Phosphine as a Biosignature Gas in Exoplanet Atmospheres

Abstract

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O${}_2$, only a handful of gases have been considered in detail. Here we evaluate phosphine (PH${}_3$). On Earth, PH${}_3$ is associated with anaerobic ecosystems, and as such is a potential biosignature gas on anoxic exoplanets. We simulate CO${}_2-$ and H${}_2-$dominated habitable terrestrial planet atmospheres. We find that PH${}_3$ can accumulate to detectable concentrations on planets with surface production fluxes of 10${}^{10}$-10${}^{14}$ cm${}^{-2}$ s${}^{-1}$ (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and UV flux. While high, such surface fluxes are comparable to the global terrestrial production rate of CH${}_4$ (10${}^{11}$ cm${}^{-2}$ s${}^{-1}$) and below the maximum local terrestrial PH${}_3$ production rate (10${}^{14}$ cm${}^{-2}$ s${}^{-1}$). As with other gases, PH${}_3$ can more readily accumulate on low-UV planets, e.g. planets orbiting quiet M-dwarfs or with a photochemical UV shield. If detected, PH${}_3$ is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets that could generate the high fluxes required for detection. PH${}_3$ also has 3 strong spectral features such that in any atmosphere scenario 1 of the 3 will be unique compared to other dominant spectroscopic molecules. PH${}_3$'s weakness as a biosignature gas is its high reactivity, requiring high outgassing for detectability. We calculate that 10s of hours of JWST time are required for a potential detection of PH${}_3$. Yet because PH${}_3$ is spectrally active in the same wavelength regions as other atmospherically important molecules (e.g., H${}_2$O and CH${}_4$), searches for PH${}_3$ can be carried out at no additional observational cost to searches for other molecules relevant to exoplanet habitability.

Accepted to Astrobiology. Expected publication info: Volume 20, Issue 2

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Cited by in corpus (34)

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• The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres
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• The Composition and Chemistry of Titan's Atmosphere
• Atmospheres of Rocky Exoplanets
• Phosphine on Venus Cannot be Explained by Conventional Processes
• Re-analysis of the 267-GHz ALMA observations of Venus: No statistically significant detection of phosphine
• Claimed detection of PH${}_3$ in the clouds of Venus is consistent with mesospheric SO${}_2$
• The Impending Opacity Challenge in Exoplanet Atmospheric Characterization
• The statistical reliability of 267 GHz JCMT observations of Venus: No significant evidence for phosphine absorption
• Super-Earths and Earth-like Exoplanets
• Phosphine in the Venusian Atmosphere: A Strict Upper Limit from SOFIA GREAT Observations
• Understanding planetary context to enable life detection on exoplanets and test the Copernican principle
• Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres
• Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
• Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres
• Chemical complexity of phosphorous bearing species in various regions of the Interstellar medium
• Coexistence of CH4, CO2 and H2O in exoplanet atmospheres
• Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures
• Large Interferometer For Exoplanets (LIFE): VIII. Where is the phosphine? Observing exoplanetary PH3 with a space based MIR nulling interferometer
• Computational Infrared Spectroscopy of 958 Phosphorus-bearing Molecules
• Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf
• A Precursor Balloon Mission for Venusian Astrobiology
• Venusian phosphine: a 'Wow!' signal in chemistry?
• H${}_2$-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets
• Fully fluorinated non-carbon compounds NF3 and SF6 as ideal technosignature gases
• Recovering Phosphine in Venus' Atmosphere from SOFIA Observations
• Revealing a main-sequence star that consumed a planet with JWST
• A Biotic Habitable Zone: Impacts of Adaptation in Biotic Temperature Regulation
• Can Carbon Fractionation Provide Evidence for Aerial Biospheres in the Atmospheres of Temperate Sub-Neptunes?
• The DRAKE mission: finding the frequency of life in the Cosmos
• Using HITRAN to Model Opacities for Planetary Atmospheres: Test case of Microwave Spectra of NH${}_3$, SO${}_2$ and PH${}_3$
• Necessary Conditions for Earthly Life Floating in the Venusian Atmosphere