Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres
Name
ast.2019.2146.pdf
Description
Published version
Size
1.93 MB
Format
Adobe PDF
Checksum (MD5)
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Author(s)
Zhan, Zhuchang
Seager, Sara
Petkowski, Janusz Jurand
Sousa-Silva, Clara
Ranjan, Sukrit
Huang, Jingcheng
Bains, William
Date Issued
2021
Journal
Astrobiology
Publisher
Mary Ann Liebert Inc
Version
Final published version
Abstract
Research for possible biosignature gases on habitable exoplanet atmosphere is
accelerating. We add isoprene, C5H8, to the roster of biosignature gases. We
found that formation of isoprene geochemical formation is highly
thermodynamically disfavored and has no known abiotic false positives. The
isoprene production rate on Earth rivals that of methane (~ 500 Tg yr-1). On
Earth, isoprene is rapidly destroyed by oxygen-containing radicals, but its
production is ubiquitous to a diverse array of evolutionarily distant
organisms, from bacteria to plants and animals-few, if any at all, volatile
secondary metabolite has a larger evolutionary reach. While non-photochemical
sinks of isoprene may exist, the destruction of isoprene in an anoxic
atmosphere is mainly driven by photochemistry. Motivated by the concept that
isoprene might accumulate in anoxic environments, we model the photochemistry
and spectroscopic detection of isoprene in habitable temperature, rocky
exoplanet anoxic atmospheres with a variety of atmosphere compositions under
different host star UV fluxes. Limited by an assumed 10 ppm instrument noise
floor, habitable atmosphere characterization using JWST is only achievable with
transit signal similar or larger than that for a super-Earth sized exoplanet
transiting an M dwarf star with an H2-dominated atmosphere. Unfortunately,
isoprene cannot accumulate to detectable abundance without entering a run-away
phase, which occurs at a very high production rate, ~ 100 times Earth's
production rate. In this run-away scenario isoprene will accumulate to > 100
ppm and its spectral features are detectable with ~ 20 JWST transits. One
caveat is that some spectral features are hard to be distinguished from that of
methane. Despite these challenges, isoprene is worth adding to the menu of
potential biosignature gases.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics
Massachusetts Institute of Technology. Department of Chemistry
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DOI of Published Version
https://doi.org/10.1089/AST.2019.2146
