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We carried out a system analysis of processes for utilization of Mars resources to support human exploration of Mars by production of propellants and life-support consumables from indigenous resources. Seven ISRU processes were analyzed to determine mass, power and propellant storage volume requirements. The major elements of each process include CO2 acquisition, chemical conversion, and storage of propellants. Based on two figures of merit [(a) the ratio of the mass of propellants that must be brought from Earth in a non-ISRU mission to the mass of the ISRU system, tanks and feedstocks that must be brought from Earth for a ISRU mission, and (b) the mission mass in LEO saved by use of ISRU] the most attractive process (by far) is one where indigenous Mars water is accessible and this is processed via Sabarier/electrolysis to methane and oxygen. These processes are technically relatively mature. Other processes with positive leverage involve reverse water gas shift and solid oxide electrolysis. These processes would be appropriate if accessible water is not available on Mars. However the technologies involved are still immature. Processes that require storage of large amounts of hydrogen were deemed infeasible because of power, volume and mass considerations. The critical interfacial issues with Mars are finding accessible sources of water and acquisition of CO2 from the atmosphere. A technology development and demonstration program is proposed that hinges heavily on the search for accessible water. A roadmap summarizes the future steps needed to implement ISRU in the human mission architecture.