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The search for life on planets outside our solar system will usespectroscopic identification of atmospheric biosignatures. The most robustremotely-detectable potential biosignature is considered to be the detection ofoxygen (O_2) or ozone (O_3) simultaneous to methane (CH_4) at levels indicatingfluxes from the planetary surface in excess of those that could be producedabiotically. Here, we use an altitude-dependent photochemical model with theenhanced lower boundary conditions necessary to carefully explore abiotic O_2and O_3 production on lifeless planets with a wide variety of volcanic gasfluxes and stellar energy distributions. On some of these worlds, we predictlimited O_2 and O_3 build up, caused by fast chemical production of thesegases. This results in detectable abiotic O_3 and CH_4 features in theUV-visible, but no detectable abiotic O_2 features. Thus, simultaneousdetection of O_3 and CH_4 by a UV-visible mission is not a strong biosignaturewithout proper contextual information. Discrimination between biological andabiotic sources of O_2 and O_3 is possible through analysis of the stellar andatmospheric context - particularly redox state and O atom inventory - of theplanet in question. Specifically, understanding the spectral characteristics ofthe star and obtaining a broad wavelength range for planetary spectra shouldallow more robust identification of false positives for life. This highlightsthe importance of wide spectral coverage for future exoplanet characterizationmissions. Specifically, discrimination between true- and false-positives mayrequire spectral observations that extend into infrared wavelengths, andprovide contextual information on the planet's atmospheric chemistry.

Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth