How could a Desert planet have an atmosphere?

Imagine a desertic planet, large dune seas and barren rocky wastes ... Mountains and the like suggesting a past geological activity and even water once exhisted on the surface.

Now though the planet is fully desertic, it has a human breatheable or partially breatheable atmosphere (for brief periods before having to rely on the use of masks breathers).

My question is the following: how could such a planet justify the atmosphere?

My idea was for example large underground water caves, may be some sort of fungi ? Lichen or else producing a sort of reaction to make it breatheable?

Volcanism and thermal activity could be acceptable eventually. The planet possibly should also have a warm or hotter climate at least on the central bands.

Thanks for any answer :).

posted over 7 years ago

Naima‭

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