Explain this atmosphere...

[whulorigan]

One could argue that you cannot have O2 in the air without plants. Oxygen too readily combines with carbon, or hydrogen or iron or, well lets face it, Oxygen gets around alot. Without photosynthetic plants, any atmospheric oxygen would soon be depleted.

Oxygen is very reactive and will vanish from the atmosphere after millenia if it is not replenished somehow. Normally plants do this, but they require precipitation or moisture from some source. Small seas should suffice.

I thought this was an interesting Article in this week's Science News:
https://www.sciencenews.org/article/exoplanet-oxygen-may-not-signal-alien-life
Exoplanet oxygen may not signal alien life

Water and ultraviolet light could create the gas without help from organisms

(Unfortunately you need to be a subscriber to read the entire article)

Essentially the article makes the case that O₂ could be produced by UV-rays on an essentially pelagic world striking water molecules in the upper atmosphere in the absence of N₂. The H₂0 breaks down into H₂ and O₂, the H₂ escapes to space, leaving the O₂ behind.