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Geology
Cold-based mountain glaciers on Mars: Western Arsia Mons
James W. Head and David R. Marchant
Geology 2003;31;641-644
doi: 10.1130/0091-7613(2003)031<0641:CMGOMW>2.0.CO;2
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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
Surface environmental conditions on Mars are currently extremely cold and hyperarid,
most equivalent to polar deserts on Earth. Coupling newly acquired Mars data with fieldbased observations regarding the flow, surface morphology, and depositional history of
polar glaciers in Antarctica, we show that the multiple facies of an extensive fan-shaped
deposit on the western flanks of Arsia Mons volcano are consistent with deposition from
cold-based mountain glaciers. An outer ridged facies that consists of multiple laterally
extensive, arcuate and parallel ridges, resting without disturbance on both well-preserved
lava flows and an impact crater, is interpreted as drop moraines formed at the margin of
an ablating and predominantly receding cold-based glacier. A knobby facies that consists
of equidimensional knobs, each to several kilometers in diameter, is inward of the ridges;
this facies is interpreted as a sublimation till derived from in situ downwasting of ashrich glacier ice. A third facies comprising distinctive convex-outward lobes with concentric
parallel ridges and aspect ratios elongated downslope likely represents rock-glacier deposits, some of which may still be underlain by a core of glacier ice. Taken together, these
surficial deposits show that the western flank of Arsia Mons was occupied by an extensive
mountain glacial system accumulating on, and emerging from, the upper slopes of the
volcano and spreading downslope to form a piedmont-like fan.
Keywords: Mars, cold-based glaciers, rock glaciers, drop moraines, ablation till, Arsia Mons.
INTRODUCTION
Arsia Mons is one of the three Tharsis
Montes shield volcanoes that cap the broad
Tharsis Rise, a huge center of volcanism and
tectonism spanning almost the entire history
of Mars. Each of the Tharsis Montes, although
largely constructed of effusive and explosive
volcanic deposits, contains a distinctive and
unusual fan-shaped deposit on their western
flanks. These deposits, as exemplified by those
on Arsia Mons (e.g., Zimbelman and Edgett,
1992; Scott and Zimbelman, 1995), usually
contain three facies (Figs. 1A, 2A, 2B, 2C).
(1) An outermost ridged facies (Fig. 2A) consists of a broad, thin sheet characterized by
numerous ridges, tens of kilometers in length
and spaced a few hundred meters to several
kilometers apart, that pass across topographic
barriers without obvious deflection. (2) A
knobby-terrain facies (Fig. 2B) forms an extensive area of chaotic terrain that is characterized by subrounded several-kilometer-diameter hills; some hills form chains that are
parallel to subparallel to the ridges in the
ridged facies. (3) A smooth facies (Fig. 2C)
contains arcuate lineations and diffuse to lobate margins; the smooth facies appears to
overlie areas of the knobby facies. Scott and
Zimbelman (1995, and references therein) described various hypotheses for the origin of
these features, including one or more of the
following: lahars, debris avalanches, landslides, pyroclastic flows, and/or causes generally related to the advance and retreat of ice
(e.g., Lucchitta, 1981).
Two new developments are the basis for the
research reported in this analysis. First, new
Mars Orbiter Laser Altimeter (MOLA) altimetry and Mars Orbiter Camera (MOC) images
from the Mars Global Surveyor mission have
permitted us to characterize the fan-shaped deposit on Arsia Mons and its relationship to the
rest of the volcano in much more detail. Second, ongoing research on polar glaciers in
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.
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; July 2003; v. 31; no. 7; p. 641–644; 2 figures.
641
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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 framework for investigating glacier ice on Mars
(Marchant et al., 2002; Denton and Marchant,
2000; Sugden et al., 1995). On the basis of
present surface temperatures on Mars and
those of the recent past, any mountain glaciers
on Arsia Mons and nearby volcanoes were
likely to be cold based and most similar to the
slow-moving, cold-based glaciers of the Dry
Valleys region of Antarctica (Schafer et al.,
2000; Cuffey et al., 2000; Marchant et al.,
2002; Atkins et al., 2002).
DESCRIPTION AND
INTERPRETATION
Collectively, the Arsia Mons fan-shaped deposits (Fig. 1A) are ;350–450 km wide and
extend ;500 km down the western flanks in
a N558W direction. They cover an area of
;180,000 km2, about the size of the state of
Washington, United States, or almost twice
the size of Iceland. Mars Global Surveyor
(MGS) MOLA data show that the summit of
Arsia Mons is at ;17.76 km above the aeroid,
rising ;16.8 km above the cratered terrain.
The base of the most distal facies of the fanshaped deposit (the ridged facies) is at ;2600
m elevation, ;15 km below the summit. The
majority of the deposits are at elevations between 2600 m and 7000 m. They are late Am-
642
azonian in age and are largely contemporaneous with the latter phases of volcanism on
Arsia Mons (interdigitate relationships with
Tharsis Montes Formation Member 5 of Scott
and Zimbelman, 1995, and possibly predating
or being partly contemporaneous with Member 6).
Ridged Facies
The ridged facies consists of a series of
.100 concentric ridges that extend several
hundred kilometers beyond the break in slope
at the base of Arsia Mons (Figs. 1A and 2A).
Ridges are typically spaced ;1 km apart, and
MOLA data show that individual ridges vary
in height: the outer prominent ridge reaches
heights of ;50 m, whereas typical inner ridges are 5–20 m high. MOC images show evidence for abundant dunes on and near the
ridges, suggesting that the ridges are composed of fine-grained material that is subject
to eolian modification. MOC images also
show that the outermost ridge is asymmetrical;
its steep side faces outward. Morphology of
the smaller ridges varies, ranging from peaked, to rounded, to flat topped. We found no
depositional or erosional evidence that might
be associated with wet-based glaciers, such as
eskers, sinuous channels, lake deposits, and/or
braided streams; likewise, we found no evi-
dence for ridge offsets that might represent
tear faults resulting from uneven compression
associated with landslide deposits, as seen in
the Olympus Mons aureole (e.g., Francis and
Waadge, 1983).
One of the most distinctive characteristics
of the ridged facies is its superposition on a
subjacent impact crater and lava flows without
apparent modification (Williams, 1978; Lucchitta, 1981; Zimbelman and Edgett, 1992;
Anguita and Moreno, 1992; Scott and Zimbelman, 1995; Helgason, 1999; Figs. 1A and
2A). We used detrended MOLA data to examine the local topographic relationships (Fig.
1B) and found that the lava flows emerging
from the edge of the fan-shaped deposit could
be readily traced inward beneath the ridged
facies and even into the area beneath the
knobby facies, apparently without major disruption and modification. The very distinctive
substrate preserved below the ridges (both a
crater, Fig. 2A, and an earlier phase of lava
flows, Fig. 1A), the blanket-like nature of the
ridged facies, and the extreme regularity of the
ridges contrast distinctly with landslide-like
features seen on the nearby Olympus Mons
aureole (Francis and Waadge, 1983). Similarly, although dunes and ridges can be produced
during pyroclastic flow emplacement, the regularity of the ridges and their great lateral ex-
GEOLOGY, July 2003
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tent would require an unexpected homogeneity in a turbulent flow. This distinctive and
delicate relationship with underlying terrain
suggests that the fan-shaped deposit was emplaced by a process that involved little interaction with the substrate. Consequently, we do
not favor the hypothesis that the ridged facies
represents the distal part of landslide deposits,
and instead support Lucchitta (1981) and subsequent workers, and explore more specific
glacial environments in which such features
might form without modification of the
substrate.
Terrestrial analogues for deposits on Arsia
Mons occur in the Dry Valleys region of Antarctica. The Dry Valleys of southern Victoria
Land (778309S, 1628E) occupy a hyperarid,
cold-polar desert. Mean annual temperatures
vary from ;235 8C in interior regions to
;214 8C near the coast. Thus, glaciers in this
environment are polar cold based (the temperature throughout the glacier is below the
pressure melting point, and deformation and
movement occur within the ice), in contrast to
temperate wet based (the temperature at the
base is at the pressure melting point, and significant slip and erosion takes place at the
base). Recent empirical and theoretical studies
show that rates of subglacial erosion beneath
cold-based glaciers in the Dry Valleys (Meserve Glacier) are as low as 9 3 1027 to 3 3
1026 m/yr (Cuffey et al., 2000) and that deposits from such glaciers (Atkins et al., 2002)
pass undisturbed across lava flows, cinder
cones (Wilch et al., 1993), and unconsolidated
ash-avalanche deposits (Marchant et al., 1993,
1994; Fig. 2D). In interior regions of the Dry
Valleys, glacier ablation is almost entirely by
sublimation (.90%), and geomorphic traces
of meltwater erosion are lacking (Marchant
and Denton, 1996). Debris that falls onto
mountain glaciers in this region is transported
supraglacially (or englacially if debris falls in
the accumulation zone) and is dropped passively at stationary ice margins to form drop
moraines (e.g., boulder-belt moraines of Denton et al., 1993) (Fig. 2D). If ice-margin retreat greatly exceeds the delivery of debris to
the glacier snout, then a thin drift sheet composed of isolated and perched clasts marks ice
recession. Drop moraines and thin drifts deposited from advance and retreat of coldbased ice in the western Dry Valleys region
are superposed with virtually no modification
of the substrate (Marchant et al., 1994).
We thus interpret the ridged facies on Arsia
Mons as a series of drop moraines, each representing a period of standstill of a cold-based
glacier followed by a phase of retreat. The extremely even distribution of the ridges and
their continuity mean that the debris distribution must have been extremely homogeneous
GEOLOGY, July 2003
and that the ice-front retreat must have been
very even and symmetrical. The evenness of
debris distribution favors widespread fine debris emplaced by pyroclastic eruptions or dust
deposited from the atmosphere, as opposed to
debris scoured from below the glacier (very
unlikely, owing to preservation of underlying
impact craters and lava flows) or deposited locally on top of the glacier by rockfalls or
landslides.
Knobby-Terrain Facies
The next innermost facies of the fan-shaped
deposit is composed of a largely chaotic assemblage of hills, some as much as several
kilometers across, that are subrounded to elongated downslope; some hills are aligned and
form arcs that in general are parallel to ridges
in the distal facies. Viking images reveal a
narrow transitional zone of .100 km between
the outer ridged facies and inner knobby facies
where both arcuate ridges and knobby terrain
are interspersed. The deposits that constitute
the knobby terrain are very homogeneous in
local areas (e.g., Fig. 2B) and show little detailed structure as a whole. There is no evidence for banking or pile-up of the deposits
behind the small number of underlying prominences. The relationship between the knobby
facies and the underlying deposits is made
clear by examination of detrended MOLA topography (Fig. 1B). Here, the distinctive underlying lava flows can be traced into the area
of the ridged facies and then farther into the
area of the knobby facies. The margins of the
underlying flows have not lost their coherence, as they probably would if the knobby
facies represented a landslide deposit. Instead,
it appears as if the knobby facies has been
deposited on top of the underlying lava flows
without marked interaction with the substrate.
Analysis of the interior of the knobby facies
and the regions around its exterior reveals little evidence for features that might indicate
melting, such as channels, ponded material, or
eskers. MOC images show abundant evidence
for eolian modification of the knobs and
mounds.
On the basis of morphologic comparisons
of the knobby facies with cold-based, debriscovered glaciers in the Dry Valleys region of
Antarctica (Fig. 2E) (Sugden et al., 1995;
Marchant et al., 2002), as well as the mapped
distribution of the knobby facies inward of the
ridged facies, we conclude that the knobby facies represents a sublimation till, likely produced by sublimation of debris-rich ice. Sublimation tills are similar to melt-out tills, but
the ice is lost by sublimation, rather than by
melting. Sublimation till formation requires
contact of the ice with the atmosphere at the
surface or through diffusion through pores,
and the sublimation process can be very slow
and nondisruptive, operating almost as a ‘‘deflation’’ and internal settling of the deposit as
the ice is progressively removed. The process
can also result in the remaining deposit being
extremely loosely packed or friable (Lundqvist, 1989) and settling with time (Marchant
et al., 2002). Grain size and particle characteristics will be inherited from the interstitial
material in the parent ice deposits. Sublimation tills on top of glacial ice in Antarctica can
reach thicknesses and porosities that insulate
underlying ice from further diffusion and sublimation. Thus, in some cases, glacier ice that
is older than 8 Ma has been shown to underlie
sublimation till in the Dry Valleys (Sugden et
al., 1995; Schafer et al., 2000; Marchant et al.,
2002).
Scott and Zimbelman (1995) interpreted the
knobby facies as having formed in a wastage
zone of an ice sheet and to include landslide
material. In their assessment, a major bedrock
scarp at the head of the knobby facies was
interpreted as a detachment surface, part of the
knobby facies representing supraglacial material derived from mass wasting and landslides. Thus, a major outstanding question in
their interpretation is the relative significance
of mass wasting in the origin of the knobby
facies. The uniform distribution of the knobby
facies over ;75,000 km2 argues against a single landslide, or series of landslides, as a probable mechanism for debris emplacement onto
the glacier. Rather, we suggest that the uniform distribution and relief of the knobby facies, as well as evidence for significant eolian
modification, point to eruptive pyroclastic deposits (air fall onto the glacier surface) as a
likely source for the material that composes
the knobby facies. We interpret the bedrock
scarp to represent supraglacial erosion, rather
than the headwall of a landslide.
In summary, the knobby facies is interpreted as a sublimation till from a cold-based
mountain glacier system on the basis of its (1)
homogeneity, (2) knobby and hummocky
morphology, (3) superposition on underlying
lava-flow topography without disruption, (4)
close association with the ridged facies, (5) superposition on the ridged facies, and (6) lack
of melting-related features. The nature of the
sublimation process means that there is a good
possibility that residual ice may underlie some
of the knobs or parts of the larger deposit.
Smooth Facies
The smooth facies inward of the knobby
terrain is characterized by a series of concentric ridges tens of meters high superposed on
broad lobes hundreds of meters thick (Fig.
2C). The heads of some lobes have depressions at their centers. The largest lobe origi-
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nates near the vicinity of a large linear depression, a location that caused Scott and
Zimbelman (1995) to interpret the facies as
pyroclastic flows emanating from a fissure
(see also Zimbelman and Edgett, 1992) or
possibly as a lahar. On the basis of the MOLA
topography, we found that the lobes tend to
be located on highs and to descend into adjacent lows, suggesting that the lobes did not
originate as pyroclastic flows originating in
deep depressions.
On the basis of the general morphology of
these deposits as revealed in the MOLA and
image data and their spatial association with
the ridged and knobby facies, we have explored glacial analogues for these features. We
find that rock-glacier deposits provide a very
compelling analogue for these lobate features
(Martin and Whalley, 1987; Whalley and Martin, 1992). Rock glaciers are commonly
capped by lobate debris-covered deposits and
are found in alpine environments. Rock glaciers range from ice plus rock mixtures to
thin, debris-covered glaciers where ice might
be preserved for considerable periods of time
owing to the insulating effects of the debris.
Rock glaciers form when a core of glacial ice
is progressively buried by a thick debris mantle; formation is favored by high debris accumulation rates and low ice velocities, conditions common in an advanced state of
glacial retreat (Whalley and Martin, 1992).
For most rock glaciers in the Dry Valleys region of Antarctica, debris over buried glacier
ice thickens progressively down ice flow (Fig.
2F). Rock glaciers move downslope as a result
of the deformation of internal ice or frozen
sediments and are characterized by surface
ridges and furrows.
On the basis of the similarity of the surface
morphology and geomorphic setting of the
smooth facies with terrestrial rock glaciers
(Steig et al., 1998), as well as its spatial relationship with knobby sublimation till and
distal moraines, we interpret this unit to be
composed of rock glaciers formed in the proximal parts of the fan-shaped deposit, perhaps
during the waning stages of the ice-sheet evolution. In this regard, the spoon-shaped depressions at the head of the rock-glacier lobes
likely reflect surface lowering due to excess
sublimation; such depressions are a common
feature of terrestrial rock glaciers given that
debris cover may be relatively thin at rockglacier heads (Potter, 1972) (Fig. 2F). In the
case of Arsia Mons, it is unlikely that such
hollows would form if the lobes were pyroclastic flows. Pyroclastic flows generally thin
progressively away from the source. The sharp
ridges at the head of major lobes on Arsia
644
Mons imply active flow and suggest the presence of buried glacier ice.
CONCLUSIONS
We interpret the unusual Amazonian-aged,
fan-shaped deposit covering ;180,000 km2 of
the western flank of Arsia Mons as the remnant of a mountain glacier. In this scenario,
the outer parallel-ridge zone is interpreted to
be distal drop moraines formed from the lateral retreat of cold-based glacier ice and the
knobby facies to be proximal hummocky drift
resulting from the sublimation, decay, and
downwasting of this ice (a sublimation till).
The arcuate lobes in the proximal zone are
interpreted to be rock-glacier deposits, formed
by flow deformation of debris-covered ice;
some deposits may still have ice cores. We
find little evidence for meltwater features in
association with any facies, and thus conclude
that the glacier ice was predominantly cold
based throughout its history and ablation was
largely by sublimation. Similar deposits are
seen on Pavonis and Ascraeus Montes.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial assistance of grants from the National Aeronautics and
Space Administration (to Head) and the National
Science Foundation, Office of Polar Programs (to
Marchant). We thank David Shean for significant
help in the data analysis and interpretation, Adam
Lewis for productive discussions on glacial processes, and James Dickson for discussions and help
in data preparation.
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Manuscript received 29 January 2003
Revised manuscript received 11 March 2003
Manuscript accepted 19 March 2003
Printed in USA
GEOLOGY, July 2003