Fuel

is synthesized in Fuel Refineries and is used to for refuelling Rockets bound for Earth, for keeping Shuttle Hubs operational and for polymer production.

Background
Using the CO2 from the Martian atmosphere and hydrogen (from water) we can produce fuel to fly our rockets back to Earth. The process uses a series of relatively simple chemical reactions to produce methane and oxygen which are used as propellant for both our rockets and shuttles."