Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres

Low-temperature highly efficient catalytic removal of odorous carbonyl sulfide by facile regulating CeO2 morphologies

Unraveling the water activation is essential in the catalytic hydrolysis of organic sulfur compounds, yet its intrinsic mechanism of the water-promoting effect is still unclear. In this work, we describe novel findings of oxygen vacancy (VO) engineering by facile regulating CeO2 nanocatalysts with different shapes (rod, octahedral, sphere, and cube) for COS hydrolysis at lower temperature, aiming at understanding the structural origin of the excellent catalytic hydrolysis activity. Unexpectedly, among CeO2 catalysts with different morphologies, spherical CeO2 (CeO2-S) catalysts can achieve completely conversion of COS at 60 ℃ and maintain 30 hours of non-deactivation, which is a significant improvement in catalytic activity and reaction temperature compared to previously reported catalysts. Through various characterizations and results analysis, it is obvious to see that the more spontaneous formation VO on CeO2-S catalysts synergistically induced the water activation and dissociation thus result in the generation of more surface active hydroxyl groups (-OH), which contributes to the enhanced performance of COS catalytic hydrolysis at lower temperature. The promoting effect of catalyst morphology changes on COS hydrolysis were furthering analyzed using in situ DRIFTS and DFT calculations, and revealed that the exposed (111) crystal plane of CeO2 exhibits the strongest adsorption capacity for COS. Notably, CeO2-S also exhibited good catalytic performance and stability towards to other typical organic sulfur compounds (COS and CS2), which is beneficial for the wide application at complex operating conditions. This study provides new insights for designing OH-rich CeO2 catalysts to remove single as well as multi-component organic sulfur compounds for different applications at lower temperatures.

Extreme smells-microbial production of volatile organic compounds at the limits of life

Microbial volatile organic compounds (MVOCs) are diverse molecules produced by microorganisms, ranging from mere waste byproducts to important signalling molecules. While the interest in MVOCs has been increasing steadily, there is a significant gap in our knowledge of MVOCs in extreme environments with e.g. extreme temperatures or acidity. Microorganisms in these conditions are subjected to additional stress compared to their counterparts in moderate environments and in many cases have evolved unique adaptations, including the production of specialized MVOCs. This review highlights the diversity of MVOCs identified in extreme environments or produced by isolated extremophiles. Furthermore, we explore potential applications already investigated and discuss broader implications for biotechnology, environmental biology, and astrobiology.

Unravelling the prebiotic origins of the simplest α-ketoacids in cometary ices: a computational investigation

We have employed the ab initio nanoreactor (AINR) and DFT calculations to explore how the soft impact of comets entering early earth's dense atmosphere could induce chemical reactions in trapped interstellar ice components, leading to the origin of glyoxylic and pyruvic acids the simplest α-ketoacids essential for prebiotic metabolic cycles.

Volatile Organic Compound Metabolism on Early Earth

Biogenic volatile organic compounds (VOCs) constitute a significant portion of gas-phase metabolites in modern ecosystems and have unique roles in moderating atmospheric oxidative capacity, solar radiation balance, and aerosol formation. It has been theorized that VOCs may account for observed geological and evolutionary phenomena during the Archaean, but the direct contribution of biology to early non-methane VOC cycling remains unexplored. Here, we provide an assessment of all potential VOCs metabolized by the last universal common ancestor (LUCA). We identify enzyme functions linked to LUCA orthologous protein groups across eight literature sources and estimate the volatility of all associated substrates to identify ancient volatile metabolites. We hone in on volatile metabolites with confirmed modern emissions that exist in conserved metabolic pathways and produce a curated list of the most likely LUCA VOCs. We introduce volatile organic metabolites associated with early life and discuss their potential influence on early carbon cycling and atmospheric chemistry.

Linking Methanogenesis in Low-Temperature Hydrothermal Vent Systems to Planetary Spectra: Methane Biosignatures on an Archean-Earth-like Exoplanet

In this work, the viability of the detection of methane produced by microbial activity in low-temperature hydrothermal vents on an Archean-Earth-like exoplanet in the habitable zone is explored via a simplified bottom-up approach using a toy model. By simulating methanogens at hydrothermal vent sites in the deep ocean, biological methane production for a range of substrate inflow rates was determined and compared to literature values. These production rates were then used, along with a range of ocean floor vent coverage fractions, to determine likely methane concentrations in the simplified atmosphere. At maximum production rates, a vent coverage of 4-15 × 10^-4 % (roughly 2000-6500 times that of modern Earth) is required to achieve 0.25% atmospheric methane. At minimum production rates, 100% vent coverage is not enough to produce 0.25% atmospheric methane. NASA's Planetary Spectrum Generator was then used to assess the detectability of methane features at various atmospheric concentrations. Even with future space-based observatory concepts (such as LUVOIR and HabEx), our results show the importance of both mirror size and distance to the observed planet. Planets with a substantial biomass of methanogens in hydrothermal vents can still lack a detectable, convincingly biological methane signature if they are beyond the scope of the chosen instrument. This work shows the value of coupling microbial ecological modeling with exoplanet science to better understand the constraints on biosignature gas production and its detectability.

The case and context for atmospheric methane as an exoplanet biosignature

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH4 that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO2 with comparatively little CO.

Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres

Ammonia (NH₃) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH₃ source. The conditions required for NH₃ to accumulate in the atmosphere are, however, stringent. NH₃'s high water solubility and high biousability likely prevent NH₃ from accumulating in the atmosphere to detectable levels unless life is a net source of NH₃ and produces enough NH₃ to saturate the surface sinks. Only then can NH₃ accumulate in the atmosphere with a reasonable surface production flux. For the highly favorable planetary scenario of terrestrial planets with hydrogen (H₂)-dominated atmospheres orbiting M dwarf stars (M5V), we find that a minimum of about 5 ppm column-averaged mixing ratio is needed for NH₃ to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH₃ (i.e., there are no NH₃-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 10¹⁰ molecules/(cm²·s), comparable with the terrestrial biological production of methane (CH₄). However, when the surface is unsaturated with NH₃, due to additional sinks present on the surface, life would have to produce NH₃ at surface flux levels on the order of 10¹⁵ molecules/(cm²·s) (∼4.5 × 10⁶ Tg/year). This value is roughly 20,000 times greater than the biological production of NH₃ on the Earth and about 10,000 times greater than Earth's CH₄ biological production. Volatile amines have similar solubilities and reactivities to NH₃ and hence share NH₃'s weaknesses and strengths as a biosignature. Finally, to establish NH₃ as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH₃'s atmospheric production.

Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres

The search for possible biosignature gases in habitable exoplanet atmospheres is accelerating, although actual observations are likely years away. This work adds isoprene, C₅H₈, to the roster of biosignature gases. We found that isoprene geochemical formation is highly thermodynamically disfavored and has no known abiotic false positives. The isoprene production rate on Earth rivals that of methane (CH₄; ∼500 Tg/year). Unlike methane, on Earth isoprene is rapidly destroyed by oxygen-containing radicals. Although isoprene is predominantly produced by deciduous trees, isoprene production is ubiquitous to a diverse array of evolutionary distant organisms, from bacteria to plants and animals-few, if any, volatile secondary metabolites have a larger evolutionary reach. Although non-photochemical sinks of isoprene may exist, such as degradation of isoprene by life or other high deposition rates, destruction of isoprene in an anoxic atmosphere is mainly driven by photochemistry. Motivated by the concept that isoprene might accumulate in anoxic environments, we model the photochemistry and spectroscopic detection of isoprene in habitable temperature, rocky exoplanet anoxic atmospheres with a variety of atmosphere compositions under different host star ultraviolet fluxes. Limited by an assumed 10 ppm instrument noise floor, habitable atmosphere characterization when using James Webb Space Telescope (JWST) is only achievable with a transit signal similar or larger than that for a super-Earth-sized exoplanet transiting an M dwarf star with an H₂-dominated atmosphere. Unfortunately, isoprene cannot accumulate to detectable abundance without entering a run-away phase, which occurs at a very high production rate, ∼100 times the Earth's production rate. In this run-away scenario, isoprene will accumulate to >100 ppm, and its spectral features are detectable with ∼20 JWST transits. One caveat is that some isoprene spectral features are hard to distinguish from those of methane and also from other hydrocarbons containing the isoprene substructure. Despite these challenges, isoprene is worth adding to the menu of potential biosignature gases.

Observational Constraints on the Great Filter

The search for spectroscopic biosignatures with the next generation of space telescopes could provide observational constraints on the abundance of exoplanets with signs of life. An extension of this spectroscopic characterization of exoplanets is the search for observational evidence of technology, known as technosignatures. Current mission concepts that would observe biosignatures from ultraviolet to near-infrared wavelengths could place upper limits on the fraction of planets in the Galaxy that host life, although such missions tend to have relatively limited capabilities of constraining the prevalence of technosignatures at mid-infrared wavelengths. Yet searching for technosignatures alongside biosignatures would provide important knowledge about the future of our civilization. If planets with technosignatures are abundant, then we can increase our confidence that the hardest step in planetary evolution-the Great Filter-is probably in our past. But if we find that life is commonplace while technosignatures are absent, then this would increase the likelihood that the Great Filter awaits to challenge us in the future.

Insights Into the Origin of Life: Did It Begin from HCN and H2O?

The seminal Urey-Miller experiments showed that molecules crucial to life such as HCN could have formed in the reducing atmosphere of the Hadean Earth and then dissolved in the oceans. Subsequent proponents of the "RNA World" hypothesis have shown aqueous HCN to be the starting point for the formation of the precursors of RNA and proteins. However, the conditions of early Earth suggest that aqueous HCN would have had to react under a significant number of constraints. Therefore, given the limiting conditions, could RNA and protein precursors still have formed from aqueous HCN? If so, what mechanistic routes would have been followed? The current computational study, with the aid of the ab initio nanoreactor (AINR), a powerful new tool in computational chemistry, addresses these crucial questions. Gratifyingly, not only do the results from the AINR approach show that aqueous HCN could indeed have been the source of RNA and protein precursors, but they also indicate that just the interaction of HCN with water would have sufficed to begin a series of reactions leading to the precursors. The current work therefore provides important missing links in the story of prebiotic chemistry and charts the road from aqueous HCN to the precursors of RNA and proteins.

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.