Even if there were “fossil fuels” on Mars, by the time it was economically feasible to extract it and get it back to Earth, there would be no point because fossil fuels will be a thing of the past by then.
any usable metals?
Ahem. Burn them to create CO2. Move a couple of water asteroids their way. Seed the planet with plant seeds.
Earth 2.0
PS. Always thought fossil fuels are not fossil fuels. I think they are asteroid juice. Star juice. Either you understand or you don’t, but coal is charcoaled forests. Gas and oil.... cometary remnants.