Annals of Glaciology
Letter
Cite this article: Butcher FEG et al. (2023).
Eskers associated with buried glaciers in Mars’
mid latitudes: recent advances and future
directions. Annals of Glaciology 1–6. https://
doi.org/10.1017/aog.2023.7
Received: 30 September 2022
Revised: 16 December 2022
Accepted: 28 January 2023
Keywords:
Debris-covered glaciers; extraterrestrial
glaciology; geomorphology
Author for correspondence:
Frances E. G. Butcher,
E-mail: f.butcher@sheffield.ac.uk
© The Author(s), 2023. Published by
Cambridge University Press on behalf of The
International Glaciological Society. This is an
Open Access article, distributed under the
terms of the Creative Commons Attribution
licence (http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted re-use,
distribution and reproduction, provided the
original article is properly cited.
cambridge.org/aog
Eskers associated with buried glaciers in Mars’
mid latitudes: recent advances and
future directions
Frances E. G. Butcher1 , Neil S. Arnold2 , Matthew R. Balme3 ,
Susan J. Conway4 , Christopher D. Clark1 , Colman Gallagher5,6 ,
Axel Hagermann7, Stephen R. Lewis3 , Alicia M. Rutledge8 ,
Robert D. Storrar9 and Savana Z. Woodley3
1Department of Geography, University of Sheffield, Sheffield, UK; 2Scott Polar Research Institute, University of
Cambridge, Cambridge, UK; 3School of Physical Sciences, The Open University, Milton Keynes, UK; 4CNRS UMR
6112, Laboratoire de Planétologie et Géosciences, Nantes Université, Nantes, France; 5UCD School of Geography,
University College Dublin, Dublin, Ireland; 6UCD Earth Institute, University College Dublin, Dublin, Ireland;
7Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology, Luleå,
Sweden; 8Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, USA and
9Department of the Natural and Built Environment, Sheffield Hallam University, Sheffield, UK
Abstract
Until recently, the influence of basal liquid water on the evolution of buried glaciers in Mars’ mid
latitudes was assumed to be negligible because the latter stages of Mars’ Amazonian period
(3 Ga to present) have long been thought to have been similarly cold and dry to today. Recent
identifications of several landforms interpreted as eskers associated with these young (100s
Ma) glaciers calls this assumption into doubt. They indicate basal melting (at least locally and
transiently) of their parent glaciers. Although rare, they demonstrate a more complex mid-to-
late Amazonian environment than was previously understood. Here, we discuss several open
questions posed by the existence of glacier-linked eskers on Mars, including on their
global-scale abundance and distribution, the drivers and dynamics of melting and drainage,
and the fate of meltwater upon reaching the ice margin. Such questions provide rich opportun-
ities for collaboration between the Mars and Earth cryosphere research communities.
Introduction and background
Present-day Mars is a cold, hyper-arid desert; liquid water is unstable at the surface under the
thin (∼6 mbar) atmosphere. Away from the polar caps, ice is also unstable at the surface and
will sublimate if exposed. Despite this, abundant ice exists in Mars’ subsurface down to lati-
tudes of ∼30°N/S, both as ground ice and massive glacial ice (e.g. Butcher, 2022 and references
therein). Putative buried glaciers (termed ‘viscous flow features’, VFFs) in Mars’ mid latitudes
(e.g. Head and others, 2005; Holt and others, 2008) are thought to have formed 10s to 100s
Myr ago due to climate changes driven by large cyclical variations in Mars’ spin-axis obliquity
(e.g. Madeleine and others, 2009). A protective lithic cover has allowed the glaciers to be pre-
served into the present day.
It is thought that Mars’ ‘Amazonian’ period (3 Ga to present) was, in general, similarly cold
and dry to the present day (current mean annual temperature ∼210 K). Mars’ geomorphology
suggests an extremely limited role of liquid water in Amazonian environments (e.g. Carr and
Head, 2010), leading to an informal baseline assumption that the mid-to-late-Amazonian-aged
VFFs have always been cold-based. However, targeted analyses of individual glacial landsystems
paint a more complex picture of VFF thermal histories, and thus an expanded range of environ-
mental and glaciological conditions in Mars’ geologically-recent past. For example, evidence of
glacial streamlining has been identified in some locations (e.g. Hubbard and others, 2011;
Gallagher and others, 2021). However, Mars does not host widespread streamlined terrains like
those generated by warm-based ice masses on Earth.
Theoretically, under lower Martian gravity, subglacial drainage might develop towards
efficient channelised drainage more readily than on Earth, reducing basal water pressures
and inhibiting sliding and bedform streamlining (Grau Galofre and others, 2022).
Therefore, Grau Galofre (2022) and others posit that the most common indicator landforms
of warm-based glaciation on Mars could be those formed by channelized subglacial meltwater,
including eskers. This is consistent with a growing number of ‘candidate glacier-linked eskers’
identified emerging from, or geomorphologically connected to, VFFs (Gallagher and Balme,
2015; Butcher and others, 2017, 2020, 2021; Woodley and others, 2022).
Candidate eskers linked to mid-latitude buried glaciers on Mars
Using ∼6 m/pixel orbital Context Camera (CTX) images (see Supplementary Table S1; Malin
and others, 2007), Gallagher and Balme (2015) identified a candidate glacier-linked esker
https://doi.org/10.1017/aog.2023.7 Published online by Cambridge University Press
assemblage in the foreland of a VFF occupying a tectonic graben
in Phlegra Montes, a mountain chain in Mars’ northern
mid-latitudes (Figs 1a and b). The candidate esker assemblage is
connected to the glacier by a corridor which appears similar to
proglacial fluvial systems on Earth (Fig 1c; Gallagher and
Balme, 2015). Butcher and others (2017) subsequently identified
a candidate esker (Fig 1d) emerging from a VFF in NW Tempe
Terra, a volcano-tectonic province ∼5000 km east of Phlegra
Montes (Fig 1e). Like the candidate esker assemblage in Phlegra
Montes, it occupies a glaciated graben (Fig 1e). Woodley and
others (2022) identified two additional candidate glacier-linked
eskers (Figs 1f and g) in the same system of grabens where it
extends into W Tempe Terra (Fig 1e). Butcher and others
(2021) also found that at least one morphological subpopulation
of sinuous ridges associated with a VFF within Chukhung crater
(in central Tempe Terra, Fig 1e) is best explained as eskers.
Chukhung crater sits between branches of a major volcano-
tectonic rift system (Fig 1e).
Planetary surface ages are estimated using the size-frequency
distributions of impact craters. The candidate glacier-linked
eskers are too small to extract direct age estimates but their min-
imum ages can be estimated from impact craters on their parent
glaciers. While affected by substantial uncertainties, all return ages
firmly in the mid-to-late Amazonian (110Ma from Butcher and
others, 2017; 150Ma from Gallagher and Balme, 2015; 220 Ma
from Woodley and others, 2022; and 330Ma from Butcher and
others, 2021).
Open questions and research directions
Discoveries of candidate eskers associated with buried glaciers in
Mars’ mid latitudes raise numerous questions. Those summarised
below are far from an exhaustive list.
Q1: What were the environmental and glaciological drivers of esker-
forming melt events?
Identifications of a handful of glacier-linked eskers do not neces-
sarily warrant global-scale re-imagination of the cold, dry climate
of Amazonian Mars (3 Ga to present). However, they demonstrate
that we have an incomplete understanding of the range of thermal
conditions during the Amazonian. We also have an incomplete
understanding of fundamentals such as ice mass-balance, which
strongly influence glacier thermal regime. For example, modelling
suggests that Mars’ low atmospheric pressure could have caused
preferential ice accumulation in topographic lows, rather than at
higher altitudes (Fastook and others, 2008).
The similarity in geologic settings between the candidate
glacier-linked eskers identified to date (within or associated
with tectonic grabens/rifts) suggests that geothermal heating
could have driven basal melting. This hypothesis was posited by
Gallagher and Balme (2015) and subsequently explored with a
1D thermal model by Butcher and others (2017), which consid-
ered strain heating for the first time. Strain heating has a highly
non-linear temperature dependence, and could be enhanced by
geothermal heat and/or by ice-flow deformation in high-relief set-
tings, such as grabens. Butcher and others (2017) found that
strain heating provided up to 14.5 K of additional heating, and
modelled basal temperatures approached 273 K for local mean
annual surface temperatures >205 K (from current ∼190 K in
mid latitudes), ice thicknesses >900 m, and geothermal heat fluxes
>50 mWm-2. Simplified global models predict that Mars’ present-
day average geothermal heat flux is ∼23–27 mWm−2, reaching
∼50 mWm−2 in confined areas (Plesa and others, 2016). Thus,
it is conceivable that localised, late-stage geothermal activity
(e.g. magmatic intrusions), combined with strain heating, drove
basal melting. A conservative scenario for esker formation is
therefore one of localised and transient polythermal glaciation,
with geothermal and strain heating driving the accumulation of
subglacial meltwater, which was initially prevented from draining
due to confinement by cold-based marginal ice. Eventual water
over-pressurisation could have driven rapid drainage towards
the ice margin (either subglacially, or by routing upwards into
an englacial position) to form eskers (e.g. Butcher and others,
2017).
Woodley and others (2022) considered the hypothesis that
regional climate change could explain the discoveries of multiple
candidate glacier-linked eskers in Tempe Terra, over a region
spanning hundreds of kilometres (Fig 1e) (Butcher and others,
2017, 2021; Woodley and others, 2022). They suggest that mul-
tiple, spatially distributed but localised geothermal heating events
in this extensively tectonised region remain the strongest explan-
ation, owing to the lack of associated evidence for past supragla-
cial melting. However, arguably, any supraglacial channels
temporally associated with esker formation would have been
erased by the kilometres-scale ice retreat that has since occurred.
Harder to explain without invoking substantial climate change
are the channels, consistent with proglacial runoff (Fig 1c), which
now separate the candidate esker in Phlegra Montes from its par-
ent glacier (Gallagher and Balme, 2015). Furthermore, Gallagher
and others (2021) also identified regionally-extensive evidence
for warm-based glacial erosion in the wider Phlegra Montes
region, including zones of areal scour, candidate tunnel valleys,
overdeepenings and groove-channel systems. If this Phlegra
Montes landsystem is genetically related to the candidate glacier-
linked esker, it complicates the hypothesis that esker formation
was controlled by localised geothermal heating, suggesting more
spatially extensive driver(s) of melting, for example a regional cli-
mate change and/or a regional geothermal hotspot (e.g. Broquet
and Andrews-Hanna, 2022).
A further unknown is the degree to which salts contributed to
basal melting of VFFs. Salts detected on Mars, such as Na, Mg or
Ca perchlorate (Hecht and others, 2009) can reduce the melting
point by tens of Kelvin. Mg perchlorate hydrate can also weaken
ice, potentially enhancing ice flow and associated strain heating
(Lenferink and others, 2013). Thus, salts could have substantially
reduced the requisite heating for basal melting; however, it is not
known whether they are/were present beneath VFFs, and in suf-
ficient concentrations to influence thermal regimes and increase
melt to levels needed for subglacial channelisation and esker
formation.
Q2: How common are glacier-linked eskers across Mars’ mid latitudes,
and what is their distribution?
To better understand the environmental implications of glacier-
linked eskers (Q1), it is necessary to constrain their global-scale
abundance and distribution. Hemispheric-to-global-scale con-
straints on esker distributions would permit comparisons to models
of palaeoclimate and geothermal heat flux, and hence assessment of
potential environmental drivers of melting. Identifications of
glacier-linked eskers are rare (5 sites) compared to the abundance
of mid-latitude VFFs (>12 000; Levy and others, 2014; Brough
and others, 2019). However, the vast majority of Mars’ glacial land-
scapes have not been analysed in detail, and there is a wealth of
imaging data left to explore, including near-global coverage of
∼6m pixel−1 CTX images (Malin and others, 2007), and more
localised 25–50 cm pixel−1 HiRISE images (McEwen and others,
2007). Supplementary Table S1 describes key image and elevation
datasets, with data-access links, and Supplementary Information
1 provides an introductory tutorial for exploring orbital images
of Mars.
2 Frances E. G. Butcher and others
https://doi.org/10.1017/aog.2023.7 Published online by Cambridge University Press
A key challenge for addressing Q2 is reliably distinguishing
between eskers and morphologically similar landforms of differ-
ent origin. Inverted fluvial palaeochannels are ridges comprising
resistant infills of fluvial channels or channel belts, exhumed by
erosion of less resistant surrounding materials (e.g. Williams
and others, 2007). While typical inverted fluvial palaeochannels
can appear distinctive from typical eskers, their morphological
ranges overlap, complicating esker identification on Mars where
landsystems consistent with both origins co-exist (e.g. in
Chukhung crater; Butcher and others, 2021).
Several studies exploring esker origins for sinuous ridges on
Mars have included comparisons to the planform morphometries
(e.g. length, sinuosity, fragmentation) of >20 000 eskers deposited
by the Laurentide Ice Sheet on Earth (Storrar and others, 2014),
identifying sinuosity as a particular source of similarity
(Butcher and others, 2016; Butcher, 2019; Woodley and others,
2022). 3D (e.g. height and width) comparisons with Earth’s eskers
(e.g. Butcher and others, 2016; Butcher, 2019) are more limited as
datasets from Earth are currently restricted to qualitative descrip-
tions, or measurements of individual eskers (e.g. Perkins and
others, 2016). The morphometries of eskers associated with
Earth’s ice caps and valley glaciers (e.g. Storrar and others,
2015) are similarly understudied, but could be more appropriate
analogues, in both scale and setting, for glacier-linked eskers on
Mars. Large-sample morphometric statistics of a range of terres-
trial analogue landforms, including those which could provide
alternative explanations for sinuous ridges (e.g. inverted palaeo-
channels), would greatly enhance our ability to explore origin
hypotheses for glacier-linked sinuous ridges on Mars.
Mineralogy could also help to distinguish between eskers and
sinuous ridges of different origin. For example, it can reveal evi-
dence for the involvement and residence times (Q3) of liquid
water. Several studies also suggest that cold and icy weathering
could have produced amorphous materials observed on Mars,
such as the mineraloids allophane and hydrated silica (e.g. Hallet,
1975; Rutledge and others, 2018; Rampe and others, 2022).
There are numerous orbital infrared datasets (Supplementary
Table S3) which can help identify and map mineral classes, at vary-
ing spatial and spectral scales, and thereby interpret whether sinu-
ous ridge formation involved liquid water and/or occurred under
climate conditions consistent with glacial origins.
Q3: What were the dynamics of esker-forming drainage and sedimentation?
To improve confidence in esker identifications on Mars (Q2), and
to constrain their palaeoenvironmental/glaciological implications
Fig. 1. a. A buried glacier in Phlegra Montes connected to the candidate esker assemblage identified by Gallagher and Balme (2015) (shown in b, 162.97°E, 32.68°N)
by a corridor of landforms similar to proglacial zones on Earth (shown in c, including channels, thermokarst-like terrain and dead ice topography). d. Candidate
glacier-linked esker (83.06°W, 46.17°N; Butcher and others (2017)) emerging from a buried glacier in NW Tempe Terra. e. Mars Orbiter Laser Altimeter (MOLA) ele-
vation map showing candidate glacier-linked esker sites in the Tempe Terra region (yellow triangles), including Chukhung crater (72.42°W, 38.47°N; Butcher and
others (2021)). Inset global elevation map shows locations of panel e (black polygon) and Phlegra Montes (red triangle). f and g. Candidate glacier-linked eskers in
W Tempe Terra (84.387°W, 43.772°N; and 83.295°W, 44.334°N; Woodley and others (2022)). Panels a, c, d, f and g are CTX images. Panel b is a High Resolution
Imaging Science Experiment (HiRISE) image. See Supplementary Table S2 for image codes.
Annals of Glaciology 3
https://doi.org/10.1017/aog.2023.7 Published online by Cambridge University Press
(Q1), it is necessary to understand how eskers on Mars and Earth
might differ morphometrically and sedimentologically, for
example due to differences in subglacial hydrology and sedimen-
tation arising from the difference in gravity and ice rheology. The
candidate glacier-linked eskers measured to date approach the
width-height ratios of eskers on Earth, but are typically wider
relative to their heights (e.g. Butcher, 2019; Butcher and others,
2020; Woodley and others, 2022). It is not yet understood whether
such differences weaken the esker hypothesis, or could be
explained by fundamental differences in esker formation dynam-
ics (e.g. due to gravity or ice rheology) and/or degradation states
(e.g. due to differences in post-depositional exposure time and
erosion rates) relative to Earth (Butcher, 2019; Woodley and
others, 2022).
Some studies have related the morphometries of eskers on
both Earth and Mars to the drainage dynamics that formed
them (e.g. Butcher and others, 2016, 2020; Perkins and others,
2016). By analogy to terrestrial eskers, Butcher and others
(2020) suggested that the complex ‘stacked’ morphology of the
candidate glacier-linked esker in NW Tempe Terra could result
from spatio-temporal variations in sediment-discharge dynamics,
either in a single drainage episode, or multiple episodes separated
by ice retreat. Analyses of esker sedimentary architecture at
exposed sections and/or using ground-penetrating radar have
enhanced efforts to reconstruct esker-forming drainage events
on Earth (e.g. Perkins and others, 2016) but are not possible
with the current instruments at Mars. Esker sedimentation on
Earth remains relatively poorly understood, as does its relation-
ship to subglacial hydrology and the morphometry of the result-
ing landforms (e.g. Stoker and others, 2021). Furthermore, despite
a considerable amount of research into eskers formed during the
last deglaciation on Earth, fewer studies focus on esker sedimen-
tation at contemporary glacier margins (e.g. Burke and others,
2008, 2010; Storrar and others, 2015). These eskers are important
analogues for Mars, and exhibit morphological differences to their
large ice sheet counterparts.
Recent advances demonstrate the potential utility of new esker
sedimentation models (Beaud and others, 2018; Hewitt and
Creyts, 2019) for relating sediment deposition in subglacial con-
duits to parameters such as bed slope (Stevens and others,
2022). As esker sedimentation modelling advances, it could help
explore the dynamics of esker sedimentation on Mars, including
potential morphological/sedimentological differences arising
from differences in gravity and ice rheology.
Q4: Where did esker-forming meltwater go?
The ultimate fate of esker-forming meltwater on Amazonian Mars
remains uncertain. Did it pond, run off, refreeze, percolate into
the ground and/or rapidly evaporate/boil? Did it accumulate in
standing (perhaps ice-covered) bodies of water such as proglacial
lakes or ponds? Constraining the contributions of these processes
would help to answer Q1 and Q3 because the behaviour of liquid
water upon exiting the subglacial environment would have been
dependent on factors such as climate and atmospheric pressure
and the duration, frequency and discharge of esker-forming
drainage.
Under atmospheric pressures similar to present day Mars,
liquid water would be stable in a pressurised, water-filled subgla-
cial conduit, but would become unstable upon reaching the ice
margin. Laboratory simulations suggest that metastable liquid
water flows could persist over short distances under Mars’ atmos-
pheric pressure (Massé and others, 2016). Their simulated beha-
viours and geomorphic imprints are quite different to flows under
Earth’s atmospheric pressure (e.g. Massé and others, 2016; Brož
and others, 2020), but it is unknown if/how they would further
differ for discharges and sediment grain-sizes typical of esker-
forming flows. Since eskers on Earth are often associated with
high-discharge meltwater drainage, we might expect
sediment-laden meltwater on Mars to be transported for some
distance beyond the ice margin before boiling, percolating into
the regolith and/or refreezing (see e.g. Brož and others, 2020).
A fan-shaped system of distributary ridges at the terminus of a
candidate glacier-linked esker identified by Woodley and others
(2022) (Fig 1f) could indicate meltwater and sediment dispersal
over a distance of ∼500 m at the outlet of a subglacial
(or englacial) conduit. However, eskers on Earth commonly
terminate abruptly, so esker-terminal fans are not expected in
all cases and, indeed, are not always observed on Mars.
Mars’ mid-latitude glacial cycles are thought to have been
linked to large variations in planetary obliquity which drove
global-scale changes to climate, aerosol and volatile cycles (e.g.
Madeleine and others, 2009), but many uncertainties remain
(e.g. Forget and others, 2017). Mars’ current atmospheric pressure
is very close to the triple point of water. If glacier-linked eskers are
related to changes in atmospheric conditions relative to the pre-
sent day, it is possible that liquid water would have been stable
upon exiting subglacial conduits. In this case, it would be neces-
sary to constrain the relative contributions of runoff, ponding,
percolation, refreezing and/or evaporation to the fate of esker-
forming meltwater. Morphological analyses of the terminal
zones of candidate eskers, and the terrains beyond them, will be
important to understand the fate of subglacial meltwater upon
entering the proglacial zone.
Conclusions and outlook
Candidate eskers have now been identified in association with
several extant buried glaciers in Mars’ mid latitudes. They indicate
that their parent glaciers produced meltwater at their beds ∼100s
Myr ago under warm-based or polythermal regimes. Geomorphic
evidence for past melting of Mars’ mid-latitude glaciers remains
rare. However, discoveries of candidate glacier-linked eskers pro-
vide a compelling basis upon which to search more extensively,
and to explore implications for geologically recent environmental
change. Additionally, they raise important questions on the past
habitability of Amazonian glacial environments for microbial
life, which requires liquid water. This is pertinent, because
Mars’ mid-latitude ice deposits are key targets for future explor-
ation, including by human missions which could use the ice for
water resources, sample it for palaeoenvironmental science, and
search for life (e.g. Morgan and others, 2021; I-MIM MDT,
2022). The key questions discussed here illustrate some critical
knowledge gaps determining the implications of glacier-linked
eskers for palaeoenvironment and habitability. For example, it
could be suboptimal for habitability if esker formation was trig-
gered by transient heat sources (Q1) that were confined to loca-
lised areas (Q2) during short-lived and/or high-energy drainage
events (Q3) delivering meltwater to cold, hyperarid, low-pressure
proglacial environments (Q1 and Q4). More favourable might be
some combination of lower-energy, longer-lived and/or recurrent
meltwater drainage, driven by more persistent and/or spatially
extensive heat sources, delivering meltwater to proglacial environ-
ments such as ponds or lakes (even if ice-covered) under more
moderate climate conditions and higher atmospheric pressure.
In striving to better understand glacier-linked eskers on Mars,
it is also necessary to advance our understanding of eskers here on
Earth (including their morphometries, mineralogies and forma-
tion dynamics; see e.g. Q2 and Q3), and their relationships to sub-
glacial hydrology, environmental change and the microbiology of
glacial environments. We therefore foresee exciting opportunities
for collaboration between the Earth and Mars research
4 Frances E. G. Butcher and others
https://doi.org/10.1017/aog.2023.7 Published online by Cambridge University Press
communities to drive forward esker science on both planets, and
we welcome correspondence from interested readers.
Supplementary material. The supplementary material for this article can
be found at https://doi.org/10.1017/aog.2023.7
Acknowledgements. This manuscript was prepared during a Leverhulme
Trust Early Career Fellowship held by FEGB. The research benefitted from
STFC grant ST/N50421X/1 (FEGB), and FEGB and CC’s membership of the
PALGLAC team, who received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme (grant agreement 787263). SJC is supported by the
French Space Agency CNES. SZW acknowledges funding from STFC grant
ST/V50693X/1. MRB and SRL are supported by the UK Space
Agency (MRB: grants ST/W002744/1 and ST/T002913/1; SRL: grants ST/
W002949/1, ST/R001405/1, ST/V005332/1 and ST/T002913/1). AMR is
funded by the NASA Solar System Workings Program (award
#80NSSC21K0908).
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