Sukrit Ranjan (UA/LPL)

A Re-Examination of Photochemical False Positives for Oxygen as a Biosignature Gas on Temperate Terrestrial Exoplanets

Exoplanet surveys have revealed temperate terrestrial planets to be common, and current (JWST) and upcoming (GMT/ELT/HWO) facilities aim to characterize their atmospheres in search of signs of life. Oxygen is the strongest single biomarker of life in Earth's atmosphere, but scenarios have been put forward whereby abiotic mechanisms like photochemistry can abiotically generate high levels of oxygen (“false positive scenarios”). Here, we re-examine two of these oxygen false positive scenarios proposed for habitable, abiotic planets with CO2-rich atmospheres: the low-outgassing scenario and the M-dwarf scenario. We use model intercomparisons to rectify long-standing model disagreements regarding these false positive scenarios. We show that discrepant assumptions regarding the near-UV absorption properties of temperate water vapor underly the low-outgassing false positive scenario, place the first-ever laboratory constraints on these cross-sections, and use these constraints to obviate this scenario. We further show that discrepant assumptions regarding planetary upper atmospheres are what drive the M-dwarf scenario, which, when addressed, elimate this false positive scenario as well. Overall, our results strengthen the biosignature gas paradigm for exoplanet life detection in general, and the case for oxygen as a biosignature in particular.

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