A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1 bar in the atmospheres of Earth1, Titan2, Jupiter3, Saturn4, Uranus and Neptune4, despite great differences in atmospheric composition, gravity, internal heat and sunlight. In all of these bodies, the tropopause separates a stratosphere with a temperature profile that is controlled by the absorption of short-wave solar radiation, from a region below characterized by convection, weather and clouds5,6. However, it is not obvious why the tropopause occurs at the specific pressure near 0.1 bar. Here we use a simple, physically based model7 to demonstrate that, at atmospheric pressures lower than 0.1 bar, transparency to thermal radiation allows short-wave heating to dominate, creating a stratosphere. At higher pressures, atmospheres become opaque to thermal radiation, causing temperatures to increase with depth and convection to ensue. A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause. We reason that a tropopause at a pressure of approximately 0.1 bar is characteristic of many thick atmospheres, including exoplanets and exomoons in our galaxy and beyond. Judicious use of this rule could help constrain the atmospheric structure, and thus the surface environments and habitability, of exoplanets.
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.
The Great Oxidation Event (GOE) was an increase in atmospheric oxygen levels from less than 1 ppm to 0.2–2% by volume 2.4–2.2 billion years ago. In atmospheric chemistry, hydrogen-bearing reduced gases, such as methane and hydrogen, are adversaries of O2. When the concentration of hydrogen-bearing reduced gases goes up, O2 declines, and vice versa. Thus, the pre-GOE atmospheric redox chemistry should have been dominated by methane and hydrogen. Before the GOE, oxygen was driven to trace levels by reactions with volcanic and metamorphic reductants, including dissolved cations (e.g., Fe2 +) in surface waters and reducing gases (H2, CH4, CO, SO2, and H2S). Rapid escape of hydrogen to space from such an atmosphere would have slowly oxidized the Earth. A ‘tipping point’ was reached when the flux of O2 associated with the burial of organic carbon exceeded O2 losses. Oxidative weathering then became significant. Models suggest that methane level fell before the GOE and such loss of greenhouse gases plausibly caused global cooling. Multiple glaciations during 2.45–2.22 Ga hint that the climate and atmospheric composition oscillated until permanent oxygenation was established. Subsequent levels of O2 were sufficient to protect the Earth’s surface from harmful ultraviolet with a stratospheric ozone layer.
We present measurements of three different organic-rich shales of varying ages prepared with eight different sample preparation protocols and identify a method with which high selenium yields are obtained for all three samples while the concentration of germanium is greatly reduced. We further investigate the quantitative importance of isobaric interferences and present new post-analytical data correction protocols. If selenium concentrations in standards and samples are matched to within 5%, the ratios of five isotopes of selenium (74Se, 76Se, 77Se, 78Se and 82Se) can be measured with precisions better than 0.2‰ for δ76/78Se, δ77/78Se and δ82/78Se and 0.5‰ for δ74/78Se, allowing analytical accuracy to be monitored with three-isotope diagrams and thus enabling the detection of any mass-independent isotopic fractionation.
We present a new dynamic thresholding method for computationally separating amygdules from their basaltic matrix in X-ray images that is based on a technique used in seismology. The technique is sensitive to the gradient of the gray-scale value, rather than an absolute threshold value often applied to an entire set of X-ray images. Additionally, we present statistical methods for extrapolating the volumetric measurement mean and standard deviation of amygdules in the measured samples to the entire population in the flow. To do so, we create additional amygdule sample sets from the original sample set in the process of ‘bootstrap’ resampling, and use the Central Limit Theorem to calculate the mean and standard deviation of the amygdule population from these sample sets. This suite of methods allows the extension of bubble-size distribution studies typically done on modern flows to the ancient rock record and potentially has many other uses in geosciences where quantitative discrimination between materials with a range of densities is required.
We find that terrestrial oxidation of pyrite by microbes using oxygen has contributed a substantial fraction of the total sulphur weathering flux since at least 2.5 Gyr ago, with probable evidence of such activity 2.7–2.8 Gyr ago. The late Archaean onset of terrestrial sulphur cycling is supported by marine molybdenum abundance data and coincides with a shift to more sulphidic ocean conditions5. We infer that significant microbial land colonization began by 2.7–2.8 Gyr ago. Our identification of pyrite oxidation at this time provides further support for the appearance6 of molecular oxygen several hundred million years before the Great Oxidation Event.
We present an analytic one-dimensional radiative-convective model of the thermal structure of planetary atmospheres. Our model assumes that thermal radiative transfer is gray and can be represented by the two-stream approximation. Model atmospheres are assumed to be in hydrostatic equilibrium, with a power-law scaling between the atmospheric pressure and the gray thermal optical depth. The convective portions of our models are taken to follow adiabats that account for condensation of volatiles through a scaling parameter to the dry adiabat. By combining these assumptions, we produce simple, analytic expressions that allow calculations of the atmospheric-pressure-temperature profile, as well as expressions for the profiles of thermal radiative flux and convective flux. We explore the general behaviors of our model. These investigations encompass (1) worlds where atmospheric attenuation of sunlight is weak, which we show tend to have relatively high radiative-convective boundaries; (2) worlds with some attenuation of sunlight throughout the atmosphere, which we show can produce either shallow or deep radiative-convective boundaries, depending on the strength of sunlight attenuation; and (3) strongly irradiated giant planets (including hot Jupiters), where we explore the conditions under which these worlds acquire detached convective regions in their mid-tropospheres. Finally, we validate our model and demonstrate its utility through comparisons to the average observed thermal structure of Venus, Jupiter, and Titan, and by comparing computed flux profiles to more complex models.
Here we show that raindrop imprints in tuffs of the Ventersdorp Supergroup, South Africa, constrain surface air density 2.7 billion years ago to less than twice modern levels. We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum. Fragmentation following raindrop flattening limits raindrop size to a maximum value independent of air density, whereas raindrop terminal velocity varies as the inverse of the square root of air density. If the Archaean raindrops reached the modern maximum measured size, air density must have been less than 2.3 kg m(-3), compared to today’s 1.2 kg m(-3), but because such drops rarely occur, air density was more probably below 1.3 kg m(-3). The upper estimate for air density renders the pressure broadening explanation possible, but it is improbable under the likely lower estimates. Our results also disallow the extreme CO(2) levels required for hot Archaean climates.
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.
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.