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Propellant is mass which is ejected from a jet engine (of which a rocket is one possible type) to produce thrust through Newton's third law of motion.

In a chemical rocket, the propellant is the reaction product of the oxidizer and the fuel. In a nuclear thermal rocket, it is normally hydrogen, since the specific impulse is dependant on the exhaust velocity of the rocket, which is higher for propellants with low molar mass. A laser ablation rocket will tend to use a combination of metal and plastic (due to their optical properties). For electrical engines, an easily ionized gas is preferred. Vaporized liquid metals are good candidates. As for nuclear rockets, the lower the molar mass the higher the exhaust velocity. However, as electric engines work on ionized gases, the ionization energy of the propellant adds complexity to the engine and favors elements that are easy to ionize.

On Mars, propellant can be produced in-situ from water and CO2. Argon from the atmosphere could be used for electrical engines, either solar or nuclear powered.