Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
Abstract
O2 is known to be one of the most detectable and robust indicators of global biological activity. Concepts for telescopes that would attempt to search for life on exoplanets all include the ability to detect O2 or its photochemical byproduct, O3. The O2 A-band at 0.76 μm has often been considered the most viable spectral feature for oxygen detection in transmission (Snellen et al., 2013) reflectance spectra (Fauchez et al., 2017). Meanwhile, Palle et al., (2009) showed that O2-O2 collision induced absorption (CIA) features at 1.06 and 1.27 μm were present in Earth's transmission spectrum during lunar eclipse and produce more absorption than the O2 A-band monomer feature. In the case of the O2-O2 CIA features, the two O2 molecules interact forming transient multipole-induced dipoles producing broad spectral features distinct from the individual underlying O2 molecule. Misra et al., (2014) showed that these CIA features may be detectable (for SNR > 3) with the James Webb Space Telescope (JWST) for a cloud-free Earth analogue orbiting an M5V star at a distance of 5~pc. Schwieterman et al., (2016) and Lustig-Yaeger et al., (2019) have shown that the 1.06 and 1.27 μm transit features could be used to identify the high O2 partial pressures predicted to be associated with abiotic O2 atmospheres, which should be significantly higher than for the modern Earth case. In this study, we identify a strong mid-infrared oxygen O2 spectral feature at 6.4 μm not previously included in exoplanet modeling studies, but which may be the most detectable O2 feature for transit observations. This feature is broad, allowing low resolving power to observe it to maximize the signal-to-noise ratio. For a potential TRAPPIST-1 analogue system within 5~pc of the Sun, we show that the 6.4 μm O2 -X CIA could be the only O2 detectable signature with JWST for a modern Earth-like cloudy atmosphere with biological quantities of O2. This feature could therefore act as a powerful signal of biotic O2, especially if detected in an atmosphere with CH4 and H2O. Also, we show that the 6.4 μm O2 -X CIA would be prominent for O2-rich desiccated atmospheres that have been postulated for abiotic O2 generation scenarios driven by massive ocean loss (Luger and Barnes, 2015) and could be detectable with JWST in just a few transits for TRAPPIST-1e.
- Publication:
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American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23534304F