Wednesday 25 Oct 2017New prospects for finding life on exoplanets: Disequilibrium biosignature metrics and their detectability with the James Webb Space Telescope

Mr. Joshua Krissansen-Totton - University of Washington

Physics, 4th Floor 14:00-15:00

Upcoming ground- and space-based telescopes will be capable of characterizing habitable exoplanets and looking for atmospheric gases produced by life. Oxygen is a promising biosignature gas, but several hypothetical scenarios have been proposed for producing abiotic oxygen or its surrogate, ozone. Furthermore, even if these scenarios can be ruled out by other observations, there is no guarantee that oxygenic photosynthesis is a common metabolism, and so oxygen biosignatures may be rare. Atmospheric chemical disequilibrium is potentially a more general biosignature. We have developed a quantitative metric for atmospheric disequilibrium and applied it to the Solar System planets. The biogenic disequilibrium in the modern Earth's atmosphere, which is attributable to the coexistence of O2, N2, and liquid water, dwarfs the photochemically-produced disequilibria of the other Solar System atmospheres. We also applied our disequilibrium metric to the Earth through time, and found that on the anoxic Archean Earth (4-2.5 billion years ago), life maintained a different disequilibrium between CO2, CH4, N2, and liquid water. Such a combination of gases would not persist without significant replenishment of CH4 from the surface, which we have shown to be difficult to explain without life. This leads us to propose the coexistence of CO2 and CH4 in the atmosphere of an ostensibly habitable exoplanet as a potential biosignature. The inference to life could be strengthened by the absence of atmospheric CO, which would be present if CH4 were outgassed and not biogenic. Finally, we evaluate the detectability of this new biosignature with the James Webb Space Telescope for nearby transiting planets such as the TRAPPIST-1 system.

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