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In the early 1970's and prior to the Viking landers, T. Owen measured the oxygen in the Martian atmosphere at 0.3%, I believe by doppler shift methodology through the Earth's atmosphere (wow). During entry, the Viking spacecraft reported 1.3%. I've heard (but not confirmed) that the MOXIE experiment used 1.3% in its modeling. Are there more recent measurements, perhaps spectral from nearby spacecraft? The source of O2 includes photolysis of CO2 (2CO2>2CO+O2), and the amount might provide some information on the sources and sinks in the Martian environment.

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The Sample Analysis at Mars (SAM) in Curiosity rover measured the air of Gale Crater and analyzed its composition: 95% by volume is carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16% molecular oxygen (O2), and 0.06% carbon monoxide (CO). Note that amount of oxygen in Mars has a seasonal variation rising upto 30% during Spring and Summer and then dropping back to levels in fall. This pattern repeated each spring implying that something was producing it and then taking it away.

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Source: https://www.nasa.gov/feature/goddard/2019/with-mars-methane-mystery-unsolved-curiosity-serves-scientists-a-new-one-oxygen

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Wikipedia xites Atmospheric Evolution on Inhabited and Lifeless Worlds giving this composition:

Carbon dioxide  95%
Nitrogen    2.8%
Argon   2%

Oxygen  0.174%

Carbon monoxide 0.0747%
Water vapor 0.03% (variable)

That's an order of magnitude lower than 1.3%

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