Which gas would be considered a potential biosignature if detected in an Earth-sized planet's atmosphere?

• A. Methane
• B. Nitrous oxide
• C. Water vapor
• D. Oxygen

Answer: (D)

Oxygen stands out as a strong potential biosignature because it is highly reactive and would be continually removed from an atmosphere unless something is actively replenishing it. On Earth, the main source of atmospheric O2 is photosynthesis by plants and microorganisms, which keeps its level sustained over long timescales. Detecting oxygen on an Earth-sized planet suggests ongoing production by life, especially if it appears with signs that the atmosphere is not in chemical equilibrium (for example, alongside reduced gases that would normally react away). Water vapor, while essential for life, isn’t a unique indicator since it depends on temperature and humidity. Methane or nitrous oxide can have non-biological sources as well. So oxygen provides the clearest link to sustained biological activity in an Earth-like atmosphere.