Icarus 201 (2009) 113–126
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Icarus
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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
b
Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA
Department of Earth Science, Boston University, 675 Commonwealth Ave., Boston, MA 02215, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Received 29 May 2008
Revised 17 October 2008
Accepted 22 December 2008
Available online 21 January 2009
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.
© 2009 Elsevier Inc. All rights reserved.
Keywords:
Mars surface
Earth
Geological processes
Ices
Regoliths
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 (∼30–50◦ 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: +1 401 863 3978.
E-mail address: joseph_levy@brown.edu (J.S. Levy).
0019-1035/$ – see front matter
doi:10.1016/j.icarus.2008.12.043
©
2009 Elsevier Inc. All rights reserved.
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
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J.S. Levy et al. / Icarus 201 (2009) 113–126
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.
2.1. 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 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 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).
2.1.1. ADV gully water sources
Soil-temperature measurements indicate that melting along the
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
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 km of
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.
2.1.2. 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 ∼100–400 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 12–24 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 (∼10–20 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).
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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.
images in the latitude bands in which the features are present.
Gullies are present in 48% of survey images between 30–55◦ S,
and in 21% of survey images between 30–55◦ N; gully–polygon
systems are present in 26% of survey images between 30–55◦ S,
and in 9% of survey images between 35–55◦ 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 30◦ –80◦ 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.
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 MROHiRISE 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_001330 to PSP_007207,
and ranging between 30–80◦ 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 001330–003824 were analyzed sequentially by orbit number
(∼530 images). Next, a subset of images from orbits 003825–
007207 were selected on the basis of geographical location within
the 30–80◦ latitude bands in order to increase the density of analyzed images in locations sparsely sampled in early orbits. A sense
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
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 ∼40–50◦ 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
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-
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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
(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.
4.2. 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
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 = 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
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 ∼200–500 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.
4.3. 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 1 × 104 –2 × 105 m2 . 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 2.8 × 105 –1.1 × 106 m2 (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
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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, 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 = 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
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.
4.4. 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 30–44◦ S predominantly
face polewards and gullies between 45–60◦ 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 30–45◦ 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
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
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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 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.
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.
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J.S. Levy et al. / Icarus 201 (2009) 113–126
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.
Geologically recent gully–polygon systems on Mars and Earth
These observations provide new challenges to the modeling
community to incorporate detailed treatment of landscape microrelief and substrate composition into water cycling models for
the martian surface. Conditions permitting localized accumulation
and peak-insolation melting of surface ice are broadly consistent with peak climate conditions modeled to have prevailed at
gully–polygon sites during the last ∼1–10 My (Forget et al., 2007;
Schon et al., 2009). Additional analysis of HiRISE images, coupled
with ongoing modeling of late Amazonian climate conditions, will
enhance our understanding of gully–polygon system morphology
as an indicator of past climate processes on Mars.
Acknowledgments
This work was made possible with support of JSL by the
Rhode Island Space Grant Consortium, by NSF Grant ANT-0338291
to D.R.M. and J.W.H., NASA MDAP Grants NNG04GJ99G and
NNG05GQ46G to J.W.H., NASA MFRP Grant NNX06AE32G to D.R.M.
and J.W.H., and NASA Applied Information Systems Research Grant
NNG05GA61G to J.W.H. Thanks are extended to Caleb Fassett and
James Dickson for HiRISE image processing and to James Dickson, Douglas Kowalewski, Gareth Morgan, David Shean, and Kate
Swanger for field support. Also, thanks to the helicopter pilots,
technicians, and ground crew of PHI, Inc., as well as to the staff of
Raytheon Polar Services Company, and the personnel of McMurdo
Station.
Supplementary data
Supplementary data for this article may be found on ScienceDirect, in the online version.
Please visit DOI: 10.1016/j.jcarus.2008.12.043.
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