Abstract
Understanding when global glaciations occur on Earth-like planets is a major challenge in climate evolution research. Most models of how greenhouse gases like CO
2
evolve with time on ...terrestrial planets are deterministic, but the complex, nonlinear nature of Earth’s climate history motivates study of nondeterministic climate models. Here a maximally simple stochastic model of CO
2
evolution and climate on an Earth-like planet with an imperfect CO
2
thermostat is investigated. It is shown that as stellar luminosity is increased in this model, the decrease in the average atmospheric CO
2
concentration renders the climate increasingly unstable, with excursions to a low-temperature state common once the received stellar flux approaches that of present-day Earth. Unless climate feedbacks always force the variance in CO
2
concentration to decline rapidly with received stellar flux, this means that terrestrial planets near the inner edge of the habitable zone may enter Snowball states quite frequently. Observations of the albedos and color variation of terrestrial-type exoplanets should allow this prediction to be tested directly in the future.
Nitrogen is the most common element in Earth's atmosphere and also appears to be present in significant amounts in the mantle. However, its long-term cycling between these two reservoirs remains ...poorly understood. Here a range of biotic and abiotic mechanisms are evaluated that could have caused nitrogen exchange between Earth's surface and interior over time. In the Archean, biological nitrogen fixation was likely strongly limited by nutrient and/or electron acceptor constraints. Abiotic fixation of dinitrogen becomes efficient in strongly reducing atmospheres, but only once temperatures exceed around 1000 K. Hence if atmospheric N2 levels really were as low as they are today 3.0–3.5 Ga, the bulk of Earth's mantle nitrogen must have been emplaced in the Hadean, most likely at a time when the surface was molten. The elevated atmospheric N content on Venus compared to Earth can be explained abiotically by a water loss redox pump mechanism, where oxygen liberated from H2O photolysis and subsequent H loss to space oxidises the mantle, causing enhanced outgassing of nitrogen. This mechanism has implications for understanding the partitioning of other Venusian volatiles and atmospheric evolution on exoplanets.
•The early evolution of atmospheric nitrogen on Earth and Venus is studied theoretically.•The biological influence on nitrogen was likely limited in the early Archean.•N fixation becomes effective in a hot reducing Hadean atmosphere.•Mantle oxidation following water loss can explain the atmospheric N2 content of Venus.
► Transient conditions for origin of life on evaporating exoplanets. ► Hydrogen greenhouse warming allows conditions for liquid water. ► Photochemistry in reducing atmosphere causes formation of ...organic precursor molecules. ► Importance of phenomenon will depend on adaptability of life to changing planetary conditions.
Exoplanets with lower equilibrium temperatures than Earth and primordial hydrogen atmospheres that evaporate after formation should pass through transient periods where oceans can form on their surfaces, as liquid water can form below a few thousand bar pressure and H2–H2 collision-induced absorption provides significant greenhouse warming. The duration of the transient period depends on the planet size, starting H2 inventory and star type, with the longest periods typically occurring for planets around M-class stars. As pre-biotic compounds readily form in the reducing chemistry of hydrogen-rich atmospheres, conditions on these planets could be favourable to the emergence of life. The ultimate fate of any emergent organisms under such conditions would depend on their ability to adapt to (or modify) their gradually cooling environment.
The oxidation of rocky planet surfaces and atmospheres, which arises from the twin forces of stellar nucleosynthesis and gravitational differentiation, is a universal process of key importance to ...habitability and exoplanet biosignature detection. Here we take a generalized approach to this phenomenon. Using a single parameter to describe the redox state, we model the evolution of terrestrial planets around nearby M stars and the Sun. Our model includes atmospheric photochemistry, diffusion and escape, line-by-line climate calculations, and interior thermodynamics and chemistry. In most cases, we find abiotic atmospheric buildup around M stars during the pre-main-sequence phase to be much less than calculated previously, because the planet's magma ocean absorbs most oxygen liberated from photolysis. However, loss of noncondensing atmospheric gases after the mantle solidifies remains a significant potential route to abiotic atmospheric subsequently. In all cases, we predict that exoplanets that receive lower stellar fluxes, such as LHS1140b and TRAPPIST-1f and g, have the lowest probability of abiotic buildup and hence may be the most interesting targets for future searches for biogenic . Key remaining uncertainties can be minimized in future by comparing our predictions for the atmospheres of hot, sterile exoplanets such as GJ1132b and TRAPPIST-1b and c with observations.
We propose that the first Neoproterozoic Snowball Earth event, the Sturtian glaciation, was initiated by the injection of sulfate aerosols into the stratosphere. Geochronological data indicate that ...the Natkusiak magmatic assemblage of the Franklin large igneous province coincided with onset of the Sturtian glaciation. The Natkusiak was emplaced into an evaporite basin and entrained significant quantities of sulfur, which would have led to extensive SO2 and H2S outgassing in hot convective plumes. The largest of these plumes could have penetrated the tropopause, leading to stratospheric sulfate aerosol formation and an albedo increase sufficient to force a Snowball. Radiative forcing was maximized by the equatorial location of the Franklin and the cool Neoproterozoic background climate, which would have lowered the tropopause height, increasing the rate of stratospheric aerosol injection. Our results have implications for understanding Phanerozoic mass extinction events, exoplanet habitability, and aerosol perturbations to the present‐day climate.
Key Points
The Franklin large igneous province was coincident with onset of the Sturtian Snowball Earth glaciation
Multiyear to decadal fissure eruptions can inject enough sulfur aerosols to the stratosphere to initiate Snowball Earth
Cold background climate states are sensitive to ice‐albedo catastrophe because tropopause height is a function of surface temperature
The evidence for abundant liquid water on early Mars despite the faint young Sun is a long‐standing problem in planetary research. Here we present new ab initio spectroscopic and line‐by‐line climate ...calculations of the warming potential of reduced atmospheres on early Mars. We show that the strength of both CO2–H2 and CO2–CH4 collision‐induced absorption (CIA) has previously been significantly underestimated. Contrary to previous expectations, methane could have acted as a powerful greenhouse gas on early Mars due to CO2–CH4 CIA in the critical 250–500 cm−1 spectral window region. In atmospheres of 0.5 bar CO2 or more, percent levels of H2 or CH4 raise annual mean surface temperatures by tens of degrees, with temperatures reaching 273 K for pressures of 1.25–2 bars and 2–10% of H2 and CH4. Methane and hydrogen produced following aqueous alteration of Mars' crust could have combined with volcanically outgassed CO2 to form transient atmospheres of this composition 4.5–3.5 Ga. Our results also suggest that inhabited exoplanets could retain surface liquid water at significant distances from their host stars.
Key Points
New ab initio and line‐by‐line calculations show that warming by H2 in CO2 atmospheres is far stronger than previously believed
Contrary to previous research, we show that methane could also have been an effective greenhouse gas on early Mars
Reducing atmospheres could have been produced transiently by a combination of aqueous alteration of Mars' crust and volcanic CO2 outgassing
The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all ...that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.
Collision-induced absorption is of great importance to the overall radiative budget in dense CO2-rich atmospheres, but its representation in climate models remains uncertain, mainly due to a lack of ...accurate experimental and theoretical data. Here we compare several parameterisations of the effect, including a new one that makes use of previously unused measurements in the 1200-1800cma1 spectral range. We find that a widely used parameterisation strongly overestimates absorption in pure CO2 atmospheres compared to later results, and propose a new approach that we believe is the most accurate possible given currently available data.
A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free ...atmosphere. Here we analyze data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogs on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called "mesoscale slope winds," which have horizontal scales of 10-100 s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models.
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