A Biomass-Based Model to Estimate the Plausibility of Exoplanet Biosignature Gases

Abstract

Biosignature gas detection is one of the ultimate future goals for exoplanet atmosphere studies. We have created a framework for linking biosignature gas detectability to biomass estimates, including atmospheric photochemistry and biological thermodynamics. The new framework is intended to liberate predictive atmosphere models from requiring fixed, Earth-like biosignature gas source fluxes. New biosignature gases can be considered with a check that the biomass estimate is physically plausible. We have validated the models on terrestrial production of NO, H 2 S, and CH 4 , CH 3 Cl, and DMS. We have applied the models to propose NH 3 as a biosignature gas on a "cold Haber World" a planet with an N 2 -H 2 atmosphere, and to demonstrate why gases such as CH 3 Cl must have too great of a biomass to be a plausible biosignature gas on planets with Earth or early-Earth-like atmospheres orbiting a sun-like star. To construct the biomass models we developed a functional classification of biosignature gases, and found that gases (such as CH 4 , H 2 S, N 2 O) produced from life that extracts energy from chemical potential energy gradients will always have false positives because geochemistry has the same gases to work with as life does, and gases (such as DMS, CH 3 Cl) produced for secondary metabolic reasons are far less likely to have false positives, but because of their highly specialized origin are more likely to be produced in small quantities. The biomass model estimates are valid to one or two orders of magnitude; the goal is an independent approach to testing whether a biosignature gas is plausible rather than on a precise quantification of atmospheric biosignature gases and their corresponding biomasses.

BIOSIGNATURE GASES IN H2-DOMINATED ATMOSPHERES ON ROCKY EXOPLANETS

“…A biomass model estimate has been developed by Seager et al (2013) that ties biomass surface density to a given biosignature gas surface source flux. The motivating rationale is that with a biomass estimate, biosignature gas source fluxes can be free parameters in model predictions and therefore provide a physical plausibility check in terms of reasonable biomass.…”

“…The biomass model estimates are valid to one or two orders of magnitude. We provide a summary of the biomass model here with the full details available in Seager et al (2013).…”

“…The variations studied include dimethylsulfide (DMS; Pilcher 2003), methyl chloride (CH 3 Cl; Segura et al 2005), and other sulfur compounds including CS 2 and COS (Domagal-Goldman et al 2011). We refer the reader to Seager et al (2012) for a review, Seager et al (2013) for a biosignature gas classification scheme, and Seager et al (2013) for a biomass model estimate intended as a plausibility check to consider biosignature gas surface fluxes different from Earth values.…”

Galactic cosmic rays on extrasolar Earth-like planets

“…False-positive detection of molecules such as CH 4 and O 3 is discussed by von . Seager et al (2013) present a biosignature gas classification. Since abiotic processes cannot be ruled out for individual molecules (e.g.…”

Planet hunters seek new ways to detect alien life

“…These include dimethyl sulfide 3 , which is produced by Earthly phytoplankton, or even ammonia 4 . On a cold alien planet, organisms might make the gas using the same chemical process as industrial manufacturers.…”