For an early, manned presence on Mars, a good source of make-up oxygen available early-on would be desirable. Losses from incomplete recovery, leakage, EVA suit venting(?), accidents, catastrophes, and others that I can't think of may not always be covered by oxygen supplies brought from Earth.
The three local sources of oxygen on Mars that I can think of are
- atmospheric CO2
- sub-surface H2O (eventually a source for fuel for return to Earth)
- ClO4- (perchlorate) in Martian regolith, order of magnitude of 1 part-per-thousand (see here and here).
Of these, which one is most likely to be used first as a source of breathable oxygen for make-up and emergency needs?
Answer should evaluate required electrical or thermal power, mass and reliability of equipment hauled from Earth, and challenges collecting the raw materials where these turn out to be limiting parameters.