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Why must nonbiological sources of oxygen and methane be ruled out before claiming evidence of life?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

In astrobiology, scientists must rule out nonbiological sources of oxygen and methane because these gases can be produced through abiotic processes, making them intrinsically ambiguous indicators of life.1

For example, atmospheric oxygen can be generated through the photodissociation of water, where hydrogen escapes into space, leaving oxygen behind. Similarly, methane can be produced by nonbiological chemical reactions, such as those between rock and water, or through geological activity.12

Because these gases can exist in a planetary atmosphere without the presence of a biosphere, detecting them is not definitive proof of life. To claim evidence of life, researchers must demonstrate that the observed concentrations of these gases cannot be explained by such nonbiological mechanisms. Relying on a single gas as a "life detector" is insufficient; therefore, scientists recommend using an array of instruments to establish or preclude the existence of life by accounting for all potential chemical and physical origins of atmospheric constituents.1