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Ozone as a Proxy for Molecular Oxygen

Presentation #102.326 in the session Poster Session.

Published onJun 20, 2022
Ozone as a Proxy for Molecular Oxygen

Molecular oxygen (O2) paired with a reducing gas is regarded as a promising biosignature pair for atmospheric characterization of exoplanets. In circumstances when O2 may not be detectable in a planetary atmosphere (for instance, in the IR wavelength region) it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy to infer the presence of O2. While O3 is not directly produced by life, it plays an important role in habitability as the ozone layer is the primary source of UV shielding for surface life on Earth. However, O3 production is known to have a nonlinear dependence on O2, along with being strongly influenced by the UV spectrum of the planet’s host star. To further evaluate the reliability of O3 as a proxy for O2 we used Atmos, a coupled 1D climate/photochemistry code, to model Earth-like atmospheres of habitable zone planets around a variety of stellar hosts (from G0V-M5V), along with modeling emission spectra of these model atmospheres with the radiative transfer code PICASO. Our models explore the O2-O3 relationship under a range of O2 abundances, along with varying amounts of biologically produced gases that contribute to the destruction of O3. We find that the O2-O3 relationship varies significantly around different stellar hosts, with planets orbiting hotter stars (G0V-K2V) reaching peak O3 levels at O2 abundances of less than 50% present atmospheric levels, while planets orbiting cooler hosts have O3 levels that decrease nonlinearly with O2 levels. Understanding both the chemistry and resulting temperature profiles of a planet’s atmosphere will be key for interpreting emission spectral features of O3 as a biosignature gase.

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