I wish I knew for sure. It is just something I read. Rockets need to maintain the pressure in the tanks as fuel is depleted. Most fuels will not vaporize fast e

I wish I knew for sure. It is just something I read. Rockets need to maintain the pressure in the tanks as fuel is depleted. Most fuels will not vaporize fast enough as they are used. Helium seems to be the best option but hard to obtain on Mars. I guess nitrogen is another choice but it is less than 2% of a very thin Mars atmosphere. I am not sure if methane will do the job by itself or if you might need to heat some up.