The Cryosphere, 8, 2047–2061, 2014
www.the-cryosphere.net/8/2047/2014/
doi:10.5194/tc-8-2047-2014
© Author(s) 2014. CC Attribution 3.0 License.
Glacier-like forms on Mars
B. Hubbard, C. Souness, and S. Brough
Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, UK
Correspondence to: B. Hubbard (byh@aber.ac.uk)
Received: 7 May 2014 – Published in The Cryosphere Discuss.: 5 June 2014
Revised: 15 September 2014 – Accepted: 26 September 2014 – Published: 5 November 2014
Abstract. More than 1300 glacier-like forms (GLFs) are located
in Mars’ mid-latitudes. These GLFs are predominantly
composed of ice–dust mixtures and are visually similar to
terrestrial valley glaciers, showing signs of downhill viscous
deformation and an expanded former extent. However, several
fundamental aspects of their behavior are virtually unknown,
including temporal and spatial variations in mass
balance, ice motion, landscape erosion and deposition, and
hydrology. Here, we investigate the physical glaciology of
martian GLFs. We use satellite images of specific examples
and case studies to build on existing knowledge relating
to (i) GLF current and former extent, exemplified via a
GLF located in Phlegra Montes; (ii) indicators of GLF motion,
focusing on the presence of surface crevasses on several
GLFs; (iii) processes of GLF debris transfer, focusing
on mapping and interpreting boulder trains on one GLF located
in Protonilus Mensae, the analysis of which suggests a
best-estimate mean GLF flow speed of 7.5 mm a−1; and (iv)
GLF hydrology, focusing on supra-GLF gulley networks. On
the basis of this information, we summarize the current state
of knowledge of the glaciology of martian GLFs and identify
future research avenues.
1 Introduction
Numerous similarities exist between ice-rich landforms on
Mars and Earth (e.g., Colaprete and Jakosky, 1998; Marchant
and Head, 2003; Forget et al., 2006). Glacier-like forms
(GLFs), which comprise one particular subgroup of these
features, are strikingly similar in planform appearance to
terrestrial valley glaciers (Fig. 1). However, despite this
similarity, the fundamental glaciology of martian GLFs remains
largely unknown. Improving this knowledge would
enhance our understanding both of the specific landforms
concerned and of broader planetary issues such as (i) how
Mars’ present-day landscape was formed, (ii) the presence
and phase state of H2O on Mars’ surface, and (iii) how Mars’
climate has changed in geologically recent times. The aim of
this paper is to summarize and develop our understanding of
the fundamental physical glaciology of Mars’ GLFs. As well
as summarizing existing knowledge, we provide new observations
and interpretations of glacial landforms on Mars and
outline potential avenues for future research. However, while
a model based on terrestrial analogues, several fundamental
controls over martian glaciation contrast sharply with those
on Earth. For example, Mars’ gravity, at ∼ 3.7 m s−2, is less
than 40 % of Earth’s. Mars’ surface temperature varies between
∼ −130 and +27 ◦C, with a mean of ∼ −60 ◦C (Read
and Lewis, 2004), ∼ 75 ◦C lower than on Earth. Finally, the
partial pressure of H2O in Mars’ near-surface atmosphere is
∼ 1 µbar, making the planet’s surface ∼ 1000 times drier than
Earth’s.
Since this paper is intended for readers who are primarily
interested in the terrestrial cryosphere, and who may not
therefore be familiar with the literature relating to the martian
cryosphere, a list of the acronyms used herein is given in
Table 1.
1.1 Background
1.1.1 GLF classification, location and form
Mars’ mid-latitude regions, between ∼ 20 and ∼ 60◦ N and
S, host numerous landforms and surface deposits that bear
a striking resemblance to small-scale terrestrial ice masses
(e.g., Souness et al., 2012). These landforms, being composed
predominantly of H2O ice (Holt et al., 2008; Plaut
et al., 2009) and exhibiting surface morphologies consistent
with viscous flow (Marchant and Head, 2003; Head et
al., 2010), have come to be known collectively as viscous
Published by Copernicus Publications on behalf of the European Geosciences Union.
2048 B. Hubbard et al.: Glacier-like forms on Mars
Figure 1. A 3-D image of a typical martian GLF (#948 in the inventory
of Souness et al., 2012), which is ∼ 4 km long and ∼ 600 m
in altitudinal range. The GLF shows evidence, through deformed
chevron-like surface ridges, of down-slope flow; is longer than it is
wide; and is bounded on all sides. This well-studied GLF is also
bounded by a series of well-defined moraine-like ridges. Image reproduced
after Hubbard et al. (2011).
Table 1. List of commonly used terms and corresponding acronym.
Term Acronym
Glacier-like form GLF
Viscous flow feature VFF
Lobate debris apron LDA
Lineated valley fill LVF
Moraine-like ridge MLR
Mars Reconnaissance Orbiter MRO
Context (Camera) CTX
High Resolution Imaging Science HiRISE
Experiment (camera)
Shallow Subsurface Radar SHARAD
flow features, or VFFs (Milliken et al., 2003; Souness and
Hubbard, 2012). Glacier-like forms, or GLFs, are a distinctive
subtype of VFF that are elongate and similar in appearance
and overall morphology to terrestrial valley glaciers.
GLFs thereby generally form in small cirque-like alcoves or
valleys, appear to flow downslope between bounding sidewalls,
and terminate in a distinctive tongue which may or
may not feed into a higher-order ice-rich terrain type. GLFs
thereby represent the lowest-order component of what Head
et al. (2010) referred to as Mars’ integrated glacial landsystem.
According to this model, GLFs flow and may merge
downslope to form broad, rampart-like lobate debris aprons
(LDAs) (Squyres, 1978, 1979). LDAs may, in turn, coalesce,
typically from opposing valley walls, to form lineated valley
fills (LVFs), which take the form of complex and contorted
surfaces that often exhibit no obvious flow direction.
Figure 2. The spatial distribution of Mars’ 1309 GLFs as identified
by Souness et al. (2012).
In their inventory of Mars’ GLFs, Souness et al. (2012)
inspected > 8000 Context Camera (CTX) images, covering
∼ 25 % of the Martian surface, and identified 1309 individual
forms, reporting the location (Fig. 2) and basic morphometry
of each. Hereafter, we refer to specific GLFs through
their classification number in this inventory, available as a
supplement accompanying Souness et al. (2012). Of the total
population, 727 GLFs (56 %) were found in the northern
hemisphere and 582 (44 %) in the southern hemisphere,
with GLFs showing a preference for the mid-latitudes (centered
on a mean latitude of 39.3◦ in the north and −40.7◦
in the south). Although Souness et al. (2012) did not normalize
their GLF count to (spatially variable) image coverage,
inspection of Fig. 2 strongly suggests that GLFs are locally
clustered in both hemispheres, for example along the
so-called “fretted terrains” (Sharp, 1973) of Deuteronilus
Mensae, Protonilus Mensae and Nili Fossae in the north and
around the Hellas Planitia impact crater in the south (Fig. 2).
GLF morphometry was found to be remarkably similar between
the two hemispheres, with a mean GLF length of
4.91 km in the north and 4.35 km in the south, and a mean
GLF width of 1.26 km in the north and 1.34 km in the south.
Similar to on Earth, a pronounced preference for a poleward
orientation was also found, with GLFs having a mean bearing
of 26.6◦ (NNE) in the northern hemisphere and 173.1◦
(SSE) in the southern hemisphere – indicating a strong sensitivity
to insolation. These interhemispheric similarities in
distribution and morphometry indicate that all martian GLFs
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B. Hubbard et al.: Glacier-like forms on Mars 2049
share a high degree of commonality in terms of composition
and formation. These are considered below.
1.1.2 GLF composition
The precise composition of GLFs is still unknown due to
the fact that they are almost ubiquitously covered in a layer
of fine-grained regolith. Debate surrounding the amount of
water ice involved in VFF composition (including GLFs)
has led to varying interpretations being advanced, including
ice-assisted talus flows (∼ 20–30% ice; Squyres, 1978,
1979), rock glaciers (∼ 30–80 % ice; Colaprete and Jakosky,
1998; Mangold, 2003), and debris-covered glaciers (> 80 %
ice; Head et al., 2005; Li et al., 2005). Since the distinctions
between these forms, and between them and “standard”
glaciers, is not sharply defined even on Earth, we are
not yet in a position to definitively attribute martian equivalents
to any or all of them. We therefore follow the convention
of much of the published literature and refer to these
forms as “glacier-like”, accepting that they may eventually,
when more information becomes available, be more accurately
reclassified as some related form such as rock glaciers
or mass flows. That said, the latter is unlikely to hold universally
on Mars since many GLFs do not show distinctive
source areas for their mass, many have lost substantial
mass since their formation (Sect. 2.2 below), and many appear
from radar data to be composed largely of water ice
(below). Hubbard et al. (2011) noted that boulder incisions
into the unconsolidated surface of GLF #948 located in the
north wall of Crater Greg, eastern Hellas, were some decimeters
deep, representing a minimum surface dust thickness at
this location. There have been very few direct observations
of the interior of GLFs, but Dundas and Byrne (2010) reported
the capture of very recent meteorite strikes that indicated
the presence of relatively clean (i.e., debris poor) massive
ice at a depth of some centimetres to metres below the
surface. Furthermore, data from the shallow subsurface radar
(SHARAD) sensor, mounted on the Mars Reconnaissance
Orbiter (MRO), suggest that many VFFs (including GLFs)
may well be composed of massive H2O ice with minimal
lithic content (Holt et al., 2008; Plaut et al., 2009). These
findings led to the widespread acceptance that H2O ice accounts
for the dominant portion of GLF mass. However, the
presence of a lithic component has been demonstrated by ice
fade through sublimation following recent impact exposures
(Dundas and Byrne, 2010), and the precise proportions of
ice–rock mixture, particularly at depth, are still unknown.
1.1.3 GLF formation
A continuing point of discussion relates to precisely how and
when GLFs formed. It is generally agreed that GLFs are now
largely relict forms dating to a past, but relatively recent,
martian ice age (see Kargel, 2004). While it is thought that
Mars’ last major ice age ceased when the planet’s obliquity
changed from ∼ 35 to ∼ 25◦ between 4 and 6 million years
ago (Laskar et al., 2004), evidence of a subsequent, Late
Amazonian ice age has been proposed (e.g., Head et al.,
2003). It is thought that obliquity still exceeded 30◦ during
periods of short-term obliquity cycles (∼ 100 ka) between
∼ 2 and ∼ 0.5 Ma BP. During these intermittent periods, increased
solar radiation led to the melting of Mars’ polar caps,
the release of moisture into the atmosphere, and its precipitation
as snow or condensation above or within the ground at
lower latitudes (e.g., Forget et al., 2006; Hudson et al., 2009;
Schon et al., 2009). This ice deposition extends well into
Mars’ mid-latitudes, where it appears to have survived, preserved
beneath surface regolith, until the present day. Still,
the mechanisms by which GLFs first accumulated sufficient
ice-rich mass to flow downslope and acquire their distinctive
surface morphologies remain uncertain.
2 The glaciological characteristics of martian GLFs
2.1 Approach and methods
Each of the following sections both summarizes published
information and supplements that information with new data
from the analysis of images acquired by MRO’s CTX, at
a resolution of ∼ 6 m per pixel, or High Resolution Imaging
Science Experiment (HiRISE) camera, at a resolution of
∼ 0.3 m per pixel. Maps were constructed from these images
using ArcMap GIS software and interpretations additionally
drew on elevation data produced by the Mars Orbiter Laser
Altimeter (MOLA), at a typical resolution of 128 pixels per
degree, mounted on the Mars Global Surveyor spacecraft.
2.2 GLF extent
Recent observations suggest that current GLFs are the remnants
of a once far larger ice mass (e.g., Dickson et al.,
2010; Sinha and Murty, 2013) that was most extensive during
a hypothesized last martian glacial maximum, or LMGM
(Souness and Hubbard, 2013). Such an expanded former extent
has been inferred from detailed regional geomorphological
reconstructions, for example identifying former ice limits
from variations in surface texture and the existence of distal
moraine-like ridges. Allied to local topography, such mapping
has allowed the reconstruction of both former ice extent
and local ice-flow directions (e.g., Dickson et al., 2008).
However, debate persists concerning both the precise timing
of the LMGM and the extent and volume of ice coverage at
the time. The complexity of this issue is compounded by the
timescales involved, with best estimates currently placing the
LMGM at ∼ 5–6 Ma BP, but possibly continuing closer to the
present day (Touma and Wisdom, 1993; Head et al., 2003)
(Sect. 1.1.3 above).
The outlines of many GLFs are clearly demarcated by the
presence along their margins and front of bounding morainelike
ridges, or MLRs (Arfstrom and Hartmann, 2005). These
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2050 B. Hubbard et al.: Glacier-like forms on Mars
Figure 3. Case study illustrations of the former extent of martian GLF #146 showing background MOLA elevation images (a and b), and a
CTX image expansion (c). The forefield of terrestrial Midre Lovénbreen, Svalbard, is shown for comparison (d).
landforms are commonly raised above the present GLF surface
and are texturally distinct from their surroundings. One
particular Amazonian-aged (∼ 10 Ma BP) GLF located in
Crater Greg, eastern Hellas (−38.15◦ N, 246.84◦ E) (#948),
has been the focus of much study (e.g., Hartmann et al., 2003,
2014; Kargel, 2004; Hubbard et al., 2011). In an analysis of
this particular GLF, Hubbard et al. (2011) described a sequence
of up to four distinct raised bounding ridges located
along the GLF’s margins. The authors also described two surface
terrain types in the GLF’s lower tongue, “linear terrain”
and “mound and tail terrain”, as being possible exposed subglacial
bedforms. Overall, this led these authors to suggest
that the GLF’s moraine-bounded outline presently represents
a glacial basin in which the lower zone now comprises an
exposed former glacier bed, while the basin’s upper zone
still hosts a degraded ice mass. In this case, therefore, the
present-day GLF outline incorporates both an ice mass and
its immediate proglacial area. This particular GLF’s multiple
bounding moraines were also interpreted in terms of a general
recession punctuated by several (at least three) episodes
of minor readvance or stillstand.
At a larger scale, GLFs form the first order of the Martian
glacial landsystem (Head et al., 2010) (Sect. 1.1 above),
which is present throughout substantial parts of the planet’s
northern and southern mid-latitudes (Milliken et al., 2003;
Souness and Hubbard, 2012). Many of the ice masses forming
this landsystem are thought to have been substantially
more advanced and thicker in the past, carrying important
implications for reconstructions of climatic variability on
Mars. For example, Dickson et al. (2008) reconstructed former
glacial limits in the Protonilus Mensae region (54.55◦ E,
40.80◦ N) based on the identification and mapping of former
glacial highstands. The analysis indicated a maximum ice
thickness of >2 km at LMGM and a downwasting of at least
920 m since then. Although reconstructed flow directions
were questioned in detail by Souness and Hubbard (2013),
this analysis indicates substantially thicker ice in the geologically
recent martian past.
Several studies have also pointed out that GLFs appear to
be distinctive from the underlying ice-rich (LDA or LVF)
material onto which they appear to have flowed (e.g., Levy
et al., 2007; Baker et al., 2010; Sinha and Murty, 2013). This
material contrast has been interpreted as signifying the possibility
of a marked age difference between the two surfaces,
suggesting two or more glacial events with at least one smallscale
or “local” glacial phase advancing over an earlier “regional”
glaciation (e.g., Head et al., 2003; Levy et al., 2007;
Dickson et al., 2008; Sinha and Murty, 2013).
Glacial activity has also been identified outside the midlatitude
regions. As well as the well-studied polar ice caps
(e.g., Seu et al., 2007; Phillips et al., 2008), degraded glacierlike
features have been described surrounding the shield
volcano of Arsia Mons (−0.31◦ N, 239.00◦ E) (Head and
Marchant, 2003) and in high-latitude (70.32◦ N, 266.45◦ E)
craters (Garvin et al., 2006). The identification of features
and landforms of glacial origin across vast areas of Mars’
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B. Hubbard et al.: Glacier-like forms on Mars 2051
present surface has also led to suggestions that continental
glaciation may once have occurred on Mars (Kargel and
Strom, 1992; Kargel et al., 1995; Fastook et al., 2014; Hobley
et al., 2014).
Case study: reconstructing former GLF extent
GLF #146 is ∼ 12 km long, ∼ 5 km wide and located in
the Phlegra Montes region of Mars’ northern hemisphere
(164.48◦ E, 34.13◦ N) (Fig. 3). This region is largely formed
from several massifs that stretch from the northeastern section
of the Elysium Volcanic Province to the dichotomy lowlands.
GLF #146 is located on the southern tip of a massif
range and converges from a wide upper basin between
two rock outcrops into an elongate lower tongue. The main
tongue of this GLF shows distinctive surface lineations and
textures that indicate the presence of three separate major
flow units. Several arcuate, linear raised features or MLRs
are located in the foreground of the current GLF. This particular
case is of interest because these MLRs are located some
distance from the GLF’s current margin, indicating formation
at some time in the past when the GLF was at a more
advanced position than at present.
The geomorphological interpretation of GLF #146
(Fig. 4), reconstructed from CTX images alone, reveals that
the region in front of the current GLF is characterized by
two distinctive terrain types. At the broadest scale, both terrains
are clearly part of the ice-rich, fretted terrain found
throughout Mars’ mid-latitudes but particularly characteristic
of Deuteronilus Mensae, Protonilus Mensae and Nili
Fossae (Sharp, 1973). However, both terrains also differ
in several important details, indicating distinctive mechanisms
of formation and/or subsequent history. The first terrain
type, “arcuate terrain”, forms a ∼ 3.3 km wide band
around the GLF’s current margin. This terrain is characterized
by arcuate ridges whose shadows indicate that they are
raised above the adjacent ground, forming distinct local topographic
highs ∼ 0.1–1.7 km long and 5–10 m wide. These
ridges show distinct similarities in morphology and spatial
relationships to MLRs identified elsewhere on Mars
(Arfstrom and Hartmann, 2005), which are the martian
equivalent of terminal moraines on Earth (e.g., Fig. 3d). The
arcuate ridges forming this terrain increase in size and coherence
away from the GLF’s margin (Fig. 3c) such that they
are almost unbroken along the terrain’s full distal edge. The
ridges are smaller and more fragmented nearer to the GLF’s
margin. The second terrain type, “smooth terrain”, extends
beyond the arcuate terrain for tens of hundreds of kilometres
into the forefield’s lower plains. This terrain appears at the
broadest scale to be visually smooth with few undulations
relative to the arcuate terrain. Close inspection also indicates
irregular mottling and a greater concentration of impact
craters on the smooth terrain than on either the arcuate terrain
or the GLF proper (Fig. 4).
The location and characteristics of the two proglacial terrain
types outlined above provide some basis for their interpretation.
We infer that the arcuate area directly in front of
the GLF represents the geologically recent former extent of
the GLF. Like on Earth, the MLRs represent the former locations
of the GLF’s terminus, with the outermost MLR representing
the maximum former extent of the GLF and each
subsequent ridge representing a former terminal position (of
minor advance or slowdown) during a period of general GLF
recession. The sequence of multiple terminal MLRs thereby
implies that the GLF has undergone a cyclic or punctuated
recession. The lower density of craters on the GLF and its
encompassing arcuate terrain relative to the outer smooth terrain
is consistent with the younger age for the deposition or
exposure of the former. Indeed, the general lack of resolvable
craters on the arcuate terrain and on the GLF itself, although
insufficient in number to analyze formally, suggests that the
feature is of a geologically very young age. On a regional
scale, other degraded ice-related features have been reported
in the northwestern region of Phlegra Montes (Dickson et al.,
2010), suggesting that regional glaciation may have occurred
at Phlegra Montes in the recent past and that we currently see
their diminished remains.
Overall, we interpret the GLF located in Phlegra Montes
as an ice-rich mass that was once much larger than its current
extent, with the outer MLR marking its former maximum
extent. The continuity of the smooth plains to the east, combined
with a lack of further evidence of ice-related processes,
suggests that the GLF reported here marks the outer limit of
glacial activity of the Phlegra Montes region. It appears that
the GLF coalesced from a wide upper basin into a narrow
tongue before spreading out onto the flatter plains, much like
a piedmont glacier on Earth. Subsequently the GLF’s terminus
has retreated ∼ 3.3 km to its current position, apparently
through periods of cyclic or punctuated standstill, particularly
early on during the period of general recession. Similar
evidence for GLFs representing degraded landforms has been
presented elsewhere in Mars’ mid-latitudes (e.g., Hubbard et
al., 2011; Souness and Hubbard, 2013), indicating it is possible
that GLFs were once a much larger feature on Mars’ surface.
Further, the appearance of deposits indicating the GLFs
former extent would also imply that GLFs have been active
and dynamic in the past.
2.3 GLF motion
While no data have yet been obtained that reveal either rates
or mechanisms of GLF movement (cf. Sect. 2.4.1 below),
such motion has been both modelled and inferred from their
overall lobate shape and the presence of flow structures on
their surface. These flow structures are typically shaped like
chevrons (e.g., Fig. 1) consistent with a transverse surface velocity
profile similar to that measured at terrestrial glaciers,
i.e., increasing inwards from the glacier’s lateral margins towards
the centerline, where velocity is highest above the
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Figure 4. Geomorphological interpretation of GLF #146 illustrated in Fig. 3c.
thickest ice. There is therefore little doubt that GLFs have
moved, at least through viscous deformation. However, there
is no evidence that mass is continuing to accumulate on
present-day GLFs, nor that they are still moving. In an effort
to shed some light on the likelihood of GLF motion, Milliken
et al. (2003) applied the multi-component constitutive relation
of Goldsby and Kohlstedt (2001) to typical ranges of
VFF temperature, slope and (assumed) ice grain size. For
an assumed 10 m thick VFF deposit, Milliken et al. (2003)
estimated shear stresses of 10−1.5–10−2.5 MPa and consequent
strain rates on the order of 10−11–10−16 s−1. Based on
these rates, the authors estimated it would take between 3 ka
and 300 Ma, respectively, to produce a shear strain of 100 %,
which was in agreement with age estimates of the VFF (105–
107 a). Although the application of this stress-strain relationship
to martian VFF conditions represented a major advance,
the model was not distributed spatially and was not therefore
applied to, nor considered, any particular VFF geometry.
Moreover, the possible presence of liquid water within or
below VFFs was (and still is) also unknown. All VFF motion
was therefore assumed to occur through deformation of
a spatially homogeneous ice–dust mixture.
2.3.1 Crevassing as an indicator of GLF motion
Fracturing is a universal diagnostic indicator of high tensile
or shear strain rates within terrestrial ice masses. Further,
the orientation of individual crevasses and the size and
shape of crevasse fields reflect the strain rate, and strain history,
of specific parcels of ice (e.g., Herzfeld and Clarke,
2001). Crevasses have been reported on a variety of icerich
surfaces on Mars. For example, fractures observed on
the floor of certain craters in Xanthe Terra formed part of
what Sato et al. (2010) described as “chaotic” terrain. Pierce
and Crown (2003) also reported transverse cracks in debris
apron deposits in eastern Hellas, and interpreted them specifically
as brittle extensional crevasses. Fractures have also
been observed at the edge of high (> 800 m) icy scarps on
Mars’ north and south polar ice caps (Byrne et al., 2013).
These fractures appear to act as planes of weakness for occasional
collapse events that have been observed in repeat
satellite images (Russell et al., 2008). Kargel (2004) made
specific reference to the presence of crevassing on martian
GLFs, where the varying size, morphology and overall state
of preservation of crevasses were interpreted in terms of formation
over a considerable period of time, possibly continuing
to the present day. This indicates that Mars’ GLFs do
appear to be, at least in some cases, still actively flowing.
Below, we present an analysis, based on new data, of the
extent and nature of crevasses visible on the surface of martian
GLFs.
2.3.2 Concentration and location of crevassed GLFs
From an overall population of ∼ 1300 GLFs (Souness et al.,
2012), surface crevasses are present on 64 individual forms
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Figure 5. The distribution of crevassed GLFs on Mars (a), with
expansions of GLF clusters in Deuteronilus Mensae (b) and western
Hellas Planitia (c).
(∼ 5 % of the total population). Of these crevassed GLFs,
37 (57.8 %) are located in the northern hemisphere and 27
(42.2 %) in the southern hemisphere (Fig. 5a). While this interhemispheric
division mirrors that of the overall GLF population
of 55.5 % on the northern hemisphere and 44.5 % in
the southern hemisphere (Fig. 2), crevassed GLFs are preferentially
clustered in certain regions relative to their parent
GLFs populations. These clusters are particularly notable
in northwestern Argyre in the southern hemisphere and in
Deuteronilus Mensae and Protonilus Mensae in the northern
hemisphere (Fig. 5). Crevassing therefore occurs, or is
at least more readily visible (i.e., exposed by the absence or
excavation of supraglacial regolith), in these specific areas.
2.3.3 Examples of GLF crevasse morphologies and their
interpretation
Crevassing occurs where tensile strain rate of ice exceeds
a critical threshold (Vaughan, 1993). Such high strain rates
can result from several factors including local changes in
mass balance, ice surface and/or bed slope, ice thickness,
and basal traction. Similar to crevassed ice masses on Earth,
many of the crevasse fields identified on martian GLFs fall
into one of a small number of repeated patterns, illustrated
below through examples of four sets of crevasses from two
martian GLFs.
Example crevasse set #1 (ECS1) (Fig. 6) is on GLF #1054,
located to the east of the large Hellas Planitia impact crater
in Mars’ southern hemisphere (102.65◦ E, −40.85◦ N). This
particular GLF exhibits two crevasse sets. ECS1 (Fig. 6c),
comprises a dense cluster of transverse linear crevasses, typically
100–250 m long and up to 50 m wide, that coincide
with an abrupt increase in slope, just down-flow of the point
at which the GLF flows out of a cirque-like alcove. The location
and transverse orientation of ECS1 conforms to longitudinal
extension associated with the acceleration of GLF
#1054 as it flows over its cirque lip and moves down a steeper
slope. The physical setting, strain regime and pattern of these
crevasses are similar to icefalls, which are commonplace on
terrestrial valley glaciers.
ECS2 (Fig. 6d) is also located on GLF #1054, but in its upper
reaches. This set of linear crevasses forms a discontinuous
band aligned adjacent and parallel to the GLF’s headwall
contact, similar to glacier bergschrunds in Earth. On Earth,
bergschrunds indicate gravity-driven ice flow away from the
headwall of a glacier where the ice surface slope is locally
sufficiently steep to induce brittle fracture (Mair and Kuhn,
1994).
ECS3 (Fig. 7) is on GLF #541, located in Deuteronilus
Mensae in Mars’ northern hemisphere (38.18◦ E, 45.14◦ N).
This crevasse field is located along the GLF’s western flank
(Fig. 7b) and consists of multiple, highly degraded fractures
that extend towards the GLF’s centerline from its lateral margin.
These crevasses are aligned slightly up-valley, and progressively
rotate towards a more transverse alignment towards
the GLF’s terminus. On Earth, such a crevasse pattern
indicates the presence of extensional lateral shear within
a glacier, caused by friction between the ice and the valley
walls. Once formed, the crevasses rotate in accordance with
a general increase in longitudinal ice velocity away from the
valley sides and towards a glacier’s centerline. The similar
morphology of the crevasses observed on GLF #541 (Fig. 7b)
and the lateral crevasses on terrestrial glaciers indicate that
martian GLFs are, or have been, characterized by a similar
geometry and velocity field to terrestrial valley glaciers. This
particular case provides evidence that GLFs both thicken towards
their centerline (where on Earth such valleys are typically
parabolic in cross section; Harbor, 1995) and that the
associated increase in ice thickness causes a corresponding
increase in ice velocity.
ECS4 (Fig. 7c) is located near the terminus of GLF
#541. This crevasse field comprises longitudinally orientated
crevasses that are located along the approximate centerline of
the GLF and diverge laterally as the terminus of the glacier
spreads to form a piedmont lobe (Fig. 7c). A series of major
ridges is also seen in this zone, located just up-flow of an
apparent bedrock protuberance. These ridges are aligned orthogonal
to the GLF’s flow direction and to ECS4. The
crevasses forming ECS4 are virtually identical in context and
shape to longitudinal crevasses on Earth’s glaciers, formed
by transverse extension. In this case, we interpret ECS4 as
forming through a combination of transverse extension, associated
with the spreading of the piedmont lobe, and longitudinal
compression as the terminus of the GLF abuts the
bedrock protuberance. This interpretation is consistent with
the transverse ridges in front of GLF #541, which are similar
to compressional ridges, or push moraines, commonly found
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2054 B. Hubbard et al.: Glacier-like forms on Mars
Figure 6. CTX image of crevassed GLF #1054, located in eastern Hellas (Fig. 5a) (a), along with its geomorphological interpretation (b)
and expansions of two crevasse sets (c and d).
Figure 7. CTX image of crevassed GLF #541, located in
Deuteronilus Mensae (Fig. 5b) (a), along with expansions of two
crevasse sets (b and c).
in the proglacial areas of valley glaciers on Earth. It is also
apparent from Fig. 7 that the edges of the crevasses forming
ECS4 are particularly sharply defined, suggesting that they
are young and have been subjected to minimal degradation
relative to other examples (e.g., ECS2; Fig. 6d).
The presence of crevasses on martian GLFs indicates that
their deformation can be achieved through brittle fracture
as well as ductile flow. The interpretations of such crevasse
fields presented above indicates that high GLF strain rates
can be caused by several factors that are similar to terrestrial
ice masses, including (i) variable bed slope, (ii) lateral
drag at shear margins (e.g., along valley sides), and
(iii) spatial variations in traction at the ice–bed interface.
These case studies also suggest that GLFs share certain geometrical
and dynamic characteristics with Earth’s glaciers,
such as a parabolic cross section and its associated transverse
velocity profile, characterized by faster motion along the centerline
than along the margins. These observations also show
that crevasses on Mars’ GLFs range from highly degraded
to sharp-edged, suggesting that crevasses have been formed
over a considerable length of time, possibly continuing to the
present day.
2.4 GLF debris transfer and deposition
The presence of moraine-like ridges (MLRs) on Mars’ GLFs
(Sect. 2.2 above) implies the entrainment, transport and deposition
of substantial volumes of debris. While very little
research has been directed specifically at evaluating how
and to what extent GLFs (or VFFs more broadly) have
shaped Mars’ landscape, some relevant information is available.
For example, lithic debris can be supplied to the uppermost
reaches of GLFs from steep bounding headwalls that
appear to be composed of weathered bedrock and unstable
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B. Hubbard et al.: Glacier-like forms on Mars 2055
boulder-rich deposits (Hubbard et al., 2011). These authors
likened this “incised headwall terrain” to ice-marginal lateral
moraines on valley glaciers on Earth. The base of this headwall
was composed of a strip of boulder-rich deposits, some
tens of metres wide, into which closely spaced parallel incisions
had been cut, similar in appearance to water-related
erosional gullies on Earth. Below this incised headwall terrain,
several boulders appeared to have rolled downslope and
come to rest on the surface of the GLF (their Fig. 7b). Such
headwalls thereby supply both coarse-scale rockfall and, if
the headwall gullies are eroded fluvially, fine-scale washed
debris to the GLF surface.
Many GLFs also appear to host medial moraines, present
as raised linear deposits, typically metres to tens of metres
wide and up to some kilometres long, which are aligned parallel
to the direction of flow. Moreover, these lineations occasionally
correspond to the common edge of two adjoining adjacent
source flow units such as a tributary flow unit joining
a glacier’s trunk or flow re-converging after splitting around
a bedrock protrusion or nunatak. Both situations closely correspond
to the most common mechanism of medial moraine
formation on Earth, as coalesced lateral moraines.
Case study: supra-GLF boulder trains
Substantial coarse debris appears to be present on the surface
of GLF #498 (Fig. 8), located on the inner edge of
the southern rim of Moreaux Crater in Protonilus Mensae
(44.06◦ E, 40.82◦ N). GLF #498 is ∼ 2 km wide and ∼ 12 km
long from terminus to confluence where two major source
areas converge into a single north-flowing trunk. Surface
landforms and textures indicate the presence of numerous
smaller source areas along the GLF’s flanks. This GLF is
unusual and interesting because it exhibits extensive surface
boulder deposits that are not confined to the GLF’s margins
but are located throughout the GLF, extending right to its
centre. Geomorphological mapping of GLF #498 (Fig. 9) reveals
that its supra-GLF debris is largely formed of bouldersized
material that protrudes conspicuously above the host
terrain, making them clearly visible on the high-resolution
HiRISE images used to map the feature. Indeed, hundreds
of individual boulders, which are commonly 1–5 m across,
can readily be identified in the images. Much of this debris
appears to belong to one of nine clusters or populations, labeled
A–I in Fig. 9. Populations A, B and C represent 1–2 km
long, elongate boulder trains located in a medial supra-GLF
position as one major tributary enters the principal tongue
from the west. All three populations are conformable with
surface lineations and raised textured areas as they together
bend northwards as they join the GLF’s main channel. Approximately
3 km down-flow of population C, population G
is notably elongate, extending for more than 2 km along the
GLF but attaining a width of no more than ∼ 20 m. This
boulder train rests on the west-facing flank of a raised supraGLF
MLR and appears as a continuation of population C. In
Figure 8. HiRISE image of GLF #498, located in Protonilus Mensae
showing background MOLA elevation images (a and b), and
an expansion of GLF #498 (c), along with two examples of surface
boulder exposures (d and e). Arrows I. and II. on panel (c) indicate
likely source areas for supra-GLF boulders illustrated in Fig. 9 and
discussed in the text. The dashed box is expanded in Fig. 9.
contrast, populations D and H to the east and E, F and I to the
west all appear to extend over ∼ 50–100 m away from steep
and rocky valley-side walls.
It is apparent from Fig. 9 that boulders have been supplied
to several locations on the surface of GLF #498 and
that they have subsequently moved across the GLF. Some of
this movement may have been by active rolling, particularly
away from immediate valley-side supply areas and towards
the GLF’s centreline, such as in the cases of populations D,
E, F, H and I. However, some or all of these populations may
also have experienced passive redistribution, with boulders
being advected with GLF motion. Without knowledge of the
precise source area and maximum reach of individual boulders,
it is not possible to determine the component of passive
advection in these cases, but it could have been anything
up to the maximum dimension of these populations,
∼ 2 km. In contrast, there appears to be no local source area
for the boulders forming trains A, B, C and G; the boulders
comprising these elongate trains are almost certainly sourced
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2056 B. Hubbard et al.: Glacier-like forms on Mars
Figure 9. Geomorphological map and interpretation of boulder
clusters A–I located on the surface of GLF #498, illustrated in
Fig. 8.
from further up-GLF. Inspection of Fig. 8c indicates that the
closest likely source areas for these trains are from along the
steep northern margin of the GLF’s trunk for populations A
and B (marked “II.” in Fig. 8c) and as a medial moraine extending
from a promontory separating the GLF trunk from
a tributary flowing into it from the south for population C
(marked “I.” in Fig. 8c). Both of these likely source areas are
at least 8 km up-flow of their corresponding supra-GLF boulder
populations. In this case, and assuming that populations
C and G are part of the same feature, then the boulders at
the far end of population G appear to have been transported
at least 15 km (8 km from the head of the tributary flow unit
to population C and a further 7 km to the distal end of population
G). In the absence of any firm age constraint on this
particular GLF, we adopt a “best-estimate” age for its formation
of 2 Ma, at the onset of the proposed “Late Amazonian”
ice age, and a likely age range from 5 Ma, the middle
of the last major ice age on Mars, to 0.5 Ma, the end of
the proposed Late Amazonian ice age (Sect. 1.1.3 above).
Thus, if boulder transport was initiated at the time of GLF
formation from point “I.” in Fig. 8c it follows that, for those
boulders to have been transported passively to the distal end
of population G, GLF #498’s minimum centreline velocity
was within the range of 3–30 mm a−1, with a best-estimate
value of 7.5 mm a−1 averaged over 2 Ma.
Finally, the nature of boulder train elongation on the surface
of GLF #498 is consistent with more rapid motion along
the approximate centerline of the GLF (populations A, B, C
and G, which are highly elongate) than at its margins (populations
D, E, F, H and I, which are less elongate), providing
independent support for the normal, plan-form flow
pattern reconstructed from crevasse patterns on other GLFs
(Sect. 2.3 above).
3 GLF hydrology
3.1 Present-day GLF hydrology
Although still debated (see Ehlmann, 2014; Haberle, 2014),
early Mars appears to have been both warmer and wetter
than at present (Kargel, 2004). Current surface conditions are
relatively cold and dry (see Sect. 1 above), and are consequently
no longer conductive to the survival of surface water.
Nonetheless, seasonal variations in temperature are sufficient
to induce occasional melting as evidenced, for example, by
the intermittent discoloration of surface slope deposits in the
southern mid-latitudes, inferred by McEwen et al. (2011) to
indicate the effects of occasional near-surface moisture. Further,
the presence of gullies incised into unconsolidated sediments
has been interpreted as the result of intermittent fluvial
erosion (Dickson and Head, 2009; Balme et al., 2006,
2013; Soare et al., 2014), as were similar gullies incised into
pro-GLF headwall materials on the well-studied GLF #948
located in Crater Greg, eastern Hellas (Hubbard et al., 2011).
Case study: supra-GLF channel networks
Despite evidence, summarized above, pointing to the intermittent
melting of near-surface ice in Mars’ mid- and low latitudes,
such melting has, to our knowledge, in only one case
been associated with GLFs. Hubbard et al. (2011) reported
the presence of numerous incisions, typically ∼ 1 m wide and
tens of metres long, linking the edges of frost or contraction
polygons (their “polygonized terrain”) on the surface of GLF
#948 (their Fig. 9). These were preferentially aligned subparallel
to the ∼ 10◦ local slope and they were interpreted as gullies
formed by fluvial erosion resulting from the occasional
melting of ice located immediately below the GLF’s unconsolidated
dust mantle. This interpretation, however, was proposed
only tentatively because the incised segments were
short and did not link up to form a coherent network, and
also because liquid water is not stable on Mars’ cold, dry and
low-pressure surface. Here, we extend this analysis to other
GLFs to evaluate the nature and degree of recurrence of this
landform in other, similar settings.
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B. Hubbard et al.: Glacier-like forms on Mars 2057
Figure 10. Surface incisions on GLF #947 imaged by HiRISE to
supplement those reported on GLF #948 by Hubbard et al. (2011).
GLF #947 (a) with an expansion of the incised terrain (b) and trace
of incised segments (c).
Because of the small scale of the polygons and incisions
involved, target GLFs were restricted to those with highresolution
HiRISE coverage, revealing at least six with extensive
areas of surface incision. While four of these are located
within or near to Hellas Planitia’s Crater Greg (GLF
# 930, 947, 948 and 951) the other two are located on the
dichotomy boundary in Protonilus Mensae (GLFs #13101
and #390). Examination of these incised supra-GLF zones,
one of which is presented for illustration in Fig. 10, reveals
similar dimensions and slope-parallel orientation to those observed
on GLF #948. Individual incised segments are also,
in all cases, limited in length to a maximum of some tens of
metres, while none of the cases identified develops a coherent
tributary-based drainage network (e.g., Fig. 10). These
similarities strongly indicate that all such incised terrains
have been formed and subsequently influenced by a similar
process set, operating widely in Mars’ mid-latitudes. These
characteristics are consistent with the formation of incisions
by occasional surface melting, perhaps beneath a thin layer
of surface dust, enhancing albedo and local energy transfer
(e.g., Nicholson and Benn, 2006), on the relatively steep
edges of surface periglacial patterned ground (Gallagher et
al., 2011). The short reach length and absence of a coherent
channel network is also consistent with the short-lived nature
of any such liquid water, evaporating before sufficient discharge
can develop to form a supra-GLF drainage network.
3.2 Former GLF hydrology
With the notable exception of the proposed intermittent
small-scale surface melting proposed above (Sect. 3.1.),
present-day GLFs show little or no sign of the presence or
influence of liquid water. For example, no evidence of proGLF
fluvial activity has been reported, and flow is almost
certainly achieved solely through ice deformation, whether
ductile or brittle (Sect. 2.3, above). Current martian GLFs
therefore appear to be cold throughout. However, this may
not have always been the case, even up to the recent geological
past. Indeed, the recently expanded extent and thickness
of GLFs during the LMGM (Sect. 2.2. above) makes
insulation of the bed beneath thick ice more likely than at
present. Although former ice thicknesses are not known, precluding
thermal modeling, large-scale regional glaciation has
been proposed (Kargel and Strom, 1992; Kargel et al., 1995;
Fastook et al., 2014) and several landforms have been interpreted
as indicative of, or consistent with, former wet-based
glaciation. These include, for example, MLRs, the formation
of which was proposed by Arfstrom and Hartmann (2005)
possibly to involve subglacial squeezing. At the larger scale
(and longer time span), sinuous and anastomosing ridge networks,
elongate bedforms and large-scale grooves located
in southern Argyre Planitia (∼ 30–55◦ S) were interpreted
by Banks and Pelletier (2008) and Banks et al. (2009) as
subglacial eskers, mega-scale glacial lineations (MSGL) and
glacial erosional grooving, respectively. The formation of
these features would have required the area to have been covered
by a large, wet-based ice mass (e.g., Bernhardt et al.,
1This particular GLF is visible in HiRISE image
ESP_019213_2210, which was acquired after the inventory of
Souness et al. (2012). We therefore give it the next-available
designation of #1310.
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2058 B. Hubbard et al.: Glacier-like forms on Mars
2013). To form MSGL, this ice mass would also have to have
been, for some period of time, sliding rapidly over its substrate,
almost certainly lubricated by subglacial water (e.g.,
Clark, 1994). Although these features are undated, Banks et
al. (2008, 2009) considered them to have been formed in preAmazonian
times.
More recently, Hubbard et al. (2011) interpreted the basin
of GLF #948 in terms of an upper zone occupied by a
remnant dust-mantled ice mass, and a lower zone now exhibiting
relict bedforms. These bedforms were classified
as “mound and tail” terrain and “linear” terrain and were
likened to terrestrial drumlins and MSGL, respectively. Since
both of these landforms are predominantly associated with
wet-based glacial conditions on Earth, these authors proposed
that such conditions may have prevailed beneath GLF
#948 at a time in the past when it had expanded and thickened
to fill its moraine-bounded basin. This interpretation, however,
was considered side by side with an alternative – not
involving wet-based glaciation – based on the mound and tail
and linear terrains representing degraded supra-GLF forms,
in this case wind-blown dune deposits and exposed longitudinal
foliation, respectively. Finally, Hubbard et al. (2011)
also reported the presence of “rectilinear ridge” terrain located
outside the current GLF basin, as enclosed by the welldefined
MLRs. This terrain was likened to moraine-mound
complexes on Earth, the formation of which again suggests
either the direct presence of water, if formed as outwash deposits
(Lukas, 2005) or crevasse fills (Sharp, 1985), or the existence
or polythermal glaciation if formed by glacial thrusting
(Hambrey et al., 2005). However, the possibility of coldbased
formation, in this case as proglacial thrust blocks, was
not entirely discounted.
4 Summary
With the aid of high-resolution imagery, particularly from the
CTX and HiRISE cameras, several major advances have been
made in a short period of time concerning Mars’ mid-latitude
GLFs. Thus, the following points are now known with some
certainty:
– Many GLFs were previously more extensive and thicker
than at present, possibly now representing the remnants
of former large ice sheets. In Sect. 2.2 (above) we identify
a distinctive proglacial zone some 3 km wide surrounding
a GLF located in Phlegra Montes. This zone,
bounded along its distal edge by MLRs is interpreted
as having been recently deglaciated and is likened to a
similar proglacial region bounding Midre Lovénbreen,
Svalbard, on Earth.
– GLFs flow slowly downslope through a combination
of ductile and (less common) brittle deformation. In
Sect. 2.3.3 (above) we identify and interpret four contrasting
sets of crevasses located on two martian GLFs
in terms of variable strain regimes. These crevasses are
also shown to range from being relatively fresh in appearance,
implying a correspondingly young age, to appearing
blunt and degraded, implying earlier formation
and possibly a relict current condition.
– GLFs have the ability to transport debris, forming large
bounding moraines and depositing boulder trains extending
for several kilometers along-GLF. In Sect. 2.4
(above) we identify an extensive supra-GLF debris
train which we interpret in terms of passive transport
from specific ice-marginal supply points. Reconstructing
boulder transport distances since GLF formation
(over the range 5.0 to 0.5 Ma, with a best-estimate age
of 2.0 Ma BP) yields an equivalent provisional GLF surface
velocity range of 3–30 mm a−1, with a best estimate
of ∼ 7.5 mm a−1.
– GLFs currently show little influence of liquid water,
confined to postulated intermittent surface melting
which is insufficient to form coherent supra-GLF
drainage. In Sect. 3.1 (above) we illustrate that such
supra-GLF incised channels occur on several GLFs and
are not confined to the single instance at which they
have hitherto been reported. However, more extensive
former GLFs, and/or their predecessor ice masses, may
have been partially wet-based.
Despite this information, many of the most fundamental
glaciological aspects of GLFs remain unknown. These include
the following:
– It is not known whether GLFs are currently active or
whether they are decaying relics of previously active
forms. Diagnostic indicators of such activity would include
any indication of motion (addressed below) and a
surface profile that is in balance with current climatic
conditions, as indicated by a spatially-distributed numerical
model of GLF flow.
– The previous extent of GLFs, and their putative parent
ice sheets, is still only poorly understood. This requirement
could be addressed through additional field
mapping at a variety of spatial scales, based on CTX
or High Resolution Stereo Camera (HRSC) images at
the regional scale to HiRISE images at a local scale.
Such mapping could be targeted at identifying markers
of former ice extent such as specific surface terrains,
subglacial deposits and ice-marginal moraines.
– The thermal regime of former GLFs is unknown, and
the possibility of partial wet-based conditions remains
unproven and their extent unevaluated. These issues
could be evaluated empirically or theoretically, ideally
through a combination of both. Empirical evidence
might include the identification of indicators diagnostic
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B. Hubbard et al.: Glacier-like forms on Mars 2059
of wet-based conditions (e.g., bedforms such as megascale
glacial lineations) or of subglacial drainage (e.g.,
meltwater channels or eskers). Theoretically, former
thermal regime could be estimated from the application
of a thermomechanically coupled ice-flow model to reconstructed
former ice mass geometries under realistic
climatic conditions for the time.
– The basic mass-balance regime of GLFs is unknown.
Whatever the spatial expression of this regime, there
is no compelling climatological reason for it to comply
with the common terrestrial valley-glacier model
of net accumulation at high elevations gradually giving
way to net ablation at low elevations. This is possibly
the most challenging unknown GLF property to elucidate,
and would likely require several lines of evidence
to be combined. Central to these might be a regional
evaluation of GLF extent in the light of corresponding
variations in meteorological conditions. A modeling approach
may also help elucidate the mass-balance regime
of GLFs, for example through comparing modeled GLF
geometries and flow with empirical data under a variety
of modeled mass-balance patterns.
– The 3-D geometry and internal structure of GLFs is
unknown. Although SHARAD radar data are available
and capable of mapping ice thickness, the data
are of fairly coarse resolution and have limited spatial
coverage. Very little information is therefore available
to allow the basal interface of GLFs to be identified
and mapped. This property is also critically important
because spatially distributed models of ice mass
flow depend sensitively on accurate bed geometry. In
this case, new and existing SHARAD data could usefully
be mined to locate intersections with known GLFs,
providing a first approximation of bed profiles. Further
to that, modeling-based sensitivity analyses (to
GLF depth) could also be used to constrain likely bed
geometries.
– Mechanisms of GLF motion are poorly known, and,
apart from the estimate of 3–30 mm a−1 presented
herein (Sect. 2.4 above), it has not yet been possible to
measure surface velocities on any Martian GLF. Further
research based on indicators of surface displacement –
such as the boulder analysis presented herein – could
usefully be used to refine the range we propose. As the
period of time between repeat HiRISE images of certain
GLFs increases, it may also become possible to identify
contemporary GLF motion on the basis of feature
or speckle tracking. Indeed, a single such measurement
would provide a major advance in our understanding of
the dynamic glaciology of martian GLFs, particularly if
the GLF concerned could also be modeled.
– GLF-related landforms such as lineations, drumlinlike
forms, surface cracks/gullies and possible eskers
remain largely unexplored and their basic morphometric
characteristics are unreported. Targeted mapping from
HiRISE images remains the best way to identify and
evaluate such landforms. The online inventory accompanying
Souness et al. (2012) would provide a suitable
starting point for identifying candidate regions of
interest.
– Although considered to be rich in water ice, the internal
composition of GLFs remains unknown, despite
these material properties having important implications
for GLF dynamics and our ability to model GLF behaviour
accurately. Apart from direct sampling in the future,
which is unlikely in the near term, SHARAD data
analysis may be combined with numerical modeling to
further constrain the internal composition of GLFs. Opportunistic
images, for example shortly following a meteorite
impact, may also continue to yield information
relevant to GLF subsurface conditions.
These issues deserve research attention to improve our
understanding of the surface features of Mars and, due to
glaciers being effective recorders of climate change, the
planet’s past environmental conditions. It is also worth noting
that the well-insulated base of thick ice masses represents
one of the most likely geologically recent environments on
Mars for the existence of the wet and relatively warm conditions
that are conducive to life.
Acknowledgements. We acknowledge the HiRISE team, University
of Arizona, and NASA/JPL for making the HiRISE
images we investigate herein available to the public. We thank
Ajay Limaye for preparing the 3-D image presented in Fig. 1.
We also thank Matthew Balme, Wilfried Haeberli, Michael
Kuhn and Sara Springman for reviewing or commenting on an
earlier version of the manuscript, and Martin Schneebeli for editing.
Edited by: M. Schneebeli
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