The question of the existence of extraterrestrial life is one of the
most important and interesting biological questions facing mankind and
has been the subject of much controversial discussion and conjecture.
Many of the quantitative, and even qualitative, environmental
constituents of the planets also are still subjects of controversy and
speculation. Best guesses about a relatively unknown planetary
environment, combined with lack of information about the capabilities of
Earth life to grow in extreme environments, do not provide the basis for
making informed scientific estimates.
Life on Earth is usually considered to be relatively limited in its
ability to grow, reproduce, or survive in extreme environmental
conditions. While many common plants and animals (including man) are
quite sensitive to, or incapable of, surviving severe chemical and
physical changes or extremes of environment, a large number of
micro-organisms are highly adapted and flourish in environments usually
considered lethal. Certain chemoautotrophic bacteria require high
concentrations of ammonia, methane, or other chemicals to grow.
Anaerobic bacteria grow only in the absence of oxygen.
Besides adapting to the extremes of environments on Earth, life is also
capable of growing and reproducing under extreme environmental
conditions not normally encountered: e.g., from a few rad of radiation
in normal habitats to 10⁶ or more rad from artificial sources, from 0.5
gauss of Earth magnetism to 167 000 gauss in manmade magnetic fields,
and from 1-g force of gravity to 110 000 g. The extreme ranges of
physical and chemical environmental factors for growth, reproduction,
and survival for Earth micro-organisms are phenomenally large.
Life is ubiquitous on Earth and is found in almost every possible
environment, including the most severe habitats, from the bottom of the
ocean to the highest mountain tops and from cold Arctic habitats to hot
springs, as well as in volcanic craters, deep wells, salt flats, and
mountain snowfields. Earth life has become adapted to, and has invaded,
nearly every habitat, no matter how severe. The physiological and
morphological adaptations of life are exceedingly diverse and complex.
Surprisingly, the extreme parameters or ranges of the physical and
chemical environmental factors permitting growth, reproduction, and
other physiological processes of Earth organisms have not been
critically compiled. A partial compilation of certain selected
environmental factors has been made by Vallentyne ([ref.76]). A
compilation of available published data on certain environmental
extremes, particularly from recent NASA-supported research (compiled by
Dale W. Jenkins, in press), is presented in tables III to VI. These data
can serve as a starting point for a more intensive literature review by
specialists, critical evaluation, standardization of end points, and
especially to point out areas where critical experimentation is urgently
needed.
This critical compilation involves a review of a very broad and complex
range of subjects involved in many different disciplines with widely
scattered literature. Since the effects of many of the specific
environmental factors are harmful, it is difficult to select a point on
a scale from no effect to death and use some criteria to say that normal
or even minimal growth and reproduction are occurring. The effects of
environmental factors are dependent on (1) the specific factor, times,
(2) the concentration or energy, times, (3) the time of exposure or
application of the factor. Many reports, especially older ones, do not
give all of the necessary data to permit proper evaluation. A
complicating factor is that the effect of each factor depends on the
other factors before, during, and after its application. The condition
of the organism itself is a great variable. Proper evaluation requires
the critical review by a variety of biological specialists, physicists,
and chemists.
To determine the potential of Earth organisms to survive or grow under
other planetary environmental conditions, a number of experiments have
been carried out attempting to simulate planetary environments,
especially of Mars, as reviewed previously. While the results are of
real interest, they do not provide much basic information. Further, as
the Martian environment is more accurately defined, the experimental
conditions are changed. In addition, some experimenters have altered
certain factors, such as water content, to allow for potential
microhabitats or for areas which might contain more water at certain
times.