Introduction The most direct method of reducing the cost of a mission to Mars is to reduce the mass of payload that is launched from Earth. If the propellants for the return phase of the mission are produced on Mars, the total Earth-launch mass could be reduced significantly. The upcoming Mars Sample Return (MSR) mission will demonstrate the feasibility of in-situ propellant production (ISPP) but it is not clear what type of ISPP system is the best choice for Mars. Five candidate ISPP systems for producing two fuels and oxygen from the Martian atmosphere are considered in this study: 1) Zirconia cell with methanol synthesis, 2) Reverse water gas shift with water electrolysis and methanol synthesis, 3) Sabatier process for methane production with water electrolysis, 4) Sabatier process with water electrolysis and partial methane pyrolysis, and 5) SabatierlRWGS combination with water electrolysis. These systems have been the subject of Regardless of the type of craft or the payload for the Mars-Earth return trip, the overwhelming majority of the vehicle launch mass will be the propellant required for the launch and insertion into the transit trajectory. If the propellant for the return phase of the mission were to be produced on Mars, the Earthlaunch mass could be reduced significantly, thereby reducing the cost of the Earth-launch system. A strong case has been made by many mission planners that l however, the major attributes of a Mars-based system will be its long-term relia