89% (by mass) of hydrogen/oxygen fuel can also be produced from Martian atmosphere, since Oxygen has an atomic mass 16 times that of hydrogen. I think Methane

89% (by mass) of hydrogen/oxygen fuel can also be produced from Martian atmosphere, since Oxygen has an atomic mass 16 times that of hydrogen. I think Methane has advantages other than a slightly larger portion producible from Martian air.