Biosignature Gases in H2-Dominated Atmospheres on Rocky Exoplanets (1309.6016v1)

Abstract: (Abridged) Super Earth exoplanets are being discovered with increasing frequency and some will be able to retain stable H2-dominated atmospheres. We study biosignature gases on exoplanets with thin H2 atmospheres and habitable surface temperatures, by using a model atmosphere with photochemistry, and biomass estimate framework for evaluating the plausibilty of a range of biosignature gas candidates. We find that photochemically produced H atoms are the most abundant reactive species in H2 atmospheres. In atmospheres with high CO2 levels, atomic O is the major destructive species for some molecules. In sun-Earth-like UV radiation environments, H (and in some cases O) will rapidly destroy nearly all biosignature gases of interest. The lower UV fluxes from UV quiet M stars would produce a lower concentration of H (or O) for the same scenario, enabling some biosignature gases to accumulate. The favorability of low-UV radiation environments to in an H2 atmosphere is closely analogous to the case of oxidized atmospheres, where photochemically produced OH is the major destructive species. Most potential biosignature gases, such as DMS and CH3Cl are therefore more favorable in low UV, as compared to solar-like UV, environments. A few promising biosignature gas candidates, including NH3 and N2O, are favorable even in solar-like UV environments, as these gases are destroyed directly by photolysis and not by H (or O). A more subtle finding is that most gases produced by life that are fully hydrogenated forms of an element, such as CH4, H2S, are not effective signs of life in an H2-rich atmosphere, because the dominant atmospheric chemistry will generate such gases abiologically, through photochemistry or geochemistry. Suitable biosignature gases in H2-rich atmospheres for super Earth exoplanets transiting M stars could potentially be detected in transmission spectra with the JWST.

• The paper demonstrates that low-UV environments enable the accumulation of biosignature gases on hydrogen-rich exoplanets.
• It employs advanced photochemical models and a biomass framework to assess NH₃ as a reliable indicator of biological activity.
• The study outlines the conditions under which reactive species like H and O limit biosignature gas build-up, guiding future telescope observations.

Analyzing Biosignature Gases in Hydrogen-Rich Atmospheres on Rocky Exoplanets

The research outlined in "Biosignature Gases in H₁₂-Dominated Atmospheres on Rocky Exoplanets" provides a comprehensive exploration into the potential for identifying biosignature gases on exoplanets with hydrogen-dominated atmospheres. This paper, seminal in its ambition, investigates the atmospheric chemistry of such worlds to identify whether certain gases can indicate the presence of life, a query propelled by the burgeoning discovery of exoplanets.

Central to this paper is the investigation of how hydrogen-dominant atmospheres support or hinder the accumulation of potential biosignature gases. Utilizing a sophisticated model inclusive of photochemistry simulations, the paper evaluates the interaction of various gas candidates with atmospheric components, primarily focusing on how significant hydrogen (H) atoms and, under specific conditions, atomic oxygen (O) serve as the major sinks for potential biosignatures due to their reactive nature in such environments.

The paper methodically details the conditions under which potential biosignature gases could accumulate or be destroyed. It places particular emphasis on low-UV radiation scenarios, such as those provided by quiescent M dwarf stars, which are found to support the build-up of detectable biosignature gases due to reduced photolytic destruction mechanisms. This contrasts with high-UV environments where biosignature gases are quickly degraded due to photochemical processes dominated by H and, occasionally, O.

Significant results from this paper include the identification of promising biosignature gases. The paper details that NH₃ (ammonia) emerges as a viable biosignature gas, particularly because it cannot easily be produced from geological processes in significant concentrations in these hydrogen-rich atmospheres, thus making it a potent indicator of biological activity. However, NH₃'s sensitivity to stellar UV radiation necessitates consideration of stellar characteristics when assessing its potential as a biosignature.

Moreover, the paper highlights a methodological approach by introducing a biomass model estimate framework. This framework is pivotal as it assesses whether the concentration of a biosignature gas required for detection corresponds to physically plausible surface biomass densities, thereby enhancing the rigor of the proposed biosignature candidates.

Practically and theoretically, this research broadens the search space for biosignatures beyond Earth-like conditions. It encourages examining planets with varying atmospheric compositions and environments, notably those orbiting M-type stars. Furthermore, the paper paves the way for future research in atmospheric science, emphasizing the potential application of the upcoming James Webb Space Telescope to examine transmission spectra of exoplanet atmospheres.

Future endeavors could extend these findings by exploring additional atmospheric compositions beyond the 90% H₂ and 10% N₂ model, potentially examining how other reducing agents might interact differently in hydrogen-rich atmospheres. Additionally, further simulation of star-exoplanet interactions, particularly for varied levels of stellar activity and UV emission spectra, would be valuable. This research delineates a rich field of inquiry into the biosignature potential of exoplanetary environments markedly different from Earth's own.