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Cold-based mountain glaciers on Mars: Western Arsia Mons James W. Head Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA David R. Marchant Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, USA ABSTRACT and spaced a few hundred meters to several Surface environmental conditions on Mars are currently extremely cold and hyperarid, kilometers apart, that pass across topographic most equivalent to polar deserts on Earth. Coupling newly acquired Mars data with ﬁeld- barriers without obvious deﬂection. (2) A based observations regarding the ﬂow, surface morphology, and depositional history of knobby-terrain facies (Fig. 2B) forms an ex- polar glaciers in Antarctica, we show that the multiple facies of an extensive fan-shaped tensive area of chaotic terrain that is charac- deposit on the western ﬂanks of Arsia Mons volcano are consistent with deposition from terized by subrounded several-kilometer-di- cold-based mountain glaciers. An outer ridged facies that consists of multiple laterally ameter hills; some hills form chains that are extensive, arcuate and parallel ridges, resting without disturbance on both well-preserved parallel to subparallel to the ridges in the lava ﬂows and an impact crater, is interpreted as drop moraines formed at the margin of ridged facies. (3) A smooth facies (Fig. 2C) an ablating and predominantly receding cold-based glacier. A knobby facies that consists contains arcuate lineations and diffuse to lo- of equidimensional knobs, each to several kilometers in diameter, is inward of the ridges; bate margins; the smooth facies appears to this facies is interpreted as a sublimation till derived from in situ downwasting of ash- overlie areas of the knobby facies. Scott and rich glacier ice. A third facies comprising distinctive convex-outward lobes with concentric Zimbelman (1995, and references therein) de- parallel ridges and aspect ratios elongated downslope likely represents rock-glacier de- scribed various hypotheses for the origin of posits, some of which may still be underlain by a core of glacier ice. Taken together, these these features, including one or more of the surﬁcial deposits show that the western ﬂank of Arsia Mons was occupied by an extensive following: lahars, debris avalanches, land- mountain glacial system accumulating on, and emerging from, the upper slopes of the slides, pyroclastic ﬂows, and/or causes gen- volcano and spreading downslope to form a piedmont-like fan. erally related to the advance and retreat of ice (e.g., Lucchitta, 1981). Keywords: Mars, cold-based glaciers, rock glaciers, drop moraines, ablation till, Arsia Mons. Two new developments are the basis for the research reported in this analysis. First, new INTRODUCTION unusual fan-shaped deposit on their western Mars Orbiter Laser Altimeter (MOLA) altim- Arsia Mons is one of the three Tharsis ﬂanks. These deposits, as exempliﬁed by those etry and Mars Orbiter Camera (MOC) images Montes shield volcanoes that cap the broad on Arsia Mons (e.g., Zimbelman and Edgett, from the Mars Global Surveyor mission have Tharsis Rise, a huge center of volcanism and 1992; Scott and Zimbelman, 1995), usually permitted us to characterize the fan-shaped de- tectonism spanning almost the entire history contain three facies (Figs. 1A, 2A, 2B, 2C). posit on Arsia Mons and its relationship to the of Mars. Each of the Tharsis Montes, although (1) An outermost ridged facies (Fig. 2A) con- rest of the volcano in much more detail. Sec- largely constructed of effusive and explosive sists of a broad, thin sheet characterized by ond, ongoing research on polar glaciers in volcanic deposits, contains a distinctive and numerous ridges, tens of kilometers in length Antarctica has resulted in depositional models Figure 1. A: Geologic sketch map of western Arsia Mons fan-shaped deposit (modiﬁed from Zimbelman and Edgett, 1992) superposed on Mars Orbiter Laser Altimeter (MOLA) topographic gradient map (fan-shaped deposits: R—ridged; K—knobby; S—smooth) (other adjacent deposits: SA—shield; SB—degraded western ﬂank; SC—smooth lower western ﬂank; CF—caldera ﬂoor; CW—caldera wall; PF—ﬂank vent ﬂows from Arsia Mons; P—undivided Tharsis plains). B: Detrended MOLA topography of western Arsia Mons (regional slopes are removed; lighter is relatively steeper topography and darker is relatively shallower topography; black—no data presented). Left arrow points to outer edge of ridged facies; right arrow points to outer edge of knobby facies. Note that lava ﬂows clearly extend underneath ridged and knobby facies and are undisturbed. 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or firstname.lastname@example.org. Geology; July 2003; v. 31; no. 7; p. 641–644; 2 ﬁgures. 641 Figure 2. Facies of fan-shaped deposit and possible terrestrial analogues. A–C: Viking Orbiter images of facies. A: Ridged facies. B: Knobby facies. C: Smooth facies. D–F: Parts of U.S. Geological Survey aerial photographs; serial numbers given in parentheses; all were taken 21 November 1993. D: Drop moraines in Antarctic Dry Valleys (TMA 3079/303). E: Sublimation tills in Antarctic Dry Valleys (TMA 3078/006). F: Rock glacier in Antarctic Dry Valleys (TMA 3080/275). that are of sufﬁcient detail to provide a frame- azonian in age and are largely contempora- dence for ridge offsets that might represent work for investigating glacier ice on Mars neous with the latter phases of volcanism on tear faults resulting from uneven compression (Marchant et al., 2002; Denton and Marchant, Arsia Mons (interdigitate relationships with associated with landslide deposits, as seen in 2000; Sugden et al., 1995). On the basis of Tharsis Montes Formation Member 5 of Scott the Olympus Mons aureole (e.g., Francis and present surface temperatures on Mars and and Zimbelman, 1995, and possibly predating Waadge, 1983). those of the recent past, any mountain glaciers or being partly contemporaneous with Mem- One of the most distinctive characteristics on Arsia Mons and nearby volcanoes were ber 6). of the ridged facies is its superposition on a likely to be cold based and most similar to the subjacent impact crater and lava ﬂows without slow-moving, cold-based glaciers of the Dry Ridged Facies apparent modiﬁcation (Williams, 1978; Luc- Valleys region of Antarctica (Schafer et al., The ridged facies consists of a series of chitta, 1981; Zimbelman and Edgett, 1992; 2000; Cuffey et al., 2000; Marchant et al., 100 concentric ridges that extend several Anguita and Moreno, 1992; Scott and Zim- 2002; Atkins et al., 2002). hundred kilometers beyond the break in slope belman, 1995; Helgason, 1999; Figs. 1A and at the base of Arsia Mons (Figs. 1A and 2A). 2A). We used detrended MOLA data to ex- DESCRIPTION AND Ridges are typically spaced 1 km apart, and amine the local topographic relationships (Fig. INTERPRETATION MOLA data show that individual ridges vary 1B) and found that the lava ﬂows emerging Collectively, the Arsia Mons fan-shaped de- in height: the outer prominent ridge reaches from the edge of the fan-shaped deposit could posits (Fig. 1A) are 350–450 km wide and heights of 50 m, whereas typical inner ridg- be readily traced inward beneath the ridged extend 500 km down the western ﬂanks in es are 5–20 m high. MOC images show evi- facies and even into the area beneath the a N55 W direction. They cover an area of dence for abundant dunes on and near the knobby facies, apparently without major dis- 180,000 km2, about the size of the state of ridges, suggesting that the ridges are com- ruption and modiﬁcation. The very distinctive Washington, United States, or almost twice posed of ﬁne-grained material that is subject substrate preserved below the ridges (both a the size of Iceland. Mars Global Surveyor to eolian modiﬁcation. MOC images also crater, Fig. 2A, and an earlier phase of lava (MGS) MOLA data show that the summit of show that the outermost ridge is asymmetrical; ﬂows, Fig. 1A), the blanket-like nature of the Arsia Mons is at 17.76 km above the aeroid, its steep side faces outward. Morphology of ridged facies, and the extreme regularity of the rising 16.8 km above the cratered terrain. the smaller ridges varies, ranging from peak- ridges contrast distinctly with landslide-like The base of the most distal facies of the fan- ed, to rounded, to ﬂat topped. We found no features seen on the nearby Olympus Mons shaped deposit (the ridged facies) is at 2600 depositional or erosional evidence that might aureole (Francis and Waadge, 1983). Similar- m elevation, 15 km below the summit. The be associated with wet-based glaciers, such as ly, although dunes and ridges can be produced majority of the deposits are at elevations be- eskers, sinuous channels, lake deposits, and/or during pyroclastic ﬂow emplacement, the reg- tween 2600 m and 7000 m. They are late Am- braided streams; likewise, we found no evi- ularity of the ridges and their great lateral ex- 642 GEOLOGY, July 2003 tent would require an unexpected homogene- and that the ice-front retreat must have been and the sublimation process can be very slow ity in a turbulent ﬂow. This distinctive and very even and symmetrical. The evenness of and nondisruptive, operating almost as a ‘‘de- delicate relationship with underlying terrain debris distribution favors widespread ﬁne de- ﬂation’’ and internal settling of the deposit as suggests that the fan-shaped deposit was em- bris emplaced by pyroclastic eruptions or dust the ice is progressively removed. The process placed by a process that involved little inter- deposited from the atmosphere, as opposed to can also result in the remaining deposit being action with the substrate. Consequently, we do debris scoured from below the glacier (very extremely loosely packed or friable (Lund- not favor the hypothesis that the ridged facies unlikely, owing to preservation of underlying qvist, 1989) and settling with time (Marchant represents the distal part of landslide deposits, impact craters and lava ﬂows) or deposited lo- et al., 2002). Grain size and particle charac- and instead support Lucchitta (1981) and sub- cally on top of the glacier by rockfalls or teristics will be inherited from the interstitial sequent workers, and explore more speciﬁc landslides. material in the parent ice deposits. Sublima- glacial environments in which such features tion tills on top of glacial ice in Antarctica can might form without modiﬁcation of the Knobby-Terrain Facies reach thicknesses and porosities that insulate substrate. The next innermost facies of the fan-shaped underlying ice from further diffusion and sub- Terrestrial analogues for deposits on Arsia deposit is composed of a largely chaotic as- limation. Thus, in some cases, glacier ice that Mons occur in the Dry Valleys region of Ant- semblage of hills, some as much as several is older than 8 Ma has been shown to underlie arctica. The Dry Valleys of southern Victoria kilometers across, that are subrounded to elon- sublimation till in the Dry Valleys (Sugden et Land (77 30 S, 162 E) occupy a hyperarid, gated downslope; some hills are aligned and al., 1995; Schafer et al., 2000; Marchant et al., cold-polar desert. Mean annual temperatures form arcs that in general are parallel to ridges 2002). vary from 35 C in interior regions to in the distal facies. Viking images reveal a Scott and Zimbelman (1995) interpreted the 14 C near the coast. Thus, glaciers in this narrow transitional zone of 100 km between knobby facies as having formed in a wastage environment are polar cold based (the tem- the outer ridged facies and inner knobby facies zone of an ice sheet and to include landslide perature throughout the glacier is below the where both arcuate ridges and knobby terrain material. In their assessment, a major bedrock pressure melting point, and deformation and are interspersed. The deposits that constitute scarp at the head of the knobby facies was movement occur within the ice), in contrast to the knobby terrain are very homogeneous in interpreted as a detachment surface, part of the temperate wet based (the temperature at the local areas (e.g., Fig. 2B) and show little de- knobby facies representing supraglacial ma- base is at the pressure melting point, and sig- tailed structure as a whole. There is no evi- terial derived from mass wasting and land- niﬁcant slip and erosion takes place at the dence for banking or pile-up of the deposits slides. Thus, a major outstanding question in base). Recent empirical and theoretical studies behind the small number of underlying prom- their interpretation is the relative signiﬁcance show that rates of subglacial erosion beneath inences. The relationship between the knobby of mass wasting in the origin of the knobby cold-based glaciers in the Dry Valleys (Mes- facies and the underlying deposits is made facies. The uniform distribution of the knobby erve Glacier) are as low as 9 10 7 to 3 clear by examination of detrended MOLA to- facies over 75,000 km2 argues against a sin- 10 6 m/yr (Cuffey et al., 2000) and that de- pography (Fig. 1B). Here, the distinctive un- gle landslide, or series of landslides, as a prob- posits from such glaciers (Atkins et al., 2002) derlying lava ﬂows can be traced into the area able mechanism for debris emplacement onto pass undisturbed across lava ﬂows, cinder of the ridged facies and then farther into the the glacier. Rather, we suggest that the uni- cones (Wilch et al., 1993), and unconsolidated area of the knobby facies. The margins of the form distribution and relief of the knobby fa- ash-avalanche deposits (Marchant et al., 1993, underlying ﬂows have not lost their coher- cies, as well as evidence for signiﬁcant eolian 1994; Fig. 2D). In interior regions of the Dry ence, as they probably would if the knobby modiﬁcation, point to eruptive pyroclastic de- Valleys, glacier ablation is almost entirely by facies represented a landslide deposit. Instead, posits (air fall onto the glacier surface) as a sublimation ( 90%), and geomorphic traces it appears as if the knobby facies has been likely source for the material that composes of meltwater erosion are lacking (Marchant deposited on top of the underlying lava ﬂows the knobby facies. We interpret the bedrock and Denton, 1996). Debris that falls onto without marked interaction with the substrate. scarp to represent supraglacial erosion, rather mountain glaciers in this region is transported Analysis of the interior of the knobby facies than the headwall of a landslide. supraglacially (or englacially if debris falls in and the regions around its exterior reveals lit- In summary, the knobby facies is interpret- the accumulation zone) and is dropped pas- tle evidence for features that might indicate ed as a sublimation till from a cold-based sively at stationary ice margins to form drop melting, such as channels, ponded material, or mountain glacier system on the basis of its (1) moraines (e.g., boulder-belt moraines of Den- eskers. MOC images show abundant evidence homogeneity, (2) knobby and hummocky ton et al., 1993) (Fig. 2D). If ice-margin re- for eolian modiﬁcation of the knobs and morphology, (3) superposition on underlying treat greatly exceeds the delivery of debris to mounds. lava-ﬂow topography without disruption, (4) the glacier snout, then a thin drift sheet com- On the basis of morphologic comparisons close association with the ridged facies, (5) su- posed of isolated and perched clasts marks ice of the knobby facies with cold-based, debris- perposition on the ridged facies, and (6) lack recession. Drop moraines and thin drifts de- covered glaciers in the Dry Valleys region of of melting-related features. The nature of the posited from advance and retreat of cold- Antarctica (Fig. 2E) (Sugden et al., 1995; sublimation process means that there is a good based ice in the western Dry Valleys region Marchant et al., 2002), as well as the mapped possibility that residual ice may underlie some are superposed with virtually no modiﬁcation distribution of the knobby facies inward of the of the knobs or parts of the larger deposit. of the substrate (Marchant et al., 1994). ridged facies, we conclude that the knobby fa- We thus interpret the ridged facies on Arsia cies represents a sublimation till, likely pro- Smooth Facies Mons as a series of drop moraines, each rep- duced by sublimation of debris-rich ice. Sub- The smooth facies inward of the knobby resenting a period of standstill of a cold-based limation tills are similar to melt-out tills, but terrain is characterized by a series of concen- glacier followed by a phase of retreat. The ex- the ice is lost by sublimation, rather than by tric ridges tens of meters high superposed on tremely even distribution of the ridges and melting. Sublimation till formation requires broad lobes hundreds of meters thick (Fig. their continuity mean that the debris distribu- contact of the ice with the atmosphere at the 2C). The heads of some lobes have depres- tion must have been extremely homogeneous surface or through diffusion through pores, sions at their centers. The largest lobe origi- GEOLOGY, July 2003 643 nates near the vicinity of a large linear de- Mons imply active ﬂow and suggest the pres- of cold climate features: Icarus, v. 45, p. 264–303. pression, a location that caused Scott and ence of buried glacier ice. Lundqvist, J., 1989, Till and glacial landforms in a Zimbelman (1995) to interpret the facies as dry, polar region: Zeitschrift fur Geomorphol- pyroclastic ﬂows emanating from a ﬁssure CONCLUSIONS ogie, v. 33, p. 27–41. (see also Zimbelman and Edgett, 1992) or Marchant, D.R., and Denton, G.H., 1996, Miocene We interpret the unusual Amazonian-aged, and Pliocene paleoclimate of the Dry Valleys possibly as a lahar. On the basis of the MOLA fan-shaped deposit covering 180,000 km2 of region, southern Victoria Land: A geomor- topography, we found that the lobes tend to the western ﬂank of Arsia Mons as the rem- phology approach: Marine Micropaleontology, be located on highs and to descend into ad- nant of a mountain glacier. In this scenario, v. 27, p. 253–271. jacent lows, suggesting that the lobes did not Marchant, D.R., Denton, G.H., Sugden, D.E., and the outer parallel-ridge zone is interpreted to Swisher, C.C., 1993, Miocene glacial stratig- originate as pyroclastic ﬂows originating in be distal drop moraines formed from the lat- raphy and landscape evolution of the western deep depressions. eral retreat of cold-based glacier ice and the Asgard Range, Antarctica: Geograﬁska An- On the basis of the general morphology of knobby facies to be proximal hummocky drift naler, v. 75, p. 303–330. Marchant, D.R., Denton, G.H., Bockheim, J.G., these deposits as revealed in the MOLA and resulting from the sublimation, decay, and Wilson, S.C., and Kerr, A.R., 1994, Quater- image data and their spatial association with downwasting of this ice (a sublimation till). nary ice-level changes of upper Taylor Glacier, the ridged and knobby facies, we have ex- The arcuate lobes in the proximal zone are Antarctica: Implications for paleoclimate and plored glacial analogues for these features. We interpreted to be rock-glacier deposits, formed ice-sheet dynamics: Boreas, v. 23, p. 29–42. Marchant, D., Lewis, A., Phillips, W., Moore, E., ﬁnd that rock-glacier deposits provide a very by ﬂow deformation of debris-covered ice; Souchez, R., Denton, G., Sugden, D., Potter, compelling analogue for these lobate features some deposits may still have ice cores. We N., and Landis, G., 2002, Formation of pat- (Martin and Whalley, 1987; Whalley and Mar- ﬁnd little evidence for meltwater features in terned ground and sublimation till over Mio- tin, 1992). Rock glaciers are commonly association with any facies, and thus conclude cene glacier ice in Beacon Valley, southern Victoria Land, Antarctica: Geological Society capped by lobate debris-covered deposits and that the glacier ice was predominantly cold of America Bulletin, v. 114, p. 718–730. are found in alpine environments. Rock gla- based throughout its history and ablation was Martin, H., and Whalley, W., 1987, Rock glaciers. ciers range from ice plus rock mixtures to largely by sublimation. Similar deposits are Part I: Rock glacier morphology, classiﬁcation seen on Pavonis and Ascraeus Montes. and distribution: Progress in Physical Geog- thin, debris-covered glaciers where ice might raphy, v. 2, p. 260–282. be preserved for considerable periods of time Potter, N., 1972, Ice-cored rock glacier Galena owing to the insulating effects of the debris. ACKNOWLEDGMENTS Creek, northern Absaroka Mountains, Wyo- Rock glaciers form when a core of glacial ice We gratefully acknowledge the ﬁnancial assis- ming: Geological Society of America Bulletin, tance of grants from the National Aeronautics and v. 83, p. 3025–3058. is progressively buried by a thick debris man- Space Administration (to Head) and the National Schafer, J.M., Bauer, H., Denton, G.H., Ivy-Ochs, tle; formation is favored by high debris ac- Science Foundation, Ofﬁce of Polar Programs (to S., Marchant, D.R., Schluchter, C., and Wieler, cumulation rates and low ice velocities, con- Marchant). We thank David Shean for signiﬁcant R., 2000, The oldest ice on Earth in Beacon ditions common in an advanced state of help in the data analysis and interpretation, Adam Valley, Antarctica: New evidence from surface Lewis for productive discussions on glacial pro- exposure dating: Earth and Planetary Science glacial retreat (Whalley and Martin, 1992). cesses, and James Dickson for discussions and help Letters, v. 179, p. 91–99. For most rock glaciers in the Dry Valleys re- in data preparation. Scott, D.H., and Zimbelman, J.R., 1995, Geologic gion of Antarctica, debris over buried glacier map of Arsia Mons volcano, Mars: U.S. Geo- REFERENCES CITED logical Survey Miscellaneous Geologic Inves- ice thickens progressively down ice ﬂow (Fig. Anguita, F., and Moreno, F., 1992, Shear-induced tigation Map I-2480, scale 1:1,000,000. 2F). 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"Cold based mountain glaciers on Mars Western Arsia Mons"