QUESTION:
My chemistry is a bit rusty but I was wondering what is the oxidizing agent
in the atmosphere or soil of Mars that accounts for the "rusty" red dust. 
My understanding is that CO2, carbon dioxide, is extremely stable and would
not be a good oxidising agent by itself, but I also understand that he
Martian atmosphere is between 90% and 99% carbon dioxide. Would life had to
have been present in order to produce the necessary oxygen?

ANSWER from Mary Urquhart on July 18, 1997:
You are right that carbon dioxide is stable by itself and will not oxidize
the surface materials on Mars.  However, carbon dioxide can be separated
into carbon and oxygen by processes such as photochemical reactions with
ultra-violet light.  Water may have played a major role in the formation of
the oxidized material as well.  Exactly what processes are or have been
important in the formation of the oxidized martian surface material is
something that we still don't really know.  Pathfinder is already yielding
some preliminary results on the mineralogy of the dust, which will help us
understand more about how it formed.

Mary Urquhart                                    
Laboratory for Atmospheric and Space Physics  
University of Colorado at Boulder       

ANSWER from Bruce Jakosky on July 21, 1997:
There's been a lot of discussion of how oxides on Mars formed, with
suggestions ranging from the presence of water to chemical reactions induced
by ultraviolet light from the sun.  A thick oxygen atmosphere is not
required, however.  Oxygen is present in CO2, the main gas in the
atmosphere.  It can be made available to react with rocks either when the
CO2 reacts directly with minerals or when oxygen is released from the CO2 by
the absorption of uv light.  Certainly, CO2 is constantly being broken up by
sunlight, and there is a small amount of oxygen present in the atmosphere as
a result (there also is CO, carbon monoxide, the other break-up product). 
The bottom line is that we don't understand the specific mechanisms
responsible for oxidizing the surface, but there is no lack of available
oxygen even without a thick oxygen-rich atmosphere.

ANSWER from Jack Farmer on July 22, 1997:
This is an excellent question. But, unfortunately the oxygen tied up in iron
oxides could have also come from the breakdown of water in the upper
atmosphere earlier in the planets' history.  We have several independent
lines of evidence supporting the view that there was once a lot of liquid
water at the surface of Mars, and an active hydrological cycle that
presumably moved lost of water into the atmosphere. A portion of the H2O
molecules in the upper atmosphere would tend to be split into hydrogen and
oxygen by a process called photolysis (simply put, molecules are split by
the sun's energy) with the subsequent losses of the much lighter hydrogen
atoms to space. The oxygen molecules liberated by this process would rapidly
combine with reduced iron present in surface rocks, forming iron oxides.
Thus, we don't need to invoke photosynthesis on Mars to account for the
oxidation of the surface because purely inorganic proceeses operating over
long spans of time will also do the job. We therefore need to find other
lines of evidence for Martian biology, and more specifically, for Martian
photosynthesis.

Jack D. Farmer, PhD.
NASA Ames Research Center