Perchlorate. This is interesting:
Lithium perchlorate, which decomposes exothermically to give oxygen, is used in oxygen "candles" on spacecraft, submarines and in other esoteric situations where a reliable backup or supplementary oxygen supply is needed.
See
Chemical oxygen generatorThis is the same stuff they use for oxygen masks on airplanes and starships.
So there's water on Mars, and there's a chemical oxygen generator too.