Biosphere False Positives from Thin Air: Abiotic Mechanisms for O2 and O3 Generation in Planetary Stratospheres
Abstract
The search for life on exoplanets must be done in the context of a rigorous understanding of "false positives" for life. These false positives are non-biological processes with the potential to produce the signals that would otherwise be interpreted as biosignatures. Here, we present studies of two such "false positives" mechanisms. The processes we present here are most relevant to transit spectroscopy, as this observational technique is likely to be the first to probe the compositions of of rocky exoplanet atmospheres via the James Webb Space Telescope and ground-based Extremely-Large Telescopes (ELTs). Because this observational technique is more sensitive to the lower-most parts of the stratosphere, we have modeled extreme ways to produce stratospheric O2 and O3, two gases that are potential biosignatures. In one mechanism, oxygen is delivered via upwelling of H2O molecules, a process that can be intensified at the sub-stellar point of tidally-locked, synchronously rotating ocean-bearing exoplanets. This delivers significant quantities of H2O to the stratosphere, which can lead to O2 and O3 accumulation. In the second mechanism, O (or H2O) is delivered from above, and ultimately sourced from O (or H2O) that escaped another planet's atmosphere. This is similar to - but an amplified version of - the influence of O from Enceladus on the atmospheric chemistry of Titan. For various reasons, both of these mechanisms should be more intense for planets in orbit around M-type stars; these are also the best observational targets for transit spectroscopy and are often highlighted as high-priority targets for both JWST and ELT's. Overall, we find that both of these mechanisms are at some level theoretically plausible. However, we also find that both mechanisms are unlikely to produce problematic false positives for multiple practical reasons. These include: the lack of detectability of features, with the unlikely exception of cloud-free atmospheres; the massive fluxes required to generate O2 or O3; limits to the reasonable magnitude of the fluxes these mechanisms could produce; and the very limited number of cases for which both CH4 and one of O2/O3 are simultaneously detectable. Thus, as with other false positives, the combined detection of O2 or O3 with CH4 remains a much stronger biosignature for planets/biospheres similar to modern-day Earth than the detection of O2 or O3 alone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMP018...01D
- Keywords:
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- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6296 Extra-solar planets;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 5455 Origin and evolution;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS