QUESTION:
I read that some of the Viking experiments did find evidence of life on Mars
(Isotope exchange experiments, for example).  Yet the Pathfinder will not do
any experiments to see if life does exist.  Right?  Even Carl Sagan thought
this was too bad. Why didn't you look any further for evidence?

ANSWER from Bruce Jakosky on June 17, 1997:
The Viking mission tested several possible mechanisms by which life could
obtain energy.  Of the three biology experiments, one showed a result that
would be interpreted as showing the presence of life.  However, one cannot
take the results of one experiment and ignore the results of the other two. 
The whole set of experiments must be understood together.  When looked at
this way, the best explanation is that there was interesting chemistry but
no biology.

Does this rule out Martian life?  Probably not.  As we know now (and didn't
know then), there are lots of chemical processes that could support life
that were NOT tested for on Viking.  Martian life could use one of these, in
which case the Viking would not have detected anything.  Of course, the
absence of organic molecules, as determined by a different Viking
experiment, makes life at those sites exceedingly unlikely to exist.

Martian life could exist in a different place on Mars.  For example, to look
for life, one would look for liquid water.  This could occur in recent
volcanic vents, for example, or in hot springs such as we find at
Yellowstone National Park on Earth.  Or, life could exist deep underground,
where temperatures are warm enough for life to exist; there is new evidence
for abundant life deep underground on the Earth.

So, how would you look for Martian life?  Many scientists feel that we don't
know enough yet about the martian chemical environment to look for life.
Rather, they think that finding life will require picking up the right rocks
and bringing them back to the Earth for detailed analysis.  The best rocks
to bring back are those in hot springs and so on, again at places where
liquid water could exist.

The Mars Pathfinder is the first step toward being able to go to Mars,
search for the best rocks, and bring them back.  In fact, the goal of the
Mars Surveyor Program of spacecraft over the next decade is exactly this--to
bring back rocks that can tell us about life on Mars.  And, whether we find
life or we find that life is absent, we are finding out something truly
fundamental about life in the universe.