This material is extracted from the review article by Chyba &
MaDonald (1995, Annual Review of Earth and Planetary Science).

 How might we tell if a future mission to another body in the solar
system had discovered life?  How do we separate living from
non-living?  A simple set of criteria  for doing so might be,
Something that is alive must (1) acquire nutrients from its
environment, (2) respond to stimuli in its environment, and 
(3) reproduce.  Unfortunately, with this definition we would conclude
that mules are not alive while fire is.  Other attempts to define
life---based on genetic, chemical, or thermodynamic criteria---suffer
from similar failings.

 A working definition used by many attempting to understand the origin
of life on the Earth is something like, "Life is a self-sustained
chemical system capable of undergoing Darwinian evolution."  (Note
that this definition, *chemical* systems, would exclude computer life
or A-life, but other definitions exist which would not.)  Again this
definition is not without its difficulties.  The emphasis on evolving
systems implicitly assumes a collection of entities; Victor
Frankenstein's creation would not have been classified as alive.
Further, how long must one wait before concluding that a system was
not evolving?  A recent definition that focusses on individual
entities is that a living organism must be (1) self-bounded, (2)
self-generating, and (3) self-perpetuating.

 Perhaps it is not possible to provide necessary and sufficient
criteria to distinguish "alive" from "not alive."  Indeed, if life can
arise from natural physical and chemical processes, there may be a
continuous spectrum of "aliveness," with some entities clearly
"alive"---humans, trees, dogs---some entities clearly "not
alive"---rocks, pop bottles---and some entities somewhere in
between---viruses.

Operationally, at our current stage of exploration of the solar
system, all of the above definitions are probably too detailed.  On
Earth, we have entities we clearly identify as "alive."  Liquid water
appears to be a requirement for these living things.  Hence, the focus
in solar system studies of life has been to target those bodies where
liquid water either is possibly now or may have once been present.