditions were met for water stability. In summary, we interpret these overprinting and cross-cutting relationships to indicate the following formational sequence. Smallscale gully fans formed by deposition of sediments over previously existing polygonally-patterned ground (consistent with MOC observations, e.g., Malin and Edgett, 2000; Heldmann et al., 2007), and crack expansion continued throughout and after fan deposition, dissecting gully fans from beneath. This implies the continuous presence of ice-cemented permafrost beneath gully fans during their development and aggradation of permafrost concurrent with the growth of gully fans. Larger fans formed from the emplacement of sediment at a rate that resulted in the burial of previously extant polygon networks, resulting in polygon development limited to fine-scale networks that are discontinuous with the surrounding polygonal network Slope orientation Although preliminary reports conflicted on the presence or absence of orientation preferences for gullies at the hemisphere scale (e.g., Malin and Edgett, 2000; Edgett et al., 2003), binning of gully orientations by latitude by Heldmann and Mellon (2004) discovered a latitude-dependence for the orientation of gullies in the southern hemisphere. Gullies between S predominantly face polewards and gullies between S generally face equatorwards (Heldmann and Mellon, 2004). This observation was verified in the Newton Crater region by Berman et al. (2005). Dickson et al. (2007a) further confirm these observations, finding that 86% of gullies in the S latitude band occur on pole-facing slopes, and noting that the few gullies mapped on equator-facing slopes are confined to above 40 S. One interpretation of the orientation data is that gullies form on protected slopes where snow/ice, if available, would tend to accumulate (Hecht, 2002; Dickson et al., 2007a; Head and Marchant, 2008; Head et al., 2008), and where protected ice reservoirs could be rapidly exposed to peak insolation, leading to melting. Hecht (2002) demonstrated that peak insolation sufficient to cause melting can be achieved on either pole- or equator-facing slopes on Mars, depending on latitude and slope inclination. In some HiRISE images gullies on polygonally-patterned surfaces can be observed on both pole-facing and equator-facing slopes (Fig. 11). Pole-facing and equator-facing are qualitative measurements of orientation, indicating that gully polygon systems were present on slopes oriented within 30 of north or south. These occur most commonly on interior crater walls. Only HiRISE images in which gully polygon systems are present on near-diametrically opposite slopes (> 150 angular separation) were included in orientation analyses. The morphology of gullies and polygons on Mars differs with slope orientation (Fig. 11). Gullies on pole-facing slopes generally have sharply-defined channels and fans, and polygons on polefacing slopes are crisply delineated. On equator-facing slopes (imaged at the same resolution) gullies and polygons have subdued

9 Geologically recent gully polygon systems on Mars and Earth 121 Fig. 8. Features interpreted to be polygon troughs annexed by gully channels. Annexed troughs are continuous and sub-linear, are present on polygonally patterned surfaces in widened and curved polygon troughs, and are preferentially filled with bright deposits. Upslope is toward image top in all panels, and annexed troughs are oriented downslope. Arrows indicate some of the annexed polygon troughs. (a) Portion of PSP_001846_2390, located at 59 N, 82 E, on a crater wall. (b) Portion of PSP_001846_2390, adjacent to part a, located at 59 N, 82 E, on a crater wall. (c) Portion of PSP_001508_2400, located at 60 N, 302 E, on a crater wall. (d) Portion of PSP_001938_2265, located at 46 N, 92 E, in Utopia Planitia. Gully features are present in a large, scalloped depression. Fig. 9. Braided annexed polygon troughs from PSP_001548_2380, located at 58 N, 292 E. (a) Small annexed troughs present within crater-slope-oriented polygons on the upper slope of a crater rim. Illumination is from the left. Boulders of various sizes are visible. (b) Larger braided channels adjacent to small channels shown in part a (box). and softened morphologies (Fig. 11). Gully channels on pole-facing slopes tend to be narrower and are bounded by steeper walls than those on equator-facing slopes. In addition, the surface texture of fans on equator-facing slopes is commonly indistinguishable from that of inter-fan surfaces. Lastly, polygons on equator-facing slopes generally lack sharp trough boundaries, and commonly grade from networks of raised mounds surrounded by low troughs, to irregular, linear albedo patterns interpreted to be incomplete and degraded crack networks. We interpret the softening of gully polygon system morphologies on equator-facing slopes to indicate removal

10 122 J.S. Levy et al. / Icarus 201 (2009) Fig. 10. Stratigraphic relationships between fans and polygons. (a b) Small, thin fans, with little topographic relief, are cut by underlying thermal contraction crack polygons. Dissecting polygons are continuous with polygons present on inter-fan surfaces. (c) A large, convex-up fan is subtly patterned with a polygon network discontinuous with the inter-fan network (inset showing white boxed region, arrows highlight two of many polygon troughs). (a) Portion of PSP_001846_2390, located at 59 N, 82 E. (b) Portion of PSP_001548_2380, located at 58 N, 292 E. (c) Portion of PSP_002368_1275, located at 52 S, 247 E. of near-surface, permafrost-cementing ice by sublimation subsequent to gully polygon system formation. 5. Discussion Analysis of gully polygon systems on Earth suggests the following stratigraphic and temporal relationships within gully polygon systems: (1) polygons pre-date alcove formation; (2) polygon troughs have been annexed by some gully channels, indicating overland flow and channel development on polygonally patterned surfaces; (3) many fans formed on a polygonally-patterned surface; and (4) polygon development has continued during fan aggradation. Identical stratigraphic and temporal relationships are observed between gullies and polygons on Mars. These results from HiRISE are consistent with stratigraphic interpretations made using MOC image data (e.g., Malin and Edgett, 2000; Costard et al., 2002; Christensen, 2003; Heldmann and Mellon, 2004; Berman et al., 2005; Balme et al., 2006; Dickson et al., 2007a; Head et al., 2008), but provide an unprecedented view of the detailed morphological relationships between martian gullies and polygons. Over half of the gullies imaged in this survey interact with underlying polygonally patterned ground, showing evidence of polygon-influenced water ice accumulation, polygonal patterning of elongate alcoves, annexation of polygon troughs by gully channels, and dissection of fans by underlying polygons. These reciprocal changes in gully and polygon morphology suggest a linked developmental history for martian gullies and polygons, analogous to that observed in terrestrial gully polygon systems. Accordingly, we interpret these spatially-linked landforms observed in HiRISE images to be gully polygon systems. Thus, understanding the effects of polygonally patterned permafrost on gully development on Earth may be important for understanding the hydrological and microclimatological processes involved in gully formation on Mars. On the basis of our observations in the Antarctic Dry Valleys and on Mars, we propose the following model for the initiation and evolution of martian gully polygon systems described in this survey (Fig. 12). On Mars, a climate-related, latitudedependent, ice-rich mantling unit composed of atmospheric dust, ice, and ice-cemented regolith is deposited regionally above 30 latitude (Mustard et al., 2001; Hecht, 2002; Christensen, 2003; Head et al., 2003; Milliken et al., 2003). This mantling unit is preferentially preserved in sheltered environments on steep slopes and at low elevations (<3 km above the datum; Hecht, 2002; Dickson et al., 2007a), and likely has an ice-free sublimation lag at its surface (Mustard et al., 2001; Hecht, 2002; Head et al., 2003; Williams et al., 2008). This mantling unit may be analogous to the unconsolidated debris layer overlying ice-cemented colluvium in the ADV. However, the mantle differs in that its primary mode of emplacement is atmospheric deposition, rather than typical colluvial transport. Next, thermal cycling generates polygons (Mellon, 1997) that, owing to relatively dry climatic conditions, may be analogous to sand-wedge and/or composite-wedge polygons (Mellon, 1997; Mangold et al., 2004; Mangold, 2005; Levy et al., 2008b; Marchant and Head, 2007). Polygons disturb mantling sediments at trough locations. Although calculations of thermal wave propagation on Mars have demonstrated the potential for wet active layers during geologically recent time (Kreslavsky et al., 2007), the lack of extensively water-related polygon structures in these units (such as markedly raised polygon shoulders), coupled with a lack of solifluction features, suggests that near-surface warming did not result in significant melting of buried ice-cemented permafrost.

11 Geologically recent gully polygon systems on Mars and Earth 123 Fig. 11. Orientation dependence of morphology in gully polygon systems. Image pairs (a and b, c and d) are portions of the same HiRISE image, with identical resolution, illumination, and signal-to-noise conditions. (a) Sharply-defined gully polygon systems on a pole-facing slope. Portion of PSP_001846_2390, located at 59 N, 82 E, on a crater wall. Illumination is from the right. (b) Softened gully features with little to no polygonal patterning present on the crater wall opposite the gullies shown in part (a). Illumination is from the left. (c) Sharply-defined gully polygon systems on a pole-facing slope. Elongate alcoves between polygon-covered topographic spurs are present. From PSP_001357_2200, located at 40 N, 105 E. Illumination is from the right. (d) Smoothed gullies with sparse polygonal patterning located on crater rim opposite gullies shown in part (c). Topographic spurs are present, but lack mantling material. Illumination is from the left. Fig. 12. Schematic diagram of a polygon-influenced model for gully initiation and evolution. (a) Initial topography is generated; in this case, the inside wall of a crater. The pole-facing slope is illustrated. (b) Thermal contraction crack polygons form in ice-cemented regolith and sediments on the crater wall. (c) Accumulation of atmospherically deposited frost or wind-blown snow occurs in sheltered polygon troughs. Localized melting of this ice is channelized by polygon troughs, creating small-scale annexed polygon trough channels that produce few to no distal fans. Small fans are readily cut by continuing thermal contraction cracking. (d) Ice deposition and localized melting continues in sheltered polygon troughs, resulting in the growth of annexed channels and the braiding of nearby channels into anastomosing groups. Growing distal fans are cut by expansion of underlying thermal contraction cracks. (e) Continued snow and ice deposition and localized melting in braided annexed channels erodes inter-channel walls, creating an elongate alcove with one or more internal channels. The distal fan is still thin enough to permit dissection by polygon growth. (f) Continued erosion of crater-mantling sediments exposes crater wall surfaces and results in rapid alcove erosion by water/ice-assisted mass-wasting. The rapid deposition of poorly sorted and large-grain-size sediments overwhelms underlying thermal contraction cracks, burying the original polygon network.

12 124 J.S. Levy et al. / Icarus 201 (2009) Subsequently, atmospherically-derived water is introduced to the gully polygon system. Topographically-depressed polygon troughs are shaded environments that could cold-trap atmospheric water frost (Hecht, 2002) and/or act as topographic obstacles, concentrating wind-blown particulate ice (Head et al., 2008) expanding the extent and thickness of seasonal frost accumulations imaged in this survey. Under appropriate obliquity- and slopedependent peak insolation conditions (Hecht, 2002; Kuzmin, 2005; Levy et al., 2007a; Morgan et al., 2008) this ice, concentrated in polygon troughs, could melt to produce short-lived, ephemeral, liquid water. Peak insolation conditions occur when the solar angle is normal to the surface slope. Gully polygon systems commonly occur on steep, 30 slopes (Dickson et al., 2007a), making them strong candidates for recent surface melting of atmospherically emplaced volatiles (Hecht, 2002). Crater-retention age dating of recent gully deposits similar to those included in this study indicates gully activity within the past 1 2 Ma, although flows may have occurred as recently as 300 ka (Riess et al., 2004; Schon et al., 2009). As in the Antarctic Dry Valleys, martian polygon troughs appear to concentrate and direct the transport of metastable surface- and near-surface, meltwater, creating annexed polygon troughs (Levy et al., 2008b). Short-lived water transport in annexed troughs (Figs. 8 and 9) could easily transport unconsolidated polygon trough and wedge sediments, contributing to sediment deposition in small terminal fans. Over time, annexed trough channels would converge towards trunk channels (Fig. 9b, compare left and right), forming braided annexed channels. Braided channels would produce larger fans than those formed from isolated annexed troughs by collecting sediment from several separate channels, and by increasing the volume of water available to move sediment. These fan deposits would still be relatively thin, and would be easily cut or dissected by the continued growth of underlying polygon cracks (Fig. 10b). Continued erosion within annexed troughs would provide an increasingly large sheltered environment for accumulation and subsequent melting of ice and windblown snow (Figs. 6 and 9b). Eventually, braided channel walls would be widened, creating an elongated alcove with one or more incised channels (Fig. 6). Material eroded from elevated alcoves would be deposited in a distal fan, which could remain thin enough to permit continuing dissection by thermal contraction crack expansion. Elongate alcoves are analogous to gully-related concavities or nivation hollows in the ADV (Figs. 1 (inset) and 2b). The presence of polygons in elongate alcoves, but not in widened alcoves (below) suggests that some ice-rich latitude-dependent mantle material remains intact in these alcoves. This process would continue for as long as climate conditions remained capable of generating liquid water or brines that remained metastable long enough to flow (e.g., Mellon and Phillips, 2001; Hecht, 2002; Costard et al., 2002; Christensen, 2003; Kreslavsky and Head, 2007; Burt et al., 2008). The longevity of briny fluids on Mars is strongly dependent on solute depression of the freezing temperature (potentially supporting liquid flow at temperatures as low as 20 to 50 C, depending on salt chemistry and concentration; Burt and Knauth, 2007) and reduction in evaporation rate to support persistent fluvial activity (potentially as slow as 0.04 mm/h at 25 C; Ingersoll, 1970; Sears and Chittenden, 2005). Eventually, alcove mantle material would be fully eroded, exposing original scarp/crater-wall surfaces (Fig. 5). Exposed crater and mantle material in these large, steep alcoves might fail in response to gravitational sliding, as well as in response to surficial fluvial erosion, producing large fans beneath leveed channels (Morgan et al., 2007b). Extensive fan deposition may bury polygons, cutting off underlying thermal contraction cracks from the fan surface exposed to seasonal thermal cycling, and leading to the generation of a network of new polygons (fine fractures present on some large fan surfaces that are discontinuous with the polygon network surrounding the fan). Other large fans lack any polygonal patterning, suggesting that fan emplacement may have been rapid enough to prevent the formation of syngenetic polygons (MacKay, 1990). As climate conditions became colder and drier (Forget et al., 2007), fluvial and erosive processes would decrease and eventually cease in the gullies. In the absence of gully flow and infiltration refreshing buried ice-cemented permafrost, enhanced sublimation on equator-facing slopes would desiccate shallow permafrost, reducing sediment cohesion, and ultimately resulting in subdued gully and polygon textures in response to aeolian erosion. In degraded gully polygon systems (e.g., Fig. 11) polygons are lost from view before gullies (owing to the larger size of gullies) suggesting that polygons may have interacted with gullies even more commonly than is currently observed. In summary, this model provides a mechanism for the development of martian gullies that occur in association with polygonally patterned ground. Morphological similarities between martian gully polygon systems and the closest morphological and climatological analog on Earth (gully polygon systems in the ADV), suggest that the martian examples may have formed and developed on slopes underlain by ice-cemented permafrost. In both cases, a top down source for gully-carving water is implied, as geomorphological evidence suggests limited melting of the underlying ice-cemented substrate. 6. Conclusions Observations of gullies and polygons from Antarctic field work and analysis of HiRISE image data suggests the following stratigraphic and temporal relationships between gullies and polygons: (1) polygons pre-date alcove excavation in some gullies; (2) polygon troughs form traps for ice and windblown snow that can become sources of meltwater for gullies; (3) polygon troughs have been annexed and eroded by some channels, indicating that channel formation occurred on a polygonally patterned surface; (4) fan embayment and dissection relationships indicate that some fans formed on polygonally patterned surfaces; and (5) polygon development continued during fan emplacement. Using morphologically similar gully polygon systems in the ADV as a guide, the stratigraphic relationships between gullies and polygons observed in the HiRISE images suggest that the martian gullies analyzed in this study developed on slopes underlain by polygonally-patterned, ice-cemented permafrost. Interactions between martian gullies and polygons are analogous to those documented in ADV gully polygon systems. No evidence was seen for significant melting of underlying icecemented permafrost on Earth or Mars. Additionally, no evidence of subsurface groundwater release (e.g., Malin and Edgett, 2000; Heldmann and Mellon, 2004; Heldmann et al., 2007) from beneath the ice table was observed at HiRISE resolution. No paired aquacludes or intensive substrate layering abutting gully channels or alcoves was observed in gully polygon system sites, which included crater rims, crater walls, and isolated central peaks, nor was scour associated with high-pressure water release observed. Rather, the locations of gullies on Mars are strongly associated with the presence of a mantling unit that is commonly polygonally-patterned. These lines of evidence suggest an atmospherically emplaced, top down source for fluids involved in martian gully evolution on polygonally-patterned surfaces, comparable to hydrological processes observed in the Antarctic Dry Valleys. On Earth and Mars, the presence of polygons is not shown to be directly causal of martian gully formation, but to be diagnostic of top down gully water sources, and to amplify the key processes of gully formation: accumulation of water ice and the channelized transport of melt water.