Unlike lunar resources, CO2 can be had by merely compressing the atmosphere. Carbon dioxide itself can be used to support plant growth at an advanced outpost. Both carbon and oxygen are important elements which have many possible uses at an outpost. There are several well understood chemical reactions that we can use to produce oxygen, methane, water, and perhaps other materials. Oxygen can be produced by passing CO2 through a zirconia electrolysis cell at 800 to 1000deg C. Twenty to thirty percent of the CO2 dissociates into oxygen and carbon monoxide.
Separation is accomplished by electrochemical transport of oxide ion through a membrane.
A prototype reactor using this chemistry has been run for over 1000 hours. Using such a scheme, we could bring a small unit to the surface of Mars which would then continuously make oxygen for life support, propellant use, or further processing. This oxygen can be converted into water if we also bring a small supply of hydrogen. Since the molecular weight of hydrogen is 2 and the molecular weight of water is 18, we can leverage 2 kilograms of hydrogen into 18 kilograms of water. The mass savings would, at some manufacturing rate, pay back the mass of the oxygen production unit.