Years before Starship ever flew, SpaceX picked a rocket fuel that only makes sense if you take Mars completely seriously. Falcon 9 burns rocket grade kerosene. Mars has no oil, so kerosene can never be made there. Hydrogen is very hard to keep cold and takes a lot of energy to produce. Methane is different. The Martian air is about 95% carbon dioxide, there is water ice in the ground, and with power from big solar fields you can turn CO2 and water into methane and oxygen. Chemists have known that recipe, the Sabatier reaction, for over a hundred years. So every Raptor engine was designed around a fuel station that doesn't exist yet, on a planet hundreds of millions of kilometers away. The ship is meant to land, refill from the air and the ice, and fly home. Elon Musk laid this out in 2016 in Guadalajara. Without making propellant on Mars, he said, a trip would cost about 500% more, and you would end up with a graveyard of ships that never come back. That's a city on Mars being planned, down to the chemistry, a decade ago.