Extraterrestrial life
The Javian systell1: the
I
Jupiter's moon Europa may carry enough water to harbour life but, argues Julian Hiscox, this does not mean that life began t
hether life is present on other
W
worlds is a question that has
occupied the minds of astronomers
and philosophers since the time of
ancient Greece. Although planets
have traditionally been considered
as abodes for life, for some years
now a growing number of scientists
have speculated that a moon of
Jupiter, Europa, might contain a
liquid water interior, and a few have
gone so far as to suggest that
where there is water there may also
be life. Whilst conditions inside
Europa might be conducive to life
based on terrestrial paradigms, this
does not mean that conditions were
suitable for an origin-of-life event.
The structure of Europa is reviewed
with relevance to its implications for
exobiology in both the context of
this solar system and others.
A
Planet or moon must satisfy a number
of conditions in order to support some
sort of life based on terrestrial paradigms. It must have liquid water and other
compounds (the so-called CHNOPS elements carbon, hydrogen, nitrogen, oxygen, phosphorous and sulphur) (McKay 1991). A planet
needs a stable climate that is, at the very minimum, conducive to the continued presence of
liquid water over geologically significant periods of time. Therefore such planets must lie
within a zone thermally compatible with life where the average global surface temperature
lies within a range from a little below 273 K up
to some value below local boiling point and
that at which a runaway greenhouse effect
occurs (i.e. what has occurred on Venus). If the
orbit of such a planet satisfies these criteria then
it is said to be within the habitable zone of its
primary star (Doyle 1996). The habitable zone
depends primarily on the luminosity of the star.
The more luminous the star, the greater the
radius of the habitable zone. In the solar system
the Earth has remained continuously habitable
2.22 A9G
for at least 3.8 billion years (Kasting 1989).
Other than the Earth, the next most likely
candidate for an origin-of-life event is thought
to be Mars. Although the surface of present-day
Mars is inimical to terrestrial life (Banin and
Mancinelli 1995, Mancinelli and Banin 1995),
photographic and mineralogical evidence suggests that liquid water was once stable on the
surface of Mars (Baker 1982; Carr 1996). This
has led to the suggestion that the environment
of ancient Mars may have been suitable for the
origin and evolution of life (McKay and Stoker
1989). Beyond the orbit of Mars it is difficult to
speculate where life might be or may have been
in residence, as planet and moon surface temperatures are too low for liquid water to exist.
In this case exobiologists need to look for
places where liquid water might be stable for a
geologically significant period of time, without
relying on the Sun's energy for warmth.
Perhaps there is one last refuge in the solar
system that can be considered: the Jovian system. Whilst Jupiter itself is an unlikel y abode
for life, (though some niches may be present
[Sagan and Salpeter 1976]), the Galilean satellites - particularly Europa - provide exciting
candidates for solar system bodies in which liquid water may have been stable for geologically significant periods of time.
A picture of Europa
The Voyager 1 and Voyager 2 space probes provided the first high-resolution reconnaissance
of the outer planets. They returned a wealth of
data and, for an exobiologist, provided tantalizing hints of where else in the solar system
conditions may be suitable for life. The
Galilean satellites of Jupiter, 10, Europa,
Ganymede and Callisto are bodies of substantial size. The smallest, Europa, is only slightly
smaller than our own Moon, and the largest is
Ganymede with a diameter slightly larger than
the planet Mercury. It is Europa that commands our attention. Europa was the most distant Galilean satellite encounter made by Voyager 1; the closest it got was to within
732270 km. Voyager 2 approached to within
204 000 km. Europa (radius 1565 km) was
April 1999 Vol 40
Extraterrestrial life
last outpost for life?
there as it did on Earth, nor that any life will exist.
shown to have a density of 3 g/cm 3, which suggests a primarily silicate bulk composition with
a small mixture of lighter materials (Squyres et
ai. 1983). The observed high albedo and
infrared spectra indicated that much of Europa
is covered with water ice or frost.
Two types of terrain were identified from
images taken by Voyager 2. First, a slightly
dark mottled terrain and, secondly, uniformly
bright terrain crossed by numerous dark linear
markings and narrow bright ridges (Smith et
ai. 1979). The ridges have very regular widths
of -10 km, and might be a few hundred metres
high. The linear markings are composed of
mainly straight, but sometimes circular bands,
that range in width between several and
70 km. In some areas the pattern formed by
intersecting bands is clearly rectangular. Only
three impact craters were identified, all
approximately 20 km in diameter. The appearance of one of the craters suggests that
Europa's surface features have been modified
by surface erosion process or internal isostatic
adjustments. A simple model for the interior of
April 1999 Vol 40
Europa was proposed, consisting of predominantly silicate body with a thin layer of water
or ice (Smith et al. 1979 ).
Analysis of the images taken by the Calileo
Orbiter has identified many different surface
features including cracks, troughs and impact
craters (Belton et ai. 1996). These tectonic features may be subdivided into sets that apparently have different structural origins. Some
are global in extent, whereas others are
restricted to local areas. Ridges are clearly visible on Europa and are 5 to 10 km wide and
rise at most only a few hundred metres above
the surrounding surface. Confirming the
images taken by Voyager, the Calileo Orbiter
showed that Europa's surface is almost devoid
of impact craters and so cannot be a primitive
surface - unlike areas of Mars, for example.
The apparent youth of Europa's surface also
suggests that a small energy input has slowed its
cooling substantially and allowed thermally driven geologic processes to continue to the present or nearly so. To explain this data Europa's
internal structure has been proposed to be com-
posed of a water-rich lithosphere (or cryosphere), possibly 100 to 200 km thick (Anderson
et al. 1997; Carr et al. 1998; Pappalardo et al.
1998 ). It is possible that a heating mechanism
caused by orbiting Jupiter (which has a strong
gravitational attraction), so-called tidal heating,
has allowed repeated eruptions of watery lava
to obscure its early history. The inference was
made that liquid water may have been stable
over geologically significantly periods of time.
Heat production from radioactive elements
may have supported a liquid water interior
beneath an outer ice layer (Reynolds et ai.
1987). However, tidal heating was shown to be
insufficient to melt a completely frozen ocean,
but may have prevented original freezing of a
water mantle, provided the tidal resonance was
established early enough (Squyres et al. 1983).
The putative mass of water on Europa
exceeds that in Earth's oceans and ice caps. The
ice cover thickness also seems to vary, and not
just systematically from equator to poles.
There is also evidence for radially contained
brine flows perhaps 10 km below the surface,
and that these being electrically conductive
may provide the observed magnetic field. If so,
then the brown spots and mottled terrain ma y
represent the peaks of silicate massifs nearly
protruding through the icy shell. Excavation of
these materials by impact or magmatic activity
may have created the dark coloration of some
areas of Europa.
The most detailed Calileo Orbiter images provided three key pieces of evidence showing that
Europa may be slushy just beneath the icy crust,
and possibly even warmer at greater depths.
The evidence included a shallow impact crater,
chunky textured surfaces like icebergs, and gaps
where new icy crust seemed to have formed
between continent-sized plates of ice. Some of
the images focused on the shallow centre of the
impact crater known as Pwyll. Impact rays and
debris scattered over a large part of the moon
showed that a meteorite hit Europa relatively
recently, about 10-100 million years ago. The
Calileo research team at Brown University suggested that the crater's shallow basin and high
set of mountain peaks may mean that subsurface ice was warm enough to collapse and fill in
the deep hole (Moore et al. 1998 ).
Europa - an abode for life?
Unlike the Earth and Mars, Europa can in no
way be considered to lie in the habitable zone
of the Sun. However, if Europa contained a liqA6)G
2.23
Extraterrestrial life
uid water ocean for a geologically significant
period of time through radioactive and tidal
heating, then the question might be how could
life not have existed? Reynolds et al. (1983)
first suggested that there could be regions of
Europa where conditions lie within the range of
adaptation of Antarctic terrestrial organisms.
Field exploration in the Antarctic dry deserts the coldest, driest, places on Earth - has shown
that perennially ice-covered lakes, which might
provide a model system for Europa, contained
microorganisms. However, as Reynolds et al.
(1983) noted, this does not imply that all conditions were suitable for an origin-of-life event.
First, the CHNOPS elements must have been
present. Second, there must be a source of liquid water - which at least appears to have been
satisfied. The precise mineral composition of
Europa is unknown, although there are strong
indications that at least silicates and water (as
ice) are present (Smith et al. 1979; Squyres et al.
1983), as well as hydrated salt minerals such as
sodium carbonates and magnesium sulphates
(McCord et al. 1998).
es which use ribonucleic acid (RNA ). Molecular phylogenetics has indicated that modernday organisms are the descendants of a single
protocell (Schopf 1989). However, the steps
from the origin of life are unknown. The oldest known fossils are approximately 3.5 billion
years old (Schopf 1993) and resemble modernday stromatolites. Analysis indicates that the
cells giving rise to the microfossils were complex, in that they possessed cell walls and were
probably capable of replication by cell division. Prior to 3.5 billion years ago all direct evidence for cellular life has been obliterated
through geological processes, although isotopic evidence hints at biogenic activity at least
3.8 billion years ago (Mojzsis et al. 1996 ).
The first stage for an origin-of-life event may
have been the synthesis of pre-biological precursor molecules, followed by the synthesis of
(Oparin 1938) (see also Fox 1965, Miller et al.
1997). The theory postulates that the seeds of
life arose in the space and the atmosphere in the
form of various combinations of the CHNOPS
elements, under the influence of electrical discharges, radiation and other sources of energy.
This material accumulated in the seas until "the
primitive oceans reached the consistency of hot
dilute soup" to quote Haldane (Wells et al.
1934). From this complex organic sludge, selfreplicating systems emerged.
Laboratory simulations by Baly of Liverpool
University (reported in Wells et al. 1934), which
underpinned Haldane's hypothesis, indicated
that under the influence of light, small quantities of sugars and other substances (some containing nitrogen) were generated from water,
carbon dioxide and ammonia. More sophisticated experiments demonstrated that certain
The possible origin of life on Europa
How life may have originated on Europa, if at
all, is unknown. Two potential pathways are
that life originated independently on Europa, or
that life was transported to Europa from elsewhere. This latter process is known as panspermia (Arrhenius 1908), and has been postulated
to be either a random process (for example
Melosh 1988) or directed by some intelligence
(Crick and Orgel 1973). Theoretical modelling
suggested that terrestrial microorganisms could
have been transported to Europa from the
Earth (Wallis and Wickramasinghe 1995). If
terrestrial-type life was identified on Europa,
then molecular genetic analysis might indicate
whether this life was derived from an independent origin-of-life event or transported from the
Earth. Certain proteins and the genes encoding
them are highly conserved throughout all kingdoms (Woese 1987). If organisms were found
on Europa then phylogenetic analysis would
probably indicate when such microorganisms
diverged from the last common terrestrial
ancestor - if such life was transported from
Earth. Otherwise, one could conclude that such
life was non-terrestrial in origin and had probably originated on Europa.
Theories of the origin of life on Earth
and their application to Europa
Modern theories of the origin of life on Earth
in the most part reject the notion of panspermia and seek to explain how life could have
arisen and evolved into what we see around us
today. All modern-day life utilizes essentially
the same genetic code to store heritable information - deoxyribonucleic acid (DNA) (Watson and Crick 1953) - apart from some virus-
2.24
A~
3: The Galileo Orbiter returned high-resolution images of Europa, such as these ridges. This image was
produced by DlR and is a three dimensional view of double ridges. The red lines indicate that the crests of
the ridge system reach elevations of greater than 300 m above the surrounding plains (blue and purple
tones). The two ridges are separated by a valley about 1.5 km wide. (Image courtesy NASAlJPL.)
self-replicating macromolecules from these
precursor molecules. The self-replicating systems would have been subject to Darwinian
selection (Eigen and Schuster 1979). At some
point these self-replicating systems were
enclosed in a membrane, although at what
stage genetics interacted with membranes is
unknown (Dyson 1985). Therefore whether an
origin-of-life event occurred on Europa boils
down to whether physical/chemical conditions
were suitable for the synthesis of the building
blocks of life, and the formation of macromolecular structures capable of selfreplication. A number of theories have gained
prominence to explain the formation of these
molecules on the Earth and these will be discussed with relevance to Europa.
Primordial (organic) soup model
The "primordial soup" hypothesis was
advanced independently by J B S Haldane and
the Russian academic Alexander Oparin
amino acids (components of proteins) and
nucleotides (components of DNA and RNA),
amongst other compounds, can be formed in
such environments (Miller 1953; Robertson and
Miller 1995). Clearly the synthesis of prebiological material in this model depends on the composition of Earth's early atmosphere and this
has been a matter of great controversy. Miller
and Urey (1953) assumed the early atmosphere
was reducing, i.e. hydrogen-rich and composed
predominately of methane, ammonia and carbon dioxide. However, methane and ammonia
may have been unstable in Earth's early atmosphere (Kasting 1982, Kasting 1993, Levine
1985, Levine et al. 1982).
Certainly in the case of Europa, there was
unlikely to have been a stable body of surface
liquid water or an atmosphere for a geologically and biologically significant period of
time. In addition, energy sources available on
the early Earth, such as ultraviolet radiation or
lightning (Chyba and Sagan 1991), would not
April 1999 Vol 40
Extraterrestrial life
be available to an internal ocean, as postulated
for Europa. Thus the primordial soup hypothesis is probably not applicable to Europa.
Hydrothermal settings
Life on Earth may have arisen without relying
upon a surface water/atmosphere interface. An
alternative energy source to surface-based systems is provided in the form of geothermal
energy at hydrothermal vents and/or hot
springs. Hydrothermal vents can be found at
the bottom of the ocean, where magma (liquid
rock) rising through the Earth's crust reacts
with sea water. These geological features can
be seen as contenders for sites for the origin of
life because conditions for redox reactions are
generated (Bock and Goode 1996; Macleod et
al. 1994). Life could have emerged via iron sulphide membranes produced at such hydrother-
Bada 1988, Miller and Bada 1991, Miller and
Lazcano 1995), although laboratory simulations
have synthesized amino acids under hydrothermal conditions (Hennet et at. 1992). The advantage of a hydrothermal vent for the synthesis of
the building blocks of life, in the absence of sunlight, for Europa is obvious. However, there is
no evidence suggesting either a primordial soup
or hydrothermal vent scenario. More than one
pathway for the origin of life may have been
used. Reynolds et at. (1983) speculated that
hydrothermal activity may have existed (or continue to function) on Europa, but that data and
modelling techniques were inadequate to assess
the probability of such an activity.
Exogenous delivery and impact
synthesis of organic material
An alternative source of pre biotic reactants to
4: Galileo
Orbiter image
of disrupted
ice crust in
the Conamara
region which
provided
further
evidence that
Europa has a
liquid water
interior.
(Image
courtesy
NASAlJPL.)
mal vents (Russell et al. 1993, Russell et al.
1994, Russell and Hall 1997). These membranes would provide a number of features
that are immediately beneficial to self-replicating systems, including catalytic sites and the
fact that the hydrothermal solution would contain all the components necessary for synthesizing the building blocks of life (Shock 1995,
Shock 1996).
A similar theory has been advanced in that
the reducing power of FeSIH 2S relative to oxidized carbon can act as an energy source.
Organic intermediates with anionic groups
become bonded to the cationic surface of pyrite
(Wachtershauser 1988; Wachtershauser 1988).
The first organized unit of life is proposed to be
a composite structure of a sphere of organic ligands around a growing cluster of pyrite. This
theory has been challenged on thermodynamic
and kinetic grounds (de Duve and Miller 1991 ),
although many of the criticisms have been
addressed (Wachtershauser 1994). One of the
key differences between the model of Wachtershauser (1988) and Russell et at. (1993) is that
in the latter theory, although a central role for
FeS is envisaged, instead the reducing power is
provided by hydrothermal hydrogen.
The idea of hydrothermal vents as origin-oflife centres has been challenged (Miller and
April 1999 Vol 40
the primordial Earth and also to Europa are
delivery by impact of asteroids or comets containing organic material (Owen and Bar-Nun
1995) or synthesis during the impact event
(Chyba and Sagan 1992, McKay and Borucki
1997). Complex organic material has been
shown to be present in samples of meteorites
found on the Earth (Wright et at. 1989) and
comets (McDonald et at. 1996). Alternatively,
material could have been injected into Europa's
surface. For example, sulphur dioxide has been
identified on Europa and may be formed when
sulphur ions in the Jovian magnetosphere are
injected into Europa's water ice surface (Lane et
at. 1981). As with Callisto and Ganymede
(McCord et at. 1997), organic material such as
tholins may have accumulated on the surface of
Europa. Tholins are a class of organic material,
containing amino acids, inferred to be present in
the atmosphere ofTitan and comets (McDonald
et al. 1996, Sagan et at. 1992). Certain terrestrial bacteria can use this class of organic material
as a sole nutrient source (Stoker et al. 1990).
However, the amount of tholins that may have
been deposited on Europa is unknown.
Clay-based life
Clearly the precursors for information macromolecules may have been synthesized by a vari-
ety of mechanisms. One way in which these
molecules may have joined together to form selfreplicating macromolecules is by random
chance under suitably energetic conditions. The
alternative mechanism is that a template catalysed the polymerization. One such catalyst or
template might have been clay (Cairns-Smith
1982) and/or minerals (Arrhenius 1984). Also,
the emergence of life may have depended on abiotically produced protonated and condensable
phosphates which may have acted as primordial
enzymes (Arrhenius et al. 1993). Cairns-Smith
(1982) argued that the simplicity and abundance
of mineral structures makes such a relationship
likely. Clay crystals are proposed to direct the
synthesis of protein molecules adsorbed to their
surfaces. The clay-based protein complexes are
then assumed to have become encapsulated in
cell membranes. The clay thus resembled a
genetic information template. Cairns-Smith
(1982) suggested that the clay mineral was gradually replaced by a nucleic acid template and
this conferred an evolutionary advantage over
clay-based life. Laboratory experiments have
shown that c1ay- catalysed glycine and diglycine
oligomerizations are possible (Bujdak and Rode
1996). No evidence for clay materials has yet
been found for Europa.
Are conditions for terrestrial life
limited to Europa?
In addition to Europa, two other Galilean
satellites may have liquid water interiors:
Ganymede and Callisto. The surface of
Ganymede includes both ancient, heavily
cratered terrain and younger, highly fractured
regions, which, like Europa, suggest internal
activity and tectonic process - possibly due to
liquid water. On the other hand, the surface of
Callisto consists entirely of geologically old,
cratered terrain, inactive since its formation
approximately 4.6 billion years ago (Greeley
and Batson 1997). Ganymede is closer to
Jupiter than Callisto, and probably received a
greater proportion of radioactive elements during its early formation as well being subjected
to greater tidal stresses. Therefore Ganymede
may provide a more suitable abode for some
form of organic life than Callisto.
Titan, the largest moon of Saturn, contains a
substantial atmosphere (1500 millibars), that
may resemble that of the Earth's earliest atmosphere before life (Owen 1983). Indeed, experimental evidence suggests that certain terrestrial bacteria can metabolize components that
may be present in Titan's atmosphere (Stoker
et al. 1990). However, the average surface temperature ofTitan is approximately 95 K, which
would preclude the growth of any life based on
terrestrial paradigms.
Recently several planets have been inferred to
orbit other stars (Goldsmith 1997, Halpern
1997). Whilst some extrasolar planets themAetG 2.25
Extraterrestrial life
5: Ray Renolds and his colleagues in 1983 were among the first to suggest that Europa could support life.
Their analysis was based upon life which existed under the surface of frozen lakes in Antarctica.
(Photograph courtesy of Chris McKay, NASA Ames.)
selves don't appear to be habitable, Williams et
al. (1997) proposed that if rocky moons orbited some of these planets then such companions
could be habitable if the planet-moon system
orbits the sun in the habitable zone. The
inferred planetary companions to the stars
16 Cygni Band 47 Ursae Majoris could satisfy
this criterion (Williams et al. 1997). The conditions are that each moon would have to be at
least 0.12 Earth masses to retain a substantial
and long-li ved atmosphere.
Based upon the model for life on Europa, it
would now seem that satellites other than
those in the habitable zone could harbour life provided a mechanism, such as tidal heating,
2.26
A~
existed to keep a liquid water interior stable
for geologically significant periods of time.•
Julian A Hiscox, Division of Molecular Biology,
IAH Comptol!, Nr Newbury, Berkshire RG20
7NN, UK. E-mail: [email protected] ...
Address from 1 M~y 1999: School of Animal and
Microbial Sciences, University of Reading,
Whiteknights, Reading, UK.
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