Cold based mountain glaciers on Mars Western Arsia Mons

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Cold based mountain glaciers on Mars Western Arsia Mons Powered By Docstoc
					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
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-
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ridges at the head of major lobes on Arsia         Lucchitta, B.K., 1981, Mars and Earth: Comparison      Printed in USA

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