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Habitability is a widely used word in the geoscience, planetary science, and astrobiology literature, but what does it mean? In this review on habitability, we define it as the ability of an environment to support the activity of at least one known organism. We adopt a binary definition of “habitability” and a “habitable environment.” An environment either can or cannot sustain a given organism. However, environments such as entire planets might be capable of supporting more or less species diversity or biomass compared with that of Earth. A clarity in understanding habitability can be obtained by defining instantaneous habitability as the conditions at any given time in a given environment required to sustain the activity of at least one known organism, and continuous planetary habitability as the capacity of a planetary body to sustain habitable conditions on some areas of its surface or within its interior over geological timescales. We also distinguish between surface liquid water worlds (such as Earth) that can sustain liquid water on their surfaces and interior liquid water worlds, such as icy moons and terrestrial-type rocky planets with liquid water only in their interiors. This distinction is important since, while the former can potentially sustain habitable conditions for oxygenic photosynthesis that leads to the rise of atmospheric oxygen and potentially complex multicellularity and intelligence over geological timescales, the latter are unlikely to. Habitable environments do not need to contain life. Although the decoupling of habitability and the presence of life may be rare on Earth, it may be important for understanding the habitability of other planetary bodies.
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Review Article
Habitability:
A Review
C.S. Cockell,
1
T. Bush,
1
C. Bryce,
1
S. Direito,
1
M. Fox-Powell,
1
J.P. Harrison,
1,
*H. Lammer,
2
H. Landenmark,
1
J. Martin-Torres,
3
N. Nicholson,
1
L. Noack,
4
J. O’Malley-James,
5
S.J. Payler,
1
A. Rushby,
6
T. Samuels,
1
P. Schwendner,
1
J. Wadsworth,
1
and M.P. Zorzano
3,7
Abstract
Habitability is a widely used word in the geoscience, planetary science, and astrobiology literature, but what
does it mean? In this review on habitability, we define it as the ability of an environment to support the activity
of at least one known organism. We adopt a binary definition of ‘‘habitability’’ and a ‘‘habitable environment.’
An environment either can or cannot sustain a given organism. However, environments such as entire planets
might be capable of supporting more or less species diversity or biomass compared with that of Earth. A clarity
in understanding habitability can be obtained by defining instantaneous habitability as the conditions at any
given time in a given environment required to sustain the activity of at least one known organism, and
continuous planetary habitability as the capacity of a planetary body to sustain habitable conditions on some
areas of its surface or within its interior over geological timescales. We also distinguish between surface liquid
water worlds (such as Earth) that can sustain liquid water on their surfaces and interior liquid water worlds,
such as icy moons and terrestrial-type rocky planets with liquid water only in their interiors. This distinction is
important since, while the former can potentially sustain habitable conditions for oxygenic photosynthesis that
leads to the rise of atmospheric oxygen and potentially complex multicellularity and intelligence over geo-
logical timescales, the latter are unlikely to. Habitable environments do not need to contain life. Although the
decoupling of habitability and the presence of life may be rare on Earth, it may be important for understanding
the habitability of other planetary bodies. Key Words: Habitability—Exoplanets—Habitat—Niche—Mars—
Exoplanets. Astrobiology 16, xxx–xxx.
1. Introduction
The terms ‘‘habitable’’ and ‘‘habitability’’ are pervasive
in astrobiology for a good reason. The investigation of
the origin of life on Earth, its persistence on the planet since
its emergence, and the search for evidence of life on other
planetary bodies all require that we define what conditions
life requires.
Astrobiologists have attempted to understand habitability
and catalogue the requirements for its presence (Kasting and
Catling, 2003; Gaidos et al., 2005; Nisbet et al., 2007;
Zahnle et al., 2007; Lammer et al., 2009; Westall et al.,
2013; Cockell, 2014a; Jaumann et al., 2014). Their focus
has been on defining the basic requirements for life to be
metabolically active or to reproduce in planetary environ-
ments and, in particular, describe processes that might be
required for these conditions to be sustained over geological
periods within the lifetimes of planetary bodies.
Efforts have also been made by ecologists to define what
constitutes a habitat (e.g., Odum, 1971; Whittaker et al.,
1973; Block and Brennan, 1993; Hall et al., 1997). This
effort has been dominated by the study of multicellular
1
UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK.
2
Austrian Academy of Sciences, Space Research Institute, Graz, Austria.
3
Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Lulea
˚University of Technology,
Kiruna, Sweden; and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain.
4
Department of Reference Systems and Planetology, Royal Observatory of Belgium, Brussels, Belgium.
5
School of Physics and Astronomy, University of St Andrews, St Andrews, UK; now at the Carl Sagan Institute, Cornell University,
Ithaca, NY, USA.
6
Centre for Ocean and Atmospheric Science (COAS), School of Environmental Sciences, University of East Anglia, Norwich, UK.
7
Centro de Astrobiologı
´a (CSIC-INTA), Torrejo
´n de Ardoz, Madrid, Spain.
*Now at Division of Microbial Ecology, Department of Microbiology and Ecosystem Science, Research Network ‘Chemistry Meets
Microbiology’, University of Vienna, A-1090 Vienna, Austria.
ASTROBIOLOGY
Volume 16, Number 1, 2016
ªMary Ann Liebert, Inc.
DOI: 10.1089/ast.2015.1295
1
organisms. Ecologists’ motivation stems from an interest in
advancing our knowledge of the interrelationships of life on
Earth, improving our understanding of wildlife ecology, and
enhancing our capacity to protect and preserve habitats.
Despite these parallel lines of thinking, there have been
few attempts to synthesize them into a consistent view of
habitability. This is unfortunate because the astrobiological
view of habitability, which extends thinking more forcefully
to microbial life and brings an extraterrestrial perspective
to bear on the understanding of habitability at the plane-
tary scale, has much to offer in helping to define the con-
cepts of habitat and habitability. In a similar vein, well-
established ecological thought has much to offer in refining
astrobiological thinking, preventing a potentially wasteful
reconsideration of long-debated topics and ensuring that a
set of ideas and definitions emerges that is consistent across
multiple scientific disciplines.
In this paper, to mark the 15
th
anniversary of the journal
Astrobiology, we provide a review of habitability from an
astrobiological perspective. This review has two purposes.
First, it is an attempt to provide clarity in definitions and
ideas using both astrobiological and ecological concepts.
Second, it is a primer for anyone entering, or already in, the
field of astrobiology who would value a review of this
pervasive but often enigmatic term.
2. What Is Habitability?
We begin by proposing a working definition of habit-
ability that we use to inform the subsequent discussions in
this paper. Definitions are important for ensuring consis-
tency in ideas and thus encouraging clarity in formulating
scientific questions. However, they are, nevertheless, human
constructs, and other authors are free to challenge, change,
or ignore this definition.
The word habitability is derived from the Latin verb, ha-
bitare, ‘‘to live or dwell.’Habitat, which means, ‘‘it lives/
dwells,’’ is the third-person singular, and habitability means a
place in which ‘‘it’’ has the ability to live/dwell. The devil,
however, is in the detail. What do we mean by ‘‘live’’?
The definition of habitat that we adopt here is ‘‘an envi-
ronment capable of supporting the activity of at least one
known organism,’’ where ‘‘activity’’ (and thus ‘‘living’’) is
metabolic activity allowing for survival, maintenance,
growth, or reproduction. This definition is binary. An en-
vironment either can or cannot sustain a given organism.
By ‘‘survival’’ we mean that life is able to use resources
in the local environment to actively maintain a state of
dormancy where the rate of molecular repair is at least equal
to the rate of molecular damage (Price and Sowers, 2004).
‘‘Maintenance’’ means activity where a cell can carry out a
range of other cell functions but has insufficient resources
or energy to reproduce. ‘‘Growth’’ means increasing body
biomass in multicellular organisms, but it is used synony-
mously with ‘‘reproduction’’ when applied to microorgan-
isms. ‘‘Reproduction’’ is multiplication of an organism.
In astrobiology, an assessment is often performed as to
whether an environment is habitable for ‘‘life’’ in general. By
this we mean that a given environment can support ‘‘at least
one known organism.’’
The definition we adopt explicitly defines habitability with
respect to a known life-form. Thus, habitability, as defined, is
a conservative term, bounded by the current state of knowl-
edge in biology. One may be able to assess certain require-
ments for the habitability of an environment independently
of known life, such as energy availability. If we make the
reasonable assumption that the Periodic Table is universal,
then we can calculate the Gibbs free energy available in a
given reduction-oxidation reaction or the energy available
in light from a star and thereby assess whether an environ-
ment could theoretically support any organism, known or
unknown, given certain assumptions about plausible energy
demands (Hoehler, 2007). Some other speculative energy
sources have been suggested, including gravitational, mag-
netic, kinetic, thermal, and radioactive energy; osmotic, ionic,
or pressure gradients; and tectonic stress (Schulze-Makuch
and Irwin, 2004). We do not know whether terrestrial life
represents a universal norm (Pace, 2001; Bains, 2004; Benner
et al., 2004). However, by constraining habitability to known
life, we avoid the term becoming inextricably linked to the
problem of defining life (Cleland and Chyba, 2002; Benner,
2010) or becoming defined by speculative capacities of
unknown organisms.
To further explain why we adopt this working definition,
it is useful to explore the existing ecological literature, since
the problem of what constitutes a habitat and a habitable
environment is not a new question. It has vexed ecologists
for decades. Odum (1971) originally referred to a habitat as
the ‘‘address’’ of an organism, its physical location. Although
this unambiguously makes it clear that habitat is a physical
space, it raises the question of what an organism must be doing
for the space to count as an ‘‘address.’’ Whittaker et al. (1973)
defined a habitat as the ‘‘range of environments or commu-
nities over which a species occurs,’’ although this usage leaves
open the question of what the organism must be doing in a
habitat for the word ‘‘occurs’’ to apply (merely being present
there seems too wide a definition, as explained shortly).
In an attempt to provide clarity, Block and Brennan (1993)
described habitat to be ‘‘the subset of physical environmental
factors that a species requires for its survival and reproduc-
tion.’’ Hall et al. (1997) similarly defined a habitat as ‘‘the
resources and conditions present in an area that produce
occupancy—including survival and reproduction—by a given
organism,’’ a definition reiterated by Krausman (1999). They
go on to remark, ‘‘wherever an organism is provided with
resources that allow it to survive, that is habitat.’’ These
definitions are broadly consistent with ours, particularly when
applied to multicellular life. To be minimally ‘‘surviving,’’
most multicellular organisms must be metabolically active.
This is also the case for dormant vegetative microbial cells,
which must have a minimum state of metabolic activity to
repair macromolecular damage against molecular degradation
(Price and Sowers, 2004). However, these definitions raise a
problem in microbiology and astrobiology when microbial
spores are considered, a particular type of survival state that is
generally metabolically inactive.
To illustrate the problem, consider a thought experiment.
Some bacterial spores are dropped deep into the nitrogen
ices on Pluto, in some sort of drilling or penetrator mission.
As they are protected from ionizing and UV radiation, they
have a good chance to remain viable in the frozen ice. For a
period of time they are surviving. Yet if mere survival of
metabolically inactive spores is a sufficient requisite for a
place to be a habitat in accordance with the definition of
2 COCKELL ET AL.
Hall et al. (1997), then Pluto has become a habitat for those
spores and, by extension, Pluto is a habitable planetary body
(it provides resources—cool temperatures and protection
from radiation under the ice—for survival of the spores).
This seems to be too wide a definition, since a habitat, and
therefore a habitable environment, would then include any
environment in the Universe where inactive bacterial spores
could survive for some defined, and potentially very short
(even seconds), time. Therefore, our working definition of a
habitat has a minimum requirement for the environment to
be able to support the metabolic activity of organisms.
Another end-member problem to address is the matter of
reproduction. Must a habitat support reproduction? Many
definitions of habitat include reproduction either as a possi-
ble or required characteristic of a habitat (e.g., the definitions
of Block and Brennan, 1993, and Hall et al., 1997, as above).
Aarts et al. (2013) defined a habitat as ‘‘a collection of re-
sources and environmental conditions (abiotic and biotic)
that determine the presence, survival, and reproduction of a
population.’
If a habitat cannot sustain reproduction, then the organism
(apart from a theoretical microbe that merely repairs itself
indefinitely) has a limited future. Thus, there might be the
inclination to include reproduction for a place to be habit-
able. To highlight the problem that lies in requiring repro-
duction to identify a place as habitable, another analogy is
useful. The rabbit that visits the lead author’s garden in
Edinburgh eats grass, but it does not reproduce there. The
garden is of insufficient size to host a rabbit warren. The
rabbit must return to the hills outside Edinburgh to repro-
duce. However, the rabbit is using the garden to gain re-
sources for growth and potentially for later reproduction. To
require that a habitat should allow for reproduction would be
to say that the garden is not a habitat for the rabbit and, by
definition, it is uninhabitable to the rabbit. This seems too
narrow a definition, since an area of a planetary surface that
contains actively metabolizing life, but where reproduction
is not occurring, would be classified as uninhabitable space.
Hall et al. (1997) resolve this problem in stating, ‘‘Thus,
migration and dispersal corridors and the land that animals
occupy during breeding and non-breeding seasons are habi-
tat.’’ As such, we consider a place that can support the
metabolic activity of an organism, even if it does not repro-
duce in that place, to be a habitat. An obviously important
caveat is that, if an organism does not reproduce in a habitat,
it must have other habitats available to it during its life cycle
that do allow for reproduction if its population is to persist.
In summary, the definition provided in this paper is broadly
consistent with earlier definitions of habitat. However, we
specify that ‘‘survival’’ must entail metabolic activity and that
reproduction is a possible, but not a necessary, activity for
an environment to be a habitat to an organism and therefore
for a place to be habitable. What about a set of habitats that
allow for an organism to be active but none of which allow
for reproduction? These environments would be described
as habitable to the organism, but clearly a population in-
troduced into them will eventually go extinct. They sustain
only instantaneous habitability but not long-term (continu-
ous planetary) habitability according to the definitions we
describe shortly.
Within the definition adopted here, a Special Region,as
used for planetary protection concerns on Mars (Rummel
et al., 2014) is a specific type of habitat in which repro-
duction (propagation) can occur.
Life itself can influence habitability by making physical
space within which given organisms can be active. On a
densely inhabited planet, biota may be habitat space. For
example, the habitat of a bird species might be a particular
forest, since the trees and their branches constitute physical
space that defines the geographical distribution of the spe-
cies (Sieving et al., 1996). Similarly, the layers of a mi-
crobial mat are habitat space for the organisms within them
(Engel et al., 2003). However, on many planets, for example
Mars, most habitat space, if it exists, is delimited by geo-
logical substrates (Southam et al., 2007).
Life can change the habitable space for other life-forms
(Block and Brennan, 1993). As organisms weather rocks,
cycle gases, and change the valence states of elements, for
example, they change the potential for an environment to be
habitable for other life-forms (Falkowski et al., 2008). On a
densely inhabited planet, life becomes inextricably woven
into defining the conditions for habitability for particular
organisms. An example of how life can create new habitat
for other types of life is successional changes in biota, where
one type of organism, for example phototrophs or nitrogen-
fixing bacteria on early lava flows, provides nutrients and
conditions required for subsequent organisms (such as higher
plants, Clarkson, 1997). An example of how life can cause
deleterious conditions to habitability would be the large-scale
oxygenation of a planetary atmosphere that causes the loss of
habitable conditions for anaerobic organisms on the surface
(Stolper et al., 2010).
The conditions required to produce organic molecules
needed for an origin of life may occur in much wider con-
ditions than those required for habitability. For example,
temperatures in impacts or spark discharge required to
produce amino acids from other compounds is likely to be
higher than the maximal temperatures for microbial growth.
However, the first organism to reproduce (and originate)
must emerge in a habitable space. The conditions required
for life to originate are a subset of the conditions for hab-
itability. Abiogenesis therefore is likely to pass through a
habitability bottleneck (Fig. 1).
2.1. Habitability and niches
An important concept embedded within the conditions for
habitability is the niche. Sometimes the word is used to
mean a physical space, in other words as a synonym for
‘‘habitat.’’ However, this is not correct.
The ecological literature contains diverse opinions on ex-
actly how the niche is defined. One approach is to consider
the niche as the role that the organism occupies within a
community (a group of interacting species within a given
locality), including its functional role. This is consistent with
Odum’s view that, if the habitat is the ‘‘address’’ of the or-
ganism, the niche is the ‘‘profession’’ of an organism (Odum,
1971). The niche as a functional definition of the relationship
of an organism to the rest of the biotic community was one
adopted originally by Elton (1927)—the Eltonian niche.
A classic definition was provided by Hutchinson (1957).
He considered the niche to be an n-dimensional abstract
hyperspace made up of environmental conditions and re-
sources that define the requirements of a known individual
HABITABILITY: A REVIEW 3
or a species to practice its mode of life. An important dis-
tinction in his concept is the ‘‘fundamental’’ niche, which is
the niche that an organism can theoretically occupy, and the
‘‘realized’’ niche, which is the niche realized in practice.
The former is more important for the theoretical assessment
of an environment as being habitable.
Another type of niche was defined by Grinnell (1917). He
considered it to be both the habitat that a species occupies
and the adaptations that allow it to be successful in that
environment (Grinnell, 1917)—the Grinnellian niche. The
Grinnellian niche, in contrast to the Eltonian niche, em-
phasizes the way in which an organism uses resources to
live in an environment rather than its functional role in the
community (Devictor et al., 2010).
In astrobiology, the habitability of an environment is the
capacity of a particular physical space to support the activity
of an organism, that is, to provide the set of resources and
conditions required for its way of life. Therefore, when as-
trobiologists speak about a habitable environment, their
meaning conforms most closely to the Grinnellian niche
concept. The Eltonian niche concept, at the current time, is
less relevant because we have not found extraterrestrial
biotic communities to be concerned about an organism’s
functional role in that community.
For microorganisms, their primary functions in the bio-
sphere are usually linked to their mode of energy acquisition
[e.g., using redox couples that mediate the biogeochemical
cycling of elements such as iron, sulfur, carbon, and others
through different valence states (Falkowski et al., 2008)].
Therefore, the niche is strongly linked to features of energy
acquisition. By contrast, as most multicellular organisms use a
single type of energy acquisition—aerobic respiration—it is
not surprising that the existing ecological literature on niches
has tended to focus on other functional roles of organisms vis-
a
`-vis their behavior (foraging, competition, predation, etc).
The differences between niche definitions have also caused
considerable controversy about whether a vacant niche can
exist. The Eltonian niche considers an organism’s place in a
community in some sense independently of the organism
itself, so that if a sulfate-reducing microorganism, for ex-
ample, became extinct, it has been thought permissible to
discuss the niche as being vacant to another sulfate-reducer.
Similarly, the Grinnellian definition of a niche, as it includes
the concept of the physical space or habitat, might also al-
low for a vacant niche, because it can be considered similar
to a physical habitat that contains resources that are vacant
for a species to use. However, Hutchinson’s abstract defi-
nition of a niche focuses on a given species and defines its
relationship to its environment and community. In this sense,
a species is required to define a niche in the first place,
making a vacant niche a logical impossibility.
The concept of a vacant niche is found in many instances
in the ecological literature. If we allow a niche to be a set
of known interactions and conditions that can provide the
resources required for a given form of life, then we could
plausibly identify a niche as being vacant, a view adopted by
many ecologists (Colwell, 1992). An extreme end-member
example of importance to astrobiology is a physical space
that contains no life but can be shown to contain all the
resources and have physical and chemical conditions required
for a known organism to be active in that space. Such a place
is an uninhabited habitat that contains a niche (or niches).
2.2. The spatial problem
The previous section raises a question on the spatial
considerations of habitability. What extent of a planetary
body must be habitable for the entire body to be defined as
‘‘habitable’’? The working definition used here makes no
prescription on habitat size. If a planetary body hosts a
habitat of any kind on any scale, it is a habitable planetary
body. The volume of Earth that is inhabited is less than 0.5%
of its total volume ( Jones and Lineweaver, 2010a, 2010b),
but we describe Earth as a habitable planet. A reductio ad
absurdum is to ask the question: ‘‘Does a planet that hosts a
one-micron cube that is habitable to a single microorganism
classify as a habitable planet?’’ Under our working defini-
tion, the answer is yes. However, a more realistic response
is to point out that such a scenario is physically unlikely. A
planetary body that has the required concatenation of con-
ditions required for an organism to be active, including
liquid water, is likely to have habitable conditions across
macroscopic areas (even if only regional) if they exist at all.
2.3. Less habitable and more habitable worlds?
If a habitat is a place that can support the activity of at
least one known organism, then it follows that it is mean-
ingless to speak of more or less habitable places. The as-
sessment is binary—either an environment can support the
activity of a given organism or it cannot.
Heller and Armstrong (2014) discussed the concept of
superhabitable worlds—worlds ‘‘generally more habit-
able’’ than the Earth. They recognized that habitability is
a binary assessment and suggested that when discussing
superhabitability they are considering the analogy of a
‘‘sow being pregnant with several furrows.’’ This notion of
superhabitability probably comes closest to the traditional
Conditions for
abiogenesis
Conditions for
production of building
blocks of life
Conditions within which
all known life can be active
Habitability
bottleneck
Two dimensional representation of
physical and chemical conditions
Time
FIG. 1. The physical and chemical conditions required to
produce the building blocks of life are broader than the
conditions for abiogenesis, which are presumed to be nar-
rower than the total physical and chemical space that can be
occupied by all life.
4 COCKELL ET AL.
ecological concept of ‘‘habitat quality,’’ which is usually
taken to be the ability of a habitat to sustain a certain biomass
of organisms. Conditions that Heller and Armstrong (2014)
identified that would lead to greater biomass on a planetary
body compared to that of Earth include greater surface area, a
larger number of water bodies, maintenance of plate tectonics
over longer times, generally warmer temperatures, older
planets, and planets around more long-lived (lower-mass)
stars. These factors are discussed in Section 6.
Taken in the binary sense, a superhabitable world would be
one that hosts a greater diversity of organisms or a greater
number of organisms with different functional capabilities
[e.g., if a planet had a more diverse range of combined
physical and chemical environmental stressors than that on
Earth (Harrison et al., 2013), it might host a more diverse set
of niches]. We do not know the extent to which the cap-
abilities of life have been fully explored on Earth. A large
number of the theoretically available redox couples on Earth
have a microorganism capable of using them to conserve
energy for growth (Kim and Gadd, 2008). Theoretical redox
couples have been successfully used to find novel microor-
ganisms, for example, the thermodynamic prediction of an-
aerobic ammonium oxidation (anammox), which led to the
subsequent discovery of organisms capable of carrying out
this transformation (Broda, 1977; Mulder et al., 1995).
2.4. Habitability is decoupled from the presence of life
If the definition of a habitable environment is one that can
support the activity of at least one known organism, then
conceptually it is possible to have an uninhabited habitat—
a place that can be shown to support the activity of a known
organism (or organisms) but contains no such organism
(Cockell et al., 2012a). These environments are rare on
Earth but can be found, for example, in fresh lava flows
(Cockell, 2014b). These environments could exist in greater
abundance on other planets where the hydrological regime
is less vigorous than on Earth, or where the atmosphere is
more inclement (e.g., high UV radiation fluxes on anoxic
planets), such that new habitats are disconnected from con-
temporaneous inhabited environments. Alternatively, if the
origin of life does not occur on a planetary body because of
the lack of suitable environments (or life is not transferred
from another life-bearing planet), but habitable conditions
exist, then the entire planet may be lifeless, despite hosting
habitable conditions. We do not know the diversity of envi-
ronments in which an origin of life can occur, and how
inevitable its occurrence is, once habitable conditions emerge,
to be able to assess the plausibility of this scenario. The
planet Mars, where habitable conditions have been re-
ported (Grotzinger et al., 2014) but deteriorating conditions
throughout its history may have made these conditions local-
ized, is one world where uninhabited habitats could plausibly
have existed or exist today (Cockell, 2014a). Several schematic
trajectories can be defined for the planet depending on whether
it was inhabited or uninhabited (Fig. 2). The timing of changes
in the relative abundance of different environments (unin-
habitable, uninhabited habitat, inhabited habitat) is one of the
key challenges in defining the history of martian habitability.
The trajectories defined for Mars circumscribe the general set
of trajectories for any Mars-like planet on which conditions
deteriorate over time.
Subsurface
Formation of Mars
(Uninhabitable)
Uninhabited Mars Inhabited Mars
Extinction
123456
Noachian
Amazonian
Time
(from Noachian to present-day)
Surface
Uninhabitable
Uninhabited habitat
Inhabited habitat
Types of environments
Time
Types of environments on Mars over time
Schematics showing changes in habitability on the surface and in the subsurface through time
Arbitrary latitude across
the surface of Mars
Arbitrary point location
on Mars into subsurface
Trajectory number
FIG. 2. Martian habitability trajectories. Examples of different trajectories of the habitability of Mars through time, beginning
with the branch point of an uninhabited and inhabited Mars. In the lower part of the diagram are shown schematics illustrating
the trajectories. The ‘‘surface’’ section represents the surface of Mars. The ‘‘subsurface point’’ is an arbitrary point through the
martian crust to an arbitrary depth of several kilometers. Color graphics available at www.liebertonline.com/ast
HABITABILITY: A REVIEW 5
3. What Are the Requirements for Habitability?
For any type of habitability to exist, there must be the
possibility of habitable planetary bodies. In the case of our
own galaxy, early star systems near the center may have
been disrupted by intense supernova activity and star for-
mation. Stars too far away from the center may exist in
regions of low metallicity, where many elements are at too
low abundance to form terrestrial-type rocky planets. Thus,
the presence of planetary bodies of relevance to this article
may be restricted to a galactic habitable zone. For the Milky
Way, this has been estimated to be an annulus of 7–9 kpc
(kiloparsecs) from the galactic center composed of stars
formed between 8 and 4 billion years ago (Gonzalez et al.,
2001; Lineweaver et al., 2004). However, this review fo-
cuses on star systems in which habitable planetary bodies
are possible and does not review the conditions required to
give rise to habitable planets in the first place. This topic
was discussed, for instance, by Gaidos et al. (2005).
3.1. Instantaneous habitability and continuous
planetary habitability
In the interests of ordering this discussion, two further
definitions are used. Instantaneous habitability is defined as
the set of conditions at any given place in an instant in time
that will support habitability. Continuous planetary habit-
ability is defined as the set of conditions on, or in, a plan-
etary body that can support habitable conditions in at least
part of the planetary body over geological time periods. In
general, instantaneous habitability applies to localized (mi-
cron to macroscopic scale) conditions on a planetary body
that support the activity of organisms. In contrast, continu-
ous planetary habitability refers to the ability of an entire
planetary body to support habitable conditions somewhere
on its surface, or within its interior, over geological time-
scales. These definitions are not merely semantic. They al-
low the separation of two distinct discussions. The rest of
this review is focused on considering the conditions that
allow for habitable environments.
4. Instantaneous Habitability
At an instant in time in a particular location, a set of
requirements for all known organisms to be active can be
identified (Fig. 3). They are
(1) A solvent.
(2) Appropriate temperature conditions and other physi-
cochemical conditions (such as water activity).
(3) Available energy.
(4) Major elements required by all known life (CHNOPS).
(5) Other elements required by a specific organism.
These requirements can be assessed for any given environ-
ment or planetary body. For an environment to be habitable,
they must be present and colocated at the scale of the or-
ganism (Fig. 3). However, it is also possible to more ge-
nerically catalogue the presence of these requirements on
planetary bodies as a first-order assessment of whether a
planetary body is likely to host instantaneously habitable
conditions on some part of its surface or interior. These
Planetary Habitability Tables are shown for Earth, Mars,
Europa, and Enceladus in Tables 1–4.
4.1. A solvent
Liquid water is the solvent required for biochemical re-
actions to occur. At the current time it is the only compound
known to be used by life as the primary biochemical solvent,
although there have been speculations about the use of
liquid ammonia, organic solvents (such as methane and
ethane), formamide, and even sulfuric acid (Benner et al.,
2004; Schulze-Makuch and Irwin, 2006). As we do not
know of organisms that use these alternative liquids, we do
not consider these possibilities further. For liquid water to
be present in any location at a given time, there must be
environmental conditions of temperature, pressure, and
chemical impurities that allow water to fall within the liquid
phase space determined by the equation of state. This is
dependent on a set of planetary conditions that are discussed
under continuous planetary habitability.
4.2. Appropriate temperature conditions and other
physicochemical conditions
The presence of liquid water is a fundamental require-
ment for any form of known life, but its presence may not
define an environment as suitable for life. Given the limi-
tations to the activity of life, only part of the phase space of
liquid water is habitable.
The lower limit for metabolic activity in microbes is
thought to be *-25C ( Junge et al., 2004; Mykytczuk
et al., 2013; Clarke, 2014). No convincing evidence for
reproduction has been demonstrated below *-15C
(Breezee et al., 2004; Wells and Deming, 2006), although
longer-term experiments and more sophisticated approaches
may well change this assumption. Liquid water can exist at
below this temperature at values that are applicable to
planetary environments. For example, eutectic solutions of
Habitable
Elemental
requirements
(CHNOPS and
other trace elements)
Energy
Solvent (water)
Appropriate
physicochemical
conditions
FIG. 3. Instantaneous habitability. A series of physico-
chemical requirements must come together at the spatial
scale of an organism to allow that organism to be active in a
given environment (adapted from Hoehler, 2007).
6 COCKELL ET AL.
Table 1. A Planetary Habitability Table for Earth
Requirement Comments and references
Liquid water Oceans, seas, and bays: 1,338,000,000 km
3
(96.54%) Source: Shiklomanov, 1993
Groundwater (fresh): 10,530,000 km
3
(0.76%)
Groundwater (saline): 12,870,000 km
3
(0.93%)
Lakes (fresh): 91,000 km
3
(0.007%)
Lakes (saline): 85,400 km
3
(0.007%)
Swamp water: 11,470 km
3
(0.0008%)
Rivers: 1,120 km
3
(0.0001%)
Main elements Plate tectonics and
atmospheric photochemical
processes continuously
supply substrates and
remove products
(Falkowski et al., 2008)
CCO
2
, CO, bicarbonates, organic compounds
HH
2
O, H
2
, organic compounds
NNH
3
,NO
3,N
2
,NO
2, NO, organic N-species
OO
2
,H
2
O, H
2
O
2
, oxides, organic compounds
PPO
3
4, ATP, phosphite, phosphides, organic P-species
SS,S
2O2
3,S
3
O
6
,H
2
S, SO 2
4, organic S-species, metal
sulfides (FeS, CuS, ZnS, NiS, etc.)
Other elements Igneous, metamorphic, and sedimentary rocks. These rocks
provide all stable elements in the periodic table.
Energy—full redox
couples
a
Electron donor Electron acceptor Kim and Gadd, 2008
Chemolithotrophy
Methanogenesis,
acetogenesis
H
2
CO
2
H
2
oxidation H
2
O
2
,SO2
4,Fe
3+
Sulfate and iron reduction,
other variable valence-state
metals
Phosphite oxidation HPO 2
3SO 2
4Oxidation of phosphite is
coupled with reduction of
sulfate
Oxidation of reduced S-
species
H
2
S, S, S2O2
3O
2
, nitrate, Fe
3+
Ammonia oxidation NH
3
O
2
Nitrite oxidation NO
2O
2
Anoxic ammonium
oxidation (AAM)
NH þ
4NO
2
Fe oxidation Fe
2+
O
2
, nitrate, perchlorate
Mn oxidation Mn
2+
O
2
Methylotrophy Methane, CO, C1 compounds O
2
Trace metal and metalloid
oxidations
Various valence states of
metals and metalloid, e.g.,
U, Se, Cr, Co, As, Tc, V
O
2
Chemoorganotrophy
Aerobic respiration Organics O
2
Fe and sulfate reduction Organics SO 2
4,Fe
3+
Nitrate reduction Organics NO
3
Trace metal and metalloid
reductions
Organics Various valence states of
metals and metalloid, e.g.,
U, Se, Cr, Co, As, Tc, V
Perchlorate reduction Organics CIO
4
Methanogenesis Formate, methanol, acetate,
methylamines, carbon
monoxide, ethanol, 2-
propanol, 2-butanol,
ketones, dimethyl sulfide
CO
2
Anaerobic respiration;
methylotrophic
methanogens use
compounds such as
methanol and
methylamines, aceticlastic
methanogens use acetate.
(continued)
HABITABILITY: A REVIEW 7
perchlorates have theoretical freezing points down to *-65C
(Chevrier et al., 2009), well below the current lower limit for
metabolic activity, although much work needs to be done to
measure the freezing points of novel salt solutions under real
environmental conditions. It remains to be seen whether the
absolute lower temperature limit for microbial metabolic ac-
tivity is coterminous with the lower limits for liquid water in
the environment.
At the other extreme, liquid water can exist at tempera-
tures well above the current upper limit for microbial growth
(122C, Kashefi and Lovley, 2003; Takai et al., 2008) at
high pressures. As chemical reaction rates (including de-
structive processes) increase exponentially with temperature
according to the Arrhenius equation, we might expect that a
point is reached when destruction of cellular structures and
noncovalent interactions caused by thermal energy exceeds
the energy that can be harnessed from the environment by
an organism to repair damage (Clarke, 2014; Corkrey et al.,
2014). This limit may be something on the order of 140–
150C (Cowan, 2004). As liquid water can exist at greater
than 300C at pressures exceeding 10 MPa, it is possible that
the upper temperature limit for life falls short of the upper
limit for liquid water availability. The potential presence of
supercritical water on exoplanets with dense atmospheres
(Elkins-Tanton and Seager, 2008) suggests that planets
could exist with liquid water above the upper temperature
limit for life defined on Earth. This suggests that the tem-
perature limits for life are an important first-order determi-
nant of habitable conditions (McKay, 2014).
Other physical and chemical conditions within an envi-
ronment must also lie within the bounds defined by available
energy (see below) and the biochemical limits of life. Life
has been found growing in extremes of different physical
and chemical stressors, such as ionizing and UV radiation,
pressure, pH, salinity, aridity, and toxic metals (Rothschild
and Mancinelli, 2001). The absolute limits for many of these
stressors are not fully known, and some of them, for ex-
ample, pH, may never achieve metabolically prohibitive
extremes within planetary environments. Extremes can ei-
ther work in synergy or compound energetic costs to or-
ganisms. We have very little information on how organisms
adapt to multiple extremes (Harrison et al., 2013).
An example of one extreme other than temperature with
well-defined limits is water activity, a
w
, for which the lower
limit is currently thought to be 0.605 (Stevenson et al.,
2014). Although such limits are unlikely to be met on a
planetary scale, as on Earth, they could be localized to spe-
cific brines.
4.3. Available energy
Whether for maintenance, growth, or reproduction, organ-
isms require energy. The process of using chemical energy
from redox processes is named chemotrophy; if the energy
Table 1. (Continued)
Requirement Comments and references
Homoacetogenesis Methanol, 2,3-butanediol,
ethylene glycol,
phenylmethylether, sugars,
lactate, methodylated
aromatics, butanol ethanol,
glycerol, betaine,
trihydroxybenzenes,
trimethoxybenzoate,
pyruvate, malonate,
vanillate
CO
2
Anaerobic respiration
Dehalorespiration
b
H
2
, formate, pyruvate, lactate,
acetate
SO 2
4,NO3
4, chloroethene,
2,4,6-TCP, PCE, PCP,
TCE, 3-CB, 2-CP, 2,6-
DCP, DCE
Anaerobic respiration
Other forms of energy
Microbial fermentations Alcoholic, homolactic, heterolactic, propionic acid, mixed
acid, butyric, butanol, caproic acid, homoacetogenic,
methanogenic; acetylene, glycerine, resorcinol,
phloroglucinol, putrescine, citric acid, aconitate, glyoxylate,
succinic acid, oxalic acid, malonic acid, benzoic acid
Anoxygenic
photosynthesis
H
2
O, Fe
3+
,S,H
2
,H
2
S, organics Photosynthesis is divided into
anoxygenic and oxygenic
photosynthesis. During the
latter O
2
is generated
Inventory of requirements for habitability on Earth (note that this table does not indicate whether the requirements for life are colocalized
in any given environment for life). Given the vast range of minerals, valence states of elements, and redox couples that have been
demonstrated, this table is non-exhaustive, but it illustrates the quantity of information on conditions for habitability compared to Tables 2–4.
a
On Earth, the number of demonstrated redox couples is vast; thus this table does not represent all identified combinations. However,
some combinations of electron donors and acceptors are more favorable over others. In addition to the electron acceptors listed above,
humic acid, (per)chlorate, iodate, organic sulfonate, and organic nitro compounds can be used as electron acceptors by anaerobic bacteria.
b
2,4,6-TCP =2,4,6-trichlorophenol; PCE =tetrachloroethene; PCP =pentachlorophenol; TCE =trichloroethene; 3-CB =3-chlorobenzoate;
2-CP =2-chlorophenol; 2,6-DCP =2,6-dichlorophenol; DCE =dichloroethene.
8 COCKELL ET AL.
Table 2. A Planetary Habitability Table for Mars
Requirement Comments and references
Liquid water Present as brines on the surface
and present in the subsurface (?)
McEwen et al., 2011; Renno
´et al.,
2009; Martin-Torres et al., 2015
Main elements
C Organics, CO, CO
2
, bicarbonate
ions from carbonates
Leshin et al., 2013; Ming et al., 2014;
Steele et al., 2012 and citations
therein; Ehlmann et al., 2008
HH
2
, organics, H
2
O Presence of serpentine on Mars
suggests H
2
production. Ehlmann et
al., 2010, 2011; Quantin et al., 2012
N Organics, N
2
, Fixed states of N—such
as NO
2, NO, NO 2
3(quantities of fixed
states of nitrogen not known)
Ming et al., 2014; Stern et al., 2015;
and the Mars N problem reviewed in
Mancinelli and Banin, 2003
O Oxygen free radicals, perchlorates, oxides Present in many oxidized species
shown under C, H, N, P, and S
PPO
3
4(in apatite and merrillite) McGlynn et al., 2012; Usui et al., 2008
SSO 2
4,S
2-
,S McLennan et al., 2014; Ming et al.,
2014; reviewed by Gaillard et al.,
2013; Karunatillake et al., 2014
Other elements Other cations and anions associated with
igneous rocks, e.g.,Ca
2+
,Mg
2+
,K
+
,
Fe
2+/3+
, many trace elements such as
Mn, Cr, Ni, Zn
e.g., McLennan et al., 2014; Meslin et
al., 2013; Stolper et al., 2013
Energy—full redox couples Electron donor Electron acceptor
Chemolithotrophy
Anaerobic iron oxidation Fe
2+
NO 2
3Distribution of NO
32-
on Mars not
known although fixed nitrogen is
inferred (Ming et al., 2014)
Anaerobic iron oxidation Fe
2+
Perchlorates Perchlorate can be used to oxidize iron,
but not shown to be used for growth
in organisms. It is included to
highlight the need for investigation
of perchlorate-containing redox
couples.
Methanogenesis, acetogenesis H
2
CO
2
Hydrogen inferred from presence of
olivine and serpentine—substrates
and products for H
2
-evolving water-
rock reactions
Iron reduction H
2
Fe
3+
As above for hydrogen
Sulfate reduction H
2
SO 2
4As above for hydrogen
Sulfur oxidation S NO 2
3Sulfur suggested at Gusev Crater
(Morris et al., 2007)
Anaerobic sulfur oxidation S Fe
3+
Occurs in acidic conditions
Anaerobic carboxydotrophy CO NO 2
3Carbon monoxide in atmosphere
Chemoorganotrophy
Iron reduction Organics Fe
3+
Distribution and quantity of organics in
different locations and depths on
Mars is not known
Sulfate reduction Organics SO 2
4As above for organics
Nitrate reduction Organics NO 2
3As above for organics and fixed
nitrogen species
Perchlorate reduction Organics Perchlorate As above for organics
Other forms of energy Photosynthesis unlikely on surface
(lack of liquid water). Fermentation
depends upon concentration of organics.
Cockell, 2014a
Inventory of requirements for habitability on Mars (note that this table does not indicate whether the requirements for life are colocalized
in any given environment for life). For energy sources, redox couple only shown if there is unequivocal evidence for both half reactions or
for one half reaction and strong evidence or likelihood of the other (adapted from Cockell, 2014a).
HABITABILITY: A REVIEW 9
Table 3. A Planetary Habitability Table for Europa
Requirement Comments and references
Liquid water Present in subsurface ocean Khurana et al., 1998
Main elements
CCO
2
, carbonate, organics Cooper et al., 2001; organics expected from
meteoritic delivery
HH
2
O, H
2
SO
4
, organics Carlson et al., 1999; Cooper et al., 2001;
organics expected from meteoritic
delivery
N? NH
3
from primordial inventory possible;
other fixed N species from surface?
OH
2
O, O
2
and other oxidants, organics Johnson et al., 2003
P ? Unknown source, but source in rocky core
possible
SSO
2
4, other sulfur oxidation states Carlson et al., 1999; Cooper et al., 2001;
McKinnon and Zolensky, 2003; Dalton,
2003; Hansen and McCord, 2008
Other elements K, Na Johnson et al., 2002
Energy—full redox couples Electron donor Electron acceptor
Chemolithotrophy
Methanogenesis, acetogenesis H
2
CO
2
H
2
plausible if there is rocky core-water
interaction
Sulfate reduction H
2
SO 2
4H
2
plausible if there is rocky core-water
interaction
Chemoorganotrophy
Sulfate reduction Organics SO 2
4Organics expected from meteoritic input
Other forms of energy Photosynthesis unlikely in ocean
as ice layer expected to block
all light. Fermentation possible
if organics entrained in ocean?
Inventory of requirements for habitability in Europa (note that this table does not indicate whether the requirements for life are
colocalized in any given environment for life). For energy sources, redox couple only shown if there is unequivocal evidence for both half
reactions or for one half reaction and strong evidence or likelihood of the other.
Table 4. A Planetary Habitability Table for Enceladus
Requirement Comments and references
Liquid water Present in subsurface ocean Waite et al., 2009
Main elements
C CO, CO
2
, carbonic acid, methane, organics Waite et al., 2009
HH
2
,
2
H, H
2
O, organics Waite et al., 2009
NN
2
, ammonia, HCN (hydrogen cyanide) Waite et al., 2009
OH
2
O, CO, CO
2
,CH
3
OH, C
2
H
4
O, C
2
H
6
O Waite et al., 2009
P?
SH
2
S Waite et al., 2009
Other elements Na, K Postberg et al., 2009
Energy—full redox couples Electron donor Electron acceptor
Chemolithotrophy
Methanogenesis, acetogenesis H
2
CO
2
Chemoorganotrophy
Sulfate reduction Organics SO 2
4Organics expected from
meteoritic input
Other forms of energy Photosynthesis unlikely in ocean
as ice layer expected to block
all light. Fermentation with organics
may be possible.
The presence of hydrogen and organics
raises the possibility of sulfate and iron
reduction if these ions are available.
Inventory of requirements for habitability in Enceladus (note that this table does not indicate whether the requirements for life are
colocalized in any given environment for life). For energy sources, redox couple only shown if there is unequivocal evidence for both half
reactions or for one half reaction and strong evidence or likelihood of the other.
10
is obtained by using inorganic electron donors, it is designated
chemolithotrophy. If the energy is obtained from organic
compounds as the electron donor, it is designated chemo-
organotrophy (chemoheterotrophy), while if the organisms
use sunlight as energy source, it is designated phototrophy.
The energy-yielding process in life involves the formation
of a proton gradient across a membrane. This itself is gen-
erated by electron transport. An electron is transferred from
a donor element or molecule to an acceptor element or
molecule with or without the help of energy available in
light. The movement of an electron through the electron
transport chain drives the pumping of protons across a cell
membrane, thus producing a proton gradient, or proton
motive force, which can be used to do work ( Mitchell,
1961). More exactly, this process produces adenosine tri-
phosphate (ATP) from the covalent bonding of phosphorus
(P
i
) to adenosine diphosphate (ADP) in a complex protein
called ATP synthase that sits across the membrane. The
energetic anhydride bond so produced in ATP can be broken
elsewhere in the cell to liberate energy for biochemical
processes. Thus, what energy cells require is quantized in
the energy available in the phosphate bond, analogous to
voltage in electric systems (Hoehler, 2007). There are a
great variety of different energy metabolisms such as the
reduction or oxidation of iron, sulfur, and nitrogen species,
manganese reduction, methanogenesis, internal reduction
and oxidation of organic compounds (designated fermenta-
tion), and aerobic respiration making use of molecular ox-
ygen as the final electron acceptor (Schulze-Makuch and
Irwin, 2004; Kim and Gadd, 2008).
For an environment to have sufficient energy to make it
habitable (assuming all other requirements are met), there
must be sufficient electron donors and acceptors thermo-
dynamically favorable to the organism, or sufficient light
(the energy supply), to generate sufficient energy for the
organism to carry out the activities it requires (maintenance,
growth, or survival). These values should be calculable from
first principles (Hoehler, 2007). The energy supply available
from a given redox couple can be calculated as a Gibbs free
energy based on chemical considerations. The energy re-
quired by the organism should be calculable if we know the
energy required to respond or adapt to certain environmental
factors. In reality, this calculation may be difficult. For ex-
ample, on the supply side, Fe
2+
, which is used as an electron
donor in biological iron oxidation, may be bound within
minerals such as olivine. Its rate of supply will depend, inter
alia, on the rate of water-rock interactions, paths of fluid
flow through the rock, and the rate of reaction of the Fe
2+
with other ions. As these conditions all feed back on one
another, it may never be possible to calculate to great ac-
curacy the energy available to organisms at micron scales.
On the demand side, energy required to adapt to different
physicochemical extremes is not merely additive, but ex-
tremes can act in a synergistic or antagonistic manner (e.g.,
an enhanced water activity tolerance when certain sea-ice
bacteria are grown at suboptimal temperatures; Nichols
et al., 1999). Given an environment with npermutations of
extremes, it is difficult to calculate the exact energy required
at any given moment by an organism; this may be impos-
sible in highly complex dynamic environments. Finally,
organisms are constrained by the evolutionary and, there-
fore, biochemical legacy of their ancestors. The mere
presence of theoretically sufficient energy to overcome a
given set of physical and chemical extremes may be con-
founded by a lack of biochemical machinery to address the
damage caused by such extremes.
Despite this, it is possible to reach a state of knowledge
where one can estimate the energy available in an envi-
ronment and estimate the energy required by an organism,
and determine whether the supply is sufficiently abundant
to make it likely that a given organism known to use that
energy supply could be active (Rogers et al., 2007). Never-
theless, a concrete determination of whether an environ-
ment is habitable requires knowledge of specific organisms’
requirements.
4.4. Major and trace elements
Of all the elements in the Periodic Table, six are ubiq-
uitous in the macromolecules of known life: C, H, N, O, P,
and S. Carbon is required as the core element in a vast
variety of macromolecules. Hydrogen is covalently linked to
carbon and other atoms in macromolecules. Nitrogen is
similarly to be found, particularly in the linkages of long-
chained molecules such as proteins and in the base pairs of
DNA. Oxygen is used in alcohols, sugars, and a variety of
molecules. Phosphorus forms part of the backbone of DNA
and is used in energy-rich molecules such as ATP. Sulfur is
used in protein bridges and a variety of iron-sulfur clusters
in molecules involved in energy acquisition. A variety of
other elements, such as magnesium and even tungsten (in
anaerobic taxa), are used by different organisms for differ-
ent functions. In a simplistic way, one can view evolution as
having used six elements to build the chassis of life and then
rummaged through the Periodic Table to find and use other
elements that have specific chemical properties of use in
particular biochemical functions or environments (Wackett
et al., 2004). The six CHNOPS elements are available to life
in a variety of forms. Some examples of these on Earth are
shown in Table 1. Their concentrations are determined by
planetary conditions. For example, more oxidized condi-
tions will favor compounds such as sulfate and ferric iron as
potential sources of S and Fe, respectively.
The forms in which some of these elements are available
are also forms that can be used as electron donors or ac-
ceptors in energy acquisition (Table 1). Therefore, there is a
tight coupling, particularly in the microbial domains of life,
between habitability with respect to the availability of
CHNOPS and redox couples for energy acquisition.
The detection of diverse chemical compounds and ele-
ments on other planetary bodies allows for the link between
CHNOPS elements and energy availability to be used in an
assessment of the habitability of other planetary bodies (for
Mars specifically, an example of this approach was ad-
dressed by Stoker et al., 2010). For example, in Tables 2–4,
detected and strongly inferred chemical species on Mars,
Europa, and in the plumes of Enceladus are used to assess
the possible presence of the components required for in-
stantaneous habitability.
For any given organism there may be other elemental
requirements (Wackett et al., 2004). For example, iron is
used widely by organisms in electron transfer proteins.
There is a reported instance of microbes not requiring iron,
the lactobacilli (Sabine and Vaselekos, 1967; Bruyneel
HABITABILITY: A REVIEW 11
et al., 1989; Weinberg, 1997). Instead, they are thought to
employ enzymes and proteins that use cobalt, magnesium,
manganese, and other cations (Elli et al., 2000).
5. Continuous Planetary Habitability
What conditions are required for at least part of a plan-
etary body to have instantaneous habitability over geologi-
cal timescales? This section explores this question.
5.1. Water: Surface liquid water worlds and interior
liquid water worlds
The distribution of liquid water can be used to recognize
broadly two types of planetary bodies. The first type is a
planet with liquid water on its surface as well as in its in-
terior, where the liquid is sustained from a combination of
internal heating and stellar energy; the latter in most cases
will be the dominant form of energy for keeping water on
the surface in a liquid state. These are ‘‘surface liquid water
worlds.’’ Earth is an example. The second type of planetary
body is one where stellar radiation is not sufficient to
maintain surface liquid water but where liquid water exists
in the interior. These are ‘‘interior liquid water worlds.’
There is a variety of this type of planetary body. It includes
icy moons with subsurface oceans (in our solar system,
examples include Enceladus, Europa, and Ganymede) and
some terrestrial-type rocky planets where stellar flux alone
is not sufficient to maintain liquid water (on the surface), but
internal sources of energy maintain liquid water ( McMahon
et al., 2013). Isolated lone planets in interstellar space have
also been suggested as locations for liquid water (Stevenson,
1999; Abbot and Switzer, 2011).
Surface liquid water worlds can transition to interior
liquid water worlds. The dry, desiccated surface of Mars, in
contrast to its more water-rich past ( Jakosky and Phillips,
2001), is an example of how conditions on the surface of a
planet can deteriorate (for reasons discussed later).
The distinction between these two types of worlds from
an astrobiological point of view may well be categorical.
In the case of surface liquid water worlds, the presence of
liquid water may occur spatially colocated with the presence
of light, thus allowing for photosynthesis. If we accept an
assumption that the emergence of multicellularity and in-
telligence is linked to the presence of high concentrations of
oxygen from oxygen photosynthesis that allows for aerobic
respiration (Catling et al., 2005), then these worlds may be
the only type capable of producing the conditions for in-
stantaneous and continuous planetary habitability required
by intelligent organisms. By contrast, interior liquid water
worlds may have habitable conditions for a range of me-
tabolisms but not conditions for photosynthesis. Some cal-
culations, however, suggest that production of surface
oxidants, if cycled into the interior, could lead to suboceanic
oxygen concentrations as high as terrestrial surface waters
(Hand et al., 2007). It is unknown whether such a scenario
could support multicellular life, let alone intelligence. In the
case of Mars or Mars-like planets, for instance, the transition
from conditions capable of sustaining surface liquid water to
those that cannot renders photosynthesis an implausible
energy supply across most, if not all, the surface. This
truncates the conditions for continuous planetary habitabil-
ity required for the large-scale production of oxygen by
oxygenic photosynthesis and thus multicellular life and in-
telligence.
5.1.1. Appropriate temperature conditions for surface liq-
uid water worlds. The window within which life can op-
erate is smaller than the total pressure-temperature phase
space for liquid water (see above, Jones et al., 2011; Jones
and Lineweaver, 2012). Thus, this smaller phase space must
also be sustained over geological lifetimes. This require-
ment implies liquid water between *-25C and 122C over
geological timescales.
Maintaining temperatures within the range required for
biological activity on the surface of a planet depends on a
sufficiently powerful greenhouse forcing. If the effective
temperature of a planetary surface exceeds certain values,
set by the energy received from its star and the greenhouse
effect caused by the gases present in its atmosphere, then
the water on the planet will evaporate at sufficient levels
to supply the upper atmosphere with a moist greenhouse
effect (Kasting, 1988). As the moist greenhouse becomes
more effective at raising the temperature, a positive feed-
back may develop between the increasing levels of atmo-
spheric humidity, the surface temperature, and rates of
evapotranspiration. The moist greenhouse may eventually
result in a runaway greenhouse effect (Walker et al., 1970;
Nakajima et al., 1992). As observed on Venus, a runaway
greenhouse will cause all loss of water from the planet,
making it uninhabitable. The moist and runaway green-
house effects therefore define an orbit that is too close to
the star for liquid water to be sustained on the surface of a
planet.
A planet too far from a star will suffer the effects that the
carbon dioxide in the atmosphere can condense onto the
surface, reducing the concentration of this greenhouse gas
and contributing to low temperatures, resulting in a frozen
surface. If CO
2
is abundant enough, scattering can contrib-
ute to ineffective heating of the surface. This in itself will
depend on the quantity of CO
2
outgassed by the planet,
linking habitability to planetary interior structure (Noack
et al., 2014). These conditions define an outer limit for
habitability, with the boundary conditions for life addition-
ally depending on the star type. Hot stars, such as F stars,
have boundary conditions for surface liquid water farther
from the star compared to our own Sun (a G star). Cooler
low-mass stars, such as K and M stars, have boundary
conditions closer in (Fig. 4). The outer limit of the habitable
zone may be considerably extended on planets with strong
greenhouse gases such as hydrogen, which theoretically
could expand the outer limit to *10 AU (Pierrehumbert and
Gaidos, 2011). In extreme cases, free-floating planets not
gravitationally bound to a star may even harbor surface
habitable conditions (Stevenson, 1999).
The so-called habitable zone is thus defined as the zone
around a star where liquid water is stable at the surface of a
planetary body (usually an Earth-mass planet) (Hart, 1979;
Kasting et al., 1993; Kasting, 1997; Franck et al., 2000;
Gonzalez, 2005; Kopparapu et al., 2013). The habitable zone
is an old concept (Huang, 1959). The idea was developed by
Dole in his book Habitable Planets for Man in which he
elaborated the idea of the circumstellar habitable zone as well
as various other determinants of planetary habitability (Dole,
1964).
12 COCKELL ET AL.
In binary star systems, the presence of a second star can
influence the habitable zone boundaries. Depending on the
dynamics of the system, the boundaries can vary over time
as a result of the gravitational interactions of the two stars
(Haghighipour and Kaltenegger, 2013), or a bright second
star can cause the boundaries to be further out than they
would in a single star system (Kaltenegger and Haghighi-
pour, 2013).
As a star’s luminosity changes over time, generally
increasing (a consequence of hydrogen burning on the
main sequence), the habitable zone will move outward.
Integrated over time, there is therefore a narrower band,
the continuously habitable zone, within which conditions
for surface liquid water are met (Hart, 1978; O’Malley-
James et al., 2013; Rushby et al., 2013). As the bound-
aries of the habitable zone move outward, a planet can
eventually cross the inner boundary, and surface temper-
atures will become too great to sustain liquid water.
Therefore, a planet remains in the habitable zone for a
given length of time, known as its ‘‘habitable lifetime.’’
This lifetime largely depends on stellar mass (but the
physical properties of a planet and its atmosphere play a
role). The more massive a main sequence star, the faster it
burns through its hydrogen fuel supply, causing a more
rapid increase in its luminosity. This pushes the habitable
zone boundaries outward at a greater rate.
The situation is different for stars at the beginning of their
lifetimes. Pre-main sequence stars are thought to have
habitable zones that move inward as stellar luminosity ini-
tially decreases as they move toward the main sequence
(Ramirez and Kaltenegger, 2015). This raises the possibility
that planetary bodies that orbit a young star lose their water
inventory because of an intense greenhouse effect (Luger
and Barnes, 2015). They later end up in the habitable zone
during the star’s main sequence phase but would lack suf-
ficient water to support life. Thus, an assessment of con-
tinuous planetary habitability must consider a planet’s
relationship to the changing location of the habitable zone
from the pre-main sequence to the end of a star’s life.
In the case of Earth, it has remained within the Sun’s
continuously habitable zone since its formation. It is expected
to leave the habitable zone after another 2–3 billion years
(O’Malley-James et al., 2013, 2014; Rushby et al., 2013). As
the inner boundary of the habitable zone moves closer to
Earth, environmental conditions on the planet are expected to
change as the planet receives more solar energy. The planet
will become hotter and drier, leading to environments that
become increasingly hostile to life, setting into motion ex-
tinction sequences that begin with the eukaryotes and end
with the extinction of extremophilic microorganisms
(O’Malley-James et al., 2013, 2014). Hence, when assessing
the habitability of a planet, it is necessary to consider the
amount of time it has spent in the habitable zone, as well as
its current position within the habitable zone.
While stellar evolution can cause the end of habitable
conditions on a planet, it might also render previously un-
inhabitable worlds habitable. For example, in the later stages
of the Sun’s evolution, when it swells to become a Red
Giant, the habitable zone will encompass Saturn’s moon
Titan—a moon that contains various organic compounds,
which has been suggested to lead to a new habitable world
(Lorenz et al., 1997).
A conceptually similar habitable zone can be formulated
for a large moon orbiting a planet, making exomoons that
have surface liquid water a viable possibility (Heller and
Barnes, 2013). The inner circumplanetary habitable edge is
defined as the orbit where a runaway greenhouse effect is
caused by illumination from the planet and tidal heating
(Heller and Barnes, 2013). The habitability of moons or-
biting planets is dependent on orbital eccentricity (Forgan
and Kipping, 2013). For example, large eccentricities can
generate strong tidal heating, which might prevent the for-
mation of stable bodies of water. Unlike tidally locked
planets, moons tidally locked to their planets will experience
variations in solar insolation, which might reduce the pos-
sibility of atmospheric freeze-out by providing combined
planetary and solar insolation on different regions of the
moon ( Joshi et al., 1997).
5.1.2. Appropriate temperature conditions for interior liq-
uid water worlds. The limitations of the habitable zone
concept can be understood at once when we consider a planet
with insufficient atmospheric conditions (or no appreciable
atmosphere) to sustain surface liquid water but a sufficient
internal source of energy to generate liquid water within the
interior (interior liquid water worlds). This source of energy
raises the possibility of planets outside the habitable zone
with habitable conditions. In the case of icy moons, if a body
has an eccentricity (noncircularity of orbit), obliquity (axial
tilt), and/or nonsynchronous rotation state, then tidal effects
can heat the interior. These conditions can melt internal ice
over geological time periods (Reynolds et al., 1987; Scharf,
2006). This phenomenon is inferred from the observations of
water plumes emanating from Enceladus (Waite et al., 2006,
2009) and inferred for the three Galilean moons Europa,
Ganymede, and Callisto based on induced magnetic fields
and thermodynamic calculations (Khurana et al., 1998;
Vance et al., 2014; Saur et al., 2015).
The Galilean moons are instructive in demonstrating how
the strength of tidal forces may play a role in regulating
habitable conditions with respect to the availability of liquid
water. Io, the closest of the moons, with a semimajor axis of
421,700 km, is so tidally active that it hosts active silicate
and sulfur volcanoes (McEwen et al., 1998). This intense
Ti
dal lock radius
Habitable zone
A
F
G
K
M
Star type
Solar System
Distance (AU)
0.01 0.1 1 10 100
Mass /Mass
star Sun
0.01
0.1
1
10
FIG. 4. The habitable zone for different spectral types.
Our own Solar System is shown. (diagram modified from
Kasting et al., 1993).
HABITABILITY: A REVIEW 13
activity likely precludes liquid water. Europa, with a semi-
major axis of 670,900 km, hosts substantial evidence for a
subsurface ocean and a young geologically active surface
(Schmidt et al., 2011). Ganymede, with a semimajor axis of
1,071,600 km, has a more ancient surface but is thought to
host a subsurface ocean (Vance et al., 2014). Callisto, the
farthest moon out, at 1,882,700 km from Jupiter, has an even
more ancient surface, although an induced magnetic mo-
ment suggests an ocean deep within the moon (Khurana
et al., 1998). Other factors aside from orbital radius, such as
the size of a body and resonances between moons, also play
a role in defining the strength of tidal interactions.
Tidal interactions enable liquid water to exist, but it
remains unknown whether the other factors required for
instantaneous habitability to exist and for these require-
ments to persist are met in these subsurface water bodies.
What is crucial is whether the body of water is in contact
with the surface or a subsurface core (Ruiz and Tejero,
2003; Hand and Chyba, 2007). From the surface, meteor-
itic matter, such as organics and various cations and an-
ions, or other matter, such as radiation-produced oxygen
and other oxidants, has a chance of being entrained within
the ocean. In the subsurface, contact with a rocky core
could provide cations and anions, which, as for the surface,
may include not only required elements for growth but
diverse half reactions required for energetic redox reac-
tions such as iron and sulfate reduction and methanogen-
esis (Chyba and Phillips, 2001; Schulze-Makuch and Irwin,
2002; Pappalardo et al., 2009). Despite the theoretical
possibilities, however, when the availability of CHNOPS
elements and half reactions for redox couples is con-
strained to measured or strongly inferred elements and
compounds, we still lack considerable knowledge about
instantaneous habitability on Europa and Enceladus (Ta-
bles 3 and 4). For Europa, surface-interior interactions are
inferred on account of such processes as putative plate tec-
tonics (Kattenhorn and Prockter, 2014), although the extent to
which these processes produce mixing between the surface
and interior ocean is not known. By contrast, for Ganymede,
the surface is ancient, and the internal liquid water is thought
to be sandwiched between high-pressure ice layers (Vance
et al., 2014), thus preventing, or at least minimizing, internal
water interactions with both the surface and an internal rocky
core. Such an arrangement might provide conditions that are
habitable to a smaller range of organisms, if any, over geo-
logical timescales.
Saturn’s moon Enceladus is thought to be a differentiated
body with a rock-metal core surrounded by a liquid water–
containing ice layer (Schubert et al., 2007). The extent of
contact or circulation between the liquid water and the
rocky core is not known. However, the detection of silica
nanoparticles in Saturn’s E-ring has been interpreted as
evidence of present-day active hydrothermal activity in the
moon in contact with the core (Hsu et al., 2015). Like Eu-
ropa, tidal forces are thought to be responsible for ice
melting, possibly aided by radioactive decay in the rocky
core, which is thought to explain the lack of similar geo-
logical activity on Saturn’s moon Mimas, which orbits
closer to Saturn and has higher eccentricity but a small rock
fraction (Schubert et al., 2007).
Even in rocky planets, internal heat from radioactive decay
could generate temperature conditions sufficient to sustain
liquid water in the subsurface far outside the boundaries of
the traditional habitable zone (McMahon et al., 2013) and
possibly in interstellar space (Abbot and Switzer, 2011).
5.2. Other physicochemical conditions
The physicochemical limits for life over geological times
are identical to those defined for instantaneous habitability
since the long-term limits are set by life’s ability to cope with
any given physical and chemical extreme at a point in time.
5.3. Available energy
For an environment or at least one location on a planet to
be continuously habitable over geological time periods, it
must provide sufficient energy for activity by a given or-
ganism. For photosynthesis to be a sustained energy source,
the surface of the planet must sustain liquid water. For other
forms of energy, the requirement is a set of chemical dis-
equilibria that provides electron donors and acceptors for
redox couples. This implies geochemical turnover or activ-
ity within the planetary crust to produce reduced and more
oxidized compounds colocated at small scales for use by life
over geological time periods.
5.4. Major elements
Over geological timescales, what are potential sources of
CHNOPS?
Carbon may always be available as CO
2
in an atmo-
sphere throughout most of the lifetime of a terrestrial-type
planet. Its presence in the atmospheres of Venus (96.5%),
Earth (*400 ppmv), and Mars (96%) shows that it is
present on most types of terrestrial-type rocky planets.
However, it is not necessarily the case that the CO
2
can be
fixed, for example by photosynthetic organisms, if its
concentrations are too low for the carbon assimilation
mechanisms used by a particular biota (e.g., the loss of
photosynthesis at below 35–45 ppm CO
2
in C3 plants;
Bauer and Martha, 1981).
The long-term presence of hydrogen depends on its
source. Serpentinization reactions (e.g., the reaction of the
mineral fayalite with water) producing H
2
cannot be sus-
tained over geological time periods unless there is geolog-
ical activity to sustain water flow through the crust (Okland
et al., 2012). Hydrogen can be obtained from organic mol-
ecules which could be endogenously produced by Urey-
Miller-type reactions (Bada, 2013). Hydrogen could also be
obtained from diverse carbon compounds in meteoritic
material (Sephton, 2002). Hydrogen is also pervasively
present in water, the latter being required for habitable
conditions to exist in the first place.
Nitrogen is present in the atmospheres of Venus (3.5%),
Earth (78.1%), and Mars (1.9%). An environment can be
habitable that contains N
2
at a partial pressure suitable for
biological nitrogen fixation (Mancinelli and Banin, 2003),
although this presupposes the existence of the energetically
expensive (16 molecules of ATP per N
2
molecule) bio-
chemical machinery to fix the gas. In the absence of bio-
logical nitrogen fixation, fixed nitrogen compounds such as
ammonia, nitrite, and nitrate must be available. Abiotic
processes such as impact events, hydrothermal activity, and
lightning discharge can generate these compounds (Brandes
14 COCKELL ET AL.
et al., 1998; Segura and Navarro-Gonza
´lez, 2005; Summers
and Khare, 2007; Manning et al., 2009). This implies the
presence of active atmospheric processes (lightning) or ex-
ogenously generated geological activity (impact events). On
planets with reducing conditions in their interior, nitrogen is
expected to be predominantly in the form of ammonia, which
is sequestered within silicates. However, in more oxidizing
conditions, such as Earth’s mantle, the element is in the form
of nitrogen gas, which is more readily degassed (Mikhail and
Sverjensky, 2014). Although more oxidizing conditions may
therefore favor a more nitrogen-rich atmosphere, the form of
nitrogen is energetically less available to life.
Oxygen is present in a wide diversity of compounds, such
as oxides. However, many of these atoms, such as oxygen
atoms bound to silicon in silicates, are not directly accessible
to life. Suitable sources of oxygen atoms include compounds
such as sulfates and iron oxides found ubiquitously on Earth
and on Mars (Bibring et al., 2007). Oxygen exists in water,
which is the source of this element for oxygen gas produced
in oxygenic photosynthesis. Oxygen atoms also exist in or-
ganic molecules within, for example, alcohol and carboxylic
acid groups. As for hydrogen, these organics can be produced
endogenously or delivered exogenously.
On Earth, phosphate is available in igneous rocks as ap-
atite and merrillite and therefore available in other rock
types as a consequence of the rock cycle. Its detection on
Mars illustrates its potentially ubiquitous availability on
terrestrial-type rocky planets (e.g., Usui et al., 2008).
Phosphorus can also be delivered to the surface of planets in
meteoritic material such as in schreibersite [(Fe, Ni)
3
P]
(Pasek et al., 2007).
Sulfur atoms are available in planetary bodies in diverse
sources such as sulfides, sulfates, and compounds of inter-
mediate oxidation state such as thiosulfate. The presence of
water can enhance the diversity and abundance of these
sources of sulfur. This is illustrated by Hesperian martian
geochemistry. The production of sulfuric acid in low water-
rock ratio interactions of water and SO
2
has led to a diverse
suite of sulfate minerals on Mars (Morris et al., 2006;
Bibring et al., 2007).
In summary, there are diverse sources of CHNOPS el-
ements available to life over geological time periods on, or
within, planetary bodies. All of them can potentially be
delivered to a surface by impact events. However, active
geological turnover (such as plate tectonics, see Section
6.1.3) or atmospheric chemistry will increase the abun-
dance and diversity of compounds in which these atoms
are available, thus enhancing the likelihood of instanta-
neous habitability for given types of organisms. Two ex-
amples are the abiotic production of oxidized nitrogen
compounds, whose diversity and abundance will be en-
hanced by greater atmospheric and geologically active
processes, and hydrogen, whose availability can be en-
hanced by geologically active processes in planetary crusts
such as serpentinization. Furthermore, the more geological
turnover there is, the greater will be the number of envi-
ronments in which CHNOPS elements, through mixing,
are likely to be colocalized at small scales to be accessible
to life. Geological turnover will ensure the constant re-
working of supplies of these elements into environments
where they may have become depleted by, for example,
leaching in hydrological processes.
Most of the observations above refer to rocky terrestrial-
type planets. The abundance and availability of CHNOPS in
subsurface oceans, for example in icy moons (a type of
interior liquid water world), is not well understood. As for
rocky planets, impacts can deliver inventories of these ele-
ments provided the material they deliver reaches, or is cy-
cled, to the interior of the oceans. The contact of water in
such oceans with a rocky core could enhance the geo-
chemical diversity and abundance of elements and com-
pounds available to life.
5.5. Other elements
The same observations for CHNOPS apply to other ele-
ments required by life. A wide variety of cations (such as
K
+
,Fe
2+/3+
,Mg
2+
) are available on a planetary surface and
interior over geological time spans in materials such as ig-
neous rocks. The combination of geological activity with a
hydrological cycle circulates them within the surface and
subsurface of a planetary body, making them available and
colocated at small spatial scales in environments where
previous water flow may have leached them and depleted
their concentrations.
Life itself will change the availability of elements on a
planetary body (including the CHNOPS elements). For
example, the oxygenation of the atmosphere in the Paleo-
proterozoic *2.4 billion years ago and again in the Neo-
proterozoic *0.7 billion years ago undoubtedly vastly
enhanced the mineral diversity of the planet by, for example,
the production of mineral oxides (Hazen et al., 2008). Large-
scale planetary oxidation can enhance the availability of some
elements (such as fixed states of nitrogen), but concomitantly
it can also reduce the availability of other elements (such as
Fe, which is less soluble in the oxidized Fe
3+
state compared
to the reduced Fe
2+
state at circumneutral pH).
6. Factors for Continuous Planetary Habitability
There are a range of planetary and astronomical factors
that influence the availability of previously described re-
quirements for continuous planetary habitability. These
factors can influence habitability in one of two ways: (1)
Existence of habitable conditions. Some factors are crucial
for determining whether a planet is habitable to any type of
life. (2) Extent of habitability. Some factors are likely to
modify whether a planet is habitable for particular types of
organisms and the time period over which continuous hab-
itability for any given type of organisms can be sustained.
Understanding these factors is important because they
may determine whether a planetary body retains continuous
habitable conditions long enough to allow certain types of
life. The comparison of some of these factors with those
associated with Earth has been proposed as a basis for
quantifying habitability (Schulze-Makuch et al., 2011).
In particular, astrobiologists are interested in factors that
control the presence of surface liquid water on a planet and
thus the potential for oxygenic photosynthesis leading to the
emergence of multicellularity and intelligence. Factors that
cause geological activity, and thus geochemical turnover,
that result in the continuous presence of diverse redox
couples and the elements required for life are also important.
Here, these factors are broadly split into planetary factors
(factors that result from the characteristics of the planetary
HABITABILITY: A REVIEW 15
body itself) and astronomical factors (factors that result
from the astronomical environment of a planet). In this
paper, we discuss four important examples of each. Most of
these factors have a profound influence on the possibility of
surface liquid water, but their relevance to interior liquid
water worlds will also be discussed. In Table 5, an example
of each of these factors and its influence on continuous
planetary habitability is shown. Figure 5 shows an example
of how some of these factors can interact.
6.1. Planetary factors
6.1.1. Planetary mass/density. The mass of the plane-
tary body influences habitability in a variety of ways. The
mass will determine whether the object retains enough pri-
mordial heat or has enough radiogenic heat to maintain a
liquid core, allowing for an atmosphere-protecting magnetic
dynamo (Breuer and Spohn, 2003). The temperature gradi-
ent through a planetary body, which is influenced by its
initial mass (influencing the energy released during accre-
tion and differentiation), will influence whether plate tec-
tonics can be initiated and sustained over its lifetime (Noack
and Breuer, 2014). Both magnetic field and tectonic activity
are discussed in more detail below.
Planetary mass will determine atmospheric composition
by influencing both the degassing of volatiles and the extent
to which a planet retains its primordial atmosphere. This
will itself determine the concentration of different types of
greenhouse gases and whether they are sufficient to sustain
liquid water on at least part of the surface of a planet
(Kasting et al., 1993; Kasting and Catling, 2003).
The study of exoplanets has revealed the considerable
complexity in understanding how planetary mass influences
the history of planetary atmospheres and thus the habit-
ability of planetary surfaces.
For example, ‘‘super-Earths’’ are planets with a size that is
larger than that of Earth and masses less than 10 Earth mass
(10 M
4
). However, many small planets have been found
to have mean densities incompatible with rocky, Earth-like
planets. In many cases, large hydrogen-dominated envelopes
and/or large amounts (up to 100%) of water are necessary to
explain the observations, which make such planets potentially
‘‘mini-Neptunes’’ instead of ‘‘super-Earths’’ (Barnes et al.,
2009; Lammer, 2013; Lammer et al., 2014; Marcy et al., 2014;
Rogers, 2014; Luger et al., 2015). This raises the question as to
what fraction of low-mass planets in the habitable zone are
indeed rocky and thus suitable for the evolution of life.
Table 5. A Non-Exhaustive Table of Examples of Factors That Influence Continuous Planetary
Habitability on the Surface of a Planetary Body (or Interior Liquid Water Worlds)
Habitability factor Example of influence on habitability
Planetary factors
Mass Insufficient mass to retain gases required for greenhouse warming and liquid water.
Insufficient mass to generate heating for subsurface ocean. Influence on size of internal
ocean and thus potential biomass.
Atmospheric composition Insufficient greenhouse gases for surface liquid water. High concentrations of greenhouse
gases lead to runaway greenhouse effect.
Presence of oxygen for multicellular life.
Production of reactants for energy and nutrients cycled into deep ocean (even in tenuous
atmosphere).
Plate tectonics Lack of plate tectonics shuts down carbonate-silicate cycle influencing surface temperature
and presence of liquid water.
Plate tectonics may enhance movement of surface material into a deep ocean.
Magnetic field Insufficient field can result in early loss of atmosphere (e.g., planets close to M stars).
Strong magnetic field can enhance longevity of atmosphere and hence habitable
conditions.
May generate radicals and other species on the surface of an icy world with implications
for energy/nutrients.
Astronomical factors
Orbital characteristics Obliquity may not critically determine habitability. Some combinations of orbital
characteristics, such as high eccentricity and tidal locking can circularize orbit outside
habitable zone.
Extremity of climatic excursions caused by high eccentricity.
Lack of tidal heating caused by tidal locking could prevent formation of habitable
subsurface water bodies.
Influences extent of tidal heating
Star type Can influence early sputtering away of atmosphere.
Influences longevity of habitable zone.
Presence of a moon Lack or presence of a moon probably not critical for presence of habitable conditions on a
planet but may influence extremity of climatic excursions caused by obliquity variations.
Impact events Frequent large impacts that sterilize oceans could prevent life emerging. Frequent impacts
create selection pressure for high-temperature tolerant/loving organisms or prevent
atmospheric oxygen buildup from photosynthesis.
Impacts may deliver material into subsurface ocean or enhance surface-subsurface
exchange of material.
Statements in italics, where appropriate, show factors that apply to habitability in interior liquid water worlds.
16 COCKELL ET AL.
The early evolution of these is also important for the tra-
jectories they take toward, or away from, being habitable.
Theoretical studies indicate that rocky planets may accumu-
late large gaseous envelopes either by accretion of gas from
the protoplanetary nebula or by outgassing (e.g.,Hayashi
et al., 1979; Elkins-Tanton and Seager, 2008; Elkins-Tanton,
2012; Lammer, 2013, and references therein). According to a
number of studies (Ikoma and Hori, 2012; Bodenheimer and
Lissauer, 2014; Lammer et al., 2014; Sto
¨kl et al.,2015),even
Earth-like planets can accumulate up to 1000 Earth ocean
equivalent amounts of hydrogen.
Modeling suggests that protoplanets with core masses that
are £1M
4
can lose their captured hydrogen envelopes during
the active X-ray and extreme ultraviolet (XUV) phase of their
young host stars (Luger et al., 2015), while rocky cores
within the so-called ‘‘super-Earth’’ domain probably cannot
get rid of their nebula-captured hydrogen envelopes during
their lifetime (Lammer et al., 2014). These results indicate
that the terrestrial planets in our solar system lost their
nebula-based early atmospheres during the intense XUV ac-
tivity phase of the young Sun or reached their final mass tens
of millions of years after the nebula gas evaporated. It has
been suggested that planets, like the terrestrial planets in our
solar system, that can lose their nebula-captured hydrogen
envelopes and keep their outgassed or impact-delivered sec-
ondary atmospheres inside the habitable zone of G-type stars
most likely have core masses with 1 0.5 M
4
and corre-
sponding radii between about 0.8 and 1.15 R
4
(Lammer
et al., 2014). Similar results have been presented by Kislya-
kova et al. (2013) and Luger et al. (2015).
However, even fast accreted Earth-like cores and ‘‘super-
Earths’’ with up to a few percent of their mass in hydrogen
and helium may still harbor conditions for liquid water
oceans. However, at higher hydrogen envelope fractions,
surface pressures in excess of more than 1 GPa (e.g.,
Choukroun and Grasset, 2007) would result in the formation
of high-pressure ices, making the planet uninhabitable.
These theoretical results show that the extent to which a
terrestrial-type rocky planet keeps it primordial gas inven-
tory may have a dramatic influence on its suitability for
surface liquid water and thus habitability.
Catastrophic outgassing of H
2
OandCO
2
is another process
that may build up massive early atmospheres around rocky
cores (Elkins-Tanton and Seager, 2008; Elkins-Tanton, 2011,
2012; Lammer, 2013). Because ‘‘super-Earths’’ have deeper
magma oceans than Earth, they are likely to outgas more
massive atmospheres than Earth-mass planets. Up to several
10
4
bar is possible (Elkins-Tanton, 2011). If the early at-
mosphere is removed from a planet, a secondary atmosphere
may subsequently build up from these processes, and the
planet may become habitable, provided the host star’s ac-
tivity has already decreased and the secondary atmosphere is
not depleted significantly. Such a scenario has been sug-
gested for early Earth (Sekiya et al., 1980). The high XUV
fluxes of active young stars can lead to significant heating
and expansion of the upper atmosphere (Tian et al., 2008;
Lammer, 2013) so that the atmospheres are no longer pro-
tected against the stellar wind (Lichtenegger et al., 2010;
Lammer et al., 2011; Lammer, 2013). A combination of
strong planetary magnetic fields (see Section 6.1.4) and
large quantities of a gas such as CO
2
may be necessary to
suppress loss of the atmosphere. If the atmospheric escape
mechanisms during the early history of a planet are too
inefficient and/or the early atmosphere is too massive, such
planets may resemble ‘‘mini-Neptunes’’ rather than terres-
trial planets (Lammer, 2013).
FIG. 5. Examples of astronomical and planetary factors that influence the presence of liquid water on the surface of a
planetary body. The figure shows examples of potential interactions that influence habitable conditions. This diagram does
not include factors influencing the habitability of interior liquid water worlds (examples shown in Table 5).
HABITABILITY: A REVIEW 17
In the case of interior liquid water worlds, mass will influ-
ence the extent of tidal distortion and thus the source of energy
for sustaining liquid water over geological time periods.
6.1.2. Atmospheric and surface characteristics. The at-
mospheric and surface characteristics of a planet will alter
habitability by changing the effective temperature resulting
from the balance between energy received from a star and the
energy lost (Kasting et al., 1993), thus influencing the pos-
sibility of surface liquid water. Two of the most important
factors are the atmospheric composition and the albedo.
Atmospheric composition will determine whether the
concentration of greenhouse gases is sufficient to maintain
liquid water on a planetary surface, thus categorically
modifying whether a planet has any habitable conditions, or
at least determining the extent of liquid water habitats. The
presence of liquid water on the surface of early Earth, for
instance, at a time when the Sun was less luminous (the
‘‘faint young Sun paradox’’) motivates research into the
type of greenhouse gases (e.g.,NH
3
,CH
4
, and CO
2
) that
could have maintained surface liquid water (Sagan and
Mullen, 1972; Pavlov et al., 2001; Haqq-Misra et al., 2008).
The removal of greenhouse gases from an atmosphere, for
example, reduction of CH
4
concentrations caused by in-
creases in atmospheric O
2
on Earth, could have had cata-
strophic effects on habitability by forcing the planet into an
ice covered ‘‘Snowball’’ state (Hoffman et al., 1998).
Atmospheric composition will also be influenced by plan-
etary mass. A large planet may retain its primordial hydrogen,
or it may have a larger reservoir of reducing gases, frustrating
the oxidation of the atmosphere and the possibility that pho-
tosynthesis can generate an oxygen-rich atmosphere required
for multicellular life. Smaller planets could potentially make
the transition to oxygen-rich conditions earlier in their history
(McKay, 1996).
The quantity of water on a planet is thought to influence
the effectiveness of the greenhouse effect. Drier planets
have been shown by modeling to be less prone to a runaway
greenhouse than wetter ones (Abe et al., 2011).
On small geological timescales (millions of years), at-
mospheric composition is one factor that influences surface
heating and thereby the surface temperature of a planet, with
implications for the frequency and severity of ice ages.
Warmer periods may lead to stagnant oceans, with the ra-
pidity of onset and emergence potentially causing extinc-
tions (Huey and Ward, 2005) by changing the distribution of
habitable conditions on a planetary surface for particular
types of organisms over relatively short time periods.
Albedo influences surface temperatures in a number of
ways. Cloud layers reflect radiation and can reduce the tem-
peratures experienced on a planetary surface by, for example,
expanding the habitable zone inward for tidally locked planets
orbiting low-mass stars (Yang et al., 2013). Ice-snow albedo
can be deleterious to habitability by providing a positive
feedback loop in the onset of Snowball conditions, whereby a
growth in ice and snow cover increases the reflection of ra-
diation, causing temperatures to drop, thus enhancing snow
and ice cover (Hoffman et al., 1998). The effects of ice-snow
albedo on conditions for habitability are thought to become
less important in the outer regions of the habitable zone
where, if CO
2
concentrations are high enough, they mask the
climatic effects of the ice-snow albedo (Shields et al., 2013).
The albedo of the surface may be further influenced by
the possible existence of life-forms that cover a large part of
the surface (as trees and other plants do on Earth). A parable
of an inhabited world inducing strong albedo feedbacks was
described as so-called ‘‘Daisyworld’’ in the work of Watson
and Lovelock (1983). In their theoretical model, two species
of daisies of different colors absorb different amounts of
radiation. The growth rate of the daisies depends directly on
the surface temperature.
Daisies with a higher albedo survive and evolve at higher
temperatures (and lead to a decrease of the surface tem-
perature), whereas daisies with a low albedo (e.g., black
daisies) have a higher growth rate at low temperatures (and
lead to an increase of the surface temperature).
This theoretical system therefore self-regulates the global
surface temperature based on albedo feedbacks and may
even develop into a Darwinian system, where species adapt
their internal physiology in response to environmental
changes (e.g., Lenton and Lovelock, 2000).
6.1.3 Plate tectonics. It is not known whether plate
tectonics as a mechanism of geological turnover is required
to ensure that supplies of CHNOPS, other elements, and
redox couples are sustained over geological time periods on,
and within, a planetary body. Geological turnover might be
achieved by magma upwellings (hot spot volcanism) or
tidally driven circulation of water in a subsurface ocean
through a rocky substrate (e.g., in icy moons) to sustain the
circulation of essential elements and chemical disequilibria,
even if only locally somewhere in, or on, a planetary body.
However, there are good reasons to suspect that plate
tectonics is an important factor in sustaining conditions for
surface liquid water over billions of years through its role in
temperature regulation. By subducting rocks over large areas,
plate tectonics provides a return pathway for CO
2
in the at-
mosphere that has been sequestered in carbonate rocks. This
is the carbonate-silicate cycle (Fig. 6), part of the carbon
cycle. The negative feedback inherent in this process [higher
CO
2
in the atmosphere generally, though not always (Tyrell,
2014), leads to warmer conditions that enhance chemical
reaction rates and increase rock weathering, thereby facili-
tating more effective drawdown of CO
2
from the atmosphere]
makes this carbonate-silicate cycle a long-term thermostat
that regulates planetary surface temperatures within a range
suitable for liquid water and biological activity (Walker et al.,
1981; Berner et al., 1983). Both Mars and Venus illustrate the
effects of the lack of this cycle. In the case of Venus, the loss
of liquid water from the greenhouse effect not only prevents
carbonate formation, but any buried carbon has long since
been heated and returned to the planet’s thick CO
2
atmo-
sphere. On Mars, on the other hand, lack of continuous vol-
canic outgassing leads to an atmosphere too thin to allow for
an efficient greenhouse effect and hence conditions suitable
for sustained surface liquid water.
Planetary mass plays a role in determining the effective-
ness of plate tectonics. Earth-mass planets or more massive
planets have sufficient radiogenic heating to maintain condi-
tions for plate tectonics over long time periods. Plate tectonics
has been proposed for ocean-covered exoplanets (Valencia
et al., 2007), with changing plate velocities linked to in-
creasing planetary mass. While some studies (Valencia
et al., 2007; van Heck and Tackley, 2011; Tackley et al.,
18 COCKELL ET AL.
2013) suggest that the greater shear stresses and thinner
plates thought to be associated with planets of higher masses
will favor subduction (the movement of plates into a plan-
etary interior) by decreasing the overall resistance to plate
motion, others maintain that a stagnant lid (an immobile
planetary crust) or episodic tectonic regime may be a more
realistic assumption because of increased internal heating
and decreased lower mantle viscosity (Stein et al., 2011) or
because of a modeled reduction in the ratio of driving to
resistive forces and increased fault strength under high
gravity (O’Neill and Lenardic, 2007; Noack and Breuer,
2014). An upper limit of planetary mass for plate tectonics
to function may be defined by the mass at which high
pressures in the mantle increase rock viscosity and prevent
plate tectonics (Noack and Breuer, 2014). Too low a mass
results in early cooling, a phenomenon seen on Mars. The
small size of the planet means that the crust long ago be-
came solidified into a single stagnant lid (Breuer and Spohn,
2003). There is therefore no process for carbonate rocks to
be subducted and to return CO
2
to the atmosphere in the
carbonate-silicate cycle. Planetary masses on the order of
one to five Earth masses may be optimum for plate tectonics
(Noack and Breuer, 2014). In summary, the presence of
plate tectonics on a planet will therefore depend upon a
range of factors including mass, age, the inventory of water,
the initial internal heat, the distribution of oceans and con-
tinents, all of which cannot be readily predicted for any
given planet in a given star system at the current time.
Although plate tectonics is crucial in driving the carbon
cycle on Earth, other cycles have been proposed for ocean-
covered planets where CO
2
-induced rock weathering is less
effective. The circulation of CO
2
or CH
4
through clathrate
reservoirs has been proposed as alternative carbon cycles
(Levi et al., 2013).
Plate tectonics may play a wider role in sustaining con-
tinuous habitability on the surface of planetary bodies. The
subduction of water-containing rocks on Earth is thought to
contribute to the fluidity of the silicate mantle, which leads to
efficient cooling of the mantle. Thus, the induced temperature
gradient at the core-mantle boundary triggers convection in
the outer core as well as chemical convection due to con-
tinuous freezing of the inner core. Both are essential for the
maintenance of long-term operation of the planetary dy-
namo (Olson and Christensen, 2006) that generates a mag-
netic field and protects the atmosphere from the solar wind,
reducing its loss.
It remains uncertain how much plate tectonics, or the lack
of it, plays a role in the lack of a magnetic field on Mars.
There is evidence for remnant magnetism in the southern
hemisphere of the planet (Connerney et al., 2005; Langlais
FIG. 6. The carbonate-silicate cycle. (A) One principal mechanism by which temperatures on the surface of Earth are
regulated through the feedback control of the greenhouse gas, CO
2
. The cycle also illustrates the link between plate tectonics
(subduction of carbonates) and habitability. (B) The carbonate-silicate cycle works by a negative feedback process.
HABITABILITY: A REVIEW 19
et al., 2010), suggesting an early magnetic field that later
shut down. The lack of plate tectonics on Mars (Nimmo and
Tanaka, 2005) may be one factor that accounts for an in-
sufficient convection to generate a dynamo.
Plate tectonics, by delivering water into the mantle,
lowers the melting point of rocks, contributing to the pro-
duction of silica-rich rocks from which the buoyant conti-
nents are made (Campbell and Taylor, 1983). Thus, the
existence of continental land masses may itself be linked to
plate tectonics and the existence of water.
Despite these observations, the presence of sedimentary
rocks on Mars that are *3.7 billion years old and their
potential habitability (Grotzinger et al., 2014) suggest that
planets can sustain liquid water and thus surface habitable
conditions over billion-year timescales in the absence of
plate tectonics. It is possible to imagine a scenario in which
Mars or a Mars-like planet has hot-spot volcanism and an
active planetary-scale hydrological cycle generating move-
ment in CHNOPS elements, other cations, and anions
through rock weathering and chemical disequilibria, allow-
ing continuous habitable conditions to be sustained in local
environments.
Plate tectonics may play a role in interior liquid water
worlds. For example, plate tectonics on icy moons (Kat-
tenhorn and Prockter, 2014) could provide a pathway for
exogenously delivered surface elements and redox couples
to be mixed into a deep interior ocean.
6.1.4. Magnetic fields. Magnetic fields act to protect at-
mospheres from being sputtered away (Lammer et al., 2008).
Planetary bodies orbiting low-mass stars, such as G and K
stars, may experience intense atmospheric sputtering if there
is no magnetic field (Lammer et al., 2009), which leads to
atmospheric loss. Early K and M stars have extreme UV
radiation emissions 3–4 and 10–100 times higher, respec-
tively, than G stars (Ribas et al., 2005); and for M stars, this
intense radiation can persist for up to a billion years (Lam-
mer et al., 2009). In these cases, a magnetic dynamo may be
indispensable for improving the longevity of a dense plan-
etary atmosphere that can support liquid water at the surface
of the planet. Compounding this problem, in the case of M
stars, planets become tidally locked in the habitable zone.
The loss of planetary rotation can weaken the magnetic
field (Grießmeier et al., 2004). These planets may lose
their magnetic field early in their history, which would
result in inclement surface conditions and limit the dura-
tion of continuous planetary habitability for surface-dwelling
organisms. Models suggest that the size of a planet can also
influence the magnetic field, with larger planets potentially
sustaining more intense magnetic fields (Lammer et al., 2009).
In the case of Mars, despite the shutdown of a dynamo in
its early history, it is apparent that liquid water was sus-
tained on, or just beneath, the planetary surface for a long
time after this event, as seen in evidence for catastrophic
outflows, some of which may be just a few million years old
(Carr, 1986, 1996; Tanaka, 1986; Burr et al., 2002; Neukum
et al., 2010). Indeed, brines may even exist on the surface
today (Renno
´et al., 2009; McEwen et al., 2011). However,
the weak magnetic field on Mars is thought to have partly
contributed to the loss of atmosphere (Luhmann et al., 1992)
and ultimately the low atmospheric pressure that prevents
sustained liquid water on its surface today.
6.2. Astronomical factors
6.2.1. Planetary rotational and orbital characteris-
tics. Planetary rotation plays a role in defining the strength
of the magnetic field (Grießmeier et al., 2004). Planets that
rotate slower, for example tidally locked planets, generally
have smaller magnetic fields and are therefore more prone to
atmospheric loss (Kasting et al., 1993; Barnes et al., 2008).
Although tidal locking might cause atmospheric freeze-out
on the dark side, if atmospheric circulation is sufficient, this
catastrophe can be averted ( Joshi et al., 1997; Joshi, 2003).
The presence of cloud layers can reduce temperatures on the
star-facing side, potentially leading to wider habitable zones
for tidally locked planets and reducing the thermal contrast
between the light and dark side of the planet (Yang et al.,
2013).
Planetary obliquity can influence habitability. There are
end-member conditions of obliquity and obliquity variations
of significance. Terrestrial planets with high obliquities at
the outer regions of the habitable zone with thick CO
2
at-
mospheres have been shown by modeling to undergo partial
atmospheric collapse, which would influence the effective-
ness of the carbonate-silicate cycle, potentially disrupting
habitability (Spiegel et al., 2009) and increasing the extremity
of climatic cycles (Williams and Kasting, 1997). However,
other modeling studies suggest that high-amplitude and high-
frequency obliquity variations can suppress the effects of the
ice-albedo feedback and thus increase the outer limit of the
habitable zone (Armstrong et al., 2014). A lack of obliquity
variation in some cases may be deleterious to habitability.
Planets orbiting close to low-mass stars may experience ‘‘tilt
erosion’’ (Heller et al., 2011) that leads to permanently low
obliquities, which with tidal locking would lead to uniform
climatic conditions on a planet with unknown consequences
for habitability.
Planetary eccentricity can influence habitability (Wil-
liams and Pollard, 2002). Eccentricity will influence the
stellar flux received by a surface liquid water world. Planets
need not be continuously within the habitable zone for liquid
water to exist on the surface. Bodies in eccentric orbits may
sustain liquid water if the average stellar flux during the
orbit is sufficient (Williams and Pollard, 2002; Dressing
et al., 2010), although temperature and climate variations in
the orbit may be extreme. In the case of highly eccentric
orbits (*0.5), high obliquity can stabilize a planetary cli-
mate against it dropping into Snowball (ice-covered) con-
ditions (Dressing et al., 2010).
For planets with large eccentricities orbiting M stars, cir-
cularization of the orbit associated with tidal locking might
move the planet to within the inner edge of the habitable zone
within a billion years (Barnes et al., 2008), rendering a planet
uninhabitable. Thus, eccentricities initially allowing for
habitable conditions coupled with the tendency toward tidal
locking can categorically modify habitability by removing a
planet from the habitable zone entirely.
In the case of icy moons (interior liquid water worlds),
eccentricity will determine the degree of internal tidal
heating and thus the extent of interior liquid water.
6.2.2. Star type. The type of star around which a planet
orbits will determine the distance of the habitable zone and
thus the semimajor axis that the planet requires to host
liquid water on its surface. For M stars, the proximity of the
20 COCKELL ET AL.
habitable zone can lead to tidal locking, with implications
for the strength of the magnetic field and the required effi-
ciency of atmospheric circulation to prevent atmospheric
freeze-out ( Joshi et al., 1997; Scalo et al., 2007; Tarter
et al., 2007).
Stellar type might influence the habitability of a planetary
surface with respect to specific metabolic groups, particu-
larly phototrophs on surface liquid water worlds. The dif-
ferent spectral quality of M stars compared to G stars, for
instance, may require phototrophs that process pigments
with different absorption characteristics to those on Earth to
match the spectrum (Kiang et al., 2007a, 2007b). Alter-
natively, novel forms of photosynthesis may be required to
capture lower-energy photons (Wolstencroft and Raven,
2002), for example, from M stars whose spectral peak is
shifted toward the red end of the spectrum.
The plasma flow from host stars controls the planetary
energy budget, the atmospheric photochemistry, and the
atmospheric mass loss from the outer layers of planetary
atmospheres (Ribas et al., 2005; Claire et al., 2012; France
et al., 2013; Lammer and Khodachenko, 2015). Stellar op-
tical and infrared radiation increases slowly during stellar
evolution. Ultraviolet radiation, including the Lyman-a
emission line (121.6 nm), that dominates the UV spectrum
of M dwarf stars controls photochemical reactions of H
2
O,
CO
2
,CH
4
, and NH
3
molecules (e.g., Lammer, 2013, and
references therein). The XUV and X-ray radiation from host
stars ionizes, dissociates, heats, and expands the upper at-
mospheres, driving atmospheric escape that is high during
the host star’s early period, which can last from tens to
hundreds of millions of years for young solar-like G-type
stars to billions of years for M-type dwarf stars.
The power of the stellar UV and X-ray fluxes depends
on stellar activity, which decays with time and is related to
the host star’s rotation period that also decreases during the
star’s evolution (Lammer and Khodachenko, 2015). Thus, the
evolution of an exoplanet’s atmosphere and its habitability
are strongly related to the evolution of its host star. Planets
within the habitable zones of active flaring stars are exposed
to intense stellar short-wavelength irradiation and extreme
particle and stellar wind conditions over long time periods.
Planets are not required to be in single star systems to be
habitable. Stable, habitable orbits can be sustained in binary
star systems, both in S-type orbits, where the planet orbits
one of the binary stars, and in P-type (circumbinary) orbits,
where the planet orbits both stars (Benest, 1988; Whitmire
et al., 1998; Dvorak et al., 2003; Haghighipour and Kalte-
negger, 2013; Kaltenegger and Haghighipour, 2013).
6.2.3 The presence of a moon. It has been suggested
that the Moon plays a fundamental role in habitability.
Modeling study results suggest that it plays a role in stabi-
lizing the obliquity of Earth (Laskar et al., 1993). However,
more recent model studies question this conclusion and
suggest instead that a moonless Earth, although exhibiting
greater obliquity changes, would still maintain obliquity
variations within a 20–25range (Lissauer et al., 2012).
Regardless, by stabilizing obliquity to some extent, the Moon
might influence variations in terrestrial climate (Waltham,
2004, 2011). Caution should be exercised in interpreting the
consequences of these processes for habitability. One might
argue that varying obliquity would select for more generalist
organisms capable of coping with frequent climatic change
caused by other perturbations. We could just as well speculate
that the stabilization of obliquity leads to the evolution of
extinction-prone specialist organisms.
6.2.4. Impact events. Highly energetic large impacts
have the potential to vaporize a planetary ocean and disrupt
the course of biological evolution. Impacts are likely to be a
universal problem for life, since no solar system–forming
process is known that remains free of leftover debris from
planetary accretion (Gomes et al., 2005). However, the scale
and frequency of impacts will depend upon orbital dynamics
and debris in any given system. Some systems, such as s
Ceti, may have more debris than our own Solar System
(Greaves et al., 2004). Giant planets can cause gravitational
scattering of small bodies, mitigating impacts (Laakso et al.,
2006). It has been suggested that the presence of Jupiter in
our planetary system reduced the perturbing effects of im-
pacts by mopping up comets (Wetherill, 1994; Horner et al.,
2010). However, giant planets may even increase the impact
flux of asteroids and other objects (Horner and Jones, 2008,
2009) and in some general cases provide minimum protec-
tion (Laakso et al., 2006).
Except for extremely large impactors capable of steriliz-
ing an entire planet (Maher and Stevenson, 1988; Sleep
et al., 1989), it seems likely that, for many planets, the
impact frequency may change the types of organisms for
which the planet is habitable (e.g., by creating a selection
pressure for thermophiles and hyperthermophiles; Sleep
et al., 1989) but not alter its long-term continuous habit-
ability for at least some organisms (i.e., for life in general).
Impact events can even improve conditions for life in the
surface and subsurface by enhancing the availability of
habitat space and/or nutrients and energy (Cockell et al.,
2012b). For interior liquid water worlds, impacts may de-
liver essential elements or redox couples or cause geo-
chemical turnover, thus improving conditions for life.
The extent to which impacts will prevent the emergence of
multicellularity or intelligence depends on their frequency or
energy on surface liquid water worlds. This is influenced by
the thickness of the atmosphere, with thicker atmospheres
disrupting impactors more effectively than thinner atmo-
spheres (Schaber et al., 1992). If sufficiently large impacts are
frequent throughout a planetary history and always periodi-
cally boil the oceans over intervals of millions of years, the
lack of sufficient oxygenic photosynthesis on the surface to
generate adequate O
2
may frustrate the formation of a highly
oxidized atmosphere and thus the emergence of multicellular
life and intelligence. Frequent large impacts could also limit
the persistence of surface-dwelling complex animal-like or-
ganisms even if they did originate. The extinction of the
dinosaurs at the Cretaceous-Paleogene boundary is an ex-
ample of how the diversity of multicellular life can be frus-
trated by impacts (Schulte et al., 2010).
6.3. Summary
In summary, the eight examples of factors that influence
continuous planetary habitability show that we still have
much to learn about which factors merely modify the hab-
itability of planetary bodies with respect to given organ-
isms and which factors categorically affect their ability to
HABITABILITY: A REVIEW 21
host any habitable conditions. The interconnectedness be-
tween them (Fig. 5) shows that more advanced atmosphere-
geosphere models of planetary bodies in their astronomical
environments, coupled with empirical observations of exo-
planets, will allow us to better model and constrain how
these factors interact on any given planetary body to influ-
ence continuous planetary habitability.
7. Technological Habitability
Environments at local or planetary scale could be changed
from uninhabitable to habitable by technological intervention
(Fogg, 1995). At the planetary scale, this process is some-
times called terraforming, although to be habitable an envi-
ronment does not strictly have to be made as similar to Earth
as possible. A planet on which such schemes may be viable is
Mars. There are a number of proposed means by which the
surface of the planet could be made habitable. The intro-
duction of chlorofluorocarbons (CFCs) or perfluorocarbons
(PFCs) into the atmosphere could be used to terraform the
planet (McKay et al., 1991). It would require *40 billion
tonnes of CFCs to meet a required warming of *60C. The
release of CO
2
to create an atmosphere of *100 mb could
make the surface suitable for plants. Over *100,000 years,
plants would generate O
2
, ultimately leading to a potentially
human-breathable atmosphere.
Technological habitability can be achieved on the local
scale, for example by covering a small area on Mars or the
Moon with a greenhouse structure, providing the require-
ments for instantaneous habitability for certain organisms
such as crop plants (Boston, 1981).
Technological habitability raises a number of questions
that bring habitability within the social sciences. Is it ethically
acceptable to deliberately modify a planet where the pres-
ence of life is uncertain (McKay and Marinova, 2001)? Is it
ethically acceptable to deliberately inoculate an uninhabited
habitat that might eventually host life from an existing bio-
sphere or eventually host life yet to originate (Cockell, 2011),
particularly if the entire planetary surface is uninhabited?
8. Conclusion
Habitability is a commonly used word. Its usage is usually
vague, yet it lies at the heart of our assessment of the con-
ditions that are required to make Earth a planet suitable for life
over its lifetime, and the potential for life elsewhere. Habit-
ability is an artificial definition. While this type of discussion
cannot be considered a goal in itself, having a clear idea of
what habitability means is at the heart of understanding the
limits of biology on Earth, and our ability to discover other
habitable worlds. In this review, we have provided some
proposed definitions while at the same time providing a re-
view of this subject for the scientist new to astrobiology.
Acknowledgments
This paper was written with support from the Science
and Technology Facilities Council (STFC), Grant No. ST/
M001261/1. H. Lammer acknowledges support from the
Austrian Research Foundation FWF NFN project S11601-
N16 ‘‘Pathways to Habitability: From Disks to Active Stars to
Planets and Life,’’ as well as the related FWF NFN subproj-
ect S11607-N16 ‘‘Particle/Radiative Interactions with Upper
Atmospheres of Planetary Bodies Under Extreme Stellar
Conditions.’’ L. Noack has been funded by the Interuniversity
Attraction Poles Programme initiated by the Belgian Science
Policy Office through the Planet Topers alliance.
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