864 Encyclopedia of the Solar System
and by inference the others as well, came from Mars. The
fourth class of Martian meteorite is represented by the sin-
gle specimen known as ALH84001. Studies of this mete-
orite indicate that it formed on Mars about 4.5 Gyr ago in
warm, reducing conditions. There are even indications that
it contains Martian organic material and appears to have
experienced aqueous alteration after formation. This rock
formed during the time period when Mars is thought to
have had a warm, wet climate capable of supporting life.
It has been suggested that ALH84001 contains evidence
for life on Mars based on four observations. (1) Polycyclic
aromatic hydrocarbons similar to molecules found in inter-
stellar space are present inside ALH84001. (2) Carbonate
globules are found in the meteorite that are enriched
in^12 C over^13 C. The isotopic shift is within the range that
on Earth, indicates organic matter derived from biogenic
activity. (3) Magnetite and iron-sulfide particles are present
that are similar to those produced by microbial activity. (4)
Features are seen that could be fossils of microbial life, ex-
cept that they are much smaller than any bacteria on Earth.
As a result of more than a decade of study, most scientists
currently prefer a nonbiological explanation for all of these
results. Only the magnetite result is generally considered
relevant, although not conclusive, evidence related to life.
ALH84001 does not provide convincing evidence of past
life on Mars when compared to the multiple lines of evi-
dence for life on Earth 3.4 Gyr ago including fossil evi-
dence. However, the ALH84001 results do provide strong
support to the suggestion that conditions suitable for life
were present on Mars early in its history. When compared
to the SNC meteorites, ALH84001 indicates that Mars ex-
perienced a transition from a warm reducing environment
with organic material present to a cold oxidizing environ-
ment in which organic material was unstable.
6.4 The Giant Planets
The “habitable zone” in the inner Solar System provides the
temperature conditions which can support liquid water on a
planetary surface, but the outer Solar System is richer in the
organic material from which life is made. This comparison
is shown in Figure 10, which shows the ratio of carbon
to heavy elements (all elements other than H and He) for
various objects in the Solar System. Earth is in fact depleted
in carbon with respect to the average Solar System value by
a factor of about 10^4. It may be interesting then to consider
life in the organic rich outer Solar System.
The giant planets Jupiter, Saturn, Uranus, and Neptune,
do not have firm surfaces on which water could accumu-
late and form a reservoir for life. Here the only clement
zone would be that region of the clouds in which temper-
atures were in the range suitable for life. Cloud droplets
would provide the only source of liquid water. Such an en-
vironment might provide the key elements needed for life
as well as an energy source in the form of sunlight. [See
Atmospheres of the Giant Planets.]
FIGURE 10 Ratio of carbon atoms to total heavy atoms (heavier
than He) for various Solar System objects illustrating the
depletion of carbon in the inner Solar System. The x-axis is not a
true distance scale but the objects are ordered by increasing
distance from the sun. Mars is not shown since the size of its
carbon reservoir is unknown.There have been speculations that life, including ad-
vanced multicellular creatures, could exist in such an en-
vironment. However, such speculations are not supported
by considerations of the biological state of clouds on Earth.
There are no organisms that have adapted themselves to
live exclusively in clouds on Earth even in locations where
clouds are virtually always present. This niche remains un-
filled on Earth and by analogy is probably unfilled elsewhere
in the Solar System.
Following this line of thought leads us to search for envi-
ronments suitable for life on planetary bodies with surfaces.
In the outer Solar System, this focuses us on the moons of
the giant planets. Of particular interest are Europa, Titan,
and Enceladus.6.4.1 EUROPA
Europa, one of the moons of Jupiter, appears to be an air-
less ice-shrouded world. However, theoretical calculations
suggest that under the ice surface of Europa there may be
a layer of liquid water sustained by tidal heating as Europa
orbits Jupiter. TheGalileospacecraft imaging showed fea-
tures in the ice consistent with a subsurface ocean and the
magnetometer indicated the presence of a global layer of
slightly salty liquid water. The surface of Europa is criss-
crossed by streaks that are slightly darker than the rest of
the icy surface. If there is an ocean beneath a relatively thin
ice layer, these streaks may represent cracks where the wa-
ter has come to the surface. [SeePlanetary Satellites.]
There are many ecosystems on Earth that thrive and
grow in water that is continuously covered by ice; these are
found in both the Arctic and Antarctic regions. In addition