# Is it feasible to have an oxidizing planetary atmosphere without the presence of an organic photosynthesis of some kind on the planet?

With the emergence of cyanobacteria on earth 3.5 billion years ago, the shift from a reducing to an oxidizing atmosphere began.

“The atmospheric O$$_\textrm{2}$$ levels (in PAL; i.e., normalized to the present atmospheric level) would have risen mainly in two geological periods in the Earth history: the Great Oxidation Event (GOE) at 2.4–2.0 Ga and the Neoproterozoic Oxygenation Event (NOE) at 0.8–0.6 Ga. Overshoot of O$$_\textrm{2}$$ occurred at 2.2–2.1 Ga and whiffs of O$$_\textrm{2}$$ occurred before the GOE. Glaciations in the Proterozoic (triangles)—especially snowball Earth glaciations (black triangles)—coincided with increases in the O$$_\textrm{2}$$ level during both GOE and NOE. Arrows with filled and open circles and bars are constraints from geochemical studies (e.g., Farquhar et al. 2007; Goto et al. 2013; Klemm 2000 ; Pavlov and Kasting 2002).”[1]

Would a non-organic based and/or non-photosynthetic derived oxidizing atmosphere be feasible on a planetary scale?

If so, given that the earth's oxidizing atmosphere is derived from the sun's thermonuclear energy over billions of years, what would be the ultimate source of energy for such a non-organic based and/or non-photosynthetic derived oxidizing atmosphere?

￼ [1] Tajika, Eiichi, and Mariko Harada. “Great Oxidation Event and Snowball Earth.” Astrobiology, 28 Feb. 2019, pp. 261–271, https://link.springer.com/chapter/10.1007%2F978-981-13-3639-3_17.

• Do you specifically mean an atmosphere sustaining oxygen-breathing life? UV light can crack water ice or vapor into oxygen and hydrogen, the latter preferentially escaping. Europa has a very sparse oxygen atmosphere due to this process. However, it doesn't recycle oxygen, it just releases it from water, so life using it as an energy source would soon run out of either water or substances to oxidize. – Christopher James Huff May 24 at 1:20

Chlorine-mediated splitting of water.

Chlorine itself is an oxidizer. In an environment with a lot of chlorine, the chlorine will dissociate oxygen from water, producing an oxygen environment.

The Wonders of Modern Chemistry, Geoffrey Martin, 1915

... Water is composed of the two gases hydrogen and oxygen. Now chlorine gas has a very powerful attraction for hydrogen, and under favourable conditions will actually decompose water, setting free the oxygen gas and forming hydrochloric acid, thus: H2O + Cl2 = 2HCl + O Water Chlorine Hydrochloric acid Oxygen This can be shown by quite a simple experiment. Chlorine gas is very soluble in water, the solution appearing yellow and tasting and smelling like the gas. Now if we fill a glass jar with this solution ,and invert it in a large basin filled likewise with chlorine water, and if we place the whole apparatus in such a way that bright sun light can fall upon it (best out of doors on a bright summer day) we shall distinctly see arising through the liquid small bubbles of oxygen gas, which gradually collect at the top…

The reader, therefore, will now perceive that if the oceans which roll over the surface of this chlorine planet are saturated with chlorine gas, then any sunlight which falls upon such waters will in the course of time cause vast masses of oxygen gradually to accumulate in the atmosphere. Thus water cannot be a stable form of matter on this new planet.

You already had an oxidizing atmosphere with all the chlorine. Where it came from is another matter. But once it is there it will liberate oxygen from any water, no biology required.

I thought it was cool that the author was speculating about a chlorine world in 1915.