Planetary and Space Science 103 (2014) 331–338
Contents lists available at ScienceDirect
Planetary and Space Science
journal homepage: www.elsevier.com/locate/pss
Preservation of ancient ice at Pavonis and Arsia Mons: Tropical
mountain glacier deposits on Mars
James W. Head n, David K. Weiss
Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
art ic l e i nf o
a b s t r a c t
Article history:
Received 2 June 2014
Received in revised form
11 August 2014
Accepted 9 September 2014
Available online 20 September 2014
Large tropical mountain glacier (TMG) deposits on the northwest flanks of the Tharsis Montes and
Olympus Mons volcanoes are interpreted to be the record of ancient climates characteristic of Mars
several hundred million years ago when planetary spin-axis obliquity was 451. During this era, polar
volatiles (predominantly H2O) were mobilized and transferred equatorward, undergoing adiabatic
cooling on the Tharsis volcano flanks, and precipitating snow and ice to form cold-based tropical
mountain glaciers up to several kilometers in thickness. Subsequent climate change resulted in retreat,
sublimation and collapse of the tropical mountain glaciers, leaving the three typical facies observed
today: (1) concentric ridges, the ridged facies, interpreted as drop moraines; (2) knobby facies,
interpreted as debris-dominated sublimation residue; and (3) the smooth facies, interpreted as remnant
alpine glacial deposits. Ring-mold craters (RMCs) are distinctive features formed by impacts into debriscovered ice. We describe a set of relatively fresh ring-mold craters superposed on the Arsia and Pavonis
Mons TMG deposits; we interpret these to indicate that the impact events penetrated a veneer of
sublimation lag and excavated buried remnant glacial ice, despite the lack of detection of buried ice by
orbital radar instruments. The diameter distribution of the RMCs suggest that the remnant ice lies at a
depth of at least 16 m. The TMG deposit ages suggest that these ice deposits date from a period in the
range of 125–220 million years before the present; the remnant ice may thus preserve records of the
ancient atmospheric gas content and microbiota, as is common in terrestrial glacial ice. Preservation of
this ice and the lack of any associated fluvial features suggest that the post-glacial climate has been cold,
and related surface temperatures have not been sufficient to bring the buried deposits to the melting
point of water.
& 2014 Elsevier Ltd. All rights reserved.
Keywords:
Mars
Surface
Remnant ice
Tropical mountain glaciers
Impact crater
Pavonis Mons
Arsia Mons
1. Introduction
Mars is currently a hyper-arid, very cold desert, with a mean
annual temperature ( 210 K) well below the freezing point of
water. Mean surface temperature and pressure (Clancy et al., 2000;
Christensen et al., 2001; Smith et al., 2001) produce near-surface
temperature–pressure conditions below the triple point of water.
Although nearly saturated with water vapor, the atmosphere is so
tenuous that it holds only five to tens of perceptible microns
(Schorn et al., 1969). Insolation is latitude-dependent; polar regions
represent cold traps for volatiles and yet higher equatorial temperatures mean that water is not stable on the surface there in any
form. Theoretical calculations (Mellon and Jakosky, 1995; Mellon
et al., 1997) and spacecraft observations (Feldman et al., 2002) show
that current conditions permit water ice in the shallow near surface
at latitudes from 451 to the poles. Studies of the spin-axis/orbital
n
Corresponding author.
E-mail address: [email protected] (J.W. Head).
http://dx.doi.org/10.1016/j.pss.2014.09.004
0032-0633/& 2014 Elsevier Ltd. All rights reserved.
parameter history of Mars provide a robust solution for the most
recent 20 Ma of martian history, but do not provide specific
predictions back into the past due to the chaotic nature of the
solutions (Laskar et al., 2004); in general, obliquities are predicted
to reach 451 and commonly spent significant time at 351, much
higher than the current 251. These variations in spin-axis/orbital
parameters in the past history of Mars have significantly modified
the insolation geometry of Mars. For example, atmospheric global
climate models (GCMs) show that at obliquities greater than the
current value, water is mobilized from the poles, transported
equatorward, and deposited at lower latitudes (Head et al., 2003;
Forget et al., 2006; Madeleine et al., 2009). Indeed, evidence is
accumulating that deposits representing significant regional glaciation in the past history of Mars occur at all latitudes (Head and
Marchant, 2003; Head et al., 2010). Analysis of such deposits
permits an assessment of the amounts and types of volatiles, their
stability and mobility, and the long-term geological record of
climate change on Mars.
Analysis of extensive fan-shaped deposits on the northwest
flanks of the equatorial Tharsis Montes with modern data has
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J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
provided compelling evidence that they represent the remnants of
tropical mountain glaciers (TMG) dating from the Late Amazonian
period of Mars history (Head and Marchant, 2003; Shean et al.,
2005, 2007; Milkovich et al., 2006; Kadish et al., 2008, 2014);
superposed impact craters show that the best-fit ages of the TMG
are in the range of 125–220 million years (Shean et al., 2005;
Kadish et al., 2014). The distinct geomorphology of the deposits,
together with updated terrestrial analogs for glaciation under
martian hyper-arid, extremely cold conditions, shows that the
tropical mountain glaciers were cold-based, a condition in which
basal melting does not occur and glacial advance occurs primarily
through internal deformation of the ice (Marchant and Head,
2007). Global climate models (GCMs) show that when obliquity
reaches 451, water-rich polar air ascends the flanks of the
Tharsis Rise, encounters the northwest flanks of the Tharsis
volcanoes, undergoes upwelling and adiabatic cooling, precipitating snow on the northwest volcano flanks (Forget et al., 2006).
Models of accumulation and glacial flow show that this scenario
can readily produce the observed tropical mountain glacier deposits (Fastook et al., 2008).
On Earth, when cold-based glaciers retreat, ablation can result in
an increase of debris on top of the glacier (sublimation residue or
till); this deposit can significantly decrease the sublimation rate and
protect the buried ice from further loss of ice, preserving it for long
periods (Kowalewski et al., 2006). At cold-based glaciers under
Mars-like conditions in the Antarctic Dry Valleys on Earth, ice buried
below sublimation till may be as old as 8 million years (Sugden et al.,
1995; Marchant et al., 2007). Is there any evidence of similar
remnant ice in the tropical mountain glaciers on Mars? We report
here on compelling evidence for the preservation of the order of
hundreds of meters of buried ice remaining from the Pavonis and
Arsia Mons tropical mountain glaciers, despite the lack of ice
detection by the SHARAD orbital radar instrument (Campbell et al.,
2013). This ice may preserve records of ancient atmosphere and
biology of Mars, and is thus an important exploration target.
Fig. 1. Mars Orbiter Laser Altimeter (MOLA) topography overlain on MOLA shaded
relief map (463 m/pixel) showing Pavonis and Arsia Mons. Smooth facies are
demarcated by red outline. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
2. Description and interpretation
The Pavonis and Arsia Mons tropical mountain glacier deposits
cover an area of 75,000 km2 and 180,000 km2, respectively
(Shean et al., 2005) and consist of three basic facies (Zimbelman
and Edgett, 1992; Head and Marchant, 2003) (Figs. 1–3). The
outermost ridged facies consists of concentric ridges interpreted
as cold-based drop moraines (Head and Marchant, 2003), deposited
during still stands while the glacier was advancing and retreating.
The intermediate knobby facies consists of hillocks and small ridges
interpreted to represent the sublimation till derived from downwasting of sediment-rich glacier ice. Because the glaciers are
predominantly cold-based and because topographic prominences
are sparse on the volcano flanks, debris is most likely derived from
pyroclastic or phreatomagmatic eruptions from the adjacent volcanoes (Wilson and Head, 1994, 2004, 2007, 2009). The proximal
smooth facies (Fig. 1; Pavonis area: 12,500 km2; Shean et al., 2005;
Arsia area: 25,500 km2) consists of lobate, relatively smoothtextured deposits interpreted as the remnants of individual coldbased glacial lobes (alpine-like glaciers), emplaced in the waning
stages of glaciation (Figs. 2 and 3). On Earth, in cold-based glacial
regions like the Antarctic Dry Valleys, such alpine-like cold-based
glaciers begin at accumulation zones, flow downhill, and lose ice in
the ablation zone (Benn and Evans, 2010; Marchant and Head,
2007; Mackay et al., under review). If not covered by debris, such
glaciers can advance and retreat with virtually no effect on macroand meso-scale geomorphology (Kleman and Hättestrand, 1999;
Borgström, 1999; Fabel et al., 2002; Marchant and Head, 2007) and
no residual evidence of their presence. Debris-covered cold-based
Fig. 2. Geologic map of Arsia Mons fan-shaped deposit superposed on Mars Orbiter
Laser Altimeter (MOLA) shaded relief map illustrating different facies interpreted as
drop moraines, sublimation till, and smooth facies. Fan-shaped deposits: ridged (R);
knobby (K); smooth (S). Other adjacent deposits: shield (SA); degraded western flank
(SB); smooth lower western flank (SC); caldera floor (CF); caldera wall (CW); flank vent
flows from Arsia Mons (PF); undivided Tharsis plains (P). Adapted from Zimbelman and
Edgett (1992) after Head and Marchant (2003) and Wilson and Head (2009).
glaciers, on the other hand, build up a protective sublimation till
derived from supraglacial and englacial debris. As the glacier
advances downslope, sublimation concentrates this debris into a
surface sublimation till, increasingly lowering ice-loss rates as a
function of distance from the source. As glacial conditions wane,
increasingly larger amounts of ice are lost in the proximal areas,
where the till is thin to non-existent, and ice is preserved longest in
the distal portions, where the insulating effect of the till is greatest
(Marchant and Head, 2007; Mackay et al., under review). This
commonly produces asymmetrical downslope profiles representing
a range of features from ice-cored terminal moraines to preserved
debris-covered lobate snouts. Similar configurations are seen in the
lobes at Pavonis and Arsia Mons (e.g., Figs. 2 and 3).
Could these lobes, morphologically and environmentally similar
to those seen on Earth, still contain remnant glacial ice from the
J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
333
7°N
4.6
Elevation (km)
2.8
6°N
5°N
4°N
116°W
115°W
114°W
113°W
Fig. 3. Geologic map of the Pavonis Mons fan-shaped Deposit. (A) MOLA gridded topography map superposed on THEMIS IR daytime global mosaic. (B) Geologic sketch map
of fan shaped deposit (tan) and smooth facies (red). Adapted from Shean et al. (2005). (For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Late Amazonian glaciation many tens of millions of years ago?
A source of information is surface morphology. For example, pitted
regions have been interpreted as evidence for the former presence
of ice (e.g., Kreslavsky and Head, 2000, 2002; Mustard et al., 2001).
Impact craters could also serve as probes into ice lying at depth
beneath a lag cover (e.g. Kress and Head, 2008). For example, a
certain type of crater morphology, known as ring-mold craters
(RMCs; Fig. 4), are concentrated almost exclusively on surfaces
thought to represent debris-covered glacial landsystems (Kress and
Head, 2008). Mangold (2003) found that bowl-shaped craters tend
to be smaller than RMCs and interpreted RMCs to be infilled and
deflated; Mangold (2003) interpreted RMCs as part of a continuum
of crater modification. Kress and Head (2008), on the other hand,
interpreted the size distribution, occurrence on debris-covered
glacial landsystems, and unusual morphology of RMCs to be the
result of impact and penetration through a till layer into an
underlying glacial deposit; smaller craters primarily form bowlshaped morphologies due to excavation exclusively in the till layer.
Experimental data indicate that impacts into porous layers can
produce compressed material and impact melt that can line the
crater cavity (Mangold et al., 2002); following deflation of surrounding material, and/or viscous relaxation of the substrate, this
more coherent material is left exposed as a saucer-shaped indurated layer (Mangold, 2003) (Fig. 5C).
Ring-mold craters (Kress and Head, 2008) in tropical mountain
glacier deposits might, therefore, represent impact probes that
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J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
0
0
1 Km
1 Km
Fig. 4. Examples of ring-mold craters from the smooth facies of the tropical mountain glaciers. (A) Fresh RMC on the Pavonis Mons smooth facies, (B) sketch map of (A).
(C) Fresh RMC on the Arsia Mons smooth facies. (D) Sketch map of (C). Sketch maps show the hummocky zone (orange), inner saucer-shaped zone (green), inward-facing
annulus zone (black), outer rim (gray), ejecta deposits (yellow), concentric fractures (black lines), high superposing topography (red) and surrounding hummocky terrain
(white). (A) CTX image D02_028015_1751, and (B) CTX image B01_010002_1852. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
sampled ice in the substrate of the glacial deposits. These observations
lead us to propose a specific model for the formation and evolution
of impact craters formed in the proximal lobate smooth deposits of the
Tharsis tropical mountain glaciers (Fig. 5): (1) Impact into a glacial
deposit overlain by meters of lag material; lag and ice excavated as
ejecta; secondary craters and melt lens form. (2) Exposed icy material
within the ejecta, melt lens and exposed inner crater wall sublimate;
inward viscous flow of the ice causes crater relaxation; concentric
flexural cracking occurs.
While RMC craters exhibiting a variety of apparent degradation
states are present on the smooth northern facies on Pavonis Mons,
we have identified seven examples that appear strikingly fresh
(Figs. 4 and 6). These fresh RMCs range from 350 m to 1.2 km
in diameter, exhibit shallow floors, and in some cases, display
prominent ejecta (Figs. 4A and 6A). We identified two examples of
fresh RMCs on the Arsia Mons smooth facies (e.g. Fig. 4C).
In general, these RMCs can be divided up into six zones from
the center outward (e.g. Fig. 4B and D). The first zone, the central
part of the floor, consists of a variety of small hummocks, mostly
randomly distributed, but with a somewhat orthogonal linear
texture. Frequently, a second zone is present in the interior of
the crater that resembles a pie-crust shaped layer in which the
inner hummocky textured deposit rests. It is broadly saucer or
bowl shaped, convex-down, and its outer margins form a crenulated pie-crust like scarp with a steep, outward-facing margin
a few meters high. This interior morphology is typical of RMCs
found in other debris-covered glacial systems (e.g. Kress and Head,
2008; Beach and Head, 2013). The third zone is 100 m wide and
forms an inward-facing annulus around the crenulated pie-crustlike scarp; we interpret this to be the remnant rim-crest. Frequently, a fourth zone, consisting of ejecta deposits, surrounds the
rim crest. The fifth zone consists of a series of one or more parallel
fractures a few decameters wide, arrayed generally concentrically
to the structure. The sixth and outermost zone represents the
generally hummocky terrain of the background deposits, but
locally this is peppered with small irregular depressions interpreted to be secondary craters from the initial impact. These
extend out to distances of 3.5 km from the center of the impact.
On the basis of the characteristics of the inner zones (unusual
pie-crust margin scarp, concentric fractures), the presence of surrounding secondary craters (confirming an impact origin and the
relative youth of the event), and the nature of RMCs elsewhere
(positive correlation with a range of landforms interpreted to be
debris-covered glaciers; Kress and Head, 2008; Beach and Head,
2013), we interpret these features to represent the remnants of
impact craters that penetrated through a meters-thick lag deposit
composed of ash/tephra, into the thick distal lobate glacial deposits.
The fresh nature of these craters (secondaries, ejecta, observable
crater cavities) suggest that they possess a young age, and that icy
remnants of the underlying glacial deposit are still present. It is
J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
CONTINUOUS ICE-RICH
EJECTA DEPOSIT
MELT
SECONDARY CRATERS
WALLS EXPOSE
FRESH ICE OUTCROPS
SUBLIMATION TILL
FORMATION
STAGE
GLACIAL ICE
SINTERED
ICY EJECTA
SUBLIMATES
GLACIAL ICE
VISCOUS
FLOW
MODIFICATION
STAGE
Zone 6;
background Zone 5;
deposit
fractures
FLEXURAL
CRACKING
Zone 1;
Zone 3;
remnant rim interior hummocks
Zone 4;
ejecta
Zone 2;
“pie-crust”
scarp
EXTENSIONAL
RIDGES
SECONDARY
PITS
DEGRADED
PRESENT
STATE
Fig. 5. Reconstruction of RMC formation. (A) Crater formation, (B) modification
stage, and (C) present configuration. Two processes, sublimation of blocky ejecta
(curved arrows) and crater wall material, and viscous relaxation of subsurface ice
play leading roles in the current morphology of ring-mold craters. Adapted from
Head and Marchant (2007).
interesting to note that smaller RMCs observed in this study (Fig. 6A
and D) and that of Kress and Head (2008) typically display one
negative annulus surrounding the crater rim, while larger RMC
craters (Figs. 4A and C and 6B and C) display extensive concentric
cracking. Indeed, the negative annulus around a fresh RMC on the
Pavonis Mons smooth facies (Fig. 6C) shows a rather sharp and
narrow annulus compared with the broader annulus around a more
degraded RMC of comparable size (Fig. 6D). We suggest that the
concentric crack generated by the viscously relaxing ice exposes the
underlying ice to differential sublimation, further widening the
crack into the commonly observed broad annulus. As crater diameter, and impact energy increases, the higher ice temperatures
generate enhanced viscous relaxation, thereby increasing the concentric cracking extent of larger craters (Figs. 4A and C and 6B and C)
relative to smaller craters (Fig. 6A and D). Higher geothermal heat
fluxes on the flanks of the Tharsis Montes may further facilitate
extensive viscous relaxation and concentric cracking in RMCs.
On the basis of these guidelines, we located and analyzed
candidate impact craters that might penetrate into these deposits
and provide clues as to whether significant ice has been preserved.
If these craters do indeed represent impacts into ice, a sizetransition between RMCs and bowl-shaped morphologies should
be evident, corresponding to the lag thickness (e.g. Kress and
Head, 2008; Beach and Head, 2013). Our analysis of the Pavonis
Mons smooth facies identified 31 candidate RMCs, 53 bowl-shaped
craters, and 47 craters that shared the inner hummocky texture of
RMCs but lack the outer textures (fractures, ejecta, secondaries,
etc.) (6) and resemble viscously relaxed craters from experiments
335
(Fig. 7; Scott, 1967); we refer to these craters as “relaxed” craters
and interpret them to be older, degraded RMCs. In line with
impact experiments into ice (Croft, 1981; Kato et al., 1995), we
scale penetration depth as 20% of the crater diameter. Comparing
the penetration depth distribution of the three different morphologies (Fig. 8) on the smooth facies of Pavonis Mons, it is apparent
that a transition from bowl-shaped to RMC occurs over a small
depth range: the maximum bowl-shaped crater depth is 40 m, and
average depth is 16 m, compared with the minimum RMC depth of
17 m. The presence of this transition indicates a change in target
properties at an average depth between 16 and 40 m. We
interpret these relationships to indicate that the average lag
thickness of the Pavonis Mons smooth facies is 16–40 m; smaller
craters (those that penetrate o16–40 m) formed exclusively in
the lag deposit and display a bowl morphology, while larger
craters (those that penetrate 416–40 m) excavate both lag material and underlying glacial ice and form RMCs.
We conducted an identical study on the Arsia Mons smooth facies,
but limited the identification of bowl-shaped and relaxed craters to
the localized smooth facies where only two examples of fresh RMCs
were found. The minimum penetration depth of RMCs is 229 m, and
the maximum bowl penetration depth is 136 m (Fig. 8). We interpret
these relationships to indicate that in this region of the Arsia Mons
smooth facies, the lag thickness is between 136 and 229 m thick. Due
to the presence of only two examples of RMCs on the smooth facies of
Arsia Mons, the geographic extent of remnant ice is not yet clear.
The thicknesses of the lag deposits on the Pavonis and Arsia
Mons TMG deposits is within the range of ash/tephra thicknesses
estimated by Wilson and Head (2009). Wilson and Head (2009)
showed that when ash/tephra accumulates to thicknesses in
excess of 2 m, they are only expected to melt several meters of
ice, and subsequent to their cooling, they form an effective barrier
to substantial sublimation. While the thickness of the lag deposits
on the Pavonis and Arsia Mons smooth facies remains uncertain
due to the small number of examples, the clear transition from
RMC to bowl-shaped morphology indicates the presence of a
transition in target properties. We suggest that this transition
results from the presence of remnant ice deposits underneath a lag
deposit that is likely to be composed of ash and tephra from
subsequent volcanic eruptions (e.g. Wilson and Head, 2009). The
thickness of the smooth facies of Pavonis Mons (Shean et al., 2005)
and Arsia Mons range from 50 to 600 m thick. While the heterogeneous nature of the lag thickness on the smooth facies prevents
a clear determination of lag thickness, it is possible that hundreds
of meters of glacial ice may still be present beneath the thick lag
material.
3. Radar data and subsurface structure
The Shallow Radar (SHARAD; Seu et al., 2007) and Mars Advanced
Radar for Subsurface and Ionospheric Sounding (MARSIS; Picardi et al.,
2005; Jordan et al., 2009) experiments provide the opportunity to
assess the nature of the surface roughness, to measure the penetration
of radar waves, to look for the presence of basal and internal reflectors
in the unit and to assess the presence of buried ice. Campbell et al.
(2013) studied the roughness and near-surface density of the Pavonis
Mons deposit using SHARAD data by combining surface-properties
analysis, subsurface sounding, and high-resolution optical images.
They showed that the Pavonis Mons fan-shaped deposit differs
significantly from lobate debris aprons which SHARAD has shown to
be ice-cored (Holt et al., 2008; Plaut et al., 2009). They found no
internal radar reflections from the smooth facies portion of the Pavonis
Mons fan-shaped deposit, and on the basis of these observations
interpreted the deposits to be either (1) quite thin or, (2) to have
little dielectric (i.e., density) contrast with the underlying terrain.
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J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
Fig. 6. Examples of the varying morphology and degradation state of ring-mold craters from the smooth facies of the tropical mountain glaciers. (A) Fresh RMC on the
Pavonis Mons smooth facies exhibiting one concentric fracture and fresh ejecta deposits. (B and C) More subdued RMC craters on the Pavonis Mons smooth facies exhibiting
numerous concentric fractures. (E) “Relaxed” crater on Pavonis Mons appears to have relaxed interior morphology and muted outer textures. (A) CTX image
G20_026076_1843, (B, C and D) CTX image B01_010002_1852.
Fig. 7. Experimentally produced viscous relaxation of crater forms in asphalt (Scott, 1967).
We examine two scenarios that might explain the lack of radar ice
detection:
(1) Minor tephra deposition on the glacial ice subsequent to
accumulation would increase the temperature of the underlying
ice due to the low albedo of the tephra, and facilitate sublimation of the ice (Wilson and Head, 2009). It is thus possible that
the ice underlying the smooth facies has sublimated away
following tephra deposition. In this scenario, the RMCs detected
in this study impacted subsequent to tephra deposition but prior
J.W. Head, D.K. Weiss / Planetary and Space Science 103 (2014) 331–338
337
(Campbell et al., 2013) may indicate that ice is not currently
present, the rapid timescales of ice sublimation appear to be at
odds with the fresh nature of the RMCs. We note that it is possible
(and perhaps more likely) that a tephra–ice mixture with no
diagnostic radar reflectors remains present underlying a metersthick tephra/debris cover.
4. Conclusion
Fig. 8. Depth distribution of RMCs, bowl-shaped craters, and “relaxed” craters for
the smooth facies of Pavonis Mons (blue triangles), and for a localized extent of the
smooth facies of Arsia Mons (red circles). Green box indicates Pavonis Mons
minimum RMC depth (top) and maximum bowl depth (bottom). Penetration depth
is interpreted to be crater depth from the surrounding terrain, and is scaled as 20%
of the crater diameter (e.g. Croft, 1981; Kato et al., 1995). Maximum Pavonis bowl
morphology depth (71 m) occurs at distal edge of the smooth facies in a
particularly hummocky zone; we thus exclude this outlying data point in our till
thickness estimate. Red box indicate Arsia Mons minimum RMC depth (top) and
maximum bowl depth (bottom). We interpret the Pavonis Mons smooth facies lag
thickness to be between 16 and 40 m, and that of Arsia Mons to range from 136 to
229 m. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
We interpret the presence of fresh ring-mold craters (RMCs) in
the smooth facies of Pavonis and Arsia Mons tropical mountain
glacier deposits to indicate the current presence of glacial ice
underlying a decameters-thick lag deposit. The overall thicknesses
of the smooth facies deposits suggest that hundreds of meters of
glacial ice may still be present. We interpret the lack of confident
detection of ice in radar data as being due to the presence of
admixed tephra deposits, a factor that reduces the radar distinctiveness. The glacial ice is likely to be 125–220 million years old,
and is a remnant of ancient climate conditions (and potentially
atmospheric chemistry) when spin-axis obliquity was 451. This
glacial deposit is one of the most equatorward ice deposits
currently observed on Mars and represents a high-priority target
for future geological, biological and climate exploration.
Acknowledgment
We gratefully acknowledge support from the NASA Mars Data
Analysis Program (NNX11AI81G) and the Mars Express High
Resolution Stereo Camera (HRSC) Team (JPL 1488322) to JWH.
References
to complete ice removal. For example, Wilson and Head (2009)
found that repetitive tephra accumulation (o 0.1 m in thickness) following ice accumulation could remove up to 3 km of ice
in as little as 50 ka.
Substantial sublimation of tephra-covered ice is only expected to
occur after snow deposition has ceased (i.e. following an
obliquity shift from 451 to lower values). This observation, in
concert with the fresh nature of the RMCs found in this study,
suggests that if one or multiple eruptions did occur, they were
not significantly older than the fresh RMCs identified in this
study (or the ice would have already sublimed away). Given the
125 Ma best-fit age of the Pavonis Mons fan-shaped deposit
(Kadish et al., 2014), this would require the fresh RMCs characterized in this study to be on the order of 125 Ma, a period
of time in which the thin ejecta deposits of the small fresh RMCs
are likely to be entirely eroded and removed (indeed, most
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(2) Alternatively, what if snow and tephra accumulation were
active concurrently? Wilson and Head (2009) noted that
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facies. The similar dielectric constants of ice and volcanic ash/
tephra could eliminate any diagnostic ice reflection.
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