QUESTION:
The analysis of the soil on mars by Pathfinder and the analysis of the rocks
does not appear to indicate the presence of carbon or carbon compounds.  Is
this due to an inability of the test instrument to find carbon or is there
no carbon?  If there is no carbon or carbon compounds in the soil, does that
indicate no life has existed in that area?  It would appear that even live
bacteria would, over a period of time, leave carbon compounds in the soil.

ANSWER from Bruce Jakosky on September 24, 1997:
As I understand it, carbon is a difficult element for the Pathfinder APXS to
observe, since the instrument is immersed in a carbon-rich atmosphere. They
may be able to pull it out with additional instrumental calibration.

Carbon in the soil can have two possible sources--it can be present as
organic molecules or as carbonate minerals.  The source of organic molecules
can be either decay of living organisms, formation on Mars under the right
chemical conditions (such as in hydrothermal areas), or supply from incoming
meteorites.  In fact, we expect a few per cent of the martian soil to be
meteoritic dust, and therefore we expect to be able to measure meteoritic
organic molecules.  When the Viking spacecraft did not detect ANY organic
molecules, this suggested that some process must be able to destroy
organics; one plausible suggestion was the presence of "superoxides" which
would combine with the organic molecules to form CO2 gas.

Carbon present in minerals in the soil could form by a chemical reaction
between the soil and the atmospheric CO2.  If there is a lot of carbonate
minerals, they could represent a substantial "sink" of atmospheric CO2, and
explain one place where a thick, early CO2 atmosphere could have gone.

Clearly, both of these are important issues in understanding the history of
Mars.  In the end, if we find none in the soil, it does not mean that there
was no life on Mars.  Rather, it means there is no life there.  Life still
could exist in the occasional place that is more suitable for life--where
liquid water might have existed for longer periods of time, or where energy
from volcanism might be able to "power" life.  If we're going to find life,
we need to look in the most plausible places--and this means "follow the
water".