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Why is free oxygen unusual in a planet’s atmosphere?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Free oxygen is unusual in a planet’s atmosphere because it is a highly reactive element that readily forms compounds with other substances. In the absence of a continuous replenishment process, any free oxygen would be quickly consumed through chemical reactions with surface materials, such as the oxidation of iron-bearing minerals (forming rust) or the oxidation of sulfur and organic carbon.12

On Earth, the presence of free oxygen is primarily the result of photosynthesis performed by green plants and algae. Early in Earth's history, the atmosphere was anoxygenic, and any oxygen produced by primitive organisms was rapidly sequestered by reacting with dissolved iron in the oceans to form banded-iron formations. It was only after these geological sinks were saturated that oxygen could accumulate in the atmosphere. Because oxygen is a byproduct of biological activity, its presence in an atmosphere is a dynamic state maintained by the oxygen cycle rather than a stable, permanent feature.34562