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How can photosynthesis or chemosynthesis be tested on samples from another planet?

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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.

To test for photosynthesis or chemosynthesis in samples from another planet, scientists utilize in situ metabolic criteria. Because these processes involve the fixation of gases—such as carbon dioxide—when exposed to specific environmental conditions, experiments are designed to detect this activity.1

In situ studies involve landing instrumentation on a planet to acquire and analyze soil or water samples. Researchers may monitor for the uptake or transformation of inorganic carbon sources, which serves as a potential indicator of autotrophic life. Additionally, scientists may employ gas chromatography and mass spectrometry to identify organic molecules that could be products of these metabolic pathways.123

However, these tests are often ambiguous. A primary challenge is the risk of contamination from the spacecraft itself, which could introduce Earth-based organisms. Furthermore, the detection of organic matter does not inherently confirm biological origin, as nonbiological processes can also produce such compounds. Consequently, researchers recommend using an array of both general and specific instruments to establish or preclude the existence of life.1