The ionized form of atomic oxygen (O+) is the dominant ion species at the altitude of maximum electron density in only one of the many ionospheres in our Solar System — Earth’s. This ionospheric composition would not be present if oxygenic photosynthesis was not an ongoing mechanism that continuously impacts the terrestrial atmosphere. We propose that dominance of ionospheric composition by O+ ions at the altitude of maximum electron density can be used to identify a planet in orbit around a solar-type star where global-scale biological activity is present. There is no absolute numerical value required for this suggestion of an atmospheric plasma biomarker — only the dominating presence of O+ ions at the altitude of peak electron density.
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This work was supported, in part, by an NSF INSPIRE grant to Boston University (Comparative Ionospheric Science: Earth, Solar System and Exoplanets; AST-1545581). We are pleased to acknowledge the thoughtful comments and suggestions by S. Chakrabarti (University of Massachusetts, Lowell). In addition, we thank the following colleagues at Boston University for helpful discussions dealing with early versions of the manuscript: J. Clarke, J. Semeter, M. Mayyasi, L. Moore, and J. Baumgardner and P. Muirhead; at the University of Massachusetts, Lowell: C. Mendillo, and S. Finn; and e-mail correspondents L. Rezac (Max-Planck Institute for Solar System Physics, Germany), J. Kasting (Penn State University), and B. Jakosky (University of Colorado, Boulder). J. Trovato helped with figure preparation.
The authors declare no competing financial interests.
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Mendillo, M., Withers, P. & Dalba, P.A. Atomic oxygen ions as ionospheric biomarkers on exoplanets. Nat Astron 2, 287–291 (2018). https://doi.org/10.1038/s41550-017-0375-y
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