QUESTION:
How will you determine whether microorganisms are living on the planet and
why?  Thank you.

ANSWER from Bruce Jakosky on November 10, 1999:
It's very difficult to imagine how we'll determine if there is life on
Mars.  Unless we see "things" that are so obviously microbial cells,
either living or dead and fossilized, the evidence is likely to be
ambiguous.  This is because, with only one example of life to study
(life on Earth, of course), we don't know what characteristics of life
would be common to all living organisms anywhere in the universe or
what characteristics would be unique to terrestrial life.

For example, we could search for DNA and RNA, which are molecules that
are very easily detected and which play fundamental roles in life on Earth.
However, we do not know if life elsewhere would use RNA and DNA molecules,
or whether it might use molecules that are similar but have slightly different
chemical compositions or structures.

Similarly, we could look for amino acids.  However, life elsewhere might not
use the same amino acids that life on Earth uses.  Also, amino acids can
be created by non-biological processes as well.

There's a tremendous research effort going on right now to try to determine
what "indicators" would tell us if there was life.  We don't know the answer
yet, but we hope to within just a few more years.

One interesting point:  All of the discussion and debate over whether there
is evidence of life in the martian meteorites ALH84001 underscores the
difficulty of knowing how to interpret the measurements we make.  We have not
been able to decide whether the features seen in that rock are evidence for
life or not!  But, because of all of the work in trying to decide how to
interpret the evidence, we've come a long way both toward knowing how little
we know and toward figuring out how to decide.