QUESTION:
How long if ever is it expected to know conclusively if the Mars meteorite
ALH84001 contains evidence of life from Mars? What are the chances that it
does?

ANSWER from Bruce Jakosky on April 21, 1997:
We may never know if the meteorite contains martian life.  In order to
prove that it does contain fossil life, we need more sophisticated
techniques than we have today.  For example, we might be able to find
cell wall structures in the possible bacteria; this would be accepted
as proof of life.  As we explore the meteorite in more depth, we will find
out more about it.  But, as can be seen from all of the scientific debate
that is going on today, there may never be complete agreement; there may
be too many uncertainties within this single rock.

Definitive proof about life on Mars may have to wait for detailed investigation
of samples that we collect and bring back to Earth.  In this case, we can
choose the best locations to look for life, where there is the highest
probability of finding it.

Also, even if we prove that there is no life in ALH84001, there still is
the possibility of finding life on Mars.  Mars has all of the ingredients
thought to be necessary for life--liquid water, access to the elements
needed for life, and a source of energy that can drive chemical reactions--
and it is very plausible that life can exist.

ANSWER from Bruce Jakosky on June 18, 1997:
Well, either ALH84001 does contain evidence of life, or it doesn't.  While
that may seem like a flip answer, that's pretty much what it comes down to. 
There is some evidence that has been interpreted as being evidence for life,
but the observations can be explained equally well by non-biological,
geological processes.  Unfortunately, that means that the evidence is
ambiguous; and, in fact, there is no agreement among scientists as to
whether there is evidence for life in the rock.

Of course, even if there turns out to be no evidence for life in the rock,
that won't change the intellectual arguments that suggest life could have
existed on Mars and could still be present.  Mars has all of the
requirements for life--liquid water, access to the biogenic elements, and a
source of energy to drive chemical reactions (in this case, geothermal
energy that derives ultimately from the planetary volcanism).

ANSWER from Bruce Jakosky on November 1, 1997:
The evidence suggesting possible fossil evidence of life in the Martian
meteorite comes from several lines of reasoning.  These include composition
and structure within the carbonates consistent with deposition by bacteria,
presence of polycyclic aromatic hydrocarbons (PAHs), magnetite grains
similar to those deposited by terrestrial bacteria, and structures similar
in appearrance to terrestrial bacteria (although 1000x smaller volume). 
There has been substantial debate about whether these properties were
produced by living organisms or by geochemical processes, and there is no
unanimous agreement.  The strongest conclusion that can be reached is that
they are consistent with both hypotheses, although some argue that they are
not even consistent with biology.  The meteorites are continuing to be
analyzed by a wide variety of researchers, including geochemists,
paleontologists, and biologists.