Geologically Recent Gully–Polygon Relationships on Mars: Insights From

Icarus 201 (2009) 113–126

Contents lists available at ScienceDirect

Icarus

www.elsevier.com/locate/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.Heada, D.R. Marchant b,J.L.Dicksona, 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: We describe the morphology and spatial relationships between composite-wedge polygons and Mars- Received 29 May 2008 like gullies (consisting of alcoves, channels, and fans) in the hyper-arid Antarctic Dry Valleys (ADV), Revised 17 October 2008 as a basis for understanding possible origins for martian gullies that also occur in association with Accepted 22 December 2008 polygonally patterned ground. Gullies in the ADV arise in part from the melting of atmospherically- Available online 21 January 2009 derived, wind-blown snow trapped in polygon troughs. Snowmelt that yields surface flow can occur Keywords: during peak southern hemisphere summer daytime insolation conditions. Ice-cemented permafrost Mars surface provides an impermeable substrate over which meltwater flows, but does not significantly contribute Earth to meltwater generation. Relationships between contraction crack polygons and sedimentary fans at the Geological processes distal ends of gullies show deposition of fan material in polygon troughs, and dissection of fans by Ices expanding polygon troughs. These observations suggest the continuous presence of meters-thick ice- Regoliths cemented 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. © 2009 Elsevier Inc. All rights reserved.

1. Introduction gullies can form by dry avalanche processes alone (Treiman, 2003; Pelletier et al., 2008). Gullies on Mars are a class of geologically young features, ini- Concurrent with advances in understanding of gully processes tially interpreted to have formed by surficial flow of released on Mars, modeling and observational studies have documented the groundwater (Malin and Edgett, 2000, 2001; Mellon and Phillips, distribution and origin of various types of martian thermal con- 2001), and which may still be active (Malin et al., 2006). Mar- traction crack polygons (Mellon, 1997; Mangold, 2005; Levy et al., tian gullies are geomorphic features composed of a recessed al- 2008a). Despite the observation of polygonally patterned ground cove, one or more sinuous channels, and a depositional fan or in gullied terrains on Mars and Earth (Malin and Edgett, 2000, apron (Malin and Edgett, 2000). Alternative hypotheses for the 2001; Bridges and Lackner, 2006), and an increasing awareness source of gully-carving fluids include obliquity-driven melting of of the importance of polygonally patterned permafrost in the de- near-surface ground ice (Costard et al., 2002), melting of dust-rich velopment of terrestrial polar fluvial systems (Fortier et al., 2007; snow deposits (Christensen, 2003), and melting of atmospherically Levy et al., 2007a; Levy et al., 2008b), there has been little analy- emplaced frost and/or snow (Hecht, 2002; Dickson et al., 2007a; sis of the interactions between thermal contraction crack polygons Head et al., 2007; Dickson and Head, 2008; Williams et al., and gullies on Mars. 2008). Complementing gully formation models, recent GCM re- In this contribution, we explore interactions between gullies sults (Forget et al., 2007) predict the deposition and potential for and polygons in the Mars-like Antarctic Dry Valleys (Marchant and melt of up to 25 mm/yr of water ice at martian northern midlat- ◦ Head, 2007), and then assess similarities and differences with fea- itudes (∼30–50 N) during obliquity conditions modeled to have tures observed on Mars. We first summarize recent research on occurred within the past 10 My (and potentially within the past the spatial distribution, formation, and modification of gullies and <1My;Laskar et al., 2004). Other workers have proposed that polygons in selected regions of the Antarctic Dry Valleys (ADV). In the next section we show how gully development on polygonally * Corresponding author. Fax: +1 401 863 3978. patterned ADV surfaces affects gully morphology and enhances E-mail address: [email protected] (J.S. Levy). water-flow processes. Further, we show how the morphology of

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 are termed equilibrium landforms (Marchant and Head, 2007). Gul- such reciprocal modification relationships as “gully–polygon sys- lies and polygons are the two dominant equilibrium landforms on tems.” inland-mixed zone valley walls. We then analyze HiRISE images that document the interplay between polygonally patterned ground, ice-cemented permafrost, 2.1. Gully–polygon systems in the ADV and gullies on Mars. If strong morphological similarities exist between gullies and polygons observed on Mars and those docu- In the inland mixed zone of the ADV, gullies are character- mented in the ADV, then this evidence would suggest that, to ized by a recessed alcove, sinuous channels with seasonally moist a first order, some martian gullies formed and were modified hyporheic zones (McKnight et al., 1999; Gooseff et al., 2002; by processes analogous to those occurring in ADV gully–polygon Levy et al., 2008b), and one or more distal fans (Figs. 1 and 2). The systems. Such morphological comparisons can help constrain the hyporheic zone is the area marginal to and beneath a stream that physical and hydrological properties of gully flow. We address exchanges water with the stream channel. Within and adjacent to concerns over equifinality (similar morphologies produced by dif- most gullies, dry, ice-free sediment overlies sediment that is ce- ferent processes) by focusing our analysis on morphological rela- mented by pore ice. The lower depth of this pore ice is unknown, tionships that illustrate specific spatial and stratigraphic relation- but its surface, called the “ice-cement table,” is fairly uniform and ships. occurs on average at about 15–20 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 chan- 2. The Antarctic Dry Valleys (ADV) nels. In ADV areas with extensive pore ice, the ground commonly The Antarctic Dry Valleys are a suitable laboratory for under- shows well-developed thermal contraction crack polygons (Berg standing the geomorphological effects of water moving through and Black, 1966). All gullies save one observed in the Wright Valley temperature-dependent phase transitions (freezing, melting, sub- study site are present on polygonally-patterned slopes (Levy et al., limation, evaporation). On the basis of summertime air tempera- 2008b; Morgan et al., 2008). Across the ADV, active and recently ture, relative humidity, soil temperature, and soil moisture con- active gullies are typically present in association with contraction- ditions, the ADV region is divided into three microclimate zones. crack polygons; relict gullies in the coldest and driest portion of The three zones include a coastal thaw zone, an inland mixed the ADV that have been inactive for up to 10 My (Lewis et al., zone, and a stable upland zone (Marchant and Denton, 1996; 2007) typically lack polygons characteristic of the Wright Valley Marchant and Head, 2007). In the inland mixed zone, melting, site. The most common polygons present in the South Fork area are evaporation, and sublimation occur, whereas in the stable up- composite-wedge polygons (Levy et al., 2008b). Composite-wedge land zone, sublimation is the dominant phase transition (Ragotzkie polygons are those in which alternating layers of sand and ice fill and Likens, 1964; Marchant et al., 2002; Kowalewski et al., 2006; thermal contraction cracks (Berg and Black, 1966). Importantly, ar- Marchant and Head, 2007). The stable upland zone is interpreted eas in the Dry Valleys that lack pore ice within the upper ∼1m to be closely analogous to Mars under current, average climate of soils tend to lack all varieties of thermal contraction crack poly- conditions, whereas the inland mixed zone may be a good analog gons (Marchant and Head, 2007). for more clement martian conditions produced by orbitally-driven climate change (Marchant and Head, 2007) or for short duration 2.1.1. ADV gully water sources peak temperature and insolation conditions. Landforms that are Soil-temperature measurements indicate that melting along the produced in equilibrium with microclimate conditions in each zone ice-cement table in the inland mixed zone is uncommon, and 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 gully systems analyzed is derived from sources lying above the im- Head, 2007; Morgan et al., 2007b). Rather, surface water arises permeable permafrost ice-cement table—there is no deep aquifer from insolation-driven snowmelt (Head et al., 2007; Morgan et component to these cold desert gullies. al., 2007a; Levy et al., 2007b). Snowbanks accumulate in polygon troughs and in gully channels through seasonal capture and 2.1.2. ADV gully–polygon morphological relationships: Alcoves, preservation of windblown snow (Dickson et al., 2007b; Head et channels, and fans al., 2007; Levy et al., 2007b; Morgan et al., 2007a). A compara- Large alcoves in the Antarctic Dry Valleys commonly form in ble volume of snow to that stored in gully channels can be stored dolerite bedrock cliffs (Fig. 1,inset;Head et al., 2007; Morgan et in polygon troughs adjacent to gully channels, and in the polygon al., 2007b). These alcoves have little to no polygonal patterning due troughs present in gully alcoves (Levy et al., 2008b). Large snow- to thin to non-existent sediment cover (Levy et al., 2007b). Large banks in gully channels and polygon troughs endure for weeks alcoves present in the ADV span ∼100–400 m in width, and can be despite high rates of summertime sublimation (see Kowalewski up to ∼400 m long, with aspect ratios of close to 1. Below large et al., 2006). Southern hemisphere peak-summer daytime insola- bedrock alcoves, small concavities are present on valley walls in tion causes melting of snowbanks that produces ephemeral water the ADV (Fig. 1; Dickson et al., 2007b; Head et al., 2007; Levy et flow capable of eroding and redistributing sediments (Dickson et al., 2007b; Morgan et al., 2007a). Concavities may form by erosion al., 2007b; Head et al., 2007; Levy et al., 2007b; Morgan et al., of colluvium by braided channels or as typical nivation hollows. 2007a). Concavities exhibit a dessicated, near-surface sediment layer over Walking surveys at the field site were conducted over ∼3kmof ice-cemented debris (Fig. 1). The concavity located in the study polygonally-patterned valley wall, rising from the valley floor to el- area is ∼290 m long, ∼150 m wide, and has an aspect ratio of evations of ∼800 m, in order to ascertain the presence or absence ∼1.6. Channels cut the concavity from upslope and emanate from of deep groundwater sources for gullies. Inspection of five gully– it. Bank erosion of braided channels is intense within and around polygon systems in the field resulted in no observations of over- the concavity. Composite-wedge polygons occur within concavities land flow associated with springs emanating from faulted bedrock as well as in adjacent colluvium, and are commonly ∼16 m in exposed at the surface or of high-pressure scouring of sediment or diameter, spanning 12–24 m in diameter, with a standard deviation bedrock due to catastrophic release from confined aquifers. of ∼2.8 m (Fig. 1; Levy et al., 2008b). In the South Fork study area of upper Wright Valley, gully– Surface-generated meltwater follows local topography and may polygon systems are overwhelmingly present on north-facing be captured in polygon troughs. Where flow is concentrated in (equator-facing) valley slopes (Morgan et al., 2007b). This distri- polygon troughs, erosion of trough/wedge sediments is enhanced bution reflects enhanced peak-summer warming and subsequent (Fig. 2c), a process termed “trough annexation” (e.g., Levy et al., melting of wind-blown snow and perennial snowbanks on warm, 2008b). Annexed polygon troughs are generally wider and deeper equator-facing slopes; shadowed, pole-facing slopes accumulate than unaffected troughs, and typically have rounded trough in- less abundant snowbanks and produce minimal volumes of sum- tersections (in contrast to angular intersections between pristine mer meltwater (Morgan et al., 2008). polygon troughs; Levy et al., 2008b). Annexed polygon troughs These observations of gully water sources in the ADV establish commonly have sandy floors, composed of layers of bedded and two important processes to gully hydrology in permafrost environ- cross-bedded sediment (Levy et al., 2008b). Water flow directly ob- ments. First, networks of polygon troughs can accumulate melt- served within annexed troughs is restricted to the ground surface water derived from broadly distributed melt source (e.g., polygon- and to the shallow subsurface (Fig. 2b); the impermeable bound- trough snowbanks) into a concentrated, channelized flow. Without ary at the top of the ice-cement table (∼10–20 cm depth) pre- the presence of polygons, snowbanks would have fewer valley wall vents deeper meltwater infiltration. Meltwater that drains to the accumulation sites, and any snowbank meltwater would simply ice-cement table may flow downslope along its surface for tens percolate downslope. Second, all the water present in the Antarctic of meters before emerging as overland flow (Levy et al., 2008b). 116 J.S. Levy et al. / Icarus 201 (2009) 113–126

Over some reaches of the gullies, meltwater derived from snow- images in the latitude bands in which the features are present. ◦ bank melting was observed to flow several meters over the surface, Gullies are present in 48% of survey images between 30–55 S, ◦ before infiltrating into the colluvium, only to emerge from the and in 21% of survey images between 30–55 N; gully–polygon ◦ sediment and resume overland flow several meters downstream systems are present in 26% of survey images between 30–55 S, ◦ (usually at small breaks in slope; Levy et al., 2007b). and in 9% of survey images between 35–55 N. These distributions At the base of ADV valley walls, fan deposits overprint and (Fig. 4) are consistent with rougher topography in the southern embay active composite-wedge polygons. Some fan material is de- highlands (e.g., Neumann et al., 2003) providing steep-sloped sur- posited in topographic lows (troughs) between high polygon cen- faces important for gully formation (Kreslavsky and Head, 2002; ters, suggesting that polygon troughs provided a topographic bar- Dickson et al., 2007a; Kreslavsky, 2008). ◦ ◦ rier to fan emplacement (Fig. 2). Other polygons are completely The distribution of gullies and polygons between 30 –80 lat- covered by fan material. These stratigraphic relationships (Fig. 2c) itude strongly correlates with the distribution of dissected and indicate that some fan material was deposited over existing poly- continuous latitude-dependent mantle terrain, a meters thick dust gons. Continued polygon development results from elevation of the and ice-rich deposit interpreted to have been emplaced during re- ice-cement table through the fan, and enables contraction cracks cent ice ages caused by spin-axis obliquity excursions (Mustard et to propagate upward, dissecting overlying fan material. This pro- al., 2001; Head et al., 2003; Fig. 4). Polygonally patterned ground cess is analogous to the formation of syngenetic contraction-crack is present on both the continuous and dissected mantles, while polygons first described by MacKay (1990) and also by Levy et al. gullies are concentrated in dissected mantle terrains. These dis- (2008b). tributions suggest that polygons form over a wide range of zonal In summary, these observations are interpreted to indicate that climate conditions, with and without gullies. In contrast, within Wright Valley gully development is strongly influenced by the the limited latitude range of gullies, the abundance of images con- presence of polygonally patterned ground. The presence of polyg- taining gullies interacting with polygons suggests that gullies may onally patterned ground does not directly cause the formation of preferentially form on polygonally patterned surfaces. Using in- gullies, however, polygons enhance the accumulation of snow that sight from the ADV, gullies and polygons may be landforms which feeds gully flow, concentrate and direct the flow of gully melt- can be used to interpret the range of cold-desert geomorphological water, and modify depositional fans. Stratigraphic relationships be- processes that have modified latitude dependent mantles on Mars. tween polygons and gully fans indicate that the Wright Valley gul- Where morphological similarities exist between spatially associ- lies studied formed on surfaces continuously underlain by meters- ated gullies and polygons on Mars and ADV gully–polygon systems, thick, ice-cemented, impermeable permafrost, effectively removing we suggest that these features may have formed by analogous pro- the possibility of groundwater contributions to gully flow. cesses.

3. Distribution of gully–polygon systems on Mars 4. Morphological relationships between gully–polygon systems on Mars Prompted by the developmental relationship between ADV gullies and polygons, we undertook a comprehensive survey of MRO- 4.1. Alcoves and polygons HiRISE images in order to assess morphological and stratigraphic relationships between martian gullies and polygons. A survey The largest gully alcoves observed in this survey, character- of HiRISE primary science phase images of the martian surface ized by lengths of ∼1000 m and widths in excess of ∼500 m, ◦ (McEwen et al., 2007), spanning orbits PSP_001330 to PSP_007207, are localized in a latitude band between ∼40–50 S and are ◦ and ranging between 30–80 north and south latitude (a region less common elsewhere. These alcoves (Fig. 5) are triangular in known for concentrations of gullies, e.g., Milliken et al., 2003; shape and have aspect ratios of <1to∼3, comparable to al- Balme et al., 2006; Dickson et al., 2007a) forms the basis for this coves described in previous surveys (e.g., Malin and Edgett, 2000; analysis. After selecting images based on latitude, images spanning Dickson et al., 2007a). These large alcoves uniformly lack poly- orbits 001330–003824 were analyzed sequentially by orbit number gons, particularly on steep slopes where mantle material has been (∼530 images). Next, a subset of images from orbits 003825– eroded, exposing bedrock (Fig. 5). 007207 were selected on the basis of geographical location within Alcoves with polygonally patterned surfaces are commonly ◦ the 30–80 latitude bands in order to increase the density of ana- elongate and have a rectangular shape (Fig. 6), comparable to lyzedimagesinlocationssparselysampledinearlyorbits.Asense the “lengthened alcoves” of Malin and Edgett (2000). Elongate al- of the magnitude of the dataset, and the degree of morphologi- coves mapped in this study average ∼820minlength(n = 29, cal detail present in each image is achieved by considering that, minimum = 250 m, maximum = 2200 m) and have high length- atypical∼1 GB HiRISE image contains approximately 200 times to-width aspect ratios (mean = 6, minimum = 4, maximum = 12). more information than a typical 5 MB Mars Orbiter Camera image, Elongate alcoves form within a surficial mantling unit (Fig. 6), resulting from increased spatial resolution within a comparable and do not generally expose underlying bedrock or crater-fractured surface footprint. Of the 722 images studied, 168 contain gullies, material and boulders (e.g., Fig. 5). One or more channels are com- and 93 contain gullies present on clearly polygonally patterned monly present within these elongate alcoves (Fig. 6), and emanate surfaces (Figs. 3 and 4; Supplementary data). Features present in out from the alcoves. HiRISE images were classified as gullies if they were composed of Thermal contraction crack polygons commonly form in the ice- at least two of the three gully structural elements defined by Malin rich sediments of the martian latitude-dependent mantle (Mustard and Edgett (2000), namely: a recessed alcove, a sinuous channel, et al., 2001; Milliken et al., 2003; Mangold, 2005)orpasted- and a distal fan or apron. on terrain within or adjacent to gully alcoves (Costard et al., Geographically, gullies are predominantly observed in HiRISE 2002; Christensen, 2003;seeFig. 4: most HiRISE images in man- ◦ images in the southern hemisphere (125 southern hemisphere oc- tled terrains polewards of 40 feature polygons). Pasted-on ter- currences compared to 43 northern hemisphere occurrences), as rain is a relatively smooth-surfaced unit typical of martian mid- are gully–polygon systems (71 southern hemisphere occurrences latitudes, that is commonly superposed on pole-facing surfaces compared to 22 northern hemisphere occurrences). We partially and is thought to have formed by atmospheric deposition of correct for targeting bias by dividing the number of occurrences ice and/or dust (Malin and Edgett, 2001; Mustard et al., 2001; of gullies and gully–polygon systems by the number of survey Christensen, 2003). Boulders present on some pasted-on terrain 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 indicates pixel DN values in processed HiRISE images that are pasted-on terrain (e.g., McEwen et al., 2007); however, the wast- several times higher than proximal pixels sampled from polygon ing of fractured crater-rim materials located upslope from pasted- centers or gully channels. These deposits may be water-ice de- on terrain may also account for the presence of boulders atop posited seasonally as frost (for images taken during winter periods; pasted-on surfaces. Polygons present in pasted-on terrain and on Mangold, 2005), salt deposits (Burt and Knauth, 2007), dusty lag mantle surfaces are commonly flat-topped, with elevated interi- deposits (Williams et al., 2008), or some other form of high-albedo, ors and depressed troughs. This morphology is consistent with particulate deposit, such as snow, that accumulates preferentially sand-wedge polygon or sublimation-polygon structures that form in shielded topographic lows (Head et al., 2008). In some im- preferentially in fine-grained and ice-rich substrates (Lachenbruch, ages, bright material is distributed broadly over surfaces containing 1962; Washburn, 1973; Maloof et al., 2002; Marchant et al., 2002; gully–polygon systems (Figs. 7a–7b); in others bright material is Marchant and Head, 2007). Analysis of 136 alcove polygons on present in polygon troughs within gully alcoves (e.g., Fig. 7c). Dust Mars, in 8 HiRISE images, indicates a mean martian alcove polygon cover is not pronounced in the analyzed images (e.g., dust ripples diameter of ∼11 m, spanning ∼5–21 m, with a standard deviation are uncommon and boulders are clearly visible), and no salts have of 3.4 m. been spectroscopically detected in the examined HiRISE images Some martian alcove polygons are outlined by bright deposits (e.g., Osterloo et al., 2008). Rather, these deposits are seasonally that are present preferentially in polygon troughs (Fig. 7). “Bright” present, and are commonly blue-toned in HiRISE color data: ob- 118 J.S. Levy et al. / Icarus 201 (2009) 113–126

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 = 174.0 : southern winter. (b) Portion of PSP_002054_1325, located at 47 S, 177 E, on a crater wall. ◦ ◦ ◦ ◦ Ls = 160.7 : southern winter. (c) Portion of PSP_001882_1410, located at 39 S, 194 E, on a crater wall. Ls = 153.7 : southern winter. servations consistent with the seasonal deposition of water ice 4.2. Channels and polygons (Gulick et al., 2008). On the basis of these observations, we interpret bright, trough-ﬁlling material present in or around alcoves, Channel-like features were observed in analyzed HiRISE images that has been imaged during winter or early spring time periods, that are (1) continuous and sub-linear; (2) present in widened, to be atmospherically-emplaced frost or, possibly, particulate ice. curved, and down-slope-oriented polygon troughs; and (3) present 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) Portionof ◦ ◦ ◦ ◦ ◦ 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 = 153.7 : 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 Polygon troughs are visible through small fan surfaces. These individually and in braided groups on polygon-surfaced slopes, and polygon troughs are typically continuous with, and are exten- can be distinguished from typical polygon troughs by variations sions of, surrounding trough networks (Fig. 10). These observations in surface texture, relief, and continuity (Fig. 9). These channels suggest that polygons have remained active through fan deposi- are commonly ∼200–500 m in length. Deposits in these linear tion, and have winnowed fan sediments into underlying polygon features are light-toned in HiRISE red-filter images (using DN- wedges. comparison of unstretched images), and are distinguished from In contrast to small-scale fans, several large fans were observed blue-toned bright deposits present in polygon troughs in alcoves in HiRISE images that show different morphological and strati- by a difference in texture (trough-channel deposits are slightly rip- graphic relationships with polygons (Fig. 10c). Large fans have pled with possible small boulders present), a lower albedo, and surface areas spanning 2.8 × 105–1.1 × 106 m2 (one to two or- different color. Given their morphological similarity to terrestrial ders of magnitude larger than small fans described above). Large gully channels that have formed through the annexation of pre- fans generally have significant topographic relief, and rise convexly existing polygon troughs (Levy et al., 2008b), we interpret these up from inter-fan slopes. Typically, these large fans lack modi- martian features to be remnants of polygon troughs annexed by fication by surface polygons, though some are characterized by incipient gully channels. We interpret in-trough deposits to be flu- an array of fine-scale fractures that superficially resemble small vially deposited sediments, similar to fan-forming sediments, de- polygon troughs. These fine fractures are not continuous with posited during periods of channelized gully flow. troughsfromadjacentpolygonnetworks(Fig. 10c). These observations suggest that large fans have buried surrounding polygon 4.3. Fans and polygons networks. Light-toned material can be seen displaced from the fan in Several gully fans observed in HiRISE images embay polygo- some HiRISE images, suggesting partial redistribution of the fine nally patterned ground (Fig. 10), consistent with observations of fraction of fan-forming sediments by subsequent aeolian processes fans overprinting polygons in MOC image analyses (e.g., Malin (Figs. 10a and 10b). The continued presence of bright fan ma- and Edgett, 2000; Heldmann et al., 2007). Some fans terminate terial in polygon centers, rather than preferential redistribution abruptly at relatively deep polygon troughs. Other fans appear to of fan material into polygon troughs, suggests that much of the wrap around elevated polygon centers and elevated outcrops of polygon-interior fan material has been preserved in place, despite patterned ground (Fig. 10a). These relationships suggest that poly- winnowing of fan sediments into polygon troughs and aeolian ero- gon topography provided a barrier to fan emplacement. sion of fines. Small gully fans on polygonally patterned slopes range in sur- Lastly, the majority of polygons present on gully fans, and face area from 1 × 104–2 × 105 m2. These fans have little topo- on surfaces topographically lower than the fans, do not display graphic relief and appear to be thin, surficial deposits (Fig. 10). morphologies characteristic of seasonal saturation of sediments 120 J.S. Levy et al. / Icarus 201 (2009) 113–126

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, sug- ◦ ◦ ◦ gesting 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 = 246.8 : southern ◦ ◦ ◦ spring. Illumination from left. (b) Portion of PSP_3511_1115, located at 69 S, 1 E. Ls = 226.7 : southern spring. Illumination from above. (c) Portion of PSP_002165_1270, ◦ ◦ ◦ located at 53 S, 28 E. Ls = 165.3 : southern winter. Illumination from left.

◦ by gully flow (e.g., Lyons et al., 2005; Levy et al., 2008b), such southern hemisphere. Gullies between 30–44 S predominantly ◦ as concentration of boulders at the surface (heaving), sorting of face polewards and gullies between 45–60 S generally face equa- sediments through cryoturbation (which might be detectable as torwards (Heldmann and Mellon, 2004). This observation was ver- changes in surface brightness or texture), or the formation of ice- ified in the Newton Crater region by Berman et al. (2005). Dickson wedge polygons with upturned shoulders. These observations sug- et al. (2007a) further confirm these observations, finding that ◦ gest that water involved in the transport and deposition of fan ∼86% of gullies in the 30–45 S latitude band occur on pole-facing sediments rapidly froze-on to the ice-cement table within the fan slopes, and noting that the few gullies mapped on equator-facing ◦ and/or sublimated until local equilibrium conditions were met for slopes are confined to above ∼40 S. One interpretation of the water stability. orientation data is that gullies form on protected slopes where In summary, we interpret these overprinting and cross-cutting snow/ice, if available, would tend to accumulate (Hecht, 2002; relationships to indicate the following formational sequence. Small- Dickson et al., 2007a; Head and Marchant, 2008; Head et al., 2008), scale gully fans formed by deposition of sediments over previously and where protected ice reservoirs could be rapidly exposed to existing polygonally-patterned ground (consistent with MOC obser- peak insolation, leading to melting. Hecht (2002) demonstrated vations, e.g., Malin and Edgett, 2000; Heldmann et al., 2007), and that peak insolation sufficient to cause melting can be achieved crack expansion continued throughout and after fan deposition, on either pole- or equator-facing slopes on Mars, depending on dissecting gully fans from beneath. This implies the continuous latitude and slope inclination. presence of ice-cemented permafrost beneath gully fans during In some HiRISE images gullies on polygonally-patterned sur- their development and aggradation of permafrost concurrent with faces can be observed on both pole-facing and equator-facing the growth of gully fans. Larger fans formed from the emplace- slopes (Fig. 11). “Pole-facing” and “equator-facing” are qualitative ment of sediment at a rate that resulted in the burial of previously measurements of orientation, indicating that gully–polygon sys- extant polygon networks, resulting in polygon development limited ◦ tems were present on slopes oriented within ∼30 of north or to fine-scale networks that are discontinuous with the surrounding polygonal network. south. These occur most commonly on interior crater walls. Only HiRISE images in which gully–polygon systems are present on ∼ ◦ 4.4. Slope orientation near-diametrically opposite slopes (> 150 angular separation) were included in orientation analyses. Although preliminary reports conflicted on the presence or ab- The morphology of gullies and polygons on Mars differs with sence of orientation preferences for gullies at the hemisphere scale slope orientation (Fig. 11). Gullies on pole-facing slopes generally (e.g., Malin and Edgett, 2000; Edgett et al., 2003), binning of gully have sharply-defined channels and fans, and polygons on pole- orientations by latitude by Heldmann and Mellon (2004) discov- facing slopes are crisply delineated. On equator-facing slopes (im- ered a latitude-dependence for the orientation of gullies in the aged at the same resolution) gullies and polygons have subdued 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 generally lack sharp trough boundaries, and commonly grade from slopes tend to be narrower and are bounded by steeper walls than networks of raised mounds surrounded by low troughs, to irreg- those on equator-facing slopes. In addition, the surface texture of ular, linear albedo patterns interpreted to be incomplete and de- fans on equator-facing slopes is commonly indistinguishable from graded crack networks. We interpret the softening of gully–polygon that of inter-fan surfaces. Lastly, polygons on equator-facing slopes system morphologies on equator-facing slopes to indicate removal 122 J.S. Levy et al. / Icarus 201 (2009) 113–126

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 subse- Earth may be important for understanding the hydrological and quent to gully–polygon system formation. microclimatological processes involved in gully formation on Mars. On the basis of our observations in the Antarctic Dry Val- 5. Discussion leys and on Mars, we propose the following model for the initiation and evolution of martian gully–polygon systems described Analysis of gully–polygon systems on Earth suggests the follow- in this survey (Fig. 12). On Mars, a climate-related, latitude- ing stratigraphic and temporal relationships within gully–polygon dependent, ice-rich mantling unit composed of atmospheric dust, ◦ systems: (1) polygons pre-date alcove formation; (2) polygon ice, and ice-cemented regolith is deposited regionally above 30 troughs have been annexed by some gully channels, indicating latitude (Mustard et al., 2001; Hecht, 2002; Christensen, 2003; overland flow and channel development on polygonally patterned Head et al., 2003; Milliken et al., 2003). This mantling unit is surfaces; (3) many fans formed on a polygonally-patterned sur- preferentially preserved in sheltered environments on steep slopes face; and (4) polygon development has continued during fan and at low elevations (<3 km above the datum; Hecht, 2002; aggradation. Identical stratigraphic and temporal relationships are Dickson et al., 2007a), and likely has an ice-free sublimation lag observed between gullies and polygons on Mars. These results at its surface (Mustard et al., 2001; Hecht, 2002; Head et al., 2003; from HiRISE are consistent with stratigraphic interpretations made Williams et al., 2008). This mantling unit may be analogous to the using MOC image data (e.g., Malin and Edgett, 2000; Costard unconsolidated debris layer overlying ice-cemented colluvium in et al., 2002; Christensen, 2003; Heldmann and Mellon, 2004; the ADV. However, the mantle differs in that its primary mode of Berman et al., 2005; Balme et al., 2006; Dickson et al., 2007a; emplacement is atmospheric deposition, rather than typical collu- Head et al., 2008), but provide an unprecedented view of the de- vial transport. tailed morphological relationships between martian gullies and Next, thermal cycling generates polygons (Mellon, 1997) that, polygons. Over half of the gullies imaged in this survey interact owing to relatively dry climatic conditions, may be analogous with underlying polygonally patterned ground, showing evidence to sand-wedge and/or composite-wedge polygons (Mellon, 1997; of polygon-influenced water ice accumulation, polygonal pattern- Mangold et al., 2004; Mangold, 2005; Levy et al., 2008b; Marchant ing of elongate alcoves, annexation of polygon troughs by gully and Head, 2007). Polygons disturb mantling sediments at trough channels, and dissection of fans by underlying polygons. These re- locations. Although calculations of thermal wave propagation on ciprocal changes in gully and polygon morphology suggest a linked Mars have demonstrated the potential for wet active layers dur- developmental history for martian gullies and polygons, analogous ing geologically recent time (Kreslavsky et al., 2007), the lack of to that observed in terrestrial gully–polygon systems. Accordingly, extensively water-related polygon structures in these units (such we interpret these spatially-linked landforms observed in HiRISE as markedly raised polygon shoulders), coupled with a lack of images to be gully–polygon systems. Thus, understanding the ef- solifluction features, suggests that near-surface warming did not fects of polygonally patterned permafrost on gully development on result in significant melting of buried ice-cemented permafrost. 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 wallofacrater.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. 124 J.S. Levy et al. / Icarus 201 (2009) 113–126

Subsequently, atmospherically-derived water is introduced to fractures present on some large fan surfaces that are discontinu- the gully–polygon system. Topographically-depressed polygon ous with the polygon network surrounding the fan). Other large troughs are shaded environments that could cold-trap atmospheric fans lack any polygonal patterning, suggesting that fan emplace- water frost (Hecht, 2002) and/or act as topographic obstacles, ment may have been rapid enough to prevent the formation of concentrating wind-blown particulate ice (Head et al., 2008)— syngenetic polygons (MacKay, 1990). expanding the extent and thickness of seasonal frost accumulations As climate conditions became colder and drier (Forget et al., imaged in this survey. Under appropriate obliquity- and slope- 2007), fluvial and erosive processes would decrease and eventually dependent peak insolation conditions (Hecht, 2002; Kuzmin, 2005; cease in the gullies. In the absence of gully flow and infiltration re- Levy et al., 2007a; Morgan et al., 2008) this ice, concentrated in freshing buried ice-cemented permafrost, enhanced sublimation on polygon troughs, could melt to produce short-lived, ephemeral, liq- equator-facing slopes would desiccate shallow permafrost, reduc- uid water. Peak insolation conditions occur when the solar angle is ing sediment cohesion, and ultimately resulting in subdued gully normal to the surface slope. Gully–polygon systems commonly oc- and polygon textures in response to aeolian erosion. In degraded ◦ cur on steep, ∼30 slopes (Dickson et al., 2007a), making them gully–polygon systems (e.g., Fig. 11) polygons are lost from view strong candidates for recent surface melting of atmospherically before gullies (owing to the larger size of gullies) suggesting that emplaced volatiles (Hecht, 2002). Crater-retention age dating of re- polygons may have interacted with gullies even more commonly cent gully deposits similar to those included in this study indicates than is currently observed. gully activity within the past 1–2 Ma, although flows may have oc- In summary, this model provides a mechanism for the devel- curred as recently as 300 ka (Riess et al., 2004; Schon et al., 2009). opment of martian gullies that occur in association with polygo- As in the Antarctic Dry Valleys, martian polygon troughs appear nally patterned ground. Morphological similarities between mar- to concentrate and direct the transport of metastable surface- and tian gully–polygon systems and the closest morphological and cli- near-surface, meltwater, creating annexed polygon troughs (Levy matological analog on Earth (gully–polygon systems in the ADV), et al., 2008b). Short-lived water transport in annexed troughs suggest that the martian examples may have formed and devel- (Figs. 8 and 9) could easily transport unconsolidated polygon oped on slopes underlain by ice-cemented permafrost. In both trough and wedge sediments, contributing to sediment deposition cases, a top–down source for gully-carving water is implied, as in small terminal fans. Over time, annexed trough channels would geomorphological evidence suggests limited melting of the under- converge towards trunk channels (Fig. 9b, compare left and right), lying ice-cemented substrate. forming braided annexed channels. Braided channels would produce larger fans than those formed from isolated annexed troughs 6. Conclusions by collecting sediment from several separate channels, and by increasing the volume of water available to move sediment. These Observations of gullies and polygons from Antarctic field work fan deposits would still be relatively thin, and would be easily cut and analysis of HiRISE image data suggests the following strati- or dissected by the continued growth of underlying polygon cracks graphic and temporal relationships between gullies and polygons: (Fig. 10b). (1) polygons pre-date alcove excavation in some gullies; (2) poly- Continued erosion within annexed troughs would provide an gon troughs form traps for ice and windblown snow that can be- increasingly large sheltered environment for accumulation and come sources of meltwater for gullies; (3) polygon troughs have subsequent melting of ice and windblown snow (Figs. 6 and 9b). been annexed and eroded by some channels, indicating that chan- Eventually, braided channel walls would be widened, creating an nel formation occurred on a polygonally patterned surface; (4) fan elongated alcove with one or more incised channels (Fig. 6). Ma- embayment and dissection relationships indicate that some fans terial eroded from elevated alcoves would be deposited in a distal formed on polygonally patterned surfaces; and (5) polygon devel- fan, which could remain thin enough to permit continuing dissec- opment continued during fan emplacement. Using morphologically tion by thermal contraction crack expansion. Elongate alcoves are similar gully–polygon systems in the ADV as a guide, the strati- analogous to gully-related concavities or nivation hollows in the graphic relationships between gullies and polygons observed in ADV (Figs. 1 (inset) and 2b). The presence of polygons in elon- the HiRISE images suggest that the martian gullies analyzed in gate alcoves, but not in widened alcoves (below) suggests that this study developed on slopes underlain by polygonally-patterned, some ice-rich latitude-dependent mantle material remains intact ice-cemented permafrost. Interactions between martian gullies and in these alcoves. polygons are analogous to those documented in ADV gully–polygon This process would continue for as long as climate conditions systems. remained capable of generating liquid water or brines that re- No evidence was seen for significant melting of underlying ice- mained metastable long enough to flow (e.g., Mellon and Phillips, cemented permafrost on Earth or Mars. Additionally, no evidence 2001; Hecht, 2002; Costard et al., 2002; Christensen, 2003; of subsurface groundwater release (e.g., Malin and Edgett, 2000; Kreslavsky and Head, 2007; Burt et al., 2008). The longevity of Heldmann and Mellon, 2004; Heldmann et al., 2007) from beneath briny fluids on Mars is strongly dependent on solute depres- the ice table was observed at HiRISE resolution. No paired aqua- sion of the freezing temperature (potentially supporting liquid cludes or intensive substrate layering abutting gully channels or al- ◦ flow at temperatures as low as −20 to −50 C, depending on coves was observed in gully–polygon system sites, which included salt chemistry and concentration; Burt and Knauth, 2007) and crater rims, crater walls, and isolated central peaks, nor was scour reduction in evaporation rate to support persistent fluvial activ- associated with high-pressure water release observed. Rather, the ◦ ity (potentially as slow as 0.04 mm/hat−25 C; Ingersoll, 1970; locations of gullies on Mars are strongly associated with the pres- Sears and Chittenden, 2005). Eventually, alcove mantle material ence of a mantling unit that is commonly polygonally-patterned. would be fully eroded, exposing original scarp/crater-wall surfaces These lines of evidence suggest an atmospherically emplaced, top– (Fig. 5). Exposed crater and mantle material in these large, steep down source for fluids involved in martian gully evolution on alcoves might fail in response to gravitational sliding, as well as in polygonally-patterned surfaces, comparable to hydrological pro- response to surficial fluvial erosion, producing large fans beneath cesses observed in the Antarctic Dry Valleys. On Earth and Mars, leveed channels (Morgan et al., 2007b). Extensive fan deposition the presence of polygons is not shown to be directly causal of mar- may bury polygons, cutting off underlying thermal contraction tian gully formation, but to be diagnostic of top–down gully water cracks from the fan surface exposed to seasonal thermal cycling, sources, and to amplify the key processes of gully formation: accu- and leading to the generation of a network of new polygons (fine mulation of water ice and the channelized transport of melt water. Geologically recent gully–polygon systems on Mars and Earth 125

These observations provide new challenges to the modeling Edgett, K.S., Malin, M.C., Williams, R.M.E., Davis, S.D., 2003. Polar- and middle- community to incorporate detailed treatment of landscape mi- latitude martian gullies: A view from MGS MOC after two Mars years in the crorelief and substrate composition into water cycling models for mapping orbit. Lunar Planet. Sci. 34. Abstract 1038. Forget, F., Montmessin, F., Levrard, B., Haberle, R.M., Head, J.W., Madeleine, J.-B., the martian surface. Conditions permitting localized accumulation 2007. Glaciers, polar caps, and ice mantling: The effect of obliquity on martian and peak-insolation melting of surface ice are broadly consis- climate. In: Seventh International Conference on Mars, Pasadena, CA. Abstract tent with peak climate conditions modeled to have prevailed at 3028. gully–polygon sites during the last ∼1–10 My (Forget et al., 2007; Fortier, D., Allard, M., Shur, Y., 2007. Observation of rapid drainage system development by thermal erosion of ice wedges on Byot Island, Canadian Arctic Schon et al., 2009). Additional analysis of HiRISE images, coupled Archipelago. Permafrost Periglac. Process. 18, 229–243. with ongoing modeling of late Amazonian climate conditions, will Gooseff, M.N., McKnight, D.M., Lyons, W.B., Blum, A.E., 2002. Weathering re- enhance our understanding of gully–polygon system morphology actions and hyporheic exchange controls on stream water chemistry in a as an indicator of past climate processes on Mars. glacial meltwater stream in the McMurdo Dry Valleys. Water Resour. Res. 38, doi:10.1029/2001WR000834. Gulick, V.C., the HiRISE Science Team, 2008. A closer look at valley, channel, and Acknowledgments gully formation on Mars with HiRISE. Lunar Planet. Sci. 39. Abstract 2411. Head, J.W., Marchant, D.R., 2008. Formation of gullies on Mars: Link to recent cli- This work was made possible with support of JSL by the mate history implicates surface water flow origin. In: Workshop on Martian Rhode Island Space Grant Consortium, by NSF Grant ANT-0338291 Gullies: Theories and Tests, Houston, TX, February 4–5. Abstract 8009. Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Recent to D.R.M. and J.W.H., NASA MDAP Grants NNG04GJ99G and ice ages on Mars. Nature 426, 797–802. NNG05GQ46G to J.W.H., NASA MFRP Grant NNX06AE32G to D.R.M. Head, J.W., Marchant, D.R., Dickson, J.L., Levy, J.S., Morgan, G.A., 2007. Mars gully and J.W.H., and NASA Applied Information Systems Research Grant analogs in the Antarctic Dry Valleys: Geological setting and processes. Lunar NNG05GA61G to J.W.H. Thanks are extended to Caleb Fassett and Planet. Sci. 38. Abstract 1617. Head, J.W., Marchant, D.R., Kreslavksy, M.A., 2008. Formation of gullies on Mars: James Dickson for HiRISE image processing and to James Dick- Link to recent climate history and insolation microenvironments implicate sur- son, Douglas Kowalewski, Gareth Morgan, David Shean, and Kate face water flow origin. Proc. Natl. Acad. Sci. USA, submitted for publication. Swanger for field support. Also, thanks to the helicopter pilots, Hecht, M.H., 2002. Metastability of water on Mars. Icarus 156, 373–386. technicians, and ground crew of PHI, Inc., as well as to the staff of Heldmann, J.L., Mellon, M.T., 2004. Observations of martian gullies and constraints Raytheon Polar Services Company, and the personnel of McMurdo on potential formation mechanisms. Icarus 168, 285–404. Heldmann, J.L., Carlsson, E., Johansson, H., Mellon, M.T., Toon, O.B., 2007. Obser- Station. vations of martian gullies and constraints on potential formation mechanisms. II. The northern hemisphere. Icarus 188, 324–344. Supplementary data Ingersoll, A.P., 1970. Mars: Occurrence of liquid water. Science 168, 972–973. Kowalewski, D.E., Marchant, D.R., Levy, J.S., Head, J.W., 2006. Quantifying summertime sublimation rates for buried glacier ice in Beacon Valley, Antarctica. Supplementary data for this article may be found on ScienceDi- Antarct. Sci. 18, 421–428. rect, in the online version. Kreslavsky, M.A., 2008. Slope steepness of channels and aprons: Implications for Please visit DOI: 10.1016/j.jcarus.2008.12.043. origin of martian gullies. In: Workshop on Martian Gullies: Theories and Tests, Houston, TX, February 4–5. Abstract 8034. Kreslavsky, M.A., Head, J.W., 2002. Mars: Nature and evolution of young, References latitude-dependent water-ice-rich mantle. Geophys. Res. Lett. 29, doi:10.1029/ 2002GL015392. Balme,M.,Mangold,N.,Baratoux,D.,Costard,F.,Gosselin,M.,Masson,P.,Pinet, Kreslavsky, M.A., Head, J.W., 2007. Slope streaks on Mars: An assessment of “wet” P., Neukum, G., 2006. Orientation and distribution of recent gullies in the scenarios and the role of concentrated brines. In: Seventh International Confer- southern hemisphere of Mars: Observations from High Resolution Stereo Cam- ence on Mars, Pasadena, CA, July 9–13. Abstract 3203. era/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor Kreslavsky, M.A., Head, J.W., Marchant, D.R., 2007. Periods of active permafrost (MOC/MGS) data. J. Geophys. Res. 111, doi:10.1029/2005JE002607. layer formation during the geological history of Mars: Implications for circum- Berg, T.E., Black, R.F., 1966. Preliminary measurements of growth of non-sorted poly- polar and mid-latitude surface processes. Planet. Space Sci. 56, doi:10.1016/ gons, Victoria Land, Antarctic. In: Tedrow, J.C.F. (Ed.), Antarctic Soils and Soil- j.pss.2006.02.010. Forming Processes. In: Antarctic Research Series, vol. 8. American Geophysical Kuzmin, R.O., 2005. Ground ice in the martian regolith. In: Tokano, T. (Ed.), Water Union, Washington, DC. 177 pp. on Mars and Life. Springer-Verlag, Berlin, pp. 155–189. Berman, D.C., Hartmann, W.K., Crown, D.A., Baker, V.R., 2005. The role of arcuate Lachenbruch, A.H., 1962. Mechanics of thermal contraction cracks and ice-wedge ridges and gullies in the degradation of craters in the Newton Basin region of polygons in permafrost. In: Geological Society of America Special Papers, vol. Mars. Icarus 178, 465–486. 70, pp. 1–69. Bockheim, J.G., Campbell, I.B., McLeod, M., 2007. Permafrost distribution and active- Laskar, J., Correia, A.C.M., Gastineau, M., Lervard, B., Robutel, P., 2004. Long term layer depths in the McMurdo Dry Valleys, Antarctica. Permafrost Periglac. Pro- evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170, cess. 18, 217–227. 343–364. Bridges, N.T., Lackner, C.N., 2006. Northern hemisphere martian gullies and man- Levy, J.S., Head, J.W., Marchant, D.R., Morgan, G.A., Dickson, J.L., 2007a. Gully– tled terrain: Implications for near-surface water migration in Mars’ recent past. polygon interactions and stratigraphy on Earth and Mars: Sand-wedge polygons J. Geophys. Res. 111, doi:10.1029/2006/JE002702. as part of cold-desert, near-surface fluvial systems. In: Seventh International Burt, D.M., Knauth, L.P., 2007. Impacts, salts, and ice on Mars: How brine flows in Conference on Mars, Pasadena, CA, July 9–13. Abstract 3059. young gullies and elsewhere could be related to impact cratering. Lunar Planet. Levy, J.S., Head, J.W., Marchant, D.R., Morgan, G.A., Dickson, J.L., 2007b. Gully surface Sci. 38. Abstract 2054. and shallow subsurface structure in the south fork of Wright Valley, Antarctic Burt, D.M., Knauth, L.P., Wohletz, K.H., 2008. Martian gullies and salty sidewalks. In: DryValleys:ImplicationsforgullyactivityonMars.LunarPlanet.Sci.38.Ab- Workshop on Martian Gullies, League City, TX, February 4–5. Abstract 1301. stract 1728. Christensen, P.R., 2003. Formation of recent martian gullies through melting of ex- Levy, J.S., Head, J.W., Marchant, D.R., Kowalewski, D.E., 2008a. Identification of tensive water-rich snow deposits. Nature 422, 45–48. sublimation-type thermal contraction crack polygons at the proposed NASA Costard, F., Forget, F., Mangold, N., Peulvast, J.-P., 2002. Formation of recent mar- Phoenix landing site: Implications for substrate properties and climate-driven tian debris flows by melting of near-surface ground ice at high obliquity. Sci- morphological evolution. Geophys. Res. Lett. 35, doi:10.1029/2007GL032813. ence 295, 110–113. Levy, J.S., Head, J.W., Marchant, D.R., 2008b. The role of thermal contraction Dickson, J.L., Head, J.W., 2008. Global synthesis of Mars gully observations: Evidence crack polygons in cold desert fluvial systems. Antarct. Sci. 20, doi:10.1017/ for top–down formation from morphology, distribution, topography, and analogs S0954102008001375. from the Antarctic Dry Valleys. In: Workshop on Martian Gullies: Theories and Lewis, A.R., Marchant, D.R., Ashworth, A.C., Hemming, S.R., Machlus, M.L., 2007. Ma- Tests, Houston, TX, February 4–5. Abstract 8010. jor middle Miocene global climate change: Evidence from East Antarctica and Dickson, J.L., Head, J.W., Kreslavsky, M.A., 2007a. Martian gullies in the southern the Transantarctic Mountains. Geol. Soc. Am. Bull. 119, 1449–1461. midlatitudes of Mars: Evidence for climate-controlled formation of young fluvial Lyons, W.B., Welch, K.A., Carey, A.E., Wall, D.H., Virginia, R.A., Fountain, A.G., Do- features based upon local and global topography. Icarus 188, 315–323. ran, P.T., Csathó, B.M., Tremper, C.M., 2005. Groundwater seeps in Taylor Valley Dickson, J.L., Head, J.W., Marchant, D.R., Morgan, G.A., Levy, J.S., 2007b. Recent gully Antarctica: An example of a subsurface melt event. Ann. Glaciol. 40, 200–206. activity on Mars: Clues from late-stage water flow in gully systems and channels Mackay, J.R., 1990. Some observations on the growth and deformation of epigenetic, in the Antarctic Dry Valleys. Lunar Planet. Sci. 38. Abstract 1678. syngenetic and antisyngenetic ice wedges. Permafrost Periglac. Proces. 1, 15–29. 126 J.S. Levy et al. / Icarus 201 (2009) 113–126

Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface Morgan, G.A., Head, J.W., Marchant, D.R., Dickson, J.L., Levy, J.S., 2007a. Gully forma- runoff on Mars. Science 288, 2330–2335. tion on Mars: Testing the snowpack hypothesis from analysis of analogs in the Malin, M.C., Edgett, K.S., 2001. Mars Global Surveyor Mars Orbiter Camera: Inter- Antarctic Dry Valleys. Lunar Planet. Sci. 38. Abstract 1656. planetary cruise through primary mission. J. Geophys. Res. 106, 23429–423540. Morgan, G.A., Head, J.W., Marchant, D.R., Dickson, J.L., Levy, J.S., 2007b. Gully forma- Malin, M.C., Edgett, K.S., Posiolova, L.V., McColley, S.M., Dobrea, E.Z.N., 2006. Present- tion on Mars: Testing the snowpack hypothesis from analysis of analogs in the day impact cratering rate and contemporary gully activity on Mars. Science 314, Antarctic Dry Valleys. In: Seventh International Conference on Mars, Pasadena, 1573–1577. CA. Abstract 1353. Maloof, A.C., Kellogg, J.B., Anders, A.M., 2002. Neoproterozoic sand wedges: Crack Morgan, G.A., Head, J.W., Marchant, D.R., Dickson, J.L., Levy, J.S., 2008. Gully for- formation in frozen soils under diurnal forcing during a snowball Earth. Earth mation and evolution in the Antarctic Dry Valleys: Implications for Mars. In: Planet. Sci. Lett. 204, 1–15. Workshop on Martian Gullies, Houston, TX. Abstract 1301. Mangold, N., 2005. High latitude patterned grounds on Mars: Classification, distri- Mustard, J.F., Cooper, D.C., Rifkin, M.K., 2001. Evidence for recent climate change on bution and climatic control. Icarus 174, 336–359. Mars from the identification of youthful near-surface ground ice. Nature 412, Mangold, N., Maurice, S., Feldman, W.C., Costard, F., Forget, F., 2004. Spatial rela- 411–414. tionships between patterned ground and ground ice detected by the neutron Neumann, G.A., Abshire, J.B., Aharonson, O., Garvin, J.B., Sun, X., Zuber, M.T., 2003. spectrometer on Mars. J. Geophys. Res. 109, doi:10.1029/2004JE002235. Mars orbiter laser altimeter pulse width measurements and footprint-scale roughness. Geophys. Res. Lett. 30, doi:10.1029/2003GL017048. Marchant, D.R., Denton, G.H., 1996. Miocene and Pliocene paleoclimate on the Dry Osterloo, M.M., Hamilton, V.E., Bandfield, J.L., Glotch, T.D., Baldridge, A.M., Chris- Valleys region, southern Victoria Land: A geomorphological approach. Mar. Mi- tensen, P.R., Tornabene, L.L., Anderson, F.S., 2008. Chloride-bearing materials in cropaleontol. 27, 253–271. the southern highlands of Mars. Science 319, 1651–1654. Marchant, D.R., Head, J.W., 2007. Antarctic Dry Valleys: Microclimate zonation, vari- Pelletier, J.D., Kolb, K.J., McEwen, A.S., Kirk, R.L., 2008. Recent bright gully deposits able geomorphic processes, and implications for assessing climate change on on Mars: Wet or dry flow? Geology 36, 211–214. Mars. Icarus 192, 187–222. Ragotzkie, R.A., Likens, G.E., 1964. The heat balance of two Antarctic lakes. Limnol. Marchant, D.R., Lewis, A.R., Phillips, W.M., Moore, E.J., Souchez, R.A., Denton, G.H., Oceanograph. 9, 412–425. Sugden, D.E., Potter Jr., N., Landis, G.P., 2002. Formation of patterned ground Riess, D., van Gasselt, S., Neukum, G., Jaumann, R., 2004. Absolute dune ages and and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria implications for the time of formation of gullies in Nirgal Valles, Mars. J. Geo- Land, Antarctica. Geol. Soc. Am. Bull. 114, 718–730. phys. Res. 109, doi:10.1029/2004JE002251. McEwen, A.S., and 32 colleagues, 2007. A closer look at water-related geologic ac- Sears, D.W.G., Chittenden, J.D., 2005. On laboratory simulation and the temperature tivity on Mars. Science 317, 1706–1709. dependence of the evaporation rate of brine on Mars. Geophys. Res. Lett. 32, McKnight, D.M., Niyogi, D.K., Alger, A.S., Bomblies, A., Conovitz, P.A., Tate, C.M., 1999. doi:10.1029/2005GL024154. Dry valley streams in Antarctica: Ecosystems waiting for water. Bioscience 49, Schon, S.C., Head, J.W., Fassett, C.I., 2009. Unique chronostratigraphic maker in de- 985–995. positional fan stratigraphy on Mars: Evidence for ∼1.25 Ma old gully activity Mellon, M.T., 1997. Small-scale polygonal features on Mars: Seasonal thermal con- and surficial meltwater origin. Geology 37, 207–210. traction cracks in permafrost. J. Geophys. Res. 102, 25617–625628. Treiman, A.H., 2003. The geologic setting for martian gullies: Implications for their Mellon, M.T., Phillips, R.J., 2001. Recent gullies on Mars and the source of liquid origins. J. Geophys. Res. 108, doi:10.1029/2002JE001900. water. J. Geophys. Res. 106, 23165–23180. Washburn, A.L., 1973. Periglacial Processes and Environments. St. Martin’s Press, Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. Viscous flow features on the surface New York, pp. 90–99. of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images. Williams, K.E., Toon, O.B., Heldmann, J.L., McKay, C., Mellon, M.T., 2008. Stability of J. Geophys. Res. 108, doi:10.1029/2002JE002005. midlatitude snowpacks on Mars. Icarus 196, 565–577.