Evidence for an Hesperian-aged South Circum-Polar Lake Margin Environment on Mars

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Planetary and Space Science 54 (2006) 251–272 www.elsevier.com/locate/pss

Evidence for an Hesperian-aged South Circum-Polar Lake Margin Environment on Mars James Dickson, James W. Head Department of Geological Sciences, Brown University, Providence, RI 02912, USA Received 6 January 2005; received in revised form 12 December 2005; accepted 14 December 2005

Abstract A broad pitted plain and an elongated low rise occur near the south pole of Mars between a region of major cavi (Cavi Angusti) and a regionally smooth and broad valley (Argentea Planum). Viking, Mars Global Surveyor (MGS), and Odyssey data reveal a densely pitted plain covering
6750 km2, and containing 4300 irregularly shaped, steep-walled and flat-floored depressions with a mean diameter of
3.5 km. At the southernmost (poleward) extent of this plain are 12 north/south trending linear valleys that are characterized by theatershaped heads abutting a major cavi within Cavi Angusti. The pitted plain, which abuts Cavi Angusti to the southwest, is separated from the floor of Argentea Planum by a smooth, elongated low rise that extends parallel to the plain for
200 km. These unusual features are all found within the Hesperian-aged circumpolar Dorsa Argentea Formation, which has been interpreted by some workers to be an icerich glacier-related deposit. We interpret the pitted plain to represent the maximum northern extent of the Angusti lobe ice deposit. The pits are analogous in morphology and distribution to terrestrial kettle holes, which form from the melting of isolated ice-blocks surrounded and partly buried by sediment, to leave hollows. The linear valleys are consistent with sapping valleys formed from the release of an elevated groundwater table, fed by meltwater lakes. On the basis of these characteristics, relationships and analogs, we interpret the marginal facies to represent an ice-sheet/lake contact environment that existed during Hesperian time. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mars; Dorsa Argentea Formation; Glacial Lake; Kettle Hole; Polar processes

1. Introduction Analysis of MOLA data (Head and Pratt, 2001a; Tanaka and Kolb, 2001) in conjunction with mapping from Viking Orbiter images (Tanaka and Scott, 1987) has revealed a complicated geologic and hydrologic evolution of the south polar region of Mars. One prominent aspect is the emplacement in the Hesperian of the circumpolar Dorsa Argentea Formation (DAF) (Hd in Fig. 1), an assemblage of smooth plains and distinct surface features that underlies much of the present-day Amazonian polar layered terrain (Tanaka and Kolb, 2001). The new data have continued the debate as to the origin of the deposits, and two primary hypotheses have been offered: massive debris flows triggered by significant volatile release (Tanaka and Kolb, 2001) and the development and decay Corresponding author. Tel.: +1 401 8632526; fax: +1 401 8633978.

E-mail address: [email protected] (J.W. Head). 0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.12.010

of an expansive circumpolar ice-sheet in the Hesperian (Head and Pratt, 2001a). Head and Pratt (2001a) argued that sinuous ridges (Fig. 2) that are unique to the DAF are eskers formed by the subglacial drainage of meltwater (see Section 2.1), an hypothesis consistent with a circumpolar ice-sheet origin for the DAF (see also Head and Hallet, 2001a, b). A concern raised by Tanaka and Kolb (2001) was that the regional terrain would not be conducive to the preservation of these ridges, and outlet channels and floodplains would be expected. Head and Pratt (2001a) suggested, however, that Argentea Planum (‘Schmidt Valley’ in Head and Pratt, 2001a), the broad regional low found at the northern termini of the ridges, shows evidence for infilling of water, ponding, and subsequent drainage into the Argyre Basin. In addition, Tanaka and Kolb (2001) noted a lack of nearby glacial landforms that would be predicted by the ice-sheet model. They did not, however, address a complicated region of enigmatic features that separates

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Fig. 1. Geologic map of the South Pole of Mars, adapted from Tanaka and Scott (1987), with the major geologic units highlighted. Schmidt Crater is shown as a reference point for regional maps (Fig. 3). The map extends to 551S. Surrounding terrain is primarily unmodified Noachian cratered terrain.

Fig. 2. MOLA topographic gradient map showing the southern extent of Argentea Planum. The sinuous ridges (interpreted to be sites for subglacial drainage (Head and Pratt, 2001a), but also as linear sand dunes (Ruff, 1992), frozen waves of mud (Jons, 1992), and wrinkle ridges (Tanaka and Scott, 1987)) follow the regional downward slope of the valley from south to north (right to left in the image), terminating to the west of Du Toit Crater. Cavi Angusti represents a portion of the Hesperian/Noachian Undivided Material (HNu), as mapped by Tanaka and Scott (1987). Du Toit Crater is given as reference point for the adjacent map (Fig. 3), to the north, following the direction of the esker-like ridges. The 1050 m topographic contour is shown by a heavy black line to illustrate the proposed boundary of a lake that existed in Argentea Planum prior to drainage.

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Fig. 3. MOLA topographic gradient map of the northern extent of Argentea Planum. Argentea Planum represents the furthest extent of the smooth planar deposits of Hd in the direction of 901W. Du Toit Crater is a reference point for Fig. 2, and Schmidt Crater is a reference point for Fig. 1. The breach at the northernmost extent of the valley (labeled Northern Breach) has been traced as a channel northward to the floor of Argyre Basin. This suggests that water that filled the valley flowed into Argyre. The region separating Argentea Planum from Cavi Angusti is characterized by irregular pits and valleys that can be seen in further detail in Fig. 5. The 1050 m topographic contour is shown by a heavy black line to illustrate the proposed boundary of a lake that existed in Argentea Planum prior to drainage.

the elevated plain of cavi (termed Cavi Angusti) from the regional low represented by Argentea Planum at 751S, 701W (Fig. 3). A distinct plain of irregular depressions and parallel linear valleys combines with an elongated low rise to separate the cavi of Angusti from the floor of Argentea Planum. Each zone is of importance for evaluating the contrasting hypotheses for the emplacement of DAF, and in this paper we document and analyze these features in detail and explore formation mechanisms and propose a geologic history for the region as a whole that is consistent with the observations.

2. Major geologic units of the south pole of Mars Units from all three periods of Martian geologic history are found within 101 of the south rotational pole (Fig. 1). Beginning from low-latitudes and extending southward towards the pole, heavily cratered Noachian terrain is embayed by smoother plains of Hesperian age (Hd) (Tanaka and Scott, 1987) (termed the DAF) between 3301W and 1151W. Interrupting these smooth plains are units of Hesperian-Noachian undivided terrain (HNu) that are found both as isolated units within Hd, and also abutting Amazonian polar layered terrain (Apl), which underlies the present-day polar cap (Api). For the purposes of this discussion, the DAF and Hesperian-Noachian undivided terrain will be described as they relate directly to the marginal facies.

2.1. Dorsa Argentea Formation Tanaka and Scott (1987) mapped the DAF from Viking data as a circumpolar deposit of Hesperian age, interpreting it to be primarily of volcanic origin. They partitioned the DAF into upper (Hdu) and lower (Hdl) units, though Head and Pratt (2001a) found stratigraphic inconsistencies that led them to consider Hdu and Hdl as one major DAF unit (Hd, Fig. 1). Later, Tanaka and Kolb (2001) also revised the original mapping by subdividing DAF into eight separate units, six of which are broad planar units. In this analysis, DAF will be treated as one major unit, as in Fig. 1. A primary morphological feature within the DAF is sinuous braided ridges of
1 km average width (Fig. 2). Data obtained from Mars Global Surveyor (MGS) has allowed for more detailed analysis of these ridges, which have been interpreted as possible linear sand dunes (Ruff, 1992), frozen waves of mud (Jons, 1987, 1992), or wrinkle ridges (Tanaka and Scott, 1987). Some workers argued that the new data supported the interpretation that these ridges are the product of subglacial drainage (Head and Hallet, 2001a, b; Head and Pratt, 2001a), an hypothesis proposed soon after the features were mapped from Viking data (Howard, 1981). This interpretation has been cited to support the ice-sheet model (Head and Pratt, 2001a). If these ridges were formed from subglacial flow, their termini provide a minimum boundary for the furthest extent of the Hesperian-aged ice sheet before recession. In

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one of the distal portions of DAF, a broad valley termed Argentea Planum (Tanaka and Kolb, 2001), the ridges terminate at the southernmost extent of the valley (Fig. 3). Head and Pratt (2001a, b) proposed that they drained into the valley, where meltwater ponded before subsequent breaching and drainage through a channel toward the regional low represented by the Argyre impact basin. An alternate interpretation proposed from MGS data is that the ridges formed from preferential eolian erosion of deposits surrounding regions of fluvial sedimentation, revealing raised-channels (Tanaka and Kolb, 2001). 2.2. Cavi Angusti Distributed around the South Pole are distinct regions of large, steep-sided, irregular-shaped depressions. A primary zone of these depressions, Cavi Angusti, is located south of Schmidt Crater (Fig. 3), to the southwest of Argentea Planum. While they fall within the unit of the Hesperianaged DAF, the specific origin of these depressions is critical to understanding the history of the region. The irregular dimensions of the cavi towards the center of Cavi Angusti give way to a more sinuous trend as they approach the northern boundary, in the direction of the regionally low Argentea Planum. These cavi terminate at the pitted plain that separates Cavi Angusti from Argentea Planum (Fig. 3). Initial inspection from Mariner data centered on eolian deflation for the origin of the cavi (Murray et al., 1972; Sharp, 1973), but examination of Viking data led Howard (1981) to propose that basal melting of ground ice is a more likely candidate. Ghatan and Head (2003) expanded upon this hypothesis, using MGS data to argue that these depressions resulted primarily from subglacial volcanism which caused melting of overlying ice-rich deposits. This interpretation is consistent with the hypothesis that a large (360,000 km2) lobe of the proposed ice sheet (Head and Pratt, 2001a) protruded north to the southern rim of Schmidt Crater. This also implies that massive amounts of meltwater would have been released from the lobe, and the topography of the region (Fig. 3) suggests that neighboring Argentea Planum would have served as a temporary basin for the meltwater (Head and Pratt, 2001a, b) before subsequent drainage north towards Argyre. 2.3. Argentea Planum Argentea Planum represents one of the broad planar units within the DAF (Tanaka and Kolb, 2001). This broad valley embays Noachian cratered terrain in all directions, with the exception of its southern margin, which slopes gradually upwards and is characterized by the previously described esker-like ridges (Figs. 2 and 3). Impact crater rims found along the perimeter of the valley are consistently breached along the valley-facing side (Fig. 3). The arcuate northern margin of the valley is at
2000 m elevation. From north to south, the floor of the valley

slopes down gradually to meet the regionally flat slope of the valley floor (at
1000 m), and the width of the valley narrows from 400 to 80 km. Further south, the valley then widens again and slopes upwards as the esker-like ridges are approached. Using the termination of the esker-like ridges as a southern boundary that defines where ponding would have likely occurred, the elevation of the valley is
1000 m. Had this been the southern margin of a major Hesperian lake, this defines a flooded region of
270,000 km2 (Head and Pratt, 2001a). According to Head and Pratt (2001a), subsequent to ponding, the meltwater breached the northern boundary and carved a channel that has been traced from Argentea Planum to the floor of Argyre Basin (Channel 1 in Head and Pratt, 2001a). Emanating from a distinct notch along the northern margin (Fig. 3), Channel 1 begins at
1000 m elevation and extends northwards for
450 km. At the rim of Argyre basin, it descends
3500 m over 400 km to the floor of the basin (Head and Pratt, 2001a). We analyzed all available MOC images taken of the floor and margin of Argentea Planum and representative terrain samples are exhibited in Fig. 4. The regionally smooth but locally hummocky texture in Fig. 4a is common throughout the valley, although more pronounced in the southern half. This texture is consistent with previous examinations of MOLA data (Head and Pratt, 2001a). Fig. 4a shows evidence of buried craters and Fig. 4b shows small-scale polygonal terrain, both ubiquitous throughout the valley. If the hypothesis that Argentea Planum was once the site of an extensive lake (Head and Pratt, 2001a) is correct, and that Cavi Angusti underwent basal melting and released significant amounts of meltwater (Ghatan and Head, 2002), then the transition zone between the two regions becomes a critical area for understanding the evolution of the region as a whole. We thus use new topographic and image data from MGS to examine this region in detail, and to test the ice-sheet model for the emplacement of the DAF. 3. Description of the marginal facies The region that separates Argentea Planum from Cavi Angusti is characterized by two distinct parallel bands that will be referred to as the pitted plain (neighboring Cavi Angusti) and the elongated rise (neighboring Argentea Planum) (Fig. 5). The contact between these bands is a narrow trough that parallels the contact with Argentea Planum (Fig. 6). 3.1. Pitted plain Bridging Cavi Angusti to the elongated rise is a plain of irregular-shaped pits (Fig. 5). Preliminary analysis of these pits from Mariner 9 data suggested an origin of eolian erosion and scouring (Murray et al., 1972; Sharp, 1973), but a thorough treatment of these features has not been offered since. Data received from Viking and MGS provide

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Fig. 4. (a) The floor of Argentea Planum is typically flat and smooth throughout, but contains a fine-scale hummocky texture. Buried craters like the one seen in the southern portion of this image are ubiquitous throughout the valley (MOC image M07/02649). (b) Networks of smallscale polygonal troughs seen on the valley floor (MOC image M08/06278). (c) Rugged terrain showing a
500 m highly degraded impact crater to the northeast of Argentea Planum, near 68.41 S, 59.31 W (MOC image R02/ 01125). (d) Cratered terrain to the east of Argentea Planum, near 70.41 S, 58.71 W (MOC image R04/01436).

new insight into these pits, and allow for an updated assessment of their formation. A major difference in size distinguishes the smaller irregular pits of the pitted plain from the cavi of Angusti (Fig. 5). Using Viking data, 310 pits were mapped. MOLA data have also been used to assess depth from the surrounding terrain. We examined images of the pitted plain obtained by MOC, and every pit imaged appears fresh and all rims are intact and slightly raised (o10 m). Regional Viking and MOLA data suggest that these properties are consistent across the plain. No impact craters are found within or around any of the pits. The floors of larger pits are flat while those of smaller ones are rounded, but pit floors appear to be smooth, consistent with the surrounding terrain and the surface of the elongated rise. No evidence for active degradation of pit

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rims or associated mass movements has been found (Figs. 7 and 8). To a first order, the pits are highly irregular in shape. Dimensions were recorded based upon a primary axis (the greatest distance across the pit) and a secondary axis (average width across the pit perpendicular to the primary axis). This, along with MOLA altimetry data, allows for detailed calculations with regard to size, distribution, volume, and elongation of the pits. Maximum width across the pits ranges from
1 km to a maximum of
18 km, although only 6 of the 310 pits are greater than 10 km. The mean value for maximum width is 3.5 km with s ¼ 2:09. Fig. 9 is a size-frequency distribution for all pits with maximum width o8.0 km. Of all pits, 67% fall within 1.25–3.75 km in maximum width, which is only 14% of the entire size spectrum. The median value for maximum width is 2.95 km. Using the mean value of the secondary axes of the pits, an approximate rectangular dimension to an average pit is estimated to be 3.5 km  1.7 km. This aspect ratio (2.05) is supportive of the initial observation of an irregular nature to the pits, and the trend of elongation is not random. When isolating the pits north of the smooth central valley with an aspect ratio 41.5, a north/northwest-south/ southeast trend is observed (Fig. 10a). Of these pits, 64% fall between 401 and 901 (01 ¼ west, 901 ¼ north), whereas a random distribution would predict 28%. This trend is regionally parallel to the pitted plain/elongated rise contact. A similar, though less distinct, north/northeast– south/southwest trend is found when isolating the pits south of the smooth central valley (Fig. 10b). Of the pits with an aspect ratio 41.5, 48% fall between 801 and 1301 (01 ¼ west, 901 ¼ north), whereas a random distribution would predict 28%. This trend is consistent with the elongation of the adjacent linear valleys, which terminate just in front of the pits (see Section 3.3). Pits are consistent in depth between 50 and 200 m (Fig. 11). Floors of the pits follow the slope of the plain, descending in the direction of the floor of Argentea Planum. MOLA data show that the pits continue into the trough separating the pitted plain and the elongated rise and give way to the subdued ridges found on the proximal slope of the elongated rise (Section 3.4). 3.2. Terrain surrounding the pits The pitted plain slopes gradually at 0.081 from Cavi Angusti (
1625 m) to the trough that separates the plain from the elongated rise (
1125 m; Fig. 11). The width of the plain tapers from north to south, where it is bounded by the elongated rise to the east and the linear valleys to the west. The plain is
90 km wide at its maximum width, extends for
150 km, yielding an approximate surface area of 6750 km2. The northern margin of the plain is superposed on an ancient crater 25 km in diameter. The plain is consistently pitted throughout, with the exception of a distinct smooth central valley (Fig. 5). This

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Fig. 5. MOLA topographic gradient map (with sketch map) of the pits and troughs that comprise the plain separating Cavi Angusti from Argentea Planum. Schmidt Crater is labeled for context within the region (lower left), shown in Fig. 3. The labels A–B, C–D, and E–F correspond to MOLA topographic profiles in Fig. 11.

region is bounded to the north and south by two large crescent-shaped depressions (each
25 km in diameter) that outline what may have been an ancient major crater. The floor of each depression shares the same elevation with the adjacent cavi to the southwest (o200 m). The 10–15 km wide valley trends from northwest to southeast for 40 km before being interrupted by another crescent-shaped depression (8 km in diameter). The terrain immediately surrounding this valley is lightly pitted in relation to more distal regions. Contour maps of the region (Fig. 6) reveal a topographic trend from the eastern most portion of the smooth central valley to the central elevated zone on the elongated rise. We used MOC and THEMIS images to examine the pits and the texture of the surrounding terrain, in conjunction with MOLA data. The texture of the plain (Fig. 12) is similar in albedo and texture to pit floors. The hummocky texture found on the floor of Argentea Planum is not found

on the plain as shown by MOLA profiles (profile A–B in Fig. 11). 3.3. Parallel linear valleys In a distinct
50 km wide region to the north of Cavi Angusti and to the south of the pitted plain are found 12 well-defined parallel valleys that extend perpendicular to the contacts that separate the facies (Figs. 5 and 13). This zone is bounded to the north by small cavi, to the east by the southernmost extent of the pitted plain, to the south by a smooth
35 km wide depression that is topographically connected to the pitted plain, and to the west by large cavi. The valleys trend downslope from Cavi Angusti to the pitted plain, although noticeable depressions are seen upslope in larger valleys, where the elevation is consistent with the elevation of the pitted plain (Figs. 13 and 14). The valleys commonly exhibit theater-shaped heads, a few of

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Fig. 6. MOLA topographic profiles across the elongated rise to the east of the pitted plain (Context provided in Fig. 5). Each profile extends from the floor of Argentea Planum (to the left) to the pitted plain (to the right). Boundaries of the elongated rise are shown on the right. Each profile is 75 km in length.

which are connected to the large cavi to the west by small notches. The larger valleys all slope gradually to the neighboring pitted plain, while a few of the shorter valleys terminate in mirroring theater-shaped heads. While being consistently parallel to each other, the valleys show a wide range in dimension. Maximum widths range from 1 to 5 km, length ranges from 10 to 40 km, and depth ranges from
75 to
300 m. The valleys are consistently deeper at their heads, a function of the upslope trend of the surrounding terrain towards Cavi Angusti. Topographic data from MOLA and albedo data from Viking Orbiter images show that the surrounding terrain is more consistent with that of the elevated terrain of Angusti than that of the distal pitted plain. None of the valleys show prominent tributaries, and the widths of the valleys vary along strike, narrowing and widening noticeably. Profiles from MOLA data (Fig. 14) reveal steep-sided walls and flat floors to the valleys, consistent with MOC images (e.g. Fig. 15; M13/01875) that pass multiple valleys.

3.4. Elongated rise Separating the floor of Argentea Planum from the pitted plain is an elongated smooth low rise that extends over 200 km from the eastern rim of Schmidt Crater to the southernmost extent of the pitted plain (Fig. 5). At its northernmost extent, the elongated rise of the contact with the floor of Argentea Planum is arcuate and relatively steep in relation to its contact with the pitted plain (profile H in Fig. 6). A 2 km wide, 12 km long channel is incised on the distal portion of the elongated rise here, fading topographically towards a regional low on the floor of Argentea Planum (Figs. 5, 6). Elevation is maintained across the flat summit of the rise to the south, although the rise descends laterally to the pitted plain (profile F in Fig. 6). The contact with the floor of Argentea Planum is conversely steep and irregular. The broad flat crest of the elongated rise widens further to the south until it adjoins the pitted plain (profile D in

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Fig. 8. Transition region from the elongated rise (to the south) to the floor of Argentea Planum (to the north). The area shown is on the eastern margin of the central elevated zone (Subframe of MOC image M11/ 01568).

Fig. 7. Thermal image of the contact between the pitted plain and the elongated rise. Location is given by context box in MOLA gradient map of the pitted plain. (Subframe of THEMIS IR image I-07480003).

Fig. 6). This marks where the elongated rise is at its widest extent (
35 km) and greatest elevation (
1060 m). An elevated zone is found atop the elongated rise here in a

north/south trend, parallel to the trend of the rise as a whole. This zone, approximately defined by the 1050 m contour in Fig. 6, is symmetric and fan-shaped, and its terminus on the Argentea Planum side is parallel to the adjacent contact between the elongated rise and the floor of Argentea Planum, which itself remains relatively steep and irregular. South of the elevated zone, the broad flat crest of the elongated rise narrows once more, where it is adjacent to a 12 km wide, 100 m deep depression separating the rise from the pitted plain (profile C in Fig. 6). The slope from the elongated rise to the floor of the depression is steeper than the gradual slope to the pitted plain further north (profile F in Fig. 6). The rise finally widens further south, where it merges with the smooth terrain south of the pitted plain. The broad crest of the rise is consistently flat-topped across all profiles and is elongated across the central elevated zone at profile D in Fig. 6. MOC and MOLA data show that the flat crest is consistently smooth and is in contrast to the hummocky floor of Argentea Planum.

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Fig. 9. Size-frequency distribution of the pits on the pitted plain (using the primary axis). Six pits (out of 310) greater than 8 km in greatest elongation were omitted from this graph to focus on the area of highest concentration.

the slope that separates the crest from the pitted plain. One impact crater (
5 km in diameter) is found upon the rise, to the west of the central elevated zone. The contact with the floor of Argentea Planum is comparatively steep and highly irregular. Only one MOC image of the rise is available (Fig. 8), but it reveals distinct mounds on the Argentea Planum side of the contact. These are observed in MOLA data (Fig. 6) and are most pronounced adjacent to the elevated zone. South of the elevated zone (between profiles B and C in Fig. 6) is a prominent v-shaped incursion that significantly narrows the rise. 4. Interpretation 4.1. Pitted plain

Fig. 10. Orientations of pits: (a) Rose diagram showing the elongation trend of pits in the north half of the pitted plain. A clear trend is apparent to the west of north, with 64% of pits (with aspect ratios greater than 1.5) falling between 401 and 901, a 28% section of the spectrum (01 ¼ W; 901 ¼ N). Context box refers to Fig. 12. (b) Rose diagram showing the elongation trend of pits in the south half of the pitted plain. A northward trend is observed, with 48% of pits (with aspect rations greater than 1.5) falling between 801 and 1301, a 28% portion of the spectrum (01 ¼ W; 901 ¼ N).

Subtle ridges are observed on the slope from the northern peak of the rise to the pitted plain (profile F in Fig. 6). This irregular topography corresponds to thermal variations observed in THEMIS data. THEMIS IR image I-07480003 (Fig. 7) shows circular features that are colder than the surrounding terrain. These features are sparsely distributed along the flat crest of the rise but densely concentrated on

Initial observations of the irregular pits of the pitted plain from Mariner 9 data suggested that they may be analogous to terrestrial deflation pits (Murray et al., 1972; Sharp, 1973). Global near-surface atmospheric circulation models produced from Viking data (Kahn, 1983, 1984), however, suggest an east–west circulation, perpendicular to the subtle north/south trend of the pits of the pitted plain. While these models apply to the present-day atmosphere, the topography in this area has not changed and a similar wind regime would be expected in the Hesperian. This, combined with the irregular shapes of the pits and their termination at the margin of the elongated rise, suggests that eolian processes have served only to modify the pits. Other workers (Head and Pratt, 2001a) have suggested a glacial origin to the pits, and we further examine these models. In glacial/polar environments on Earth, three primary processes are known to yield isolated depressions: (1) preferential calving of blanketed ice exposed through surface crevasses of a debris-layer (supraglacial lakes); (2) melting of an underlying ice-layer beneath deposited sediment (collapse pits); and (3) sediment surrounding isolated ice-blocks that subsequently melt/sublime (kettle holes). We now test the glacial hypothesis by investigating these processes and determining the most likely candidates for the formation of the martian pits.

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Fig. 11. MOLA topographic profiles across the elongated rise and the pitted plain. Pit walls are consistently symmetric in slope in all profiles. Pit floors generally follow the same topographic trend in relation to each other as the surrounding terrain, with the exception of some of the smaller pits (particularly in profile A–B). The dichotomy in slopes on either side of the elongated rise is apparent, particularly in profiles A–B and C–D. The contact between the elongated rise and the floor of Argentea Planum is a relatively abrupt scarp, whereas the rise trends gradually downwards to the trough that separates it from the pitted plain. Vertical exaggeration for all profiles is 38  .

4.1.1. Supraglacial lakes Found upon debris-covered glaciers in varying sizes, and also referred to as perched lakes (Benn et al., 2001), supraglacial lakes are most abundant on Earth in highaltitude equatorial regions that undergo significant seasonal climatic variations. Those found upon the debriscovered glaciers of the Himalayas are the most well studied (Sakai et al., 2000; Reynolds, 2000; Ageta et al., 2000; Benn et al., 2000, 2001), as they are of high concern for potential glacial lake outburst floods. They are initiated by crevasses among the debris blanket that expose portions of the underlying ice, which yield calving from the increased atmospheric heat and direct solar radiation (Benn and Evans, 1998). These are preferentially found in regions of highly irregular terrain (Benn et al., 2001), where local debris slides result in the development of the triggering crevasses (Fig. 16a). Larger supraglacial lakes are likely to form from coalescence of smaller ponds on regionally level

glaciers (o21 regional slope (Reynolds, 2000)), as there is little to inhibit the backwasting of an ice-wall once it has begun. There is also a clear distinction in slope between the side of the lake that is backwasting (a steep ice cliff) and the side that slopes gradually to the surrounding terrain (Fig. 16b). From these properties of terrestrial supraglacial lakes, it would be expected for those on Mars to hold similar morphologic properties had they been formed by a similar mechanism: (1) hummocky terrain surrounding the pits that would yield crevasses among the surface, (2) clear coalescence of smaller pits, and (3) noticeable contrasts in slope between opposing sides of the depression. Data, however, show that these properties are not found within or around the Martian pits. (1) The texture of the plain in which the pits are found is smooth and regular in all data sets acquired (Fig. 12), and formation of crevasses of the size necessary to form the observed

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Fig. 13. MOLA topographic gradient map of the linear valleys at the southern extent of the pitted plain. The largest valley shows a distinct breach in its head that is connected to the large cavi behind it (arrows), and breaches are found at the heads of other valleys as well. A–A0 is the topographic profile in Fig. 14.

Fig. 12. Representative pit morphologies throughout the pitted plain. Pits are highly irregular in shape and have intact walls. Coalescence is clear at the southernmost extent of the frame. The texture of pit floors and the surrounding plain is comparably smooth, and evidence for multiple concentric faulting is not observed (MOC image M07/00889). Context provided in Fig. 10a.

depressions would be unlikely on such terrain. (2) While several pits do appear to be the product of coalescence of smaller pits, the small distances separating the pits today would require an unusual mechanism to explain why they did not continue to coalesce. The pitted plain is locally flat and there would have been little resistance to backwasting of exposed ice cliffs. (3) There is also no evidence from either photometric or topographic data to show that there is a slope dichotomy between opposite walls (Figs. 8 and 11). Backwasting of exposed ice-walls upon debris-covered glaciers is well-documented on Earth and could eventually serve as a powerful analog for small depressions on Mars, but the key morphologic distinctions with the pitted plain would require too many additional mechanisms to make it a reasonable hypothesis for pit formation.

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Fig. 14. MOLA topographic profile through three of the primary linear valleys (arrows).

Fig. 15. MOC image across two primary valleys. Valley walls appear to be intact, and valley floors appear to be as smooth as the surrounding terrain. If groundwater sapping eroded these valleys, the valley heads would have migrated from the top of this picture to the bottom (Subframe of MOC image M13/01875).

4.1.2. Collapse pits Pits form from subsidence in glacial environments on Earth when melting erosion from a glacier blankets ice with sediment that collapses from the melting of the underlying ice. This occurs when the underlying ice is a consistent sheet (Sanford, 1959; McDonald and Shilts, 1975) and also when independent ice-blocks are buried (Branney and Gilbert, 1995; Maizels, 1992). While these are referred to as kettle holes, they are morphologically distinct from kettle holes that are the result of partial burial of independent iceblocks, as described in the following section. For clarity, pits formed from complete burial of ice will be referred to hereafter as collapse pits. Collapse pits most commonly occur in regions that exhibit both glaciation and volcanism (Branney and Gilbert, 1995), where magmatic activity can serve as a

catalyst for the melting of buried ice. Assuming that this region was volcanically active during the Hesperian (Ghatan and Head, 2002, 2003), this makes collapse pits an intriguing hypothesis for the pitted plain. Sanford (1959) conducted lab experiments to analyze the faulting that occurs when sections of a buried layer of ice melts to cause subsidence in the overlying sediment layer. It was observed that near-vertical shear fractures occurred towards the base of the ice layer, and these propagated upwards into the sediment blanket, where they curved to produce reverse faults at the surface (Fig. 17). These are then surrounded by concentric faults away from the primary reverse fault. This is consistent with large-scale collapse pits (e.g. Woburn, Quebec (McDonald and Shilts, 1975)), and is applicable to isolated ice-blocks buried by sediment, like

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Fig. 16. Supraglacial lakes and ponds. (a) Three supraglacial lakes upon Ngozumpa Glacier, Khumbu Himial, Nepal, that eventually coalesced. Several prominent features of supraglacial lakes are observed: steep ice-cliffs on one side of the lake, hummocky surrounding terrain, and coalescence among the lakes (frame is
300 m in width) (from Benn et al., 2001). (b) Small supraglacial pond on the Lirung Glacier in the Himalayas, Nepal. The contrast in slope between the receding ice-cliff and the gradual slope to the host glacier is apparent (from Sakai et al., 2000).

Fig. 17. Collapse pit schematic from Sanford (1959). Steep reverse faults in the ice-substrate curve to more gradual reverse faults at the surface, which are surrounded by concentric normal faults. Terrain between faults is likely to collapse and collect at the base of the pit.

those found in the 1991 lahar deposits of Volca´n Hudson in the Patagonian Cordillera of Chile (Branney and Gilbert, 1995). Collapse pits of each form are characterized by hummocky floors (Fig. 18), and the models developed by Sanford (1959), and documented by fieldwork (McDonald and Shilts, 1975) suggest that those on Mars should show evidence for concentric faults surrounding the pit. Pits

Fig. 18. Funnel-shaped collapse pit in Volca´n Hudson, Chile, as found by Branney and Gilbert (1995), formed from the melting of a buried ice-block (diameter E10 m). The hummocky nature of the pit floor is contrasted by the generally smooth texture of the surrounding plain. The initial reverse faults predicted by Sanford (1959) yielded collapse to the floor of the pit, leaving a steep normal fault that surrounds the depression. C: extensional crevasse; R: ring fractures; O: base; E: extensional fractures. From Branney and Gilbert (1995).

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lacking concentric faults should show evidence for collection of debris on the floor from massive collapse of pit walls that were bounded by the interior concentric faults. While proximity to volcanic edifices make this an attractive hypothesis, several morphologic properties suggest that these are not collapse features: (1) MOLA data show that few pits exhibit walls that could be interpreted as reverse faults; (2) no pits have been observed that show any evidence for concentric faulting; (3) MOC images show that pit floors are smooth (Fig. 12); and (4) martian pits are highly irregular in shape, whereas terrestrial collapse pits are generally circular. Despite the favorable proximity of the pits to Cavi Angusti, these morphologic factors strongly suggest that subsidence is insufficient to account for the formation of the pits. 4.1.3. Kettle holes Kettle holes are closely associated to collapse pits, in that they include the surrounding of an ice-block with sediment, as opposed to just the burial of ice. While the amount of sediment that delineates surrounding from burial can be slight, the subsequent morphologies of the final pits contrast dramatically. Classic evidence for pit formation from the surrounding of ice-blocks with sediment is found both in morphology

and distribution. Kettle holes generally exhibit irregular shapes, steep walls, flat floors, and smooth surrounding terrain (Benn and Evans, 1998), and are generally larger in size than collapse pits, as the steep walls shed sediment as the ice is removed (Flint, 1971). The parent ice-block provides the initial supply of water upon melting, which is recharged in temperate environments, yielding freshwater kettle lakes. Kettle holes are distributed within deposits of stratified drift and till across proglacial outwash plains, yielding a pitted sandar (Benn and Evans, 1998), where isolated iceblocks are either detached from the receding glacier due to differential ablation (e.g. Rich, 1943) or delivered to the plain by jo¨kulhlaup-type floods (e.g. Maizels, 1992; Embleton and King, 1975; Hobbs, 1931). The parent ice can exist at a variety of scales, from isolated ice-blocks to kilometers-wide belts of glacier ice calving from the glacier terminus, which is then broken up by meltwater from the host glacier (Flint, 1971). Various terrestrial pitted landscapes have been deposited in this manner, including those in New Hampshire and Maine in the USA (Lougee, 1940) and in Greenland (Hobbs, 1931). A classic terrestrial example of a pitted sandar is the Mashpee pitted plain of Cape Cod, Massachusetts (Fig. 19). Cape Cod, itself, was constructed by the Cape

Fig. 19. Mashpee Pitted Plain, Cape Cod, Massachusetts: (a) Location map with general geologic units of Cape Cod, at the easternmost end of Massachusetts (Strahler, 1966). Cape Cod Bay was once the site of the Cape Cod Lobe of the late Wisconsin Laurentide ice-sheet, which constructed Sandwich Moraine and deposited the sediment that created the outwash plains (Mashpee, Barnstable, and Harwich) (Strahler, 1966). (b) Topographic map of the Mashpee Pitted Plains, location of the highest concentration of kettle ponds on Cape Cod. The kettle ponds are highly irregular in shape and often interact with the linear valleys (pamets) that characterize the mid to distal portion of the plain. Data provided by MassGIS.

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Fig. 20. Topographic profile across two water-filled kettle holes (arrows) within the Mashpee Pitted Plain. Notice the symmetric slopes of the walls of the pits. The terrain between the kettles is uncharacteristically rugged for the pitted plain, which itself is very smooth (Fig. 19). Each kettle has a slightly raised rim, an indication that the parent ice-blocks did host some sediment within them (Maizels, 1992). Data provided by MassGIS.

Cod lobe of the late Wisconsinan Laurentide ice sheet
18,000 years ago, which formed the Sandwich moraine that today serves as the northern margin of western Cape Cod (Strahler, 1966; Oldale, 1982; Chamberlain, 1964). The smooth plain to the south was formed from outwash from the receding Cape Cod lobe, and is marked by irregular shaped depressions that range from
200 to
3600 m in greatest diameter, most filled with water. Smaller pits are generally circular, which is more indicative of water-filled collapse pits (Branney and Gilbert, 1995), as less sediment would be required to completely bury the iceblock. Larger pits are more irregular, and those in the Mashpee pitted plain show elongation trends radial to Cape Cod Bay. Profiles across the pits of Cape Cod show slightly raised rims (o1 m; Fig. 20). New high-resolution topographic data show few instances of concentric faults surrounding the kettles of Cape Cod, suggesting that very few of the ice-blocks were completely buried by sediment. Maizels (1992) conducted experiments to measure the sediment concentration within the parent ice-blocks. Sediment-poor ice-blocks yielded pits with flat rims, while increasing sediment concentration resulted in rimmed kettles that formed from deposition of sediment from within the ice-block upon melting. Upon melting, sediment-poor ice-blocks yield flatfloored pits with generally steep and symmetric walls (Maizels, 1992), which is consistent with the topographic data obtained of the rims of the kettles of Cape Cod (Fig. 20). Terrain surrounding kettle holes is flat from the consistent layer of sediment that was deposited to form the sandar. On the basis of the properties of terrestrial kettle holes, we find similarities to the properties of the pits on Mars. The irregular shapes of the Martian pits are consistent with the formation of kettle holes on Earth, as is the smooth texture of both the floors of the pits and the surrounding

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plain. Martian pits are on average larger than terrestrial kettles, suggesting that the parent ice-blocks were greater in volume, and that coalescence was a more common occurrence. MOLA data show that rims of the pits are slightly raised, which suggests that the original ice-blocks were ice-rich and sediment-poor (Maizels, 1992). As outlined above, evidence for collapse of sediment above buried ice-blocks is not found for the pits of Mars, which provides constraints on the amount of sediment that would have been deposited on the pitted plain: enough sediment would be required to surround the ice-blocks, but not enough to bury them. Head and Pratt (2001b) suggested that the hummocky nature of the floor of Argentea Planum is a remnant of freezing and sublimation of water that once filled the valley. The floors of the pits are uniformly smoother than the floor of the valley, suggesting that (a) the ice-blocks initially melted and subsequently evaporated or (b) the ice-blocks sublimed and pit floors were resurfaced. The method by which the ice-blocks were emplaced on the pitted plain can be attributed to one of two possibilities, and in both cases the neighboring elongated rise is of importance: (a) supraglacial flow from the Angusti Lobe deposited ice-blocks across the plain. This would be consistent with the earlier analysis and conclusion that the elongated rise was deposited subaqueously; and (b) the pitted plain was once covered by ice that underwent decay, producing isolated ice-blocks from bottom-up melting. In the second scenario, the pitted plain represents the furthest extent of the Angusti lobe of a circumpolar ice-sheet on the ground, and the elongated rise received sediment from Cavi Angusti, but there were no ice-blocks to surround to form the pits. This is consistent with our earlier analysis of the elongated rise, which suggests that the Angusti Lobe protruded to the contact between the pitted plain and the elongated rise. We thus favor the hypothesis of pit formation via melting of sediment-surrounded ice-blocks, which had been deposited by the decay of the stagnant Angusti Lobe. 4.2. Linear valleys Linear valleys form on Earth from a variety of processes, including volcanism, eolian activity, tectonism, and fluvial activity. On Mars, however, valleys that share the general morphologic properties and scale of the valleys of the pitted plain are most commonly attributed to groundwater sapping and fluvial erosion (Squyres, 1989; Schumm and Phillips, 1986; Pieri, 1976). For this reason we focus on groundwater sapping as an erosive agent, and as an analog we examine the linear valleys on the proglacial outwash plains of Cape Cod, Massachusetts. 4.2.1. Origin of the relict valleys of Cape Cod, Massachusetts The pitted sandar of Cape Cod described above are also characterized by parallel linear valleys (Fig. 19), referred to

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locally as pamets, after one particular valley within the Wellfleet plains. These pamets are most prominent on the distal portion of the Mashpee Pitted Plains on Cape Cod and within the Wellfleet Plain Deposits on eastern Cape Cod (Oldale and Barlow, 1986). In each region, the valleys trend radially from Cape Cod Bay, which was once the location of the Cape Cod Lobe of the Late Wisconsinan Laurentide Ice Sheet (Strahler, 1966). The pamets are best defined on the mid to distal ends of the outwash plains, and the valleys of the Mashpee Pitted Plain all open into Nantucket Sound to the south. Newly acquired digital terrain models of Cape Cod show that the topographic trends defined by the valleys can be traced northward through the outwash plains to kettle ponds and to breaches in both Sandwich and Buzzards Bay moraines (Fig. 19b). Unlike the valleys of Mars, the relict pamets of Cape Cod show distinct tributaries that first appear short (Uchupi and Oldale, 1994), but can also be traced to neighboring kettle holes and other pamets. The pamets of Cape Cod are smaller than those of Mars, ranging from tens to hundreds of meters in width and 5–20 km in length in the most well defined regions. The flat floors are comprised primarily of sand and gravel, while the walls are relatively steep (4–171) and symmetric. The radial trend of the pamets from Cape Cod Bay has made surface runoff an attractive hypothesis for the formation of the linear valleys (Strahler, 1966), and until recently it was the only viable hypothesis that had been proposed. Uchupi and Oldale (1994) argued, however, that certain properties of the pamets (poorly developed tributaries, generally linear courses, flat floors, concentration at the mid to distal ends of the outwash plains) suggested an alternate mechanism. They proposed that the morphologies were more consistent with formation via groundwater sapping, with the pamet heads migrating up the outwash plains. In its broadest definition, groundwater sapping is the emergence and release of groundwater that erodes soils and rocks, and the point of release can range in size from a distinct spring (‘‘spring sapping’’) to a distributed lateral release (Howard, 1988a). Laity (1988) outlined the major requirements necessary for groundwater sapping on Earth: A permeable aquifer, a rechargeable groundwater system, a free face at which subsurface water can emerge, some form of structural or lithologic inhomogeneity, and a means of transporting material released from the scarp face. Erosion of sapping valleys is often supplemented by runoff erosion, resulting in composite channels. This method has been used to account for the Canterbury Plain channels in New Zealand (Schumm and Phillips, 1986). This will yield valleys with several general morphologic properties, including theater-shaped heads, generally linear courses, and short and stubby tributaries (Fig. 21). Applying the model of composite channels proposed by Schumm and Phillips (1986), after formation of the Sandwich Moraine, the Cape Cod Lobe retreated and meltwater fed Glacial Lake Cape Cod, now represented by

Fig. 21. Photograph of a laboratory experiment conducted by Howard (1988b). As water was added behind the slightly cohesive coarse sand (top), the steep-sided valleys eroded headward from a scarp at the bottom of the image. Shallow rills accompany these valleys on the higher surface, which could be analogous to the breaches and paths found along the heads of some of the valleys.

Cape Cod Bay (‘‘proglacial lake’’ in Fig. 22). Sandwich Moraine served as a dam, but the high permeability of the outwash plains allowed for the groundwater level to elevate as Glacial Lake Cape Cod was recharged. As the groundwater level was elevated, valley heads migrated up the outwash plains to the furthest elevation of the water table. This model predicts that the heads of each Cape Cod pamet would be found at a generally consistent elevation, representing the greatest elevation of the water table. Newly acquired digital terrain models allow us to test this model. The new data suggest that additional mechanisms are necessary to account for these features. Uchupi and Oldale (1994) acknowledged that several of the pamets emanate from kettle holes within the pitted plain. The new data show that all but one pamet within the pitted plain, including associated tributaries, can be traced to other pamets or prominent kettle holes. These kettles show a dominant elongation trend in the direction of runoff, indicating that flow likely continued after the parent iceblocks had already melted. In turn, these kettles can be

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Fig. 22. Schematic cross-section of valley formation within the Mashpee Pitted Plain, as outlined by Uchupi and Oldale (1994). The glacier would have fed the proglacial lake, elevating the water table, which would seep out through groundwater seeps along the free surface.

Fig. 23. Topographic map of the Mashpee Pitted Plains of Cape Cod with superposed paths of water across the plain. Prominent breaches are found along both Sandwich Moraine (bordering to the north) and Buzzards Bay Moraine (bordering to the west). These breaches can be traced topographically (and through the use of 1/2 m resolution color orthophotos) to the prominent kettle ponds on the plain, which can in turn be traced topographically to the linear valleys which make up the mid to distal end of the outwash plain. Data provided by MassGIS.

traced to significant breaches in both Sandwich and Buzzards Bay moraines (Fig. 23), which had previously gone unrecognized in geologic maps of the region (Oldale

and Barlow, 1986). These breaches indicate that outwash from the Cape Cod Lobe was an erosive agent, but it is also insufficient in terms of accounting for all of the features of

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the valleys. Specifically, it fails to explain why the pamets are well defined at the mid to distal ends of the outwash plains, but poorly defined closer to Cape Cod Bay. Channels formed through a combination of groundwater sapping and surface runoff have been documented in other coastal environments on Earth (Schumm and Phillips, 1986), and this model offers the most complete explanation for the relationship between the pamets, the kettles, and the abutting moraines. Different configurations of ice, moraines, outwash plains, and sea level can produce different settings, as in the Finnish Salpausselka¨ (e.g. Glu¨ckert, 1975, 1995; Kujansuu et al., 1995; Rainio, 1995).

collapse of adjacent terrain, forming the parallel troughs with mirroring u-shaped heads. Regional topography indicates that material eroded to form the troughs would have been transported downslope to the pitted plain, or deposited directly into Argentea Planum. The present downslope of the plain in the direction of Argentea Planum is consistent with an origin by groundwater sapping, like the pamets of Cape Cod (Uchupi and Oldale, 1994) and the composite channels of Canterbury Plain, New Zealand (Schumm and Phillips, 1986). 4.3. Elongated rise

4.2.2. Application to Martian linear troughs Whereas the pamets of Cape Cod are interrupted by kettle holes within the Mashpee Outwash Plain (Fig. 19), the linear troughs of the pitted plain occur in front of a large cavi toward the pole (Fig. 13). Several of these troughs have small notches at the head that suggest that if this cavi was once a meltwater lake, it would have drained subaerially through these valleys. The lack of prominent tributaries and the existence of well defined u-shaped trough heads suggest that surface drainage from Cavi Angusti only served to modify the troughs, not carve them. Considering the proximity of the linear troughs (running downslope to the pitted plain) and their respective morphologies, groundwater sapping through distinct seeps, aided by surface drainage from Cavi Angusti, is the strongest analog that can account for these features. Sapping has been used to partially account for the formation of other valleys on Mars (Schumm and Phillips, 1986), most notably southern-highland valley networks (Pieri, 1976; Squyres, 1989). While not all of Laity’s (1988) conditions are immediately testable for the linear troughs of the marginal facies, they provide a guide by which the hypothesis may be evaluated. The general concept of groundwater transport in the south polar region has been used to suggest that slow migration of liquid water from high-latitudes to low-latitudes in the Hesperian could have occurred (Carr, 1996), and to account for the valley networks (Pieri, 1976; Squyres, 1989). A source for recharging the aquifer is meltwater from a decaying ice-sheet that would have initially filled the cavi (Ghatan and Head, 2002, 2003). The large number of positive features interpreted to be subglacial volcanoes implies that the cavi would have been filled, depleted, and recharged with meltwater. Terraced interiors, centrally located edifice-like features, and lava flow-like structures on some cavi floors strongly suggest that some of the cavi formed as a result of magmatic intrusion and extrusion, causing heating and melting of a volatile-rich substrate and drainage and loss of liquid water (Ghatan and Head, 2003). This process, accompanied by possible sub-polar cap basal melting in the region (Clifford, 1987, 1993), could have recharged the subsurface aquifer, further eroded the valleys, and caused the valley heads to migrate up the slope of the plain. Drainage of this aquifer also led to

Hypotheses for the formation of this rise have not been offered in previous accounts of the region. Among possible candidate origins are eolian or fluvial erosion or deposition, ice-related or circum-polar processes (e.g., Head and Pratt, 2001a) and massive debris flows triggered by significant volatile release (e.g., Tanaka and Kolb, 2001). Although eolian and fluvial processes cannot be ruled out in the modification of these features, few of the features of the elongated rise are suggestive of these as a primary origin. The hypothesis of the origin of the DAF by massive debris flows triggered by significant volatile release (e.g., Tanaka and Kolb, 2001) does not specifically account for these features. The proximity to the polar region, and the close association with the Argentea Planum floor and Cavi Angusti, suggest that ice-related and circum-polar processes were involved. We thus further explored analogs associated with proglacial sedimentation processes where ice-sheets/glaciers terminate on Earth. Interaction between glaciers/ice-sheets and proglacial lakes is common on Earth due to multiple factors, including the formation of regional lows from loading and marginal flexure and construction of moraines as boundaries for a lake. A broad partition distinguishes the two major types of environments that result from this type of ice-sheet/lake contact relationship: (1) non-glacier contact water bodies, where the lake is only being fed by meltwater from the ice-sheet; and (2) Glacier-contact water bodies, where the ice-sheet extends to or beyond the margin of the lake (Benn and Evans, 1998). The scenario of a glacier-contact water body (2) is far more complex in terms of sediment deposition from beneath, within, and above the glacier/ice-sheet directly into the proglacial lake. Little is known concerning the tectonic interaction between a glacier/ice-sheet with a lake that is frozen (Benn and Evans, 1998) due to the difficulty in observing the processes below the ice surface, but the interaction at marine terminating ice-sheet margins is well documented at temperate regions on Earth (Ashley et al., 1991; Hunter and Smith, 2001; McCabe and O’Cofaigh, 1995; Seramur et al., 1997; Cai et al., 1997; Bennett et al., 2000; Powell et al., 1996; Huddart and Peacock, 1990; Hunter et al., 1996; Powell, 1983). Consistent among all contact environments of this type is a grounding line, or the

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zone across a glacial terminus where the protruding ice shelf begins to float, or where the vertical ice-cliff terminates. Various grounding-line processes (Hunter et al., 1996) contribute to the deposition of sediment in front of the terminus, including dumping of supraglacial debris from surface melting, deposition of bedload material from subglacial/englacial streams (generally in the form of subaqueous outwash fans (or grounding-line fans)), and settling of overflow plumes of sediment (Hunter et al., 1996). Given sufficient sediment volumes, these processes combine to form morainal banks, elongate sedimentary deposits that extend parallel to the glacier/ice-sheet terminus. Deposition of sediment across the morainal bank is first controlled by the nature of the contact itself: vertical ice-cliffs will yield a higher concentration on the ice-proximal side of the bank, whereas a floating ice-shelf will deposit more sediment on the mid to distal portions of the bank. Second, deposition is controlled by the relative input from each individual process. A combination of these processes that are common at grounding lines of marine-terminating glaciers on Earth account for the features described above for the elongated rise at the contact between Argentea Planum and the pitted plain. The symmetric and fan-shaped distribution of the central elevated zone and its proximity to where the rise adjoins the pitted plain indicate that this is an ice-contact delta, or an exposed grounding-line fan that may be associated with the smooth central linear valley within the pitted plain. Ice-contact deltas on Earth (e.g. Hunter and Smith, 2001) are characterized by having distal slopes steeper than proximal slopes due to erosion from the proglacial lake, which is consistent with the central elevated zone of the elongated rise. The distal side of the central elevated zone, which would have been susceptible to erosion from the water filling Argentea Planum, shows evidence for mass movements in MOC and MOLA data (Figs. 6 and 8), contributing to the steep slope. Given enough sediment deposition from this conduit, this may have also contributed to the arcuate northern margin of the v-shaped incursion. Assuming that the point where the elongated rise adjoins the pitted plain was the mouth of a significant ice-contact delta, we interpret this to be the location of a grounding line, or the furthest extent of the Angusti Lobe on the ground. This does not discount, however, the possibility that an ice-shelf extended above the lake, past the grounding line. Physical constraints prevent extensive study of grounding lines below floating ice-shelves on Earth, but Powell et al. (1996) successfully deployed a submersible below the tongue of Mackay Glacier, a floating terminus on the margin of the Ross Sea, Antarctica. Their study confirmed hypothetical models that suggested that basal debris contributes strongly to sediment deposition, and that regions most recently exposed exhibit the least sediment drape (Powell et al., 1996). Additionally, they noted that terrain beneath the ice-

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shelf is more irregular than the sediment drape in front of the tongue, due in part to calving of debris-rich ice-blocks that sink to the floor, then are surrounded/buried with sediment to yield subaqueous kettle holes. We feel that the irregular topography and thermal variations on the gradual slope from the flat crest of the elongated rise to the pitted plain (Fig. 7) could be evidence for subaqueous kettle hole formation. Supraglacial and englacial debris delivered through meltwater streams could account for the deposition of sediment in front of the ice-shelf, yielding the even distribution of material across the rise. This interpretation is consistent with the hypothesis that a major lobe of ice protruded on the ground to the extent of the proposed grounding-line, represented presently by a narrow trough (Fig. 11) at the eastern margin of the pitted plain. In this interpretation, a marginal ice-shelf extended above the level of water in Argentea Planum, approximately to the present 1000 m contour on the ice-proximal side of the rise (Fig. 6). Supraglacial dumping distributed sediment evenly across the entire rise, accounting for the evenness exhibited in MOLA data (Fig. 6). Sediment was also deposited below the ice-shelf from basal debris. Simultaneously, a massive grounding line fan was deposited at the center of the rise, which is now represented by an ice-contact delta. Sediment on the distal side of the rise was subsequently eroded by the lake that filled Argentea Planum. 5. Geologic history of the region The above analyses provide a framework for formulating a candidate geologic history of the marginal facies and the surrounding regions in order to test for consistency of the hypothesis. 1. Deposition of Elongated Rise: The circumpolar Hesperian ice-sheet proposed by Head and Pratt (2001a) included a significant lobe that protruded over the region now known as Cavi Angusti (Fig. 1), the eastern terminus of which occurred at the present-day contact between the pitted plain and the elongated rise (Fig. 5). In this hypothesis, this was a lobe that was in contact with a major body of water that filled Argentea Planum. Sediment was deposited in front of the grounding line through a variety of processes common among water body-terminating glaciers on Earth. We find that the characteristics of the elongated rise are consistent with this hypothesis and we interpret the even distribution of material across the rise as due to settling of sediment deposited from supraglacial runoff, and the fan-shaped central elevated zone (Fig. 5) as a major ice-contact delta deposited from subglacial sediment transport. 2. Continued Eruption of subglacial volcanoes beneath the Angusti Lobe: Volcanic activity beneath the center of the Angusti Lobe (Ghatan and Head, 2003) contributed to the decay of the overlying ice and constructed meltwater basins that are interpreted to be represented today by

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major cavi, facilitating the development of major meltwater streams. The cavi were sites for massive meltwater lakes that raised the local water table. 3. Retreat of eastern terminus/erosion of linear valleys: Significant decay of the eastern terminus deposited isolated sediment-poor ice-blocks across the region today represented by the pitted plain. The meltwater lakes within Angusti recharged the water table, which subsequently drained through distinct seeps to the west. As the meltwater lakes were recharged by decay of the Angusti Lobe (Ghatan and Head, 2003), the water table was elevated, yielding parallel linear valleys as the theater-shaped heads eroded upslope. Distinct notches in the heads of several of the valleys also indicate that the meltwater lakes also drained subaerially. 4. Deposition of proglacial sandur: Supraglacial and subglacial drainage from the decaying Angusti Lobe deposited water and sediment in large amounts across the pitted plain in the direction of Argentea Planum. A proglacial outwash plain was formed in front of the retreating Angusti Lobe, with sediment being deposited in layers around the isolated ice-blocks. 5. Melting of isolated ice-blocks: Volcanism beneath the Angusti Lobe decreased or ceased, slowing the water supply to the meltwater lakes. Groundwater level decreased, terminating the erosion of the linear valleys, and sediment deposition to the proglacial sandur abated. Kettle holes formed from the melting/sublimation of the isolated ice-blocks, and sediment from within the ice-blocks was deposited to form minor rims around each depression. Sediment within the blocks was also deposited on the floors of the depressions. This produced the major components of the pitted sandur that is observed today.

glacier/lake contact environments on Earth. The elongated low rise adjacent to the floor of Argentea Planum is interpreted to be a morainal bank, deposited both supraglacially and subglacially. The central elevated zone on this bank is topographically connected to the adjacent pitted plain, and is interpreted to be a massive ice-contact delta. The pitted plain is interpreted to be a proglacial pitted sandur, formed from the deposition of sediment around isolated ice-blocks that were deposited by ablation at the lobe’s terminus. The parallel valleys were formed by sapping of an elevated water table in front of a large meltwater lake. Further questions remain about specific properties of the features studied here, but we feel that this model can serve as a basis for future investigation and testing. Any account of the DAF and the evolution of the south polar region of Mars should include a formation mechanism for these facies and the unique features that they enclose. The global significance of this region must also be addressed. If water was ubiquitous across the surface of Mars at points in the Hesperian, where did the water come from? Head and Pratt (2001a) suggested that water filled Argentea Planum and drained northward to Argyre. If water filled Argentea Planum, where did that water come from? Our finding is that much of it came from the decay of the Angusti Lobe of a circum-polar ice-sheet, due primarily to the eruption of multiple subglacial volcanoes, as proposed by Ghatan and Head (2003) and possibly to significant poleward ice accumulation and basal melting (e.g., Clifford, 1987, 1993). This study serves to underline the importance of this region as it relates to the global hydrologic cycle of Mars. Acknowledgments

On the basis of this generally consistent set of geological relationships and historical sequence, we conclude that the new data support the general concept of the marginal facies representing an ancient ice-sheet/lake contact environment. 6. Discussion and conclusions Evidence has been presented that a major Hesperianaged circumpolar ice-sheet once existed at the south pole of Mars (Head and Pratt, 2001a) and is responsible for the emplacement of the DAF. On the basis of remapping of the south polar region with new data, however, Tanaka and Kolb (2001) concluded that no glacial features existed, and instead focused on debris flows triggered by volatile release to account for the emplacement of the DAF. As a test of the ice-sheet model, we have examined the facies that occur in the region separating Cavi Angusti from Argentea Planum. We found that the features of the marginal facies studied here are not readily explained by eolian, fluvial or debrisflow processes, but that they show many comparable features in terms of morphology and distribution to

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