QUESTION: 
Why is there carbon dioxide underground and in the rocks in Mars?

ANSWER from Bruce Jakosky on April 16, 1997:
We know that there is carbon dioxide (CO2) present in the martian
atmosphere; we can detect its presence using earthbased telescopes and
using spacecraft measurements.  One of the main questions for Mars is,
How much CO2 might be present on Mars but not be in its atmosphere?

CO2 gas can diffuse from the atmosphere into the ground, moving through
open spaces or cracks in the rocks and soil.  In addition, if there is
any liquid water on Mars, the CO2 can dissolve in the water and be carried
underground that way.  (CO2 also can be present in the polar regions, frozen
out as dry ice.)

Once CO2 is present beneath the surface, it can attach itself to the soil
grains, or it can form carbonate minerals by interacting chemically with
the soil that is there.  Because CO2 can diffuse into the ground, we know
that this must be happenning.  And, in the meteorites that come from Mars,
we do find carbonate minerals.  These minerals appear to have been deposited
from liquid water that was flowing through the rocks; similar processes are
very common on the Earth.

What we do not know is how much CO2 is present in the form of carbonate minerals
beneath the surface of Mars.  We believe that Mars once had a thick atmosphere,
because we see geological features that were formed by liquid water flowing over
the surface; because temperatures at Mars are below the freezing point of water,
though, something like a greenhouse atmosphere would be needed to raise
the temperature enough to allow liquid water.  A thick CO2 greenhouse
atmopsphere would do the trick, trapping enough heat to raise the
temperatures.  The question, then, is where did this CO2 go?  The two best
places for the CO2 to have gone are (i) to have been lost to space, and
(ii) to have gone into the crust as carbonate minerals.  If we can understand
how much CO2 is in the crust today, then we can better understand the
history of the martian climate.