The Use of Oxygen as a Biosignature in the Search for Extraterrestrial Life

Credit: L. Hustak and J. Olmsted (STScl).

The search for signs of life on exoplanets relies heavily on our ability to detect key biosignatures within a given planet’s atmosphere. Among potential markers for life in exoplanet atmospheres, molecular oxygen (O2) and ozone (O3, a byproduct of photochemical processes in Earth’s stratosphere) are the focuses of many space- and ground-based spectroscopic studies because their abundances in Earth’s atmosphere are strongly controlled by biology. Important factors in determining the appropriate use of oxygen-based biosignatures in the search for extraterrestrial life include how Earth’s oxygen-rich atmosphere formed and how it will evolve and decay in the future.

Recent modeling by Kazumi Ozaki of Toho University, Japan, and Christopher Reinhard of Georgia Tech combines biogeochemistry and climate models to investigate the geologic timescale of the oxygen-rich atmosphere on Earth. This work builds upon previous studies that tracked carbon, oxygen, phosphorus, and sulfur by incorporating a coupled global methane cycle and its radiative impact as a greenhouse gas. Additionally, the model considers redox reactions between Earth’s crust and mantle in order to assess ongoing planetary processes such as plate tectonics and mantle degassing as additional factors affecting atmospheric oxygen levels. Their model results suggest that in one billion years, oxygen levels in Earth’s atmosphere will be less than 10% of today’s value, and possibly even less than 1%, largely due to surface heating from increased solar luminosity. The authors also note that their model begins with a robust terrestrial biosphere, which may not be representative of many habitable exoplanets. However, model results from analyses without terrestrial biospheres did not significantly deviate from their original findings of a one-billion-year future lifespan for Earth’s oxygen-rich atmosphere.

These results imply that the habitability marker of molecular oxygen in an oxygen-rich atmosphere may only be detectable during a fraction of time throughout an exoplanet’s history, e.g., approximately 20-30% of Earth’s total habitable lifetime. In other words, the absence of oxygen-rich atmospheric markers in remote detections from exoplanets does not necessarily rule out the potential for past or future life and could lead to a false-negative conclusion in the search for life. In this way, the study highlights the importance for improved models of the redox balance between planetary interiors and surface biospheres, as well as evaluating the potential habitability of exoplanets without obvious oxygen-related biosignatures in their present-day atmospheres. READ MORE