The speculative Spacex manned mission to Mars would involve Falcon Heavy Rockets to send Dragon capsules on Mars and being refuelled, and then perhaps being lifted back to orbit atop boosters running on Mars locally-produced methane.
The martian atmospheric composition will allow the Mars astronauts to take advantage of in-situ resource utilization to provide them with life support reserves as well as the propellant required by the MAV. The martian atmosphere is composed of approximately 95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen, 0.08% carbon monoxide, and trace amounts of water, nitrogen oxide, neon, krypton and xenon. By utilizing simple reactions between martian carbon dioxide and imported hydrogen, the astronauts will be able to produce methane, water, and oxygen. Direct atmospheric extraction of nitrogen and argon will also be possible.
The Sabatier process involves the reaction of hydrogen with carbon dioxide at elevated temperatures to produce methane and water. The ISRU module will react imported hydrogen with atmospheric carbon dioxide in order to achieve this. For each metric ton (tonne) of imported hydrogen that is reacted, 2 tonnes of methane and 4.5 tonnes of water will be produced.
Ice has been found in large quantities on Mars. Water ice can have its hydrogen extracted.
Landing small nuclear power modules on Mars would allow for all local materials to be used to produce rocket fuel and other resources to enable a very light Mars Mission.