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Project Progress

This research is divided into two main areas, the development and identification of novel biosignatures, and the in depth study of the robustness of known biosignatures by searching for possible false positives, i.e. planetary processes that may mimic a known biosignature.

In novel biosignatures, Schwieterman and Meadows collaborated with Cockell of the UK Center for Astrobiology to conduct an interdisciplinary study of the diversity and detectability of non-photosynthetic pigments as biosignatures, especially for halophiles (Schwieterman et al., 2015a). This study included reflectance spectra measurements of a diverse collection of pigmented non-photosynthetic organisms and an analysis of the remote detectability of analogs of these organisms in disk-averaged spectra, with a planetary environment that includes other surfaces, an atmosphere and clouds. These spectra are now available through the existing VPL Biological Pigments Database (http://vplapps.astro.washington.edu/pigments).

In other innovative work on biosignature definition, Krissansen-Totton, Catling and colleague tested the idea that atmospheric chemical disequilibrium can act as a biosignature. They performed the first detailed quantification of thermodynamic disequilibrium in Solar System planetary atmospheres, presenting their work at AbSciCon 2015 (Krissansen Totton et al., 2015 and eventually published as (Krissansen-Totton et al., 2016). The work involved calculating the equilibrium abundances for atmospheric gases using Gibbs free energy minimization, and then calculating the Gibbs energy difference between observed and equilibrium states of an atmosphere to quantify the degree of disequilibrium. They showed that the Earth has ~20 times the thermodynamic chemical disequilibrium of other planets because of the biosphere. They identified that most of the disequilibrium comes from the coexistence of O₂, N₂ and (liquid) H₂O instead of thermodynamically more stable nitric acid. This strong disequilibrium is due to biology: photosynthesis supplies the O₂ directly while N₂ is produced indirectly through denitrification that uses organic matter.

Work is continuing on the topic of biosignatures and disequilibrium with an effort to identify “anti-biosignatures”, where the free energy of abiotic disequilibrium is present but there is no life on a planet’s surface to exploit the free energy. They began by examining the minimum biosphere compatible with carbon monoxide (CO) on Mars, which ordinarily is consumed by a large range of microbes with overall reaction CO + H₂O → CO₂ + H₂. This is done by introducing a biological sink to a 1D Mars photochemical model and finding the maximum possible biological sink consistent with the observed mixing ratios (Fig. 2). This biological sink can then be converted to an approximate biomass. Preliminary results suggest the maximum Martian biomass that could be supported by CO metabolism in the top 1 km of regolith is 0.01 cells/cm³. This biomass is approximately equivalent to less than 100 blue whales across the entire surface of Mars, which is vanishingly small compared to the Earth’s biomass. This work also involved graduate student Steven Sholes, who has been running 1D Mars photochemical models. Future work will examine kinetic measures of disequilibrium rather than thermodynamic ones.

In the area of false positives for previously considered robust biosignatures, this past year, three separate modeling investigations found cases in which O₂ and/or O₃ could build up abiotically in an exoplanet’s atmosphere. This builds on our work in other tasks (Living Planet, Observer, and Earth Through Time) which has shown us how planetary system components act and interact in complex ways. Gao et al. (2015) used VPL models to show that for a planet orbiting an M dwarf with a high FUV/NUV flux ratio—if the planetary atmosphere is dry and CO₂-dominated—then recombination of the products of CO₂ photolysis is inhibited. The atmosphere can then build up CO and O₂ in 103-104 years, with abundances of abiotic O₂ and O₃ rivaling that of modern Earth (Gao et al. 2015). In this case, the abiotic production of the O₂ is given away by the lack of water in the planetary spectra. However, via the same mechanism of CO₂ photolysis, Harman et al. (2015) did a comparison of stellar types, and showed that for planets with liquid water, while abiotic O₂ should not accumulate to detectable levels around F and G stars, with K and M stars, the low near-UV flux may allow build-up of O₂ if the sinks for O₂ are low; meanwhile, O₃ could be detectable for a wider range of stars (Harman et al. 2015). Instead of CO₂, Luger and Barnes (2015) looked at extreme atmospheric H₂O loss during the early high luminosity phase of M dwarfs for planets. For planets that form at the position that will become the 5-Gyr habitable zone, atmospheric evolution is sufficiently severe that the planet can generate large amounts of atmospheric O₂ while retaining liquid water (Luger and Barnes, 2015). Thus O₂ and O₃ detection alone are not robust biosignatures but must be accompanied by knowledge of stellar parameters and a more comprehensive census of atmospheric composition and conditions on terrestrial exoplanets to rule out false positives.

As the direct detection of nitrogen would provide a means to characterize the bulk atmosphere of potentially habitable exoplanets and constrain the likelihood of oxygen production by abiotic processes, Schwieterman et al. (2015) explored a novel way to detect and quantify N₂ in planetary atmospheres. Although the N₂ molecule is extremely challenging to observe in exoplanet spectra, N₂ has a collisional-induced absorption band near 4.2 µm, which is significant in Earth’s spectrum and potentially in those of Earth-like exoplanets with similarly N₂-dominated atmospheres. The VPL team quantified the potential magnitude of this spectral signature by producing synthetic transit transmission and radiance spectra using VPL radiative transfer models (Schwieterman et al. 2015b).

To determine how to best discriminate abiotic accumulation of O₂/O₃ from photochemical and atmospheric escape, VPL team members Schwieterman, Meadows, Domagal-Goldman, Deming, Arney, Lugar, Harman, Misra and Barnes collaborated on work that produces the first self-consistent transmission spectra of potentially habitable planets with abiotic O₂/O₃, including estimates of noise using a JWST simulator (Schwieterman et al., 2016). They found that detection of CO (at 2.35, 4.6 um) with CO₂ (1.65, 2.0, and 4.3 um) or O₄ (1.06, 1.27 um) in transmission spectra would be indicative of abiotic O₂ accumulation and could be detected by JWST assuming photon-limited noise. Strong O₄ bands seen in reflected light (at 0.345, 0.36, 0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by future direct-imaging telescopes would indicate an oxygen atmosphere too massive to be biologically produced. This kind of work strengthens our ability to identify true biosignatures by discriminating against the “false positives” when the first spectroscopic studies of rocky habitable zone planets begin.

Finally, we leveraged the knowledge on this task for critical inputs to multiple mission teams, which we anticipate continuing in coming years. This includes the direct participation of VPL team members Meadows and Domagal-Goldman on the Exo-C and Exo-S teams, respectively, and the participation of Domagal-Goldman on the ATLAST Study Team. It also includes presentations by team members Arney, Schwieterman, and Lustig-Jaeger to the ATLAST team. Finally, there were formal publications on telescope architectures and detectors (France et al., 2015; Raucher et al., 2015; Stark et al., 2015) on which we directly participated, or on which our work was acknowledged (e.g., “From Cosmic Birth to Living Earths”, AURA Report, 2015).