it’s even really difficult technically, because you have to keep the propellant cool on the surface of mars while you stay there, which is really difficult. in outer space, it’s possible because there’s only sunlight as a heat source, and if you create a shadow, then it’s chilly as fuck, but on the planet surface, winds will carry heat to the fuel deposit and eventually warm it up, and cooling that amount of propellant is expensive (energy-wise) as fuck.