We recognize that our own evolution may represent only one pattern among many and are therefore examining environments where a different type of life may have appeared. However, our understanding of the chemical characteristics of life on our own planet provides a valuable starting point of a search for life elsewhere. Even on Earth, life survives and sometimes thrives under unexpected conditions. For example, some microbes flourish at temperatures of 113°C. Others exist only in caustic acids. Others survive intense radiation in nuclear reactors.
So while
Earth organisms are composed of nearly identical macromolecules, these can
evolve to cope with a broad range of physical and chemical conditions. This
gives us some indication of how life on other planets could differ from that on
Earth. Nevertheless, life itself has certain general principles. Living systems
must capture energy in some form and obtain nutrients from the environment. They
use these inputs to generate the molecular structures they need to maintain and
reproduce themselves, and to survive in the face of
varying challenges from the environment. Part of the search for extraterrestrial
life is to investigate alternative ways that living organisms can implement
these functions.
Our initial approach
is to look for life that is carbon based and makes use of chemical reactions in
the presence of liquid water. Carbon is very broadly dispersed throughout the
universe. By forming long chains it is ideally suited to create the complex
molecules that sustain life. Water is made up of the commonest element in the
universe, hydrogen, and the third most common, oxygen. (The second most common
is helium, which is inert.) Strong bonds between water molecules cause them to
form networks that enable it to wet surfaces and move through capillaries of
organisms. The bonds are formed by electrical charges on opposite sides of the
molecule, which can break up other molecules without changing their composition,
bringing them into solution to take part in chemical reactions.
Water also has the
property of discriminating strongly between fats and water-soluble molecules so
as to favor the creation of cell membranes and boundaries. In this way it makes
an essential contribution to the formation of cell walls that can enclose and
protect groups of interacting biological
molecules. While free-floating molecular complexes may have been an early form
of life, the formation of cells provided a way of stabilizing molecular systems
and enabled them to evolve more complex forms. A fixed unit of evolution is then
ready to capture energy from star light or local energy sources, obtain
nutrients, and maintain and reproduce itself. The
cellular enclosure provides a constant environment where molecules coding the
instructions for reproduction can avoid all but the most severe environmental
insults. Water also has the valuable property that it remains in liquid form
over a wide range of temperatures suitable for both activity and stability of
molecules important to life. When it solidifies into ice, the ice floats on top
of the water, sheltering life forms in the water below.
The presence of
oxygen in a planetary atmosphere is an important indicator of the possible
presence of life. It is one of the most distinctive features of Earth’s
atmosphere. The chemical surface of a planet will probably remove oxygen from
the atmosphere (as rust, for example) unless there is an active process such as
plant photosynthesis to replace it. However, by itself the presence of oxygen
can be misleading. There are traces of oxygen in the atmosphere of Europa, a
moon of Jupiter, but this is thought to be produced in the upper atmosphere by
radiation splitting water molecules into oxygen and hydrogen. The hydrogen, being lighter then escapes. Life requires
oxygen in the lower atmosphere, which can be difficult to detect directly. A
more reliable indication of conditions favoring life is the detection of carbon
dioxide, ozone and water appearing together.
Programs searching
for extraterrestrial life on planets orbiting other stars initially seek to determine whether
a planet is capable of supporting life. These programs represent our
the start of our effort to reach a
deeper understanding of life and how it arises, and to throw light on whether it
is unique to one planet or widely represented in the universe. If we find
earth-like planets with conditions suitable for life, a further search for
evidence of communications or for industrial gases in their atmospheres may
indicate whether a civilization has been present.