Exobiology




The Columbia Encyclopedia, Sixth Edition




exobiology or astrobiology, search for extraterrestrial life
within the solar system and throughout the universe. Philosophical
speculation that there might be other worlds similar to ours dates back
to the ancient Chinese and Greeks. However, the achievements of space
exploration and molecular biology have turned speculation into
experimentation.

Parameters of a Suitable Environment for Life


There are six basic parameters that determine whether an
environment is suitable for life as we know it: temperature, pressure,
salinity, acidity, water availability, and oxygen content. Advanced
life is restricted to a narrow range of these parameters, but primitive
microorganisms exist over a much wider range. Data already collected by
space probes essentially rule out advanced life on other planets of our
solar system; however, given the potential number of planetary systems
in the galaxy, there may be as many as 50,000 planets that have
earthlike conditions, a fraction of which could have cultures as
technologically advanced as our own. Three decades of passive listening
with radio telescopes have produced no signals comparable to those
radiating from earth, but we do not know if another civilization would
produce electromagnetic radiation in detectable amounts.

The Search for Primitive Life


A continuing effort is being made to detect primitive life within
our solar system. According to the Russian biologist A. I. Oparin, life
can appear as the result of progressive development of organic matter
from nonorganic. The principal constituents of organic matter—hydrogen,
carbon, nitrogen, and oxygen—are among the most abundant atomic
elements in the universe. Oparin assumed that on earth these elements
combined to form simple hydrocarbons and that the hydrocarbons combined
to form the precursors of life, such as amino and nucleic acids. Once
these precursor molecules existed in the earth's primitive seas, they
spontaneously interacted to form increasingly complex structures, until
self-replicating molecules like deoxyribonucleic acid (DNA) were
created, leading the way to protein synthesis.

American chemists Stanley L. Miller and Harold C. Urey provided
experimental support for Oparin's theory, by discovering that when a
mixture of methane, ammonia, water, and hydrogen is exposed to an
electric discharge, amino acids are formed. The composition of this gas
mixture is similar to the atmosphere of Jupiter. The same result has
been obtained by exposing the gas mixture to ultraviolet radiation,
which exists in outer space. Further support for Oparin's theory came
with the discovery of organic molecules like ammonia and formaldehyde
in the interstellar medium. A labeled-release experiment contained on
the Viking landers which analyzed the surface of Mars in 1976 detected
what could be organic activity.

Evidence of organic material and, possibly, fossils of microscopic
bacteria have been found in certain carbonaceous chondrite meteorites,
most notably the Orgeuil meteorite, which fell in France in 1864, the
Murchison meteorite, which fell in Australia in 1969, and the Allan
Hills martian meteorite, found in Antarctica in 1984, but most
scientists believe either that such organic traces result from
terrestrial contamination or that the data have been misinterpreted.
Some scientists believe that the moon, Mars, and Venus have already
been contaminated by microorganisms carried on space probes.
Conversely, fears that returning Apollo astronauts could introduce
destructive alien organisms into earth's biosphere led NASA to
quarantine them for as long as two weeks, and lunar rock samples were
kept carefully isolated.