A number of isolated mountains mapped in the south polar region of Mars are documented using Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) data. These mountains have average separation distances of ∼175 km, they are typically 30–40 km in diameter and 1000–1500 m high, and their bases fall near an elevation of ∼1200 m. A significant number of the population are located on or very close to a 660 km long line extending toward the south pole. The summits of a number of these mountains have unusual shapes: flat-topped, flat-topped with a cone, and large summit craters relative to the summit diameter. Sinuous channels are found in association with the margins of several of the mountains. Several modes of origin are considered for these mountains, including impact, tectonic, and volcanic. These mountains occur in intimate association with the Hesperian-aged Dorsa Argentea Formation, a unit containing features and structures interpreted to be related to a thick, areally extensive ice sheet. On the basis of these observations and analyses it is concluded that the most plausible origin for these mountains is volcanic and that they represent extrusion of lava from vents, many of which lay underneath an ice sheet. The unusual shape of many of the mountains is consistent with construction of a volcanic edifice underneath an ice sheet and melting of adjacent ice-rich material, sometimes forming drainage channels. The topography of these mountains suggests that the ice sheet averaged at least 1.4 km thick at the time of eruption. These features appear to be associated with Early Hesperian-aged ridged plains (Hr).
 In their mapping of the south polar region of Mars, Tanaka and Scott  documented 17 features located between 20° and 340°W (Figure 1a) as (m), described as mountains of a high and rugged nature, primarily found within the Dorsa Argentea Formation (Hd). These mountains were first observed in Mariner 9 images [Murray et al., 1972] and were mapped by several authors [Peterson, 1977; Condit and Soderblom, 1978; Scott and Carr, 1978]. Due to the resolution of Mariner images and lack of high-resolution topography, a definite age and origin for the mountains were not able to be determined. Several possible origins were proposed, such as remnants of crater rims, central peaks, and volcanoes [Murray et al., 1972; Peterson, 1977; Condit and Soderblom, 1978; Scott and Carr, 1978]. Even with the high-resolution images from the Viking mission, a more detailed assessment of the age and origin of the mountains was not possible [Tanaka and Scott, 1987]. Also mapped by Tanaka and Scott  in the south polar region were several features labeled as (v), confidently interpreted as volcanoes, and (v?), features that appeared to be volcanoes but whose origin was less certain. The ages of these volcanoes were also unknown. A single occurrence of (v?) was mapped in the region containing the mountains (Figure 1a).
 Using new Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) data from the Mars Global Surveyor Mission, we have been able to reexamine these features at much higher resolution than previously possible. We have performed morphometric and morphologic analyses of the mountains and the feature interpreted to be a volcano in the region that reveal in great detail their shapes, dimensions, and slopes [Ghatan and Head, 2001; Head and Ghatan, 2001]. We have further investigated the relationship of the structures to the surrounding plains, along with their association with nearby features, and examined their detailed characteristics where MOC images were available.
 We examine a range of hypotheses for the origin of the mountains (e.g., impact cratering, tectonism, volcanism), addressing those origins which were previously proposed by other authors. Detailed mapping of the region strongly suggests that the middle Hesperian Dorsa Argentea Formation (Hd) mapped by Tanaka and Scott  is a volatile-rich unit and represents the formerly large extent of a south polar ice sheet [Head and Pratt, 2001]. Since the age and origin of the mountains are unknown, there may be a relationship between them and this deposit. We find that a number of the mountains are excellent candidates for subglacial volcanoes, having most likely erupted into the Hesperian-aged ice sheet. Evidence exists for (1) a sequence of morphologies related to interaction with the overlying ice-rich layer and (2) drainage of some meltwater into adjacent low-lying areas.
2. Description and Characterization
2.1. The Mountains
 The 17 mountains mapped by Tanaka and Scott  occur within an area 930 km × 250 km, centered at 65°S and 355°W (Figure 1a). Using MOLA data, it is possible to construct a view of the topography of the region in which the mountains are located (Figure 1b). From this view, four additional features that meet the criteria of Tanaka and Scott  are found that were not discernable from original Viking images, bringing the total number of mountains to 21 (Figure 1a). Also, the feature mapped as (v?) in this region does not appear in the high-resolution topographic data. Distances between the mountains range from 64 to 430 km, with an average separation from their nearest neighbor of 175 km. The mountains are predominantly located within a broad, regional low, roughly corresponding to the local deposits of the Dorsa Argentea Formation (Figures 1a and 1b).
 A clear linearity exists in the location of the mountains (Figures 1b and 2a). Specifically, mountains 2, 4, 5, 6, 7, 11, and 13 form a line ∼660 km in length, with most of the remaining mountains positioned within ∼100 km of the line. This line, trending slightly NW-SE (58°S, 358°W to 69°S, 355°W), extends poleward to a large mountainous area ∼100 km south of South crater. The large mountainous area, about 60 km × 200 km in dimension, is located mostly in the formation Tanaka and Scott  mapped as Hesperian-Noachian undivided (HNu) (Figure 1a).
 From the MOLA data the physical characteristics of the mountains were measured (Table 1). The heights of the mountains are generally centered between 1 and 1.5 km (mean of 1.17 km), with some minor variation above and below this central range (Figure 3a). The lowest of the group are mountains 1, 2, 9, 16, and 17. Mountains 1 and 2 are the features found farthest from the cap but are part of the alignment. Mountains 9, 16, and 17 are the three features found farthest from the alignment, situated 392, 680, and 530 km away, respectively. Mountain 13 is the tallest, with a height of 2.2 km, and is located near the middle of the Dorsa Argentea Formation.
Table 1. Physical Characteristics of the Mountains
Approximate Volume, km3
Average Slope, deg
Approximate Height, km
Approximate Width, km
Summit Elevation, m
Base Elevation, m
cone-shaped with summit crater
flat-topped with cone
cone-shaped with summit crater
flat-topped with cone
cone-shaped with summit crater
cone-shaped with summit crater
 The basal diameters of the mountains range from ∼20 to 70 km (mean of 40.3 km) and tend to be clustered between 30 and 40 km (Figure 3b). The main exceptions are mountains 3, 16, 18, 20, and 21. Mountains 3 and 18 are the two widest features and lie outside of the Dorsa Argentea Formation boundaries. Mountain 18 was unmapped by Tanaka and Scott , has a width of 70 km, and is situated 415 km away from the alignment. Mountain 21, also unmapped, has a width of 49 km. It falls at the edge of the Dorsa Argentea Formation adjacent to an impact crater that is 25 km in diameter and is located 160 km away from the alignment. Mountain 16 has a width of 52 km. Mountain 20 has the smallest diameter, 20 km, and sits 190 km away from the alignment.
 The mountains generally have a narrow range of basal elevations centered around 1200 m (mean of 1176 m) (Figure 3c). The two exceptions are mountains 12 and 16, which have basal elevations of 1400 and 200 m, respectively. Mountain 12, whose basal elevation is near 1200 m, is located 160 km from the alignment. The level of the surrounding regional plains, located at 1200 m elevation, consists of the Dorsa Argentea Formation (Hd) and a unit of unconsolidated material mapped as HNu.
 The range of summit elevations is between 750 and ∼3100 m (mean of 2345 m), but over 60% have summit elevations between 2 and 3 km (Figure 3d). Here again the main exception is mountain 16, which has a summit elevation of only 750 m.
 Volumes of the mountains were determined from the MOLA data. Volumes range from 75 to 1964 km3 and average around 700 km3, with some variation above and below. The main exceptions are mountains 1, 2, and 20, with volumes around 150 km3, and mountains 15, 18, and 21, with volumes of 1935, 1964, and 1134 km3, respectively. Mean flank slopes were also measured for the mountains, with values ranging from 2.4° to 13.7° and clustering between 6° and 11°. The three exceptions are mountains 2, 9, and 16, with slopes of 2.4°, 3.96° and 4°, respectively.
 To summarize the above physical descriptions (Table 1), the heights of the mountains are generally between 1 and 1.5 km (Figure 3a), while the widths are clustered between 30 and 40 km (Figure 3b). The basal elevations are clustered at ∼1.2 km (Figure 3c), and the summit elevations are clustered between 2 and 3 km (Figure 3d). Typical volumes are ∼700 km3, and flank slopes are between 6° and 11°. Those mountains that exhibit some variation from one or more of the above trends tend to be separate from the main alignment of the mountains (for example, mountain 16) or are farther away from the cap (for example, mountains 1 and 2).
 While remarkably similar in morphometric characteristics, the mountains do display some variation in their morphology. The various morphologies present can be seen in MOLA shaded relief views (Figure 4) and the corresponding profiles (Figure 5) as well as in MOC wide-angle images (Figure 6). The overall shapes of the mountains fall into five categories: (1) low-domed, (2) flat-topped, (3) flat-topped with a cone, (4) cone-shaped with a summit crater, and (5) cone-shaped. The mountains predominantly fall into one of the cone-shaped varieties (4 and 5). The low-dome mountains are found only outside the boundary of the Dorsa Argentea Formation. This variety contains two of the three mountains that are farthest away from the main alignment (mountains 9 and 16) as well as those farthest from the cap (mountains 1 and 2). The mountains that are cone-shaped with a summit crater are found either just inside or just outside the boundary of the Hd. The summit craters tend to be large relative to the summit diameter of the mountain and reach depths up to ∼200 m. The cone-shaped mountains are located within the center of the Dorsa Argentea Formation of the region. The exception to this is mountain 17, which is located just outside Hd in the unit mapped as Npl [Tanaka and Scott, 1987]. The flat-topped varieties are scattered, falling both within and outside of Hd. However, all are located north of 66°S (Figures 1a, 1b, 2a, and 7).
2.2. Surrounding Features and Associations
 Surrounding many of the mountains are what appear to be shallow depressions, circularly shaped and extending for about one diameter of the mountain in every direction. This is most clearly seen among mountains 2, 4, 5, 6, 7, 10, 11, and 17 (Figure 2b). At their deepest the depressions are 50–100 m below the level of the regional plains. The depressions then gently grade up into the level of the plains. The exception is mountain 17, with a depression reaching depths of 250–300 m. Also, the depression around mountain 11 extends out from the mountain for only ∼10 km. There is no lip to the “rim” of the depressions.
 Seen extending away from many of the mountains are channels that are carved into the adjacent regional plains (for example, the Dorsa Argentea Formation). The best example of these channels is seen with mountains 5, 6, and 7 (Figure 2b). The channel (indicated by the arrows) appears to breach a narrow, rocky rim surrounding the three mountains and to extend away from it for ∼250 km. In addition, areas of low-lying terrain are seen in Figure 2b surrounding mountains 8, 9, 10, 18, and 21. These marginal troughs are about as deep as the circular depressions and also gently grade back into the level of the surrounding plains. However, the troughs are more expansive and are not restricted to the symmetrical shape of the circular depressions.
 Surrounding some of the mountains, fine-scale polygonal patterns can be found in MOC images. The polygons are <100 m across. MOC image coverage of the mountains is sparse, but from the available images, polygons appear to be present in the low-lying areas, especially in the shallow depressions surrounding the mountains. This is best seen with mountain 11 (Figure 8), for which a MOC image traverses part of the surrounding depression, and then onto some of the surrounding plains. Polygons are seen only in the depression.
3. Discussion and Interpretation
 In order to interpret the origin and evolution of the mountains, we consider features that are found on Mars with similar physical characteristics and spacing. What Martian phenomena could lead to a series of topographic highs, aligned as many of the mountains are? Possible candidates that have been previously proposed are remnants of impact crater rims, central peaks, and volcanoes [Murray et al., 1972; Peterson, 1977; Condit and Soderblom, 1978; Scott and Carr, 1978]. A fourth possible origin, not previously considered, is rootless volcanic cones (often referred to as pseudocraters). Each shall be assessed in terms of its ability to explain the three key factors pertaining to the mountains: their morphologies, their morphometric characteristics, and their spacing and alignment.
3.1. Remnants of Impact Crater Rims
 Impact craters on Mars show a wide range of sizes and complexities [e.g., Mouginis-Mark, 1979; Barlow, 1988, 1990; Strom et al., 1992], dependent on the size, speed, and angle of the impactor, along with the material composing the bolide and the substrate [e.g., Melosh, 1989; Strom et al., 1992]. The generally circular rim can be degraded through fluvial, eolian, tectonic, or volcanic means or through bombardment by later impacts. Such degradation could remove all but the most resistant parts of a crater rim, leaving behind an isolated mesa. Craters may also be buried beneath sediment and then later exhumed, leaving much of the rim intact [Grant and Schultz, 1990].
 Remnants of impact crater rims do not compare well to the characteristics of the mountains. On the basis of the physical dimensions of the mountains, if these features formed as parts of crater rims, then the craters would have had to have been rather large in size; a crater several hundred kilometers in diameter would be necessary to create a rim at least as wide as the mountains, and little evidence is seen of the former presence of such features.
 For some of a crater rim to remain while the rest erodes away is indicative that the remaining part of the rim is composed of a more resistant material. The chance that the number of large impacts required to form the mountains would have occurred in the same region of Mars at approximately the same time, and that resistant parts of the various crater rims would have been aligned to lead to the present alignment of the mountains, seems highly unlikely. Further, almost all the mountains display a clear circular outline to their structure and decrease in diameter approximately symmetrically up the structure toward its peak. This degree of regularity seems unlikely for remnants of a crater rim.
3.2. Central Peaks
 As crater size increases, the physical characteristics of a crater become more complex [Pike, 1980] with the onset of central peaks, flat floors, and wall terraces (hence their designation as a complex crater). The onset diameter for craters displaying central peaks on Mars ranges from 2 km [Wood et al., 1978] to 5 km [Cintala, 1977] but is generally is about 3–3.5 km [Pike, 1980].
 Upon initial inspection, central peaks might well appear to be a suitable explanation for the mountains. They are clearly topographic highs and are broadly similar in shape to the mountains (Figure 9a). However, detailed examination shows that the dimensions of the mountains are different from those of Martian central peaks [Strom et al., 1992; Hale and Head, 1981; Pike, 1985]. Estimates of the diameter of the craters that should be associated with central peaks similar in size to the mountains [Hale and Head, 1981] are several tens of kilometers greater than the diameter of the circular depression found around many of the mountains (Table 2). For some mountains the expected crater diameter is greater than three times the diameter of the depression. If the circular depressions are the remains of crater rims, they are severely degraded. It would be highly unlikely that the source of the erosion of the crater rims would leave the central peaks as well preserved as the mountains are found to be. Degraded craters are found in the surrounding plains that are of the size necessary to produce central peaks having the width of the mountains; however, all of these craters lack any peaks. Those craters that do contain peaks are crisp, <100 km in diameter, and have peaks that are much smaller in size than the mountains (Figure 9a).
Table 2. Comparison of the Circular Depressions to Estimated Crater Diameter
 The depth of the expected craters would be estimated to be between 1 and 3 km [Strom et al., 1992], significantly deeper than the depth of the circular depressions, which are only 100 m at their deepest. Also, the alignment of the mountains is not explained by a central peak origin (Figures 1a and 2a); it would be improbable to have as many craters of the size required to produce such peaks lined up as they would need to be. Further, the mountains stand hundreds of meters higher (Figures 1b and 2a) than any possible “rim,” not a usual characteristic of central peaks (Figure 9a).
 For all the above reasons, a central peak origin does not appear consistent with the physical parameters of the vast majority of the mountains. The only exception might be mountain 17 (Figure 9b). This mountain is one of the farthest of the mountains from the alignment (530 km) and is not located within the Dorsa Argentea Formation but is situated in the Noachian cratered Unit (Npl) (Figure 1a), surrounded by craters of similar size containing central peaks. However, the peaks found within the craters near mountain 17 are smaller in size than mountain 17. Furthermore, the depth of the circular depression around mountain 17 is only a few hundred meters, whereas the depths of nearby craters of diameters similar in size to that of the circular depression around mountain 17 are over 1 km. If this mountain is a central peak remnant, significant degradation of the crater has occurred and the central peak must have been enlarged through subsequent deposition or construction.
 Volcanoes on Mars come in many different shapes and sizes, some of which are similar to the mountains. Specifically, Uranius [Hodges and Moore, 1994] and Zephyria Tholi [Stewart and Head, 2001] (Figure 10) have similar morphometric characteristics to the mountains, their heights, widths, and volumes all falling within the ranges established for the mountains (Figure 11). The summit craters found among at least four of the mountains (Table 1) could be interpreted as collapse calderas [Crumpler et al., 1996]. The alignment of some of the mountains could also be explained by dike swarms propagating to the surface [Wilson and Head, 1994, 1998, 2000, 2001a].
 Many volcanic edifices show evidence for lateral dike propagation, commonly in the form of flanking rift zones and flank eruptions. If the mountains are volcanoes, the lack of a significant edifice in the vicinity suggests that they and their alignment represent primarily vertical dike propagation from depth, and are not the result of lateral dike propagation from a major edifice reservoir [e.g., Wilson and Head, 1994]. The spacing and total distance covered by the mountains are empirically similar to those of some of the smaller Tharsis Montes [Scott and Tanaka, 1986] and volcanic edifices observed in Iceland [Bjornsson and Einarsson, 1990].
 An interpretation related to volcanic construction appears to explain both the morphometry of the mountains and their relatively isolated state from each other. However, as mentioned earlier, while similar in most morphometric descriptions, the mountains do display some variation in their morphology from each other (Figures 4, 5, and 6). In addition, they display some variation in their morphology from the above mentioned Martian volcanoes (Figure 10). Specifically, the slopes of a number of the mountains tend to be steeper than those of similar sized Martian volcanoes [Kortz and Head, 2001; Head et al., 1998; Smith et al., 1998], leading to broader summits for the non-cone-shaped mountains and high width-to-height ratios; this also leads to many mountains that are quite conical. The exceptions to this are the low-domed mountains, which together account for the lowest four slopes among the mountains. In addition, for those mountains containing summit craters, the craters tend to be large relative to the mountain [Crumpler et al., 1996], in some cases dominating the entire summit. Also, the depths of the summit craters reach depths of only ∼200 m. On the basis of these characteristics and comparisons, we believe that the interpretation of the mountains as volcanoes is most consistent with the data. The several morphological variations from other Martian volcanoes of similar size, as well as the surrounding features found in association with the mountains, may hint at an environment of eruption different from that typical of the subaerial edifices elsewhere on Mars.
 The location of these mountains and their surrounding environment may provide clues as to their origin. Most of the mountains are situated within the Hesperian-aged Dorsa Argentea Formation (Hd), whose origin has previously been attributed to eolian mantle or lava flows [Tanaka and Scott, 1987]. The relation of the mountains to Hd is intimate in that they are predominantly surrounded by it (Figure 1) and the common basal elevation shared by the mountains, ∼1.2 km, is the same as the level of the top of surrounding plains, largely the Dorsa Argentea Formation. Recent detailed mapping of the Dorsa Argentea Formation [Head and Pratt, 2001] supports the interpretation that it is a volatile-rich unit, and it has been interpreted as representing the remnants of a formerly larger south polar ice sheet. This interpretation is supported by the presence of esker-like ridge systems [Head and Hallet, 2001a, 2001b], large-scale collapse features, flat marginal areas interpreted to be ponded debris, and sinuous channels leading from the margins of the deposit and interpreted to be drainage from meltback products [e.g., Head et al., 2001; Milkovich et al., 2002]. An alternative interpretation is presented by Kolb and Tanaka .
 Here we use the terms “ice cap” and “ice sheet” according to Head and Pratt , following the terrestrial convention [e.g., Benn and Evans, 1998]. Ice cap will refer to an ice-rich body <50,000 km2 in area, such as the current Amazonian-aged south polar ice cap. Ice sheet will refer to an ice-rich body >50,000 km2 in area, such as the Hesperian-aged ice sheet. The composition of the residual ice (Api) and layered terrain (Apl) of the Amazonian-aged Martian ice caps is not definitely known [Thomas et al., 1992]. It is known that there is a volatile component and a dust component. The volatile component has been argued to be mostly water ice [Clifford, 1987, 1993; Mellon, 1996], mostly CO2 ice, or any ratio in between, including CO2 clathrates [Kargel, 1998; Hoffman et al., 2002]. However, it is generally accepted that water is the dominant volatile component. The dust component is inferred from albedo [Herkenhoff and Murray, 1990; Thomas et al., 1992] and from images such as those of the layered terrain [Murray et al., 1972; Cutts, 1973], which clearly show alternating dark and light layers. This is mentioned because a Martian ice cap or ice sheet is not necessarily a pure ice body as may be thought to be the case with their terrestrial analogs. In a relative sense, it is accepted that Api has a high ice-to-dust ratio [Kieffer, 1990; Mellon, 1996] and Apl has a lower, but still high, ice-to-dust ratio [Cutts et al., 1979].
 What might be said about the volatile component of the Hesperian-aged south polar ice sheet? What is left of this ice sheet is what has been mapped as the Dorsa Argentea Formation and related unit HNu and will be referred to as the deposits of the ice sheet. These units have been shown by Head and Pratt  to be volatile rich, and as mentioned above, evidence exists for meltback and drainage. The meltback that occurred indicates that volatiles were lost and that the larger ice sheet once had a higher ice-to-dust ratio than the deposits now have. What exactly this ratio was is unknown. However, estimates made by Head and Pratt  of the thickness of the ice sheet suggest that in places it was as much as 2500 m thick. Such a voluminous loss of material from the area suggests that the majority of the loss was volatile in nature and that the Hesperian-aged ice sheet likely had a high ice-to-dust ratio.
 The presence of an expansive ice sheet at the south pole during the Hesperian may have influenced the construction and evolution of the mountains. Since the ages of the mountains are unknown, the possibility exists that these features interpreted to be volcanic edifices formed by eruption into or beneath the formerly larger Hesperian ice sheet. We now examine this possibility to see if it might be able to explain the variations of the morphologies of the mountains from some Martian volcanoes of similar size.
 The morphological form of the structure built by subglacial eruptions is controlled by a number of factors [e.g., Wilson and Head, 2001b], such as the thickness of the ice sheet [Smellie and Skilling, 1994], type of ice sheet (polar versus temperate), the volume of meltwater produced and confined by the ice [Smellie, 2001], type of eruption (localized vent versus fissure eruption), size of cauldron formed on the ice surface, composition of the magma, rate of eruption, and volume of magma erupted [Hickson, 2000; Smellie, 2000]. Such differences in morphology are documented even for the same eruption, the 1934 eruption of Grimsvotn, Iceland, where two eruptive centers within a few kilometers of each other produced edifices of different morphologies [Nielson, 1937].
 Much of a subglacial eruption actually takes place subaqueously. A general sequence of events for a subglacial eruption is as follows (e.g., Figure 12). During the initial stages of eruption, large volumes of meltwater are produced by the increase in magmatic heat. This leads to formation of a depression in the surface of the overlying ice [Bjornsson, 1974; Gudmundsson et al., 1997]. Glacier hydrology is predominantly controlled by the topography of the ice surface [Nye, 1976; Benn and Evans, 1998]. The depression in the ice surface will cause the meltwater to drain toward the site of eruption and to collect around the forming edifice as an englacial lake [Bjornsson, 1975; Smellie, 2001]. The englacial lake is enclosed on all sides by the ice wall barriers.
 Further eruptions occur into the englacial lake, which quenches the lava, makes it less mobile, and causes it to either crumble or form pillows [Gudmundsson et al., 1997; Smellie and Skilling, 1994; Skilling, 1994; Smellie 2000, 2001]. The quenched lava acts as a good insulator, keeping further erupted lava out of contact with the ice. The easiest way for lava to flow in this englacial lake is sideways, where it is confined by the ice walls, thus causing the mound to have high width-to-height ratios and relatively steep slopes [Einarsson, 1966]. If the mound reaches a height sufficient to breach the level of the englacial lake, effusive subaerial capping flows will occur, leading to a relatively flat top, making the edifice less susceptible to erosion [Smellie and Skilling, 1994; Skilling, 1994; Smellie 2000, 2001]. If the eruption turns subaerial, pyroclastic activity can also occur, and a cone of material can be formed on top of the otherwise low, and often flat-topped, mound [Allen, 1979]. This pyroclastic activity could be due to either release of volatiles from the magma or phreatomagmatic interactions with the surrounding meltwater. Alternatively, it may date to a time after the edifice has been constructed when the ice sheet has retreated [Allen, 1979]. If the surface of the englacial lake is of sufficient height not to be breached, an entirely conical feature can be formed [Hickson, 2000]. Important in the constructional history of the volcano are the stability of the englacial lake and how the meltwater is drained.
 The general structure of an ice sheet is that of an impermeable ice layer overlain by a permeable layer composed of snow and firn. The permeable layer is referred to as the passage zone [Smellie and Skilling, 1994]. On the Earth the passage zone is located within ∼150 m of the ice sheet surface [Smellie, 2001]. There exist two main mechanisms by which the englacial lake can drain, discussed in detail by Smellie and Skilling  and Smellie  and briefly summarized here. The first mechanism involves floating of the surrounding ice sheet. If the hydrostatic pressure at the base of the englacial lake reaches the magnitude at the base of the surrounding ice, the ice sheet will be lifted off its bed, and the meltwater will drain subglacially, underneath the ice along the basement topography, and will carve out channels. This is presumably the mechanism by which the majority of meltwater drains from underneath the Vatnajokull ice cap in Iceland [Bjornsson, 1974, 1975, 1992]. In order for this mechanism to occur, the height of the englacial lake required to reach the critical hydrostatic pressure must be less than the height required to reach the passage zone. If the passage zone is reached before the critical hydrostatic pressure is reached, the second mechanism of drainage results. Meltwater accumulates within the ice chamber, increasing the height of the englacial lake. When the level of the passage zone is reached, the overflow of meltwater drains supraglacially, through the permeable firn and snow. This leads to a more stable lake level than if drainage were to occur subglacially. Some subglacial drainage may still occur occasionally if lava melts the ice at the base of the ice sheet [Bjornsson, 1975; Bjornsson and Kristmannsdottir, 1984].
 Supraglacial drainage through the passage zone requires a relatively thick permeable layer and is thus thought to be the predominant mechanism of drainage in thick glaciers and ice sheets. Estimates of the thickness of the Hesperian-aged ice sheet performed by Head and Pratt  suggest that in places it was as much as 2500 m thick. Also, the lower gravity of Mars relative to that of Earth would suggest that there would be less compaction of snow into ice in a Martian ice sheet, and thus a Martian ice sheet would be expected to have a thicker passage zone relative to a terrestrial one. Together, the estimates of the ice sheet thickness, as well as the lower gravity of Mars, suggest that the predominant mechanism of drainage of meltwater produced from eruption into the Hesperian ice sheet would have been supraglacial drainage through the passage zone.
 After the subglacial eruptions have ceased, meltwater can still accumulate from the increase in geothermal heat near the erupted structure [Bjornsson and Gudmundsson, 1993; Gudmundsson et al., 1995]. Thus jokulhlaups can continue to form, leading to continued drainage of water along the basement topography and the carving out of channels. Once the geothermal heat level returns to pre-eruption conditions, the ice is able to reform around and upon the volcano. Such glacial activity could lead to erosion of the flanks of the edifice [Van Bemmelen and Rutten, 1955]. As pointed out by Chapman and Tanaka , the postconstructional environment provides ample opportunity for degradation of the ideal tuya form. The identification of well-preserved examples may be indicative of an ice sheet that was melted and did not reform.
 The heights of the mountains can be used as an estimate of the thickness of the ice sheet at the time of construction of the volcanoes. The conically shaped mountains would have erupted entirely within the englacial lake, while the flat-topped varieties would have just breached the surface of the lake. Since the lake surface corresponds to the level of the passage zone, which is below the surface of the ice sheet, estimates made from the heights of the volcanoes would be minimum estimates. These observations provide a basis for comparison to the characteristics of the Martian mountains interpreted to be volcanoes erupted in the proximity of the Hesperian-aged ice sheet [Head and Pratt, 2001].
4.2. The Mountains
 Many of the morphological differences observed between the mountains and Martian subaerial volcanoes of similar size could be explained by the processes involved in subglacial eruptions, as described above (Figure 12). The high width-to-height ratios of the mountains (Figure 11), along with their relatively steep slopes, and in some cases flat tops, are very similar to those predicted by the sequence of events outlined for a subglacial eruption (Figure 12).
 Some of the mountains display a conical feature on top of an otherwise flat-topped summit (mountains 6 and 11) (Figure 5). Such a conical feature could be accounted for by the transition in the eruption from a subaqueous phase to a subaerial phase, or more simply from subaerial eruptions at a time when the ice sheet had retreated from the area [Allen, 1979]. Most of the mountains do display a conical shape, although some are irregular in outline. How is this morphology accounted for among the others attributed to subglacial origin? As documented by Mathews , there are several conical volcanoes interspersed among table mountains in northern British Columbia. These conical features likely erupted at a time when the overlying ice was sufficiently thick that breach of the englacial lake was not possible by the erupting structure. In such a situation the ice walls would have confined accumulation of the pillow material and hyaloclastites. A conical feature would result and is a likely scenario for the origin of the more conical and irregularly conical mountains under discussion here. The cone-shaped mountains all fall within the center of the regional deposits of the Dorsa Argentea Formation (Figure 7). Such locations are consistent with the origin just outlined, as the formerly large Hesperian-aged ice sheet [Head and Pratt, 2001] would be expected to be have been thicker toward the pole and thinner farther away. The only exception is mountain 17, which, as discussed earlier, is a likely candidate for an impact crater central peak.
 An interesting terrestrial analog for the spacing and alignment of the mountains is a trio of volcanic edifices erupted subglacially in Iceland (Figure 13). This trio consists of Kverkfjoll in the northeast, Grimsvotn in the middle, and Thordarhyrna and the Laki craters in the southwest [Grondvold and Johannesson, 1983; Bjornsson and Einarsson, 1990]. The alignment spans ∼120–130 km, with spacing between the different centers of 30–70 km. A similar trio can be found within the alignment of the Martian mountains, specifically, mountains 5, 6, and 7, which span ∼123 km (Figure 2b). As indicated by the Icelandic features, eruptions at one center do not require simultaneous eruption at one of the other centers. Additionally, the morphologies among the Icelandic structures in this trio are not constant, nor are those exhibited by mountains 5, 6, and 7. It should be noted that while this Icelandic analog serves to show that different morphologies can be constructed underneath the same ice sheet, the conditions at Iceland are certainly different from the Martian volcanoes. Specifically, there is no indication of a rift system, as is the case with Iceland, other than the alignment of the mountains.
 The relatively large size of the summit calderas found on some of the mountains may also have a relationship to a subglacial origin. During times between eruptions, meltwater may have collected within the summit craters in a fashion similar to that occurring with the Grimsvotn caldera in Iceland today [Bjornsson, 1983]. In Grimsvotn the meltwaer lake has served to both erode away at the banks of the summit crater as well as fill in the crater with eroded material [Gudmundsson, 1989]. The summit craters found on at least four of the south polar mountains may share a similar evolutionary history to that of the Grimsvotn cladera.
 The low-domed mountains could likely have erupted entirely subaerially. Together they account for the four shallowest slopes and lowest heights among the mountains (Table 1). Also, all four of these mountains are located outside the boundary of the Dorsa Argentea Formation (Figure 7).
 Finally, the interpretation of the mountains as subglacial volcanoes is also supported by the surrounding features discussed earlier. The channels found extending away from some of the mountains may be indicative of some basal meltwater drainage (Figure 2b). Such meltwater could have accumulated during the constructional history of the mountains and been released as jokulhlaups by melting of the surrounding ice walls along the basement. As mentioned in section 4.1, the main mechanism of meltwater drainage is inferred to be supraglacial drainage through the passage zone [Smellie, 2001].
 The circular depressions located around several of the mountains are remarkably similar to depressions observed around some volcanoes in the north polar region of Mars [Grosfils and Sakimoto, 2001]. These volcanoes are slightly smaller than the south polar mountains, but their corresponding circular depressions extend outward for about one diameter of the volcano and are at most ∼100 m deep, similar to the observations made of the circular depressions around the south polar mountains. Grosfils and Sakimoto  interpreted the north polar depressions as resulting from volatile loss from the surrounding plains and ground subsidence due to emplacement of the volcanoes, and inferred underlying sills. While there is no evidence for underlying sills beneath the south polar mountains, the approximate central location of the mountains within the circular depressions suggests that the two are related and that melting and loss of volatiles could have resulted from heat conduction from the magma reservoir as well as sill emplacement [Head and Wilson, 2002]. A volatile loss origin for the circular depressions is consistent with the volatile-rich nature of the Dorsa Argentea Formation and the former presence of an ice sheet [Head and Pratt, 2001].
 Polygonal patterns are found primarily in the circular depressions and marginal troughs surrounding many of the mountains. If these date from the Hesperian period, the polygons may have formed in the process of formation of the circular depressions, when volatiles were removed or when the ground subsided. Other possible origins for the cracking are from either mud desiccation or ice-wedge cracking [Seibert and Kargel, 2001] following pooling of meltwater in the depressions around the mountains or in some other low-lying areas acting as basins for meltwater drainage. Alternatively, the polygonal terrain could be due to more recent cycling of a volatile-rich upper layer [Lane and Christensen, 2000].
 On the basis of the above detailed analysis of the mountains and surrounding features found in association with them, we interpret several of the mountains as having been constructed into the Hesperian-aged ice sheet. The well-preserved state of the mountains indicates that little postconstructional erosion has occurred. This suggests that once the ice was melted and removed from the vicinity of the mountains, the ice sheet did not reform [Chapman and Tanaka, 2001]. Therefore the mountains could have likely played a significant role in local meltback of the Hesperian-aged ice sheet. This is consistent with the work of Milkovich et al. , which suggests that meltback of the Hesperian ice sheet likely resulted from bottom-up, rather than the top-down, melting.
 The Dorsa Argentea Formation, the deposits of the Hesperian ice sheet, dates to the middle part of the Hesperian Period [Tanaka and Scott, 1987], suggesting that the larger ice sheet had melted back and retreated by that time. Therefore the sources of the melting must have operated prior to that time. Among the most prominent candidates for bottom-up melting in this region is the emplacement of the Hesperian-aged ridged plains (Hr), which have been mapped by Tanaka and Scott  throughout much of the south polar region. The ridged plains material forms numerous patches of “broad, planar, moderately cratered surfaces marked by long, linear to sinuous ridges., embays all adjacent units.,” and is interpreted as “extensive flows of low-viscosity lava erupted at high rates from local sources.” [Tanaka and Scott, 1987]. According to Tanaka and Scott , Hr lies largely stratigraphically below the Dorsa Argentea Formation and was emplaced during the Early Hesperian. Thus, if the Dorsa Argentea Formation represents the remnants of a previously larger volatile-rich ice sheet unit, then the emplacement of volcanic deposits of the ridged plains (Hr) during the early part of the Hesperian could have served as a heat source to melt portions of the ice sheet.
 Indeed, such melting could have occurred in several different modes. Recent mapping of the northern lowlands has shown evidence for the widespread occurrence of Hr, lying below the Vastitas Borealis Formation (Late Hesperian) and resurfacing the northern lowlands [Head et al., 2002]. This new northern lowlands occurrence brings the total area of Hr to ∼30% of the surface of Mars, strengthening arguments that this phase of volcanism and tectonism may have represented a significant global event on the planet [e.g., Watters, 1993]. Such important global events could have involved a period of enhanced global heat flow, potentially leading to enhanced basal melting of thick volatile-rich deposits. Second, local intrusion of Hr into volatile-rich deposits or ice sheets could obviously lead to the equivalent of terrestrial supraglacial, intraglacial, and subglacial melting, in which features such as tuyas might have formed. Thus the widespread nature and style of occurrence of the Early Hesperian ridged plains, together with their geographic and temporal proximity, mean that the emplacement of Hr is a clear candidate for a source of bottom-up melting of the unit that became the Dorsa Argentea Formation. Furthermore, the individual occurrences of the mountain structures could well mark the manifestations of an Hr source vent erupted under an ice sheet, producing these unusual morphologic features, rather than laterally extensive volcanic plains.
4.3. Estimates of the Thickness of the Ice Sheet
 If these features are correctly interpreted to be volcanic eruptions beneath and in the vicinity of an ancient ice sheet, then the thickness of the ice sheet can be estimated from the heights and morphologies of the mountains. Specifically, the heights of the flat-topped mountains and cone-shaped mountains can be used to develop constraints on the minimum thickness of the ice sheet into which the volcanoes erupted. The flat-topped mountains are interpreted to have erupted up to about the level of the englacial lake. Different authors have proposed various connections between the level of the lake and the thickness of the cap [e.g., Smellie and Skilling, 1994]. However, it can be agreed that the maximum level of the lake is less than the thickness of the overlying ice. Therefore an estimate of the meltwater lake level can provide a minimum value for the thickness of the ice. This is especially true for Mars, where the passage zone, corresponding to the lake level, would be expected to be farther from the surface of the ice sheet. The heights of the flat-topped south polar mountains provide an estimate of between 700 and 1300 m.
 As discussed in section 4.2, the mountains that are more cone-shaped could have erupted entirely within the englacial lake. Discounting mountain 17, which may well be a central peak, the remaining cone-shaped mountains provide ice sheet thickness estimates ranging from 1200 to 2200 m. Interestingly, the tallest of the cone-shaped mountains (13) is found in the middle of the regional deposit of the Dorsa Argentea Formation. The remaining cone-shaped mountains decrease in height closer toward the edge of the deposit (Figure 7, Table 1).
 The above estimates combine to give a minimum estimate of the thickness of the overlying ice sheet, ranging from 700 m to 2.2 km and averaging to ∼1.4 km. Thicker estimates generally come from mountains closer toward the middle of the regional deposit of Hd, and smaller estimates come from mountains found closer toward the boundary of the deposit. This trend is consistent with the expectation that the ice sheet would have thinned toward its edges. The estimates are also consistent with the thickness of the formerly large Hesperian-aged south polar ice sheet as estimated by Head and Pratt .
 An alternative explanation for the different ice sheet thicknesses obtained from the heights of the mountains is a variation in ice sheet thickness with time rather than simply in location. If the mountains were constructed at somewhat different times, then the heights of the mountains may be indicating the thickness of the ice sheet at these times, implying that it was not constant. However, there does not appear to be any good evidence to suggest a significant age variation among the mountains. The simplest explanation for the varying heights seems to be a variation in ice sheet thickness with location.
4.4. Other Candidate Martian Subglacial Volcanoes
 To date, other features on Mars interpreted as subglacial eruptions have been low, elongated, steep-sided mounds, considered to be analogous to terrestrial table mountains [Hodges and Moore, 1978; Allen, 1979; Chapman, 1994; Chapman and Tanaka, 2001]. To some degree, this has been due to the limited resolution of Viking images. However, planetwide surveys in search of volcano-ice interaction have also been restricted to features resembling Icelandic table mountains and moberg ridges. Such features are not the sole type of subglacial volcanoes found on Earth and therefore should not be considered the sole type that might be found on Mars.
 The majority of the table mountains found on Mars are located in the northern lowlands. The structures are generally smaller than the mountains found near the south pole and longer than they are wide, with widths ranging from 1 to 16 km in diameter and heights reaching up to 1 km (Figure 11). An exception to this is a feature in Hebes Chasma which stands 4 km above the chasma floor [Chapman and Tanaka, 2001]. They have steep slopes, and many are flat-topped [Hodges and Moore, 1978; Allen, 1979; Chapman, 1994; Chapman and Tanaka, 2001]. What reason is likely to explain the difference in morphology and morphometry between the south polar features and the table mountains of the northern lowlands?
 A possible explanation might be found with the thickness and type of ice layer into which the volcanoes erupted [Smellie, 2000]. Allen  proposes that the table mountains he describes erupted into a subsurface ground ice layer. Chapman  proposes that the table mountains found in Utopia erupted into a frozen paleolake, of thickness estimated to be ∼180 m. The south polar mountains would have erupted into a formerly large south polar ice sheet. The heights of the flat-topped and conical features have been used to arrive at a minimum value of 1.4 km for the average thickness of the ice sheet. Clearly, a different set of constraining variables would have been involved in the formation of the south polar mountains and the table mountains found elsewhere. Additionally, the state of the base of the ice sheet (wet or dry) would have played a role in the eruption [Smellie and Skilling, 1994]. Unfortunately, there is no method by which to determine such conditions on Mars at this time.
 Several features previously mapped as mountains in the south polar region of Mars [Tanaka and Scott, 1987] have been reexamined using MOLA data. From the spacing, alignment, and morphometries of the mountains, and from comparison to other Mars features, we interpret the mountains as volcanoes. However, their morphologies show some variation from other similarly sized Martian volcanoes. The mountains are located within the Dorsa Argentea Formation, a volatile-rich unit interpreted as the deposits of a larger Hesperian-aged ice sheet [Head and Pratt, 2000]. Their location within the Dorsa Argentea Formation, the correlation of their basal elevations with the elevation of Hd, and the unknown age of the mountains hint at a possible interaction between the volcanoes and the formerly large south polar ice sheet. Comparisons were made between the south polar mountains of Mars and terrestrial subglacial volcanic constructs. Subglacial eruptions on Earth are known to produce structures of varying sizes and shapes, depending on the specific eruption conditions. The morphologies of the mountains fit well with a plausible subglacial origin. The association of the mountains with surrounding features supports this scenario. Channels extending away from the volcanoes may represent meltwater drainage, and cracking found in shallow depressions surrounding the structures may date from the time of the melting and could represent pooling of water followed by desiccation, ice wedging, or volatile overturn.
 If these interpretations are correct, thickness estimates of the larger Hesperian-aged south polar ice sheet interpreted to be represented by the Dorsa Argentea Formation [Head and Pratt, 2001] can be made from the heights of the flat-topped and cone-shaped mountains. Such estimates suggest a minimum average thickness of ∼1.4 km for the ice sheet. Eruption into ice of this thickness is an important difference from the eruption conditions inferred for other Martian subglacial constructs [Allen, 1979; Chapman, 1994]. The table mountains interpreted to occur on Mars appear to have erupted into ice bodies significantly thinner than that of the formerly large south polar ice sheet. This difference appears to account for the distinct morphologies observed between the south polar mountains and the Martian table mountains. The well-preserved state of the mountains suggests that little postcontructional erosion has taken place and could mean that the mountains are responsible for local meltback of the Hesperian ice. In addition, as pointed out by Head and Pratt , if life originated in the early history of Mars, then the Dorsa Argentea Formation and related units may provide an excellent location to explore for fossil evidence [e.g., McKay and Stoker, 1989]. Specifically, volcanic eruptions in this water-rich environment, such as those described here, should be considered carefully as possible sites for further detailed analysis.
 We gratefully acknowledge fruitful discussions with John Smellie, Ian Skilling, Ken Tanaka, Mary Chapman, and Susan Sakimoto. Thanks are extended to the Rhode Island NASA Space Grant (P. H. Schultz, PI) for providing the internships during which this work was accomplished. We gratefully acknowledge NASA grants NAG5-9780 and NAG5-10690 to J.W.H., which helped fund the work and attendance at the Lunar and Planetary Science Conference to present these results. Comments from an anonymous reviewer significantly improved this work. We thank Steve Pratt for advice on data processing and volume calculations. Peter Neivert and Anne Côté assisted with preparation of the figures and manuscript. We also wish to thank the Brown MOLA Science Group, with special thanks extended to Harry Hiesinger and Brad Thomson. The authors acknowledge the use of Mars Orbiter Camera images processed by Malin Space Science Systems that are available at http://www.msss.com/moc_gallery/.