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Subglacial Environments and the Search for Life Beyond the Earth
Charles S. Cockell,
1
Elizabeth Bagshaw,
2
Matt Balme,
1
Peter Doran,
3
Christopher P. McKay,
4
Katarina Miljkovic,
1
David Pearce,
5
Martin J. Siegert,
6
Martyn Tranter,
2
Mary Voytek,
7
and Jemma Wadham
2
One of the most remarkable discoveries resulting from the robotic and remote
sensing exploration of space is the inferred presence of bodies of liquid water under
ice deposits on other planetary bodies: extraterrestrial subglacial environments.
Most prominent among these are the ice-covered ocean of the Jovian moon,
Europa, and the Saturnian moon, Enceladus. On Mars, although there is no current
evidence for subglacial liquid water today, conditions may have been more favor-
able for liquid water during periods of higher obliquity. Data on these extraterres-
trial environments show that while they share similarities with some subglacial
environments on the Earth, they are very different in their combined physicochem-
ical conditions. Extraterrestrial environments may provide three new types of
subglacial settings for study: (1) uninhabitable environments that are more extreme
and life-limiting than terrestrial subglacial environments, (2) environments that are
habitable but are uninhabited, which can be compared to similar biotically influ-
enced subglacial environments on the Earth, and (3) environments with examples
of life, which will provide new opportunities to investigate the interactions between
a biota and glacial environments.
1. INTRODUCTION
The robotic exploration of the solar system has revealed an
increasing number of glacial environments. They include, for
example, glacial deposits on Mars [Head and Marchant,
2003], which were originally thought to be restricted to the
polar regions but have now been identified at lower latitudes;
oceans under ice covers on moons orbiting Jupiter, including
Europa, Ganymede, and Callisto [Carr et al., 1998; Baker
et al., 2005]; and moons orbiting Saturn, including Encela-
dus [Parkinson et al., 2008] and, potentially, Titan [Grindrod
et al., 2008].
Of the known extraterrestrial glacial environments, in one
case, the subglacial environment has been sampled. The icy
plumes produced in the southern polar regions of Enceladus
were sampled in a flyby by the Cassini spacecraft in 2005
and found to contain water ice, CO, CO
2
,N
2
,CH
4
, organics
[Waite et al., 2006, 2009; Matson et al., 2007], and more
recently, NH
3
has been interpreted [Waite et al., 2009]. It is
not known at what depth these plumes emanate.
At the time of writing, liquid water has only been indirectly
measured in some of these subglacial environments. The
most substantial liquid water body associated with an extra-
terrestrial subglacial environment is the ocean of Europa
[Carr et al., 1998], which, based on the volume of the moon,
1
Planetary and Space Sciences Research Institute, Open
University, Milton Keynes, UK.
2
School of Geographical Sciences, University of Bristol,
Bristol, UK.
3
Department of Earth and Environmental Sciences, University
of Illinois at Chicago, Chicago, Illinois, USA.
4
NASA Ames Research Center, Mountain View, California, USA.
5
British Antarctic Survey, Cambridge, UK.
6
School of GeoSciences, University of Edinburgh, Edinburgh, UK.
7
NASA Headquarters, Washington, D. C., USA.
Antarctic Subglacial Aquatic Environments
Geophysical Monograph Series 192
Copyright 2011 by the American Geophysical Union
10.1029/2010GM000939
129
contains more water than the terrestrial oceans. Here we
review the various subglacial environments (any environ-
ments under ice sheets or glaciers) that are likely to exist in
the solar system.
Scientific interest in extraterrestrial subglacial environ-
ments stems from two motivations. The first is the interest
in understanding the major physical and chemical processes
that drive surface and subsurface evolution on other plane-
tary bodies. The melting, refreezing, and movement of ices,
which entrain volatiles and salts, are the result of, or result in,
geologically active processes. The investigation of these
environments provides insights into the early conditions in
the solar system. The second motivation is the search for life.
Liquid water is presumed to be a basic requirement for life
(in addition to many other requirements, such as an energy
source). The discovery of glacial environments on other
planetary bodies that may harbor subglacial liquid water
suggests promising possibilities for the search for extrater-
restrial life. However, the habitability of these environments
can only be properly assessed when the presence of liquid
water has been confirmed and their physical and chemical
conditions have been determined.
Extraterrestrial environments might yield three new types
of subglacial environments for study:
1. These environments could be completely different from
those known on the Earth and could be uninhabitable. Many
extraterrestrial subglacial environments are likely to have very
different physical and chemical conditions to subglacial en-
vironments on the Earth. Different redox environments, dif-
ferent absolute and fluctuating conditions of radiation
(ionizing and UV radiation), temperature, and pH will yield
different environmental conditions. These environments will
expand the known physical and chemical parameter space of
subglacial environments and improve knowledge of the
known boundaries for habitability.
2. These environments could be habitable but uninhab-
ited. A habitable subglacial environment in a planetary
location where there is no life to take advantage of it (a
plausible example could be a localized and transient impact
melting of glacial ice on Mars) would allow biogeoche-
mists to examine how geochemical cycles operate in gla-
cial environments without a biota. This research might
therefore provide “control”environments in which only
geochemical cycles occur without the influence of a biota,
which can be compared to terrestrial environments to un-
derstand better the role of a biota in shaping glacial envi-
ronments on the Earth.
3. These environments could be inhabited. These envir-
onments would provide new data points to study the inter-
action of a biota and its subglacial environment and to
investigate new examples of life.
These three possibilities are conceptually illustrated in
Figure 1.
In this chapter, our objectives are to (1) briefly review what
we know about subglacial environments on the Earth and
their major physical, chemical, and biological characteristics,
(2) review the current state of knowledge of some of the
major extraterrestrial subglacial environments that have been
examined to date, (3) discuss parallels between analog en-
vironments on the Earth and extraterrestrial subglacial envi-
ronments, and (4) summarize current plans for the future
exploration of extraterrestrial subglacial environments.
2. DISTRIBUTION AND BIOLOGICAL POTENTIAL
OF SUBGLACIAL HABITATS ON THE EARTH
Ice covers between 11% and 18% of the Earth’s surface
during Quaternary glacial cycles and may have been even
more widespread in ancient periods of the Earth’s history such
as the Neoproterozoic [Schrag and Hoffman, 2001]. The
Antarctic Ice Sheet (>80% of world glacier area) and Green-
land Ice Sheet (10% of world glacier area) currently dominate
the distribution of subglacial environments on Earth.
In contrast to other parts of the biosphere, the composition
and function of microbial communities in deep, cold envi-
ronments is poorly understood, since they were once be-
lieved to be devoid of life, and direct access is hampered
by the overlying ice cover. The temperature profile and
substrate of the basal environment of glaciers and ice sheets
has a major bearing on the rates and pathways of microbial
activity. The most biologically active subglacial environ-
ments are those where liquid water is present. Here physical
erosion of the bedrock may also promote the accumulation of
reactive debris, which acts as a substrate for microbes, in
addition to a source of energy, organic carbon, and nutrients
[Tranter et al., 2005; Wadham et al., 2010]. Temperate valley
glaciers have ice at the pressure melting point throughout and
possess dynamic subglacial hydrological systems. Here sig-
nificant concentrations (10
4
–10
7
cells/mL) of active micro-
organisms have been reported [Sharp et al., 1999; Skidmore
et al., 2000, 2005; Botrell and Tranter, 2002; Foght et al.,
2004]. Colder polar systems may present a more challenging
environment for microbial communities, since a proportion
of glacier bed ice is below the pressure melting point, and
there are restrictions on the availability of liquid water.
Small, thin polar glaciers are entirely composed of “cold”
ice, and there is little or no free liquid water at the glacier bed.
The larger “polythermal”systems display a layer of cold ice
at the surface and around the margins [Paterson, 1994]. Here
microbial activity prevails in the “warm”core of the glacier,
where ice at the pressure melting point and liquid water is
present [Skidmore et al., 2000, 2005; Wadham et al., 2004].
130 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
In certain cases, the basal liquid water can be brine, support-
ing microbial communities, as at Taylor Glacier, Antarctica
[Mikucki and Priscu, 2007; Mikucki et al., 2009]. Less is
known about microbial communities in sub-ice sheet envi-
ronments, despite their large areal extent on Earth. Satellite
and geodetic data have begun to reveal the nature of sub-ice
sheet hydrological environments, demonstrating the wide-
spread presence of liquid water at the ice sheet bed. For
example, there is rapid drainage of surface meltwater to the
bed in Greenland [Zwally et al., 2002; Das et al., 2008], and
the transfer of kilometer cubed volumes of meltwater be-
tween Antarctic subglacial lakes [Wingham et al., 2006;
Fricker et al., 2007]. A mounting body of data demonstrates
the existence of microbial communities beneath ice sheets
[Miteva and Brenchley, 2005; Lanoil et al., 2009], though
further information regarding their composition, distribution,
and function awaits direct access of subglacial aquatic envir-
onments (e.g., Antarctic subglacial lakes) [Priscu et al.,2005;
Lake Ellsworth Consortium, 2007] via deep drilling cam-
paigns. A major control on the diversity of these microbiolog-
ical communities and the biogeochemical processes they
are involved with is the prevalent redox environment at
the bed.
Redox conditions at glacier beds, as in any aquatic envi-
ronment at or near the Earth surface, are controlled by the
rate of supply of oxidants versus the rate of oxidation [Tran-
ter, 2004]. The types of oxidants that can be supplied at
glacier beds include O
2
, Fe(III), Mn(IV), NO
3
, and SO
4
2
.
The main electron donors are often sulfides and organic
matter [Wadham et al., 2008].
Oxidants are derived from any surface meltwaters, which
reach the glacier bed, basal ice melt, and comminuted rock
flour. Oxygenated water flows from the surface during peri-
ods of snow and ice melting through moulins, crevasses, and
other englacial channels to the bed in many smaller ice
masses and near the margins of most of the larger ice masses
[Tranter et al., 2005]. The oxygen content of the waters
depends on the altitude from which the waters were sourced
and any overpressuring via interactions with entrained air
that may occur during descent to the bed. The type of
drainage system receiving these waters at the bed is usually
a channelized or low-pressure drainage system, which is
Figure 1. Conceptual illustration of the contribution of the study of extraterrestrial environments to subglacial studies.
Extraterrestrial subglacial environments might be so extreme as to be uninhabitable and lie outside the boundaries of
habitable conditions, providing new insights into the physical and chemical conditions suitable for life. They might
provide new examples of extreme but habitable subglacial environments. If they are inhabited, then they will provide new
examples of subglacial life to study. If they are habitable but are uninhabited, then they might be used as “control”
environments to study geochemical cycles in habitable environments without the influence of life.
COCKELL ET AL. 131
characterized by relatively short residence times (hours to
days) and low rock-water ratios (<1 g/L). Water flow rates
are also relatively high (~1 m/s), and so much of the fine
sediment to which the biomass is attached is suspended in
the water column. Hence, waters flowing through channel-
ized drainage systems are usually oxic because of the high
oxygen supply relative to the potential for oxidation [Tranter
et al., 2002]. Lower oxygen levels may be found in the
channel marginal zone that flanks the main channels. Here
water floods the subglacial till during rising water levels and
drains out during falling levels. Hence, the water residence
time is higher, the rock-water ratio is higher. Reduced com-
pounds such as organic matter, Fe (II), and Mn (II) on the
surfaces of comminuted mineral grains and sulfide minerals,
such as pyrite, become depleted over time by microbial
activity, although surface organic matter may be washed in
during rising water levels [Tranter et al., 2005].
The channelized or low-pressure drainage system and its
marginal zone (similar to the hyporeic zone in streams and
rivers) are flanked by the much more pervasive distributed, or
high-pressure, drainage system. Water flows approximately
in the direction of ice flow in the low-pressure drainage
system, whereas it flows across the direction of ice flow in a
high-pressured drainage system. Waters flow quite slowly in
the distributed drainage system (<0.1 m/s), and so rock-water
contact times are higher. Glaciers and ice sheets often have
areas of the bed that are draped with subglacial till, and where
this is unfrozen, flow within the water-laden till is part of the
distributed drainage system. Hence, rock-water ratios are also
higher in the distributed system, and so too is the biomass as a
consequence. Oxidation of organic matter and sulfide miner-
als with molecular oxygen is the most thermodynamically
favorable redox reaction to occur in till-rich environments
within the distributed drainage system. Should organic matter
and sulfide minerals, which are reducing agents, be plentiful,
microbially catalyzed reactions may deplete the oxygen
along the water flow path to such an extent that all the waters
become anoxic and other oxidizing agents, such as NO
3
,
Fe(III), Mn (IV), and SO
4
2
, are utilized to oxidize organic
matter and sulfide minerals. Should sufficient reactive organ-
ic matter be present, for example, in the form of overridden
paleosoils, then methanogenesis may occur [Tranter et al.,
2002, 2005; Wadham et al., 2008].
Sulfides are common components of many rocks, and
Fe(II) is a common component of many primarily silicate
minerals. Hence, glacial comminution of bedrock produces a
ready supply of reducing agents. Organic matter is also found
in many rocks. Beneath the interior of ice sheets, where no
surface meltwater reaches the bed, it is highly probable that
the supply of O
2
(and other oxidizing agents such as NO
3
and SO
4
2
) from the melting of basal ice, either as a conse-
quence of geothermal heating, frictional/deformational heat-
ing or regelation, is less than the supply of reducing agents
from subglacial comminution. Hence, it is likely that water
flowing through subglacial till beneath thick ice becomes
anoxic and that the biomass is dominated by communities
capable of existing at low Eh conditions [Wadham et al.,
2008]. Types of anoxic environments might include the till
beneath ice streams [Tulaczyk et al., 2000] and in the areas
between hydrologically connected subglacial lakes, which
may transmit water periodically and at least partially freeze
between connection events [Wingham et al., 2006]. By con-
trast, the larger, hydrologically closed subglacial lakes, such
as Vostok Subglacial Lake, are more likely to be oxic, since
the input of comminuted glacial debris is more limited, and
there is continual oxic recharge of the lake with meteoric ice
melt [Siegert et al., 2003], although biotic and abiotic sinks
for O
2
do not rule out anoxia even in these systems. Hence,
ice sheet beds are likely to display a wide spectrum of redox
conditions, that are connected to the type of drainage system
and the quantity and nature of the till present. Consequently,
ice sheet beds are also likely to be colonized by a diverse
spectrum of microorganisms [Miteva and Brenchley, 2005;
Skidmore et al., 2005; Tung et al., 2005; Christner et al.,
2008; Lanoil et al., 2009; Mikucki et al., 2009; Skidmore
et al., 2010].
3. EXTRATERRESTRIAL SUBGLACIAL
ENVIRONMENTS
3.1. Mars
3.1.1. Introduction. Mars is the fourth planet from the Sun
with an equatorial radius of 3397 km. Mars is rocky with a thin
(~6 mbar; about 1/200th that of the Earth) with an atmosphere
comprised primarily of CO
2
. The absence of a thick atmo-
sphere means that Martian surface temperatures are highly
variable: daytime temperatures can be higher than 20-Catthe
equator, but nighttime temperatures are tens of degrees Celsius
below zero. A similar latitudinal control of temperature exists
as the Earth, with polar regions being the coldest. At about
25.1-,Mars’obliquity is similar to that of the Earth, meaning
that Mars also experiences seasonal climate variations.
In many ways, Mars is the most similar planet in the solar
system to Earth, and decades of research have allowed a
detailed picture of Mars’geological history to be con-
structed. A particular focus of Mars missions has been trac-
ing the history of water, and Mars is thought to have once
been “warmer and wetter”than today, as demonstrated by
observations of ancient valley networks [e.g., Fanale et al.,
1992; Mangold et al., 2004], outflow channels [Baker,
1982], possible deltas [Pondrelli et al., 2008], and in situ
132 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
identifications of minerals interpreted to have formed by
groundwater processes [Squyres et al., 2004]. Mars might
also have once possessed a frozen ocean [Parker et al., 1989;
Taylor Perron et al., 2007] that occupied its low-standing
northern hemisphere, although debate over the existence of
such an extensive, long-lived body of water is still ongoing
[e.g., Carr and Head, 2003].
The upper few kilometers of the Martian crust contains
large amounts of water-ice [Squyres et al., 1992]. At high
latitudes, the surface regolith can contain more than 50% ice
[Boynton et al., 2002; Feldman et al., 2004] by volume and
is covered by only a few centimeters of ice-rich dust [Smith
et al., 2009]. Models (summarized by Squyres et al. [1992])
suggest that such ice persists to depths of several kilometers,
at which point the melting isotherm (the depth at which the
geothermal temperatures are high enough to melt ice) causes
the ice to become liquid water. Nearer the equator, the top of
the ice table is driven deeper, but the base of the ice table is
shallower, because of warmer year-round surface tempera-
tures [Fanale, 1976].
The most recent high-resolution imaging data from Mars
have shown that the action of liquid water on the surface has
not been confined to the ancient past. Images of fluvial-like
gullies [e.g., Malin and Edgett, 2000], geologically recent
outflow channels [Burr et al., 2002b], and low-latitude peri-
glacial landforms [Balme and Gallagher,2009;Page, 2007]
have demonstrated the action of liquid water at the surface in
the past few millions years. Further studies [e.g., Costard et
al., 2002] have linked gully formation with changes in the
Martian climate, which are driven, in turn, by changes in
Mars’obliquity [Head et al.,2003a;Kieffer and Zent,1992;
Schorghofer, 2007]. The Martian obliquity varies periodically
by more than 20-in an ~125,000-year cycle [Laskar et al.,
2004]. This suggests that recent cycles of deposition, removal,
and even perhaps thaw of ice have controlled the Martian
surface environment over the past few million years. Recently,
a variety of hydrated minerals have been identified on Mars
that provide further evidence for groundwater [Gendrin et al.,
2005; Poulet et al.,2005].
3.1.2. Physicochemical conditions and the prospects for
life. Mars hosts a range of terrains and environments that, in
a similar way to the Earth, have evolved over geological
time, often leaving only morphological or geological traces
of their existence. Therefore, it is impossible to summarize
all the possible habitats that might have come and gone over
the planet’s history. Chemically, the Martian crust is poorly
documented, for a blanket of fine dust, dominated by sili-
cates and iron, calcium, aluminum, and magnesium oxides,
drapes much of the surface and makes remote sensing studies
difficult.
Martian environments under ice covers, such as lake ice
covers, have for a long time been recognized to be potential
habitats for life, based on studies of analogous ice-covered
habitats on Earth [McKay et al., 1985]. One subsurface
candidate for a stable subglacial environment is the location
of the melting isotherm, several kilometers beneath the sur-
face of the Martian cryosphere. This perhaps provides the
environment most conducive for life, for water here could
remain liquid for geologically significant time periods. Sec-
ond, Mars’extensive, kilometer-thick perennial polar caps
[Phillips et al., 2008] are mainly water-ice, and it has been
suggested that pressure-induced melting or geothermal ac-
tion could lead to the formation of pockets of liquid at the
base of the ice [Clifford, 1987], in a similar way to the
preservation of subglacial lakes in the Antarctic. Indeed, it
has also been suggested that Chasma Boreale, a large reen-
trant and valley system within the north polar cap, was
formed by catastrophic flooding from just such a subglacial
liquid reservoir [Clifford, 1987; Fishbaugh and Head, 2003].
Although simulations suggest that pressure melting is un-
likely [Greve et al., 2004], the presence of salts that depress
the melting point of water (such as perchlorates, recently
discovered at the Phoenix Landing site) [Hechtetal.,
2009] could allow melt to form. If this were the case, then
the margins of the north polar cap would form an attractive
target for study of past subglacial environments.
Radar studies of massive ice deposits on Mars have not
revealed liquid water [e.g., Holt et al., 2008]. It is plausible
that the Martian water table is hidden and that attenuating
material within ice might hide aquifers beneath ice deposits
[Farrell et al., 2009]. However, at the time of writing, it
seems that most Martian ice deposits are more similar to cold
terrestrial polar glaciers with little, if any, water in the sub-
glacial environment. However, thin layers of water at the
base of glaciers would not be easily visible to radar analysis.
Near-surface candidate subglacial environments can be
split into either (1) locations beneath extant surface ice or
(2) regions in which ice was recently present at the surface
but has since been removed. Although there is good evidence
for extant surface ice (usually dust or debris covered) in the
form of tropical mountain glaciers [Head and Marchant,
2003], ice-rich and glacier-like flows [Lucchitta, 1981;
Pierce and Crown, 2003], midlatitude ice-rich dust mantles
[Mustard et al., 2001], and high-latitude patterned ground
[e.g., Mangold, 2005], most authors have found no evidence
for liquid water in these systems. For example, the Martian
glaciers seen today are inferred to be cold-based with no
basal melting [Head and Marchant, 2003; Shean et al.,
2005], similar to cold polar glaciers on the Earth (section 3).
The evidence for recent wet-based glaciers in Mars’past is
also somewhat equivocal. Observations of possible eskers
COCKELL ET AL. 133
(ridge-like landforms caused by deposition of sediments in
sub- or intraglacial fluvial channels) and associated land-
forms in highlands surrounding the Argyre and Hellas im-
pact basins point to possible wet-based glaciation [e.g.,
Banks et al., 2008; Kargel and Strom, 1991], although it is
likely that these features are a billion years old or more in
age.
Another example of a surface morphology that might indi-
cate a recent subglacial environment comes from the recent
description of an equatorial “frozen sea”in the Elysium
Planitia region of Mars [Murray et al., 2005]. These depos-
its occur at the end of what is probably the youngest, large-
scale outflow channel on Mars (Athabasca Vallis), which
might have been active only a few million years ago [Burr
et al., 2002b]. Although some authors have argued that the
“sea”is now occupied by flood lavas, the pattern of inter-
linked basins and channels in which it sits strongly suggest
that there was a water-filled basin here at some point
[Balme et al., 2010]. Like many other flood channels, Atha-
basca Vallis was carved by liquids emanating from a deep
tectonic fracture [Burr et al., 2002a; Head et al., 2003b].
Given that the source of the floods is probably long-lived,
subsurface aquifers, the slowly freezing water that occupied
the basin at the termination of the channel could plausibly
have provided a transient subglacial habitat for any organ-
isms that were once present in the aquifer, deep below
ground.
The only geologically recent environments in which ice
and water are likely to coexist near the surface appear to be
periglacial, rather than subglacial. Evidence for very recent
thaw of ground ice on Mars is amassing [e.g., Balme and
Gallagher, 2009; Balme et al., 2009; Costard et al., 2002;
Levy et al., 2009; Page, 2007; Soare et al., 2008] and
includes both the well-known gullies and also more contro-
versial features such as thermokarst, pingos, and sorted pat-
terned ground. Again, though, these environments probably
only contain(ed) transient liquid water, and in many cases,
the actual amount of water was likely to be very small.
Periods of higher obliquity in the past may also have caused
glacial melting [Jakosky et al., 2003]. Although these envi-
ronments may not be conducive to life today, they might be
plausible sites to search for past life on Mars.
Currently, there are no extant subglacial Martian environ-
ments that can be easily observed. The astrobiologically
most promising subglacial habitats are at the base of the
polar ice caps and deep within the crust, at the base of the
cryosphere. It is likely that deep drilling will be required to
analyze either of these environments. Debris-rich basal ice in
the marginal zones of the ice caps, especially the North Polar
cap, are also promising from an astrobiological perspective
and do not require deep drilling [Skidmore et al., 2000].
Assuming that melting can, or could, occur in Martian
subglacial environments, then they are likely to be anaerobic,
consistent with Martian atmospheric composition. There-
fore, the closest terrestrial analogs in terms of available redox
couples are likely to be anaerobic zones in subglacial envi-
ronments (section 3). A range of electron acceptors found in
terrestrial subglacial environments (section 3) are available
on Mars including Fe(III) and probably Mn(IV), both from
comminuted oxidized basaltic rocks. Despite the probable
lack of photosynthesis to create a ready supply of other
oxidized elements for use as electron acceptors, there are
plentiful supplies of sulfate in salts detected across the Mar-
tian surface [Clark et al., 1982; Rieder et al., 1997; Gendrin
et al., 2005; Langevin et al., 2005; Squyres et al., 2006].
Perchlorate, identified in the Martian soil by the Phoenix
Lander [Hechtetal., 2009], is also a microbial electron
acceptor. Electron donors in subglacial environments could
plausibly include Fe(II), again produced from comminuted
Martian basalts, and possibly organic material delivered
exogenously in meteorites and either directly delivered into
the subsurface through glaciers or leached there by melting
in the past.
Other elements required for life are likely to be present in
Martian subglacial environments, including trace elements
such as Zn, Cu, Ni, and other elements found in glacially
comminuted basaltic rocks and phosphorus from apatite.
More uncertain is the source of nitrogen to sustain a Martian
subglacial biota. Without a biological nitrogen cycle, fixed
nitrogen will be produced in low abundance on the present-
day subsurface or surface. However, fixed nitrogen could
have been produced by volcanic or impact processing in the
early history of the planet [Segura and Navarro-Gonzalez,
2005; Summers and Khare, 2007; Manning et al., 2009],
with nitrate-containing minerals subsequently made avail-
able by glacial comminution.
The limited, or lack of, supraglacial melting today would
limit the movement of water through glaciers to generate the
flow paths of nutrients observed in present-day subglacial
settings on the Earth (section 3), meaning that subglacial
environments on present-day Mars are more likely to reach
chemical equilibrium and have unfavorable conditions for
the persistence or replenishment of redox couples for life.
These considerations show that Martian subglacial envi-
ronments would be favorable places for life at any point in
time at which melting could occur, driving fluid movement
to supply nutrients and generate chemical disequilibria in
analogy to present-day subglacial environments on the Earth.
Although these conditions cannot be ruled out today, they are
morelikelytohaveoccurredduringgeologicallyrecent
obliquity changes or in the more distant past history of the
planet. Thus, the search for extant life in Martian subglacial
134 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
environments is a valid objective, but subglacial environ-
ments are also particularly favorable locations to search for
past life on Mars.
3.2. Europa
3.2.1. Introduction. At least three of the Jovian moons
may harbor liquid water oceans (Callisto, Ganymede, and
Europa) [Baker et al., 2005]. The extent of any putative
oceans or their state (frozen or liquid) in Ganymede or
Callisto is not known [Spohn and Schubert, 2003]. Greatest
attention has been given to Europa, which will be discussed
in detail here. Europa is the sixth moon of Jupiter and has a
radius of 1550 km. The moon was discovered in 1610 by
Galileo Galilei. It is the smallest of the four Galilean moons.
Europa has a similar bulk composition to the terrestrial
planets. The surface has a high albedo caused by water ice
and few surface impact craters, suggesting a young reworked
surface, possibly of 20 to 180 million years old.
Europa has a variety of features, which suggest a relatively
active geology. The moon’s surface is crisscrossed with lines
(lineae), which are dark streaks across its surface (Figure 2).
The lines appear to be cracks in the ice on either side of
which sheets of ice move relative to one another. Cross-
sections of these features reveal a ridge-like morphology.
Chaos terrain, differently interpreted as the result of diapir-
ism or melting of the ice, is observed on the surface [Riley et
al., 2000; Greeley et al., 2004].
Europa is thought to host a liquid water ocean beneath its
icy crust, a supposition resulting from three observations: (1)
the presence of an induced magnetic field as the moon passes
through Jupiter’s magnetic field, detected by the Galileo
spacecraft provides evidence for a conducting medium
[Khurana et al., 1998; Kivelson et al., 2000], (2) the active
geological nature of its surface, which suggests a mobile
medium beneath the ice [Carr et al., 1998], and (3) the
asynchronous rotation of Europa, which suggests a decou-
pling of its silicate core and icy surface. The extent of the
ocean remains debated. The ice layer above it may be kilo-
meters to tens of kilometers thick and the ocean about 80–
170 km deep [Anderson et al., 1998; Turtle and Pierazzo,
2001; Greeley et al., 2004]. Nevertheless, even with lower
estimates, the ocean would still contain substantially more
water than the Earth’s oceans.
3.2.2. Physicochemical conditions and the prospects for
life. The physical conditions at the surface of the Europan ice
are better constrained than the ocean. It is comprised primar-
ily of water ice and has temperatures of 86–132 K. Particle
bombardment from Jupiter’s magnetosphere delivers H, S,
and O, and also drives complex chemistry resulting in the
formation of compounds such as O
2
,SO
2
, and H
2
O
2
[Kargel,
1998; Carlson et al., 1999, 2002; Hand et al., 2006], which
have been observed. The radiolysis of water contributes to
the formation of oxygen at the surface of Europa. The en-
trapment of oxygen within the surface ice and its exposure to
radiation may also generate O
3
, but the spectral feature of
this compound has remained controversial [Johnson et al.,
2003] because it may be mixed with other absorption fea-
tures such as those caused by –OH or organics [Johnson
et al., 2003].
Models suggest that sulfuric acid, hydrated salts, and other
compounds should be present. Different lines of evidence
support the presence of sulfates on Europa and in its oceans.
The infrared signatures on Europa’s surface can be explained
with sulfates [McCord et al., 2002], and sulfate might be
produced radiolytically [Johnson et al., 2003]. The sulfate
concentrations of Europa’s oceans were modeled by McKin-
non and Zolensky [2003]. They derive an upper limit of 10%.
Other chemical parameters of the Europan ocean are poorly
constrained. For example, the pH of the ocean is unknown; it
may have a low pH [Kargel et al., 2000].
In addition to salts, simple organic molecules should also
be produced on the surface, and the dark lineations on the
surface of the ice may well contain more complex organic
chemistry. Meteoritic and cometary infall to the surface
would be expected to deliver organic molecules [Pierazzo
and Chyba, 1999, 2002].
The presence of a subsurface ocean on Europa has made
the moon the focus of many astrobiological investigations as
a potential habitat for life [Reynolds et al., 1983; Jakosky and
Shock, 1998; Gaidos et al., 1999; Greenberg et al., 2000;
Chyba and Hand, 2001; Chyba and Phillips, 2001; 2002;
Schulze-Makuch and Irwin, 2002; Pierazzo and Chyba,
2002]. The various potential habitats that could exist in and
Figure 2. Conamara Chaos on Europa. Visible are cracks in the ice
of the Europan ice shell. The image is about 100 km across.
COCKELL ET AL. 135
under the Europan ice sheet have led to a proposed taphon-
omy of Europa, with suggestions on the best locations to
search for preserved life [Lipps and Rieboldt, 2005].
Assessing the ocean as an abode for life depends critically
on knowledge of the physicochemical properties and their
many factors, important for biochemistry, which are still
poorly constrained. A number of studies have attempted to
assess potential sources of energy in a Europan ocean. En-
ergy might come from the surface in the form of organics or
oxidants produced during interactions of radiation with the
surface [Chyba, 2000; Chyba and Hand, 2001; Cooper et al.,
2001, 2003]. These forms of energy would require an active
connection between the surface and the Europan ocean.
In the ocean, phototrophy would be unlikely, since solar-
derived light will be reduced to below the minimum required
for photosynthesis in the first few meters of the Europan ice
layer [Cockell, 2000], although phototrophy could plausibly
occur using geothermal energy from hydrothermal vent-like
environments [Beatty et al., 2005].
Depending on the oxidation state of the ocean, life might
be able to use a variety of other redox couples available in the
Europan ocean. There are a diversity of plausible candidates
including H
2
and CO
2
used in methanogenesis. H
2
would be
derived from serpentinization of ultramaficrocksinthe
Europan silicate core and CO
2
derived from the primordial
inventory. Fe
3+
and H
2
could act as a redox couple for iron
reduction, with Fe
3+
derived from the silicate core [Schulze-
Makuch and Irwin, 2002].
McKinnon and Zolensky [2003] point out the critical lack
of information on sulfur evolution in Europa, which has
implications for the assessment of its habitability. The oxi-
dation state of the sulfur in the ocean and its concentration
will have an important influence on the extent to which the
ocean is in direct circulatory contact with a silicate core. For
example, if sulfur is present as thick beds of sulfur at the
bottom of the ocean, these beds would impede any possibil-
ity of life analogous to hydrothermal vents in the Earth’s
deep oceans.
Schultze-Makuch and Irwin [2002] considered speculative
organisms other than those that use traditional redox couples
as possible inhabitants of a Europan ocean. Their study was
an investigation of unconventional energy acquisition path-
ways that organisms might evolve in an environment where
chemical energy is limited. They considered organisms using
thermal energy, kinetic energy, osmotic gradients, magnetic
fields, and gravitational energy. Of these various potential
energy sources, they concluded that kinetic energy and os-
motic energy might be the most promising candidates
[Schultze-Makuch and Irwin, 2002].
A critical parameter still not well constrained is the tem-
perature within the ocean. Marion et al. [2003] investigated
temperature and salinity in model Europan oceans and sug-
gested that temperatures within the ocean might be too low
for life (<253 K) and salinity high. Thermal diapirs within
the ice crust might yield more favorable environments for
life [Ruiz et al., 2007].
All of these studies illustrate that at the current time, there
is insufficient data on the ocean composition and that future
missions will dramatically improve the basis with which to
assess the habitability of Europa.
Accessing the subglacial environment on Europa to search
for life will be hugely challenging on account of the need to
penetrate the ice layer that may be many kilometers thick.
However, if ocean-surface connection exists in lineae and
chaotic terrains, then biosignatures might be sought on the
surface of Europa [Dalton et al. 2003].
3.3. Enceladus
3.3.1. Introduction. Enceladus is a small Saturnian moon
with a mean radius of 252 km. It was first observed in 1789
by William Herschel. The first investigations of the moon
were carried out by the Voyager robotic craft, which deter-
mined it had a high albedo, probably caused by water ice,
and an association with the Saturnian E ring. Voyager 2 also
determined that the surface of the moon was comprised of
terrains of different ages.
The Cassini spacecraft, which first visited the moon in
2005, revealed the presence of multiple gas plumes emanat-
ing from the south polar terrain of the moon, which coalesce
into a giant plume over 80 km from the surface of the moon.
It was this phenomenon that heightened astrobiological in-
terest in the moon.
The region from which the plumes are ejected has charac-
teristic “stripes”(named “tiger stripes,”Figure 3), several
hundred meters wide and hundreds of kilometers long. They
are morphologically analogous to the ridges observed in the
ice sheet of Europa. The plumes are associated with an
anomalous source of heat suggesting temperatures near the
plumes in the near surface of 190 K and ~6 GW of energy
[Spencer et al., 2006], implying even higher temperatures
(250–273 K) [Parkinson et al., 2008] in the deeper subsur-
face. The plume material is responsible for the formation of
the Saturnian E ring.
3.3.2. Physicochemical conditions and the prospects for
life. Active geological turnover within the plume-generating
region could occur. If the tiger stripes are formed in a mech-
anism analogous to the spreading at terrestrial mid-ocean
ridges, then a regional heat flux sufficient to generate the
observed thermal anomaly could be created (~250 mW
m
2
), with recycling of the crust on a 1 to 5 Ma time scale,
136 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
in which case, the heat source would occur from within the
ice. An alternative hypothesis is that the heat is generated
within a silicate core [Castillo-Rogez et al., 2007], and it
might be driven by tidal heating of the moon by Saturn. At
the current time, the exact source of the heat and its geolog-
ical history is not well understood. The extent of differenti-
ation of the moon is also not fully known. The core might be
~150 km in diameter [Parkinson et al., 2008].
The composition of the plumes provides tantalizing in-
sights into the possible composition of Enceladus’interior
(Figure 4). The plumes contain water ice and vapor (91%),
which may come from a subsurface (subglacial) ocean, or it
may be ice entrained from the surface of the moon [Porco et
al., 2006; Kieffer et al., 2006].
Intriguing is the observation of N
2
(~4%) and CH
4
(~1.6%) in the plume. There are many plausible explanations
for the presence of these gases [Waite et al., 2006, 2009;
Matson et al., 2007; Glein et al., 2008]. The gas production
may be linked to ancient sources including primordial ther-
mal degradation of ammonia into N
2
and organics into CH
4
,
which would be consistent with the presence of propane and
acetylene in the plumes, although the latter could also be
formed by photolysis of methane [Parkinson et al., 2008].
Alternatively, these gases are primary in source but are
trapped within the ice as clathrates and are steadily released
in the plumes. Although there were originally no reports of
ammonia in the present-day plumes (Figure 4), new studies
of the data suggest it is present [Waite et al., 2009].
From an astrobiological point of view, a significant obser-
vation is the lack of salts in the plumes, since that might imply
a lack of available cations and anions required as nutrients in
life. However, as McKay et al. [2008] point out, this may
reflect preferential retention of salts in deeper water bodies
with ices nearer the surface remaining “fresher,”analogous to
perennially ice-covered lakes in Antarctica, whose ice covers
are low in salt but whose deeper layers contain salts. The same
situation has also been proposed for Vostok Subglacial Lake
[McKay et al., 2003], where gas buildup in the lake may occur
as a consequence of gassed water input to the lake via mete-
oric ice and the removal of water as pure ice into the ice base
accretion [Siegert et al., this volume].
Many of the discussions on habitability elaborated for
Europa apply to Enceladus. At the time of writing, there are
many key unknowns about the moon that preclude accurate
modeling of the conditions for life on Enceladus. Key among
these questions are the following: (1) Is there exchange be-
tween the surface of Enceladus and its subsurface? (2) Are
there salts in the deeper layers of ice or in deep water bodies?
(3) How can point 2 be reconciled with the presence of a sili-
cate core, and is a core linked to any putative subsurface water
body? (4) What are the sources and sinks of redox couples?
McKay et al. [2008] discuss a range of possible terrestrial
analogous ecosystems for Enceladus. The production of
methane from CO
2
and H
2
by methanogens is one plausible
reaction scheme. The CO
2
would be derived from primordial
CO
2
entrainedwithintheice(whichisobservedinthe
plumes) [Matson et al., 2007] and the H
2
from serpentiniza-
tion reactions occurring in Enceladus’core. Ultimately, the
recycling of these redox couples would be achieved by
thermal degradation of the methane produced.
Figure 3. Tiger stripes in the southern polar region of Enceladus.
Figure 4. Composition of the plumes of Enceladus showing range
of cometary values (N
2
is not included).
COCKELL ET AL. 137
Indeed the discussions by McKay et al. [2008] underline,
as with all subglacial environments, how critical the link is
between subglacial liquid water and silicate rocks for life.
Silicate cores open other possibilities for the generation of
redox couples [Parkinson et al., 2007], including the radio-
lytic production of H
2
from H
2
O[Lin et al., 2006], which can
be used in methanogenesis as above or in sulfate reduction
(Figure 5).
Aside from redox couples, silicate rocks provide a range of
other elements (Mg, Ca, K, etc.) required by all organisms on
the Earth and trace elements (Ni, Cu, Zn, Mo, etc.), which
are used by organisms. It is beyond the scope of this paper to
speculate on what “other”life would use among the range of
available elements, but it can be assumed that at least some
subset of them is required to do any meaningful complex
biochemistry.
If the plumes on Enceladus are directly emanating from
subsurface liquid water reservoirs, then the lack of observed
salts suggests that at least the water observed is not directly
in contact with the silicate core and is depauperate in bio-
logically useful elements. However, organisms can use ex-
tremely low levels of elements. Further, many organisms can
extract essential nutrients from silicate grains. Even if the
silicate content of the plumes was less than 1% [Parkinson et
al., 2008], then they would provide a source of many ele-
ments. However, if the grains were covered in almost pure
water ice, then silicate grains would be difficult to detect. A
thorough investigation of habitability awaits a more com-
plete and detailed inventory of elements within the plumes
and their component ices/grains.
Unlike Europa, the supraglacial environment of Enceladus
is not as heavily processed by radiation. However, Parkinson
et al. [2008] propose a mechanism whereby water in the
Saturnian E ring (itself from Enceladus) would be affected
by UV radiation and ionizing radiation to generate oxidants
such as H
2
O
2
, which would then be swept up by the moon on
its surface. Whether these compounds would have any astro-
biological significance would depend on their circulation
into the interior of the moon.
One important factor that distinguishes the environment of
Enceladus from terrestrial subglacial environments is the
possible presence of ammonia in the ice at over 10% mass
[Squyres et al., 1983]. Ammonium is used as an effective
biologically available source of N by terrestrial organisms,
and in some subglacial environments, nitrate is ultimately
produced by N fixation and nitrification in the supraglacial
environment (section 3). However, the effects of long-term
concentrations of high ammonia concentrations to biochem-
ical systems are not well known.
The ejection of plumes into outer space from Enceladus
offers remarkable possibilities for future astrobiology mis-
sions, since they provide an opportunity for technically
“easy”sampling of an extraterrestrial subglacial environ-
ment with the minimum chance for forward contamination
of the environment (see below). Already, the data we have on
these plumes was achieved by a flybywiththeCassini
spacecraft, which was not designed for this task, showing
the readily achieved sampling of the plumes. Future missions
dedicated to astrobiology might study the isotopic composi-
tion of the CH
4
in the plumes to attempt to determine a biotic
or abiotic source [McKay et al., 2008]. An investigation of
higher carbon compounds would also yield information on
pathways of organic complexification in the subsurface of
Enceladus.
Figure 5. Hypothetical scheme for sulfate reduction in a subglacial environment using the radiolysis of water to generate
hydrogen as an electron donor and oxidants to regenerate sulfate [after McKay et al., 2008].
138 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
3.4. Titan
3.4.1. Introduction. Titan is the largest Saturnian moon
with a radius of 2575 km. It was first observed in 1655 by
Christian Huygens. Titan is composed primarily of rock and
water ice and hosts lakes of liquid hydrocarbons, the only
body other than the Earth known to possess stable liquid on
its surface. The atmosphere of the moon is 98.4% nitrogen,
the remainder is comprised of methane and trace gases
including ethane, diacetylene, methylacetylene, acetylene,
and propane [Nieman et al., 2005]. The surface pressure is
~1.5 bar. The atmosphere is opaque to many wavelengths,
and a complete reflectance spectrum of its surface cannot be
obtained from present space measurements.
3.4.2. Physicochemical conditions and the prospects for
life. Fortes [2000] presents a model for a Titan with a water-
ammonia ocean early in its history. The formation of a thick
N
2
atmosphere combined with loss of other atmospheric
components would cause this ocean to roof over with frozen
volatiles. The depth of any putative ocean is uncertain;
estimates have ranged from an initial depth of 50 km to a
present-day depth of 200 km [Grasset and Sotin, 1996]. The
temperature of this putative ocean may be ~235 K, set by the
eutectic temperature of the H
2
O–NH
3
system. It is not clear
that if an ocean was present, it would be in contact with a
silicate core. Titan is thought to have such a core, but the
ocean could be separated from it by a thick layer of H
2
O–
NH
3
ice.
Fortes [2000] examines a range of physical and chemical
parameters that might be found in a Titan subglacial ocean.
These considerations are similar to those that have more
recently been applied to Europa and Enceladus. Nutrients
are presumed to be made available from early chondritic
input, including the input of major elements and organic
carbon. The latter was suggested to provide a potential
source of energy for heterotrophs.
Insofar as the reaction of acetylene, ethane, and organic
solids with hydrogen is thermodynamically favorable, these
substrates could also provide a redox couple for any pro-
posed Titan biosphere [McKay and Smith, 2005]. The detec-
tion of life on Titan could focus on the use of carbon isotopic
analysis of organic material or methane [e.g., Fortes, 2000]
or the anomalous depletion of gases in the atmosphere or on
the surface [McKay and Smith, 2005].
4. TERRESTRIAL ANALOGS: PARALLELS
AND LIMITATIONS
Finding analogs on the Earth with which to assess the
habitability of extraterrestrial subglacial environments is se-
verely limited by a lack of comprehensive information on the
physicochemical conditions in extraterrestrial subglacial en-
vironments. Marion et al. [2003] consider the case of Europa
and split their discussion into three sections: the ice layer, the
brine ocean, and the seafloor environment. They point out
that the choice of analog environment depends upon the
biological factor under consideration, suggesting that deep
ice cores might be good analogs for understanding the pres-
ervation of biosignatures in Europan ice, but that ecosystem
structure and constraints to life in ice are best investigated in
the perennially ice-covered lakes of Antarctica. Assuming
that salinity, acidity, and temperature will be three of the
most important factors limiting life in aqueous environments,
Marion et al. [2003] list a number of analogs for understand-
ing different types of brine systems and their influence on a
biota. Deep brine basins such as Orca Basin, the Gulf of
Mexico, and Mediterranean deep brine basins are suggested
as possible analog environments for investigating Europan
ocean seafloor environments.
Analog environments can provide a set of observations
concerning life that can be tested by exploring extraterrestrial
subglacial environments, and they can also be used as tech-
nology test beds for developing methods to be used to
explore extraterrestrial environments.
Greatest attention has been given to the most extreme
terrestrial environments as analogs. Subglacial lakes have
previously been investigated as analogs for extraterrestrial
subglacial water bodies. For example, Vostok Subglacial
Lake, located ~4 km deep in the East Antarctic ice sheet and
isolated from the atmosphere for ~15 Ma (although ancient
atmospheric gases are continuously being delivered to the
lake through basal pressure melting) has previously been
discussed as an analog for extraterrestrial subglacial envi-
ronments [e.g., Karl et al., 1999; Lipps and Rieboldt, 2005;
Siegert et al., this volume]. Deep ice core drilling has yielded
organisms and environmental data from the accretion ice just
above the lake. The accretion ice had two- to sevenfold
higher bacterial numbers than the ice above it, suggesting
that the lake is a source of bacteria. Members of the beta,
gamma, and delta subdivisions of “Proteobacteria”were
identified [Christner et al., 2006]. In addition to a focus on
biological data, the study of Vostok Subglacial Lake has
focused on many of its physical characteristics, which influ-
ence the biota [Wells and Wettlaufer, 2008]. For example, the
extent of mixing is important for determining the nutrients
and redox couples available to life. The lake also receives
mineral and biological input from the Antarctic ice. Models
developed for understanding the mixing regimen in subgla-
cial lakes will prove valuable for calculating the mixing
regimens in extraterrestrial water bodies when sufficient data
exists to constrain them [see Siegert et al., this volume].
COCKELL ET AL. 139
The drilling of a subglacial volcanic lake in Iceland (under
the Vatnajokull ice cap) provided evidence of a community
containing a diversity of chemolithotrophs using sulfide,
sulfur, or hydrogen as electron donors or oxygen, sulfate, or
CO
2
as electron acceptors [Gaidos et al., 2004]. The lake
water is fresh and slightly acidic, and the lake chemistry is
dominated by glacial melt. Between 1922 and 1991, 78% of
the lake water is estimated to have been supplied by basal
melting of the ice sheet. Microbial numbers in the water
column were 2 10
4
mL
1
. The microbial diversity was
explained by the mixing of sulfidic and oxygenated water.
Some of these redox couples could provide analogies to
extraterrestrial redox couples. However, insofar as the oxy-
genated water is linked to the surface aerobic environment,
the lake chemistry and its biota is strongly coupled to the
terrestrial aerobic biosphere and is likely to limit the analog
[Gaidos et al., 1999].
It is also clear that the best analogs for extraterrestrial
subglacial environments may not be subglacial environments
on the Earth. Prieto-Ballesteros et al. [2003] describe Tirez
Lake, a briny lake in La Mancha, Spain. The lake contains a
Mg-Na-SO
4
-Cl brine. Although the lake contains a photo-
trophic population, which is not considered relevant to Eu-
ropa, it also contains a population of sulfate reducers and
methanogens, which the authors discuss as a potential analog
for the anaerobic use of the sulfate or methanogenesis in a
Europan ocean.
It is possible that no environment on the Earth can truly
represent analogous conditions to any extraterrestrial subgla-
cial environments (Table 1). Apart from putative ecosystems
operating entirely independently from the surface photosyn-
thetic biosphere, on the Earth, even anaerobic subglacial
environments are linked to the availability of redox couples,
e.g., sulfate and nitrate, generated, in large part, in the aerobic
biosphere (section 3). Understanding fluxes of solutes into
and out of extraterrestrial subglacial environments can only
be properly achieved by directly taking measurements in situ;
only then can their astrobiological potential be assessed.
A case in point is again the Europan ocean. Gaidos et al.
[1999] discuss the biotic potential of the ocean. Many of the
redox couples associated with terrestrial subglacial environ-
ments may be difficult or impossible to sustain. For example,
more reducing conditions in the silicate core of Europa may
mean that carbon is primarily outgassed as methane rather
than carbon dioxide, preventing methanogenesis (although
other substrates such as acetate and methanol can support
methanogenesis. Furthermore, methane is itself a substrate
for methanotrophy). Similarly, sulfur, produced as sulfides,
might deny life a source of oxidized electron acceptors. If
turnover with the icy surface of the moon, where radiation
bombardment could produce electron acceptors such as per-
oxide, is insufficient, then the Europan ocean may provide an
extremely energy-poor environment for life. The turnover
within the ocean will also determine the extent to which any
redox couples reach equilibrium or whether geochemical
disequilibria can be maintained over geologically long time
scales to provide energy and nutrients for life. It is not yet
clear to what extent this is the case for many of the extrater-
restrial subglacial environments discussed here.
5. RISK OF CONTAMINATION
Concern about the contamination of extraterrestrial bodies
that might be capable of sustaining life has led to substantial
considerations on measures to prevent forward contamina-
tion. Apart from the threat to an indigenous biota, it is
conceivable that extraterrestrial subglacial environments
might locally have habitable conditions for life, but these
environments are not inhabited because life has not origi-
nated on that body, been transferred to it, or had the
opportunity to move into highly localized conditions for
habitability (e.g., transient liquid water). Contaminating
these sites might prejudice an ability to study habitable but
abiotic environments.
For Mars, detailed consideration has been given to the
environmental parameters that would define regions as “spe-
cial regions,”i.e., locations in which conditions might be
suitable for the replication of terrestrial organisms. To ad-
dress what these parameters are, an extensive workshop was
held to synthesize our existing knowledge of the limits of
terrestrial life with a special focus on low temperature and
water activity limits [Kminek et al., 2010]. It was concluded
that any region experiencing temperatures >25-C for a few
hours a year and a water activity >0.5 can potentially allow
the replication of terrestrial microorganisms. These con-
straints were based on the addition of a 5-C buffer to a range
of data that suggest that metabolic activity, let along repro-
duction, does not occur below ~20-C and the known limits
of water activity in microorganisms, which generally cannot
grow at water activities below ~0.6. Physical features on
Mars that can be interpreted as meeting these conditions
constitute a Mars Special Region. Based on current knowl-
edge of the Martian environment and the conservative nature
of planetary protection, these regions include gullies and
bright streaks associated with them, pasted-on terrain, deep
subsurface, dark streaks only on a case by case basis, and
others to be determined. The deep subsurface could include
subglacial environments.
Insofar as this study considered available knowledge on the
low temperature and water activity limits for life, then
its conclusions apply to other extraterrestrial subglacial envi-
ronments, accepting, of course, that life needs much more than
140 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
just high water activity and liquid water above 25-Ctogrow.
In the absence of detailed information on the chemistry of the
Europan ocean or the source of the plumes of Enceladus, then
initial missions to characterize these moons that involve direct
contact with the surfaces will require stringent planetary pro-
tection protocols to prevent forward contamination.
Perhaps one of the most useful contributions of subglacial
exploration on the Earth is the development of technologies that
will improve the ability to access and explore extraterrestrial
subglacial environments. Doran and Vincent [this volume],
discuss the growing procedures and guidelines developed for
responsible stewardship of subglacial environments on Earth.
Table 1. Some Differences and Similarities Between Terrestrial and Extraterrestrial Subglacial Environments
a
Factor Comments Consequences for Life
Example Subglacial
Environment Reference
Differences
Ammonia High ammonia concentrations
in some extraterrestrial
subglacial environments
High concentrations of
ammonia potentially
toxic
Enceladus and Titan Fortes [2000], Mitri
et al. [2008], and
McKay et al. [2008]
Highly reducing
conditions
Lack of source of oxidants Shortage of electron
acceptors
Europa Gaidos et al. [1999]
and Schulze-Makuch
and Irwin [2002]
Lack of organics No biological input of
exogenous organics
Heterotrophic modes of
production limited
Potentially many
extraterrestrial
subglacial
environments
Gaidos et al. [1999]
Thick ice covers Reduced geochemical exchange
with supraglacial environment
and reduced light penetration
to liquid water below
Limitation in supply of useful
compounds, e.g., meteoritic
organics and radiation-
formed oxidants; prevention
of phototrophy
Enceladus and Europa Anderson et al. [1998]
and Turtle and
Pierazzo [2001]
Low temperatures Temperatures may be well
below limit of metabolic
activity in organisms
Low-temperature limitation
to life
All extraterrestrial
subglacial
environments
Marion et al. [2003]
Salt concentrations Briny conditions may create
water activity below
minimum in which
metabolic activity can occur
Water activity limit to life Europa Marion et al. [2003]
Similarities
Connection to
silicate surfaces
Silicate minerals weather to
produce redox couples and
nutrients/trace elements
Large range of elements/
mineral available to drive
redox reactions and
biochemistry
Mars, Europa, and
Enceladus
Parkinson et al.
[2008]
Geological activity Turnover in environments with
subglacial oceans in contact
with a silicate core
Prevents system running to
equilibrium with respect
to redox couples
Europa and Enceladus Parkinson et al.
[2008]
Organic input Meteoritic input in
extraterrestrial environments
Source of basic organic
compounds and source
of electron donors
Potentially all
extraterrestrial
subglacial
environments
Pierazzo and Chyba
[1999, 2002]
Liquid water Indirect evidence for presence
of liquid water in many
extraterrestrial subglacial
environments
Required for biochemical
reactions
Potentially all
extraterrestrial
subglacial
environments;
Mars at higher
obliquity than today
Carr et al. [1998],
Parkinson et al.
[2008], and McKay
et al. [2008]
a
For Encaladus, Titan, and Europa, the table refers to the ocean environment.
COCKELL ET AL. 141
Doran et al. [2008] developed an entry approach for a pristine
ice-sealed lake in the McMurdo Dry Valleys that used a
combination of filtration and UV radiation to cleanly enter the
lake for the first time [Doran et al., 2008]. Gaidos et al.
[2004] discuss the mitigation of exogenous contamination of
the Grímsvötn volcanic caldera by the use of hot water in their
water drilling operation. The use of UV radiation to kill
microorganisms within a water stream during hot water dril-
ling, filter sterilization of drilling water, and other approaches
to minimize contamination, for example, those proposed to be
used to access the Ellsworth Lake, will provide further in-
sights into methods for accessing extraterrestrial subglacial
environments. Similar entry techniques are currently being
developed for the United States-planned sampling during the
Whillans Ice Stream Subglacial Access Research Drilling
project (P. Doran, personal communication, 2010).
6. PLANS FOR EXPLORATION
The possible astrobiological significance of extraterrestrial
subglacial environments has encouraged interest in their
exploration. A number of missions have been proposed for
the exploration of these environments, and development of
tools and strategies has begun. Martian subglacial environ-
ments have received little attention. However, ice drilling at
the Martian poles was considered in the mid-1970s using
designs based on Viking technology [Staehle et al., 1976,
1977]. The growing recognition of the wide distribution of
glacial environments on Mars and the possibility that they
might harbor liquid water during periods of high obliquity is
likely to encourage greater efforts to explore them. In 2009,
the Phoenix Lander was the first craft to characterize a
Martian supraglacial environment [Smith et al., 2009; Mellon
et al., 2009].
At the time of writing, the greatest efforts have been
directed toward Europa and Enceladus.
6.1. Europa Jupiter System Mission (EJSM)
In early February 2009, NASA and the European Space
Agency (ESA) announced a joint mission to Jupiter’ssystem
including the Laplace mission and NASA’s mission studies.
From this date, the mission has been referred to as the EJSM
[Lebreton, 2009]. The EJSM would consist of a Jupiter Gan-
ymede Orbiter (JGO), developed by ESA, and a Jupiter Euro-
pa Orbiter (JEO), developed by NASA. It is possible that
JAXA, the Japanese Space Agency, will also contribute with
a Jupiter Magnetospheric Orbiter (JMO) for the investigation
of plasma physics in the Jupiter system. The mission could
also be enhanced by a Russian Lander mission [Martynov et
al., 2009] and/or a penetrator.
In the mission proposal by Blanc et al. [2009a], three key
questions were emphasized: How was the Jupiter system
formed? How does it work? Does Europa belong to the
habitable zone (including, does it harbor life)? These ques-
tions were formulated into the EJSM mission science objec-
tives according to Lebreton [2009] as (1) the investigation
of the subsurface oceans and their relation to a deeper inte-
rior; (2) characterization of the ice shell and subsurface water,
including heterogeneity of ice and the surface-ice-ocean ex-
change; (3) deep internal structure, differentiation history,
and the magnetic field of Ganymede; (4) exospheres, plasma
environments, and magnetic interactions; global surface
composition and chemistry, especially related to habitability;
and (5) formation of subsurface features and sites of recent or
current activity to help identify candidate sites for future in
situ exploration. JEO and JGO would explore and character-
ize different objects and parts in the Jupiter system, which
will allow comparative science (Europa versus Ganymede
versus Callisto) [Blanc et al., 2009b]. According to the
currently planned trajectory, it will then take 6 years for the
spacecraft to reach the Jovian system. Upon arrival, the JGO
should tour the Jupiter system for ~28 months and include 21
Callisto flybys, 9 Ganymede flybys, and a Ganymede orbital
phase of 260 days. The JEO tour should last 30 months in the
Jupiter system, out of which 4 Io, 6 Ganymede, 6 Europa,
and 9 Callisto flybys are planned, in addition to a 9-month
orbital phase around Europa, first at 200 km and then at
100 km orbital altitudes [Clark et al., 2009].
6.2. Titan Saturn System Mission (TSSM)
After Cassini-Huygens mission revealed the amazing two
worlds, Titan and Enceladus, a mission called the Titan and
Enceladus Mission (TandEM) to investigate these Saturnian
moons in greater detail was proposed. In January 2009,
ESA’s TandEM merged with NASA’s Titan Explorer 2007
study to create the TSSM [NASA/European Space Agency,
2009]. The TSSM mission includes a Titan orbiter, Titan in
situ elements such as a hot air balloon (~265 kg), three probe/
landers (~500 kg each), as well as minipenetrators and an
Enceladus penetrator to be deployed to the geologically
active south polar region on Enceladus. The orbiter would
perform several flybys of both moons and in the early mis-
sion would deliver landers/penetrators.
The scientific objectives at Enceladus encompass the in-
vestigations of origin, nature and properties of the plume,
subsurface liquid water, surface, interior, and global dynam-
ics as well as signs of past and/or present life that includes
organic inventory, molecular chirality, etc. It is also important
to investigate the influence of Enceladus on the magneto-
sphere, other satellites, and ring structure. At Titan, the
142 SUBGLACIAL ENVIRONMENTS AND THE SEARCH FOR LIFE BEYOND EARTH
primary science goals are to study its atmosphere, image,
sample, and analyze the surface features such as hydrocarbon
lakes, dunes, rivers, impact craters, mountain ranges, and
volcanoes and determine if there is a subsurface liquid ocean.
On both Enceladus and Titan, it is important to determine the
pre- and protobiotic chemistry and to understand the origin
and evolution of these bodies both individually and in the
Saturnian system as a whole [Coustenis et al., 2009]. Since
NASA and ESA jointly announced that the EJSM would be
the candidate for the first L mission, the studies concerning
the TandEM mission concept within ESA were halted.
7. CONCLUSION
Extraterrestrial environments in the inner and outer solar
system host a diversity of icy substrates that offer the oppor-
tunity to expand the known physical and chemical conditions
that can be produced in subglacial environments. The robotic
and human exploration of these environments is immensely
challenging, but their investigation will yield new insights
into the potential for life elsewhere and the physical and
chemical parameters that define the boundaries of habitabil-
ity in subglacial environments. Although mission designs,
including those reviewed here, will change in the future,
there can be little doubt in saying that the exploration of
extraterrestrial subglacial environments will play an increas-
ingly important role in space exploration. Insofar as most
extraterrestrial environments of biological interest are cold
(Mars, Europa, Enceladus, etc.), then the search for extrater-
restrial life is a development of glaciology and its allied
sciences. Terrestrial analog environments will provide valu-
able insights into the physics, chemistry and biology of
extraterrestrial subglacial environments. Ultimately, however,
a comparison can only be made by directly sampling extrater-
restrial subglacial environments.
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