858 Encyclopedia of the Solar System
TABLE 7 Limits to LifeParameter Limit NoteLower Temperature ∼− 15 ◦C Liquid water
Upper Temperature 113 ◦C Thermal denaturing of proteins
Low Light ∼ 10 −^4 S Algae under ice and deep sea
pH 1–11
Salinity Saturated NaCl Depends on the salt
Water Activity 0.6 Yeasts and molds
0.8 Bacteria
Radiation 1–2 Mrad May be higher for dry or frozen statebeen able to reach on this planet with respect to environ-
mental conditions. Life does not exist everywhere on Earth.
There are environments on Earth in which life has not been
able to effectively colonize even though these environments
could be suitable for life. Perhaps the largest life-free zone
on Earth is in the polar ice sheets, where there is abundant
energy, carbon, and nutrients (from atmospheric deposi-
tion) to support life. However, water is available only in
the solid form. No organism on Earth has adapted to using
metabolic energy to liberate water from ice, even though
the energy required per molecule is only∼1% of the energy
produced by photosynthesis per molecule. Table 7 lists the
limits to life as we currently know them. The lower tem-
perature limit clearly ties to the presence of liquid water,
while the higher temperature limit seems to be determined
by the stability of proteins, also in liquid water. Life can
survive at extremely low light levels corresponding to 100
AU, roughly three times the distance between Pluto and
the Sun. Salinity and pH also allow for a wide range. Water
activity, effectively a measure of the relative humidity of a
solution or vapor, can support life only for values above 0.6
for yeasts, lichens and molds. Bacteria require levels above
0.8. Radiation resistant organisms such asDeinococcus ra-
dioduranscan easily survive radiation doses of 1–2 Mrad
and higher when in a dehydrated or frozen state.
6. Life in the Solar System
Because our knowledge of life is restricted to the unique
but varied case found here on Earth, the most practical ap-
proach to the search for life on the other planets has been to
proceed by way of analogy with life on Earth. The argument
for the origin of life on another world is then based on the
similarity of other planetary environments with the postu-
lated environments on early Earth. Whatever process led
to the establishment of life in one of these environments
on Earth could then be logically expected to have led to
the origin of life on this comparable world. The more ex-
act the comparison between the early Earth and another
planet, the more compelling is the argument by analogy.
This comparative process should be valid for all the theo-
ries for the origin of life, ranging from panspermia to the
standard theory, listed in Figure 6.
Following this line of reasoning further, we can conclude
that if similar environments existed on two worlds and life
arose in both of them then these life forms should be com-
parable in their broad ecological characteristics. If sunlight
was the available energy source, CO 2 the available carbon
source, and liquid water the solvent, then one could expect
phototrophic autotrophs using sunlight to fix carbon diox-
ide with water as the medium for chemical reactions. Our
knowledge of the Solar System suggests that such an en-
vironment could have existed on Mars early in its history
as well as on Earth early in its history. While life forms in-
dependently originating on these two planets would have
different biochemical details, they would be recognizably
similar in many fundamental attributes. This approach by
analogy to Earth life and the early Earth provides a specific
search strategy for life elsewhere in the Solar System. The
key element of that strategy is the search for liquid water
habitats.
Spacecraft have now visited or flown past comets, as-
teroids, and most of the large worlds in the Solar System
except Pluto; however, a spacecraft is en route to Pluto at
the time of this writing. Observatory missions have stud-
ied all of the major objects in the Solar System as well. We
can do a preliminary assessment of the occurrence of liquid
water habitats, and indirectly life, in the Solar System.6.1 Mercury and the Moon
Mercury and the Moon appear to have few prospects for
liquid water, now or anytime in the past. These virtually
airless worlds have negligible amounts of the volatiles (such
as water and carbon dioxide) essential for life. There are no
geomorphological features that indicate fluid flow. There is
speculation that permanently shaded regions of the polar
areas on Mercury can act as traps for water ice. Recent radar
data support this hypothesis. However, there is no indication
that the pressure and temperature were ever high enough
for liquid water to exist at the surface. [SeeMercury.]