The fuel processor is designed to produce methane and water, then breaking down the water to make hydrogen and oxygen: the process only produces twice the oxygen as methane. As it is used to oxidize the methane for the return trip to Earth, a lot more oxygen is needed: four and a half to one is the ideal ratio.
Another way to produce oxygen is to distill it directly from carbon dioxide, making one part oxygen and the other part carbon monoxide. Normally, you wouldn’t need the carbon monoxide and would discard it back into the atmosphere, but in the spirit of living off the land, one of the mission coordinators came up with a novel use for it: inflating the greenhouse’s superstructure. Not the interior where the plants will be located, but the exterior structural envelope of the greenhouse.
Shaped like a Quonset hut, the greenhouse is an arched structure made up of three layers of inflatable, transparent, Mylar tubes. As an air mattress contains tubes running back and forth from head to foot, the tubes of the greenhouse run up from the ground on one side of the canopy, over the open area, and down to the ground on the other side. By inflating these tubes with carbon monoxide, it is a way of using the non-breathable gas and not dumping it back into Mars’ atmosphere. Steel cables running over the structure every meter, and anchored on the ends, will keep the building from blowing away in a windstorm.
If the outer layer were to be breached, another layer can be moved into place on the inside and inflated. The breached layer can be left in place for quite a while before needing to be removed, and that can be done in pieces. As long as it lets in the light, keeps in the air, and keeps out the cold, all is well.