Astrobiology 865
to the polar oceans where sea ice diatoms perform pho-
tosynthesis under the ice cover, there are perennially ice-
covered lakes in the Antarctic continent in which microbial
mats based on photosynthesis are found in the water be-
neath a 4-m ice cover. The light penetrating these thick
ice covers is minimal, about 1% of the incident light. Using
these Earth-based systems as a guide, it is possible that sun-
light penetrating through the cracks (the observed streaks)
in the ice of Europa could support a transient photosyn-
thetic community. Alternatively, if there are hydrothermal
sites on the bottom of the Europan ocean, it may be possible
that chemosynthetic life could survive there—by analogy to
life at hydrothermal vent sites at the bottom of the Earth’s
oceans. The biochemistry of hydrothermal sites on Earth
does depend on O 2 produced at the Earth’s surface. On
Europa, a chemical scheme like that suggested for subsur-
face life on Mars would be appropriate (H 2 +CO 2 ).
The main problem with life on Europa is the question
of its origin. Lacking a complete theory for the origin of
life, and lacking any laboratory synthesis of life, we must
base our understanding of the origin of life on other plan-
ets on analogy with the Earth. It has been suggested that
hydrothermal vents may have been the site for the origin of
life on Earth and if this is the case improves the prospects
for life in a putative ocean on Europa. However, the early
Earth contained many environments other than hydrother-
mal vents, such as surface hot springs, volcanoes, lake and
ocean shores, tidal pools, and salt flats. If any of these en-
vironments were the locale for the origin of the first life
on Earth, the case for an origin on Europa is weakened
considerably.
6.4.2 TITAN
Titan, the largest moon of the planet Saturn, has a substan-
tial atmosphere composed primarily of N 2 and CH 4 with
many other organic molecules present. The temperature
at the surface is close to 94◦K and the surface pressure is
1.5 times the pressure of Earth at sea level. The surface does
not appear to have expansive oceans as once suggested but
numerous small lakes have been discovered in the north
polar region. However, the ground beneath the Huygens
Probe was wet with liquid CH 4 , which was heated by the
problem and formed vapor. [SeeTitan.]
The spacecraft data from theVoyagerand Cassini/
Huygensmissions, as well as ground-based studies, in-
dicate that there is an optically thick haze in the upper
atmosphere. The haze is composed of organic material,
and the atmosphere contains many organic molecules
heavier than CH 4. Photochemical models suggest that
these organics are produced from CH 4 and N 2 through
chemical reactions driven by solar photons and by magne-
tospheric electrons. The observed organic species and even
heavier organic molecules are predicted to result from
these chemical transformations. Laboratory simulations of
organic reactions in Titan-like gas mixtures produce solid
refractory organic substances (tholin) and similar processes
are expected to occur in Titan’s atmosphere.
Conditions on Titan are much too cold for liquid water
to exist, although the pressure is in an acceptable regime.
For this reason, it is unlikely that Earth-like life could orig-
inate or survive there. The organic material in Titan’s atmo-
sphere provides a potential source of energy and the liquid
methane on the surface provides a possible liquid medium
for life. Life in liquid methane could use active transport
and large size to overcome the low solubility of organics
in liquid methane and enzymes to catalyze reactions at the
low temperatures. If carbon-based life in liquid methane ex-
isted on Titan, it could be widespread. With or without life,
Titan remains interesting because it is a naturally occurring
Miller-Urey experiment in which simple compounds are
transformed into more complex organics. A detailed study
of this process may yield valuable insight into how such a
mechanism might have operated on the early Earth.
There is also some speculation that under unusual con-
ditions Titan may have liquid water on or near the sur-
face. This could have occurred early in its formation when
the gravitational energy released by the formation of Titan
would have heated it to high temperatures. More recently,
impacts could conceivably melt local regions generating
warm subsurface temperatures that could last for thousands
of years. Whether such brief episodes of liquid water could
have led to water-based life remains to be tested.6.4.3 ENCELADUS
The Cassini mission has recently documented geysers
erupting from the south polar region of Enceladus. [See
Planetary Satellites.] Associated with this outflow of
water, CH 4 is present but no NH 3. The source of the wa-
ter is considered to be a subsurface liquid water reser-
voir heated and pressurized by subsurface heat flow. Such
a subsurface habitat could support the sort of anaerobic
chemoautotrophic life that has been found on Earth. These
systems are based on methanogens that consume H 2 pro-
duced by geochemical reactions or by radioactive decay.
The age or lifetime of any subsurface liquid water on Ence-
ladus is not known, which adds uncertainly to speculations
about the origin of life. The theories for the origin of life on
Earth, shown in Figure 6, that would apply to Enceladus are
panspermia and a chemosynthetic origin of life. The same
that would apply to Europa.
If there is subsurface life in the liquid water reservoirs
on Enceladus, then the geysers would be carrying these
organisms out into space. Here they would quickly become
dormant in the cold vacuum of space and would then be
killed by solar ultraviolet radiation. But these dead, frozen
microbes would still retain the biochemical and genetic
molecules of the living forms. Thus aStardust-like mission
moving through the plume of Enceladus’ geysers might col-
lect lifeforms for return to Earth, which might provide the
easiest way to get a sample of a second genesis of life.