It is not possible to present completely convincing evidence for the
existence of extraterrestrial life. The problem often reduces to
probabilities and to estimates of observational reliability. In almost
all cases the evidence is optimistically considered strongly suggestive
of—or, at the worst, not inconsistent with—the existence of
extraterrestrial life. Alternatively, there is a pessimistic view that
the evidence advanced for extraterrestrial life is unconvincing,
irrelevant, or has another, nonbiological explanation.
In studies of the laboratory synthesis of life-related compounds and its
significance concerning the origin of life, several results seem to
suggest that organochemical synthesis is a general process, occurring
perhaps on all planets which retain a reducing atmosphere. The
temperature ranges must be such that precursors and reaction products
are not thermally dissociated. The reaction rates for the synthesis of
more complex organic molecules diminish to a negligible value when the
temperature range is below 100° C.
Besides the planetary parameter of temperature, an even more fundamental
necessity for a living state exists—a liquid solvent system. For
terrestrial life forms, water serves this purpose. Water has this and
other properties of biological significance because of hydrogen bonding
between adjacent molecules in the liquid state.
Ultraviolet radiation could serve as an extraterrestrial energy source
for organic synthesis. Research shows that, while an atmosphere is
important, living systems can survive a wide range of ambient pressures
and are little affected by a wide range of magnetic field strengths.
Oxygen is not a prerequisite for all living systems. While it is
sometimes concluded that free oxygen is needed for all but the simplest
organisms, less efficient metabolic processes coupled with higher food
collection efficiency—or a more sluggish metabolism—would seem to do
just as well. Earth is the only planet in the solar system on which
molecular oxygen is known to be present in large amounts. Since plant
photosynthesis is the primary source of atmospheric oxygen, it seems
safe to infer that no other planet has large-scale plant photosynthesis
accompanied by the production of oxygen.
The possibility of the existence of extraterrestrial life raises the
important question of man’s being able to detect it. Research on
extraterrestrial life detection is predicated on the ability to develop
ways to detect it even when the living systems are based on principles
entirely different from those on Earth.
The substitution of various molecules for those of known biological
significance to living organisms as we know them has been investigated;
the substitution of NH₂ for OH in ammonia-rich environments leads to a
diverse, and biologically very promising, chemistry. The hypothesis that
silicon may replace carbon does not support the construction of
extraterrestrial genetics based on silicon compounds. (Silicon compounds
participate in redistribution reactions which tend to maximize the
randomness of silicon bonding, and the stable retention of genetic
information over long time periods is thus very improbable.)
Evidence relevant to life on Mars has been summarized by Sagan (ch. 1 of
[ref.10]):
_The Origin of Life_
In the past decade, considerable advances have been made in our
knowledge of the probable processes leading to the origin of
life on Earth. A succession of laboratory experiments has shown
that essentially all the organic building blocks of contemporary
terrestrial organisms can be synthesized by supplying energy to
a mixture of the hydrogen-rich gases of the primitive
terrestrial atmosphere. It now seems likely that the laboratory
synthesis of a self-replicating molecular system is only a short
time away from realization. The syntheses of similar systems in
the primitive terrestrial oceans must have occurred—collections
of molecules which were so constructed that, by the laws of
physics and chemistry, they forced the production of identical
copies of themselves out of the building blocks in the
surrounding medium. Such a system satisfies many of the criteria
for Darwinian natural selection, and the long evolutionary path
from molecule to advanced organism can then be understood. Since
nothing except very general primitive atmospheric conditions and
energy sources are required for such syntheses, it is possible
that similar events occurred in the early history of Mars and
that life may have come into being on that planet several
billions of years ago. Its subsequent evolution, in response to
the changing Martian environment, would have produced organisms
quite different from those which now inhabit Earth.
_Simulation Experiments_
Experiments have been performed in which terrestrial
micro-organisms have been introduced into simulated Martian
environments, with atmospheres composed of nitrogen and carbon
dioxide, no oxygen, very little water, a daily temperature
variation from +20° to -60° C, and high ultraviolet fluxes. It
was found that in every sample of terrestrial soil used there
were a few varieties of micro-organisms which easily survived on
"Mars." When the local abundance of water was increased,
terrestrial micro-organisms were able to grow. Indigenous
Martian organisms may be even more efficient in coping with the
apparent rigors of their environment. These findings underscore
the necessity for sterilizing Mars entry vehicles so as not to
perform accidental biological contamination of that planet and
obscure the subsequent search for extraterrestrial life.
_Direct Searches for Life on Mars_
The early evidence for life on Mars—namely, reports of vivid
green coloration and the so-called "canals"—are now known to be
largely illusory. There are three major areas of contemporary
investigation: visual, polarimetric, and spectrographic.
As the Martian polar ice cap recedes each spring, a wave of
darkening propagates through the Martian dark areas, sharpening
their outlines and increasing their contrast with the
surrounding deserts. These changes occur during periods of
relatively high humidity and relatively high daytime
temperatures. A related dark collar, not due to simple dampening
of the soil, follows the edge of the polar cap in its
regression. Occasional nonseasonal changes in the form of the
Martian dark regions have been observed and sometimes cover vast
areas of surface.
Observations of the polarization of sunlight reflected from the
Martian dark areas indicate that the small particles covering
the dark areas change their size distribution in the spring,
while the particles covering the bright areas _do_ not show any
analogous changes.
Finally, infrared spectroscopic observations of the Martian dark
areas show three spectral features which, to date, seem to be
interpretable only in terms of organic matter, the particular
molecules giving rise to the absorptions being hydrocarbons and
aldehydes. [However, see p. 7 and Rea et al. ([ref.9]).]
Taken together, these observations suggest, but do not
conclusively prove, that the Martian dark areas are covered with
small organisms composed of familiar types of organic matter,
which change their size and darkness in response to the moisture
and heat of the Martian spring. We have no evidence either for
or against the existence of more advanced life forms. There is
much more information which _can_ be garnered from the ground,
balloons, Earth satellites, Mars flybys, and Mars orbiters, but
the critical tests for life on Mars can only be made from
landing vehicles equipped with experimental packages....
Results of Kaplan et al. ([ref.31]) indicate that Mars has no detectable
oxygen, but does contain small amounts of water vapor, more abundant
carbon dioxide, possibly a large surface flux of solar ultraviolet
radiation, and estimated daily temperature variations of 100° C at many
latitudes. Studies have shown that terrestrial micro-organisms can
survive these extremely harsh environments. Furthermore, a variety of
physiological and ecological adaptations might enable the biota to
survive the low nighttime temperatures and intracellular ice
crystallization.
Less evidence is available to support the possibility of
extraterrestrial life on other planets. The Moon has no atmosphere, and
extremes of temperature characterize its surface. However, the Moon
could have a layer of subsurface permafrost beneath which liquid water
might be trapped. The temperatures of these strata might be biologically
moderate.
Studies by Davis and Libby ([ref.32]) on the atmosphere of Jupiter
support the possibility of the production of organic matter in its
atmosphere in a manner analogous to the processes which may have led to
the synthesis of organic molecules in the Earth’s early history. It is
difficult to assess the possibility that life has evolved on Jupiter
during the 4- or 5-billion-year period in which the planet has retained
a reducing atmosphere.
The question of extraterrestrial life and of the origin of life is
interwoven. Discovery of the first and analysis of its nature may very
well elucidate the second.
The oldest form of fossil known today is that of a microscopic plant
similar in form to common algae found in ponds and lakes. Scientists
know that similar organisms flourished in the ancient seas over 2
billion years ago. However, since algae are a relatively complex form of
life, life in some simpler form could have originated much earlier.
Organic material similar to that found in modern organisms can be
detected in these ancient deposits as well as in much older Precambrian
rocks.
Although the planets now have differing atmospheres, in their early
stages the atmospheres of all the planets may have been essentially the
same. The most widely held theory of the origin of the solar system
states that the planets were formed from vast clouds of material
containing the elements in their cosmic distribution.
It is believed that the synthesis of organic compounds preceding the
origin of life on Earth occurred before its atmosphere was transformed
from hydrogen and hydrides to oxygen and nitrogen. This theory is
supported by laboratory experiments of Calvin ([ref.16]), Miller
([ref.33]), and Oró ([ref.34]).
The Earth’s present atmosphere consists of nitrogen and oxygen in
addition to relatively small amounts of other gases; most of the oxygen
is of biological origin. Some of the atmospheric gases, in spite of
their low amounts, are crucial for life. The ultraviolet-absorbing ozone
in the upper atmosphere and carbon dioxide are examples of such gases.
Significant in the search for extraterrestrial life are the data (e.g.,
planet’s temperature) transmitted by Mariner II, which was launched from
Cape Canaveral on August 27, 1962, and flew past Venus on December 14,
1962. Mariner II’s measurements showed temperatures on the surface of
Venus of the order of 800° F, too hot for life as known on Earth.
The question "Is life limited to this planet?" can be considered on a
statistical basis. Although the size of the sample (one planet) is
small, the statistical argument for life elsewhere is believed by many
to be very strong. While Mars is generally considered the only other
likely habitat of life in our solar system, Shapley ([ref.35]) has
calculated that more than 100 million stars have planets sufficiently
similar in composition and environment to Earth to support life. Of
course, yet unknown factors may significantly reduce or even eliminate
this probability.