Advancements in our understanding of exoplanetary atmospheres, from massive gas giants down to rocky worlds, depend on the constructive challenges between observations and models. We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize the atmospheric structure, composition, and circulation of these worlds. These improvements stem from significant investments in new missions and facilities, such as JWST and the several planned ground-based extremely large telescopes. However, while exoplanet science currently has a wide range of sophisticated models that can be applied to the tide of forthcoming observations, the trajectory for preparing these models for the upcoming observational challenges is unclear. Thus, our ability to maximize the insights gained from the next generation of observatories is not certain.
Here we present four new coupled carbon and quadruple sulphur isotope records from distal, time equivalent (2.7–2.5 Ga), sedimentary successions from South Africa and Western Australia. These extended records reveal similar chemostratigraphic trends, supporting a dynamic terminal-Neoarchaean atmosphere, oscillating between a hazy state at elevated methane concentrations, and a haze-free anoxic background state. We suggest these atmospheric aberrations were related to heightened biogenic methane fluxes fuelled by enhanced nutrient delivery from climatically or weathering induced feedbacks. These data question the canonical view of a simple, unidirectional planetary oxygenation and signify that the overture to the GOE was governed by complex feedbacks within the Earth–biosphere system.
We present a new method to probe atmospheric pressure on Earth-like planets using (O2-O2) dimers in the near-infrared. We also show that dimer features could be the most readily detectable biosignatures for Earth-like atmospheres and may even be detectable in transit transmission with the James Webb Space Telescope (JWST). The absorption by dimers changes more rapidly with pressure and density than that of monomers and can therefore provide additional information about atmospheric pressures.
Here we argue that the second condition was the oxidation of the surface and crust to the point where O2 became more stable than competing reduced gases such as CH4. The cause of Earth’s surface oxidation would be the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum would have been determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O2 became favored. We suggest that continental growth has been influenced by hydrogen escape, and we speculate that, if there must be an external bias to biological evolution, hydrogen escape can be that bias.
Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.
The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star’s main sequence lifetime. We describe the time that a planet spends within the HZ as its “habitable zone lifetime.” The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the “classic” HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a “hybrid” HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79×109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns. Key Words: Exoplanet habitability metrics—Continuously habitable zone—Stellar evolution—Planetary habitability. Astrobiology 13, 833–849.
We present multiple sulfur isotopes for 2.71 Ga pyritic black shales derived from the Kidd Creek area, Ontario, Canada. These samples display high positive Δ33S values up to 3.8‰ and the typical late Archean slope in Δ36S/Δ33S of −0.9. In contrast, the time period before (3.2–2.73 Ga) is characterized by greatly attenuated MIF-S magnitudes and a slope in Δ36S/Δ33S of −1.5. We attribute the increase in Δ33S magnitude as well as the contemporaneous change in the slope of Δ36S/Δ33S to changes in the relative reaction rate of different MIF-S source reactions and changes in atmospheric sulfur exit channels. Both of these are dependent on atmospheric CH4:CO2 and O2 mixing ratios. We propose a distinct change in atmospheric composition at 2.7 Ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed Neoarchean MIF-S record. Our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2.7 billion years ago.
We find evidence for oxygen production in microbial mats and localized oxygenation of surface waters. Carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane, consistent with the prediction of a hydrocarbon haze. We use a photochemical model to corroborate our geochemical data. Our simulations predict transitions between two stable atmospheric states, one with organic haze and the other haze-free. The transitions are presumably governed by variations in the amount of biological methane production during the Archaean eon. We find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the Archaean.
We used one-dimensional photochemical and radiative transfer models to study the potential of organic sulfur compounds (CS2, OCS, CH3SH, CH3SCH3, and CH3S2CH3) to act as remotely detectable biosignatures in anoxic exoplanetary atmospheres. Concentrations of organic sulfur gases were predicted for various biogenic sulfur fluxes into anoxic atmospheres and were found to increase with decreasing UV fluxes. Dimethyl sulfide (CH3SCH3, or DMS) and dimethyl disulfide (CH3S2CH3, or DMDS) concentrations could increase to remotely detectable levels, but only in cases of extremely low UV fluxes, which may occur in the habitable zone of an inactive M dwarf. The most detectable feature of organic sulfur gases is an indirect one that results from an increase in ethane (C2H6) over that which would be predicted based on the planet’s methane (CH4) concentration. Thus, a characterization mission could detect these organic sulfur gases—and therefore the life that produces them—if it could sufficiently quantify the ethane and methane in the exoplanet’s atmosphere. Key Words: Exoplanets—Biosignatures—Anoxic atmospheres—Planetary atmospheres—Remote life detection—Photochemistry. Astrobiology 11, 419–441.
Despite the importance of the anaerobic oxidation of methane (AOM) to global biogeochemical cycles, the relationship between sulfate concentration and the rate of AOM has not been previously experimentally constrained. Here, we present measurements showing substantial methane oxidation at low sulfate concentrations, with no significant decrease in the rate of AOM until sulfate levels are well below 1 mm. At sulfate levels below 1 mm, there appears to be a strong decoupling of AOM and sulfate reduction, with a 13C‐label transferred from methane to carbon dioxide occurring at a rate almost an order of magnitude faster than the observed rate of sulfate reduction. These results allow for the possibility that high rates of AOM occurred in the Archean oceans and that high rates of AOM may be found in freshwater environments (lakes, rivers, etc.) and deep ocean sediments today.
Because perchlorate‐rich deposits in the Atacama desert are closest in abundance to perchlorate measured at NASA’s Phoenix Lander site, we made a preliminary study of the means to produce Atacama perchlorate to help shed light on the origin of Martian perchlorate. We investigated gas phase pathways using a 1‐D photochemical model. We found that perchlorate can be produced in sufficient quantities to explain the abundance of perchlorate in the Atacama from a proposed gas phase oxidation of chlorine volatiles to perchloric acid. The feasibility of gas phase production for the Atacama provides justification for future investigations of gas phase photochemistry as a possible source for Martian perchlorate.
Here we use a radiative–convective climate model to show that more N2 in the atmosphere would have increased the warming effect of existing greenhouse gases by broadening their absorption lines. With the atmospheric CO2 and CH4 levels estimated for 2.5 billion years ago, a doubling of the present atmospheric nitrogen (PAN) level would cause a warming of 4.4 ∘C. Our new budget of Earth’s geological nitrogen reservoirs indicates that there is a sufficient quantity of nitrogen in the crust (0.5 PAN) and mantle (>1.4 PAN) to have supported this, and that this nitrogen was previously in the atmosphere. In the mantle, N correlates with 40Ar, the daughter product of 40K, indicating that the source of mantle N is subducted crustal rocks in which NH4+ has been substituted for K+. We thus conclude that a higher nitrogen level probably helped warm the early Earth, and suggest that the effects of N2 should be considered in assessing the habitable zone for terrestrial planets.
We consider time‐dependent fluxes that include organic carbon burial and associated oxygen production, reducing gases from metamorphic and volcanic sources, oxidative weathering, and the escape of hydrogen to space. We find that the oxic transition occurs in a geologically short time when the O2‐consuming flux of reducing gases falls below the flux of organic carbon burial that produces O2. A short timescale for the oxic transition is enhanced by a positive feedback due to decreasing destruction of O2 as stratospheric ozone forms, which is captured in our atmospheric chemistry parameterization. We show that one numerically self‐consistent solution for the rise of O2 involves a decline in flux of reducing gases driven by irreversible secular oxidation of the crust caused by time‐integrated hydrogen escape to space in the preoxic atmosphere, and that this is compatible with constraints from the geological record. In this model, the timing of the oxic transition is strongly affected by buffers of reduced materials, particularly iron, in the continental crust. An alternative version of the model, where greater fluxes of reduced hydrothermal cations from the Archean seafloor consume O2, produces a similar history of O2 and CH4. When climate and biosphere feedbacks are included in our model of the oxic transition, we find that multiple ‘Snowball Earth’ events are simulated under certain circumstances, as methane collapses and rises repeatedly before reaching a new steady‐state.