The same geological principles that produce massive amounts of abiotic methane here o Earth are at work in Mars. Thats the source of the methane that has been detected on Mars. This also means that there should be massive amounts of methane hydrates hidden under the surface. If that is true, Elon Musk may be in luck as his Raptor engines use methane as a fuel. He would only have to synthesize oxygen either from CO2 or from water. Be sure you take a drilling rig with you when you go to the red planet.
Scientists have come to realize that, just below the surface, about one third of Mars is covered in ice. We study this ice to learn about Mars' ancient climate and astronauts' future water supplies.