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Phosphine and the importance of unusual biosignatures

Presentation #1028 in the session “Open Engagement Session A”.

Published onMar 17, 2021
Phosphine and the importance of unusual biosignatures

At the edge of our present scientific frontier lies the question: “Can we identify the signs of life on an exoplanet?”. Establishing whether a planet is habitable, or inhabited, relies both on the detection of potential biosignature gases and, crucially, the interpretation of their presence. Beyond the most discussed biosignature gas O2, only a handful of gases have been considered in detail. Here we present a broad and thorough investigation on the biosignature potential of phosphine (PH3), as an example of an unusual, but promising biosignature gas.

We first explore phosphine’s ecology and biochemistry. On Earth, PH3 is extremely toxic to aerobic life, but is produced in anoxic ecosystems. We then analyze the biosignature potential of PH3, considering not just its spectroscopic potential, but also its photochemistry, thermodynamics, and detectability.

An ideal biosignature gas a) lacks abiotic false positives, b) has distinguishable spectral features, and c) is sufficiently unreactive to build up to detectable concentrations in exoplanet atmospheres. Phosphine fulfills the first two criteria: on Earth, PH3 is only known to be associated with life and geochemical false positives for PH3 generation on potentially habitable planets are highly unlikely; PH3 possesses three strong features in that are distinguishable from common outgassed species that may be present in terrestrial exoplanet atmospheres, such as CO2, H2O, CO, CH4, NH3, and H2S. Phosphine’s weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We simulate the atmospheres of habitable terrestrial planets with CO2- and H2-dominated atmospheres and find that PH3 can accumulate to detectable concentrations on planets with surface production fluxes of 1010 to 1014 cm-2 s-1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and UV irradiation. While high, these PH3 surface flux values are comparable to the global terrestrial production rate of methane or CH4 (1011 cm-2 s-1) and below the maximum local terrestrial PH3 production rate (1014 cm-2 s-1). The main conclusion of the presented work is that PH3 is a promising biosignature on oxygen-poor rocky planets, where it has no known abiotic false positives from any source that could generate the high fluxes required for detection.

Whether alien life will produce familiar gases (e.g., oxygen) or exotic biosignatures (e.g., phosphine), painting a confident picture of a potential biosphere will require a holistic interpretation of an atmosphere and its molecules. In this talk we will describe the ongoing efforts to decipher potential biospheres through the identification of volatile molecules, in particular those that might be produced by non-Earth-like life on exoplanets.

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