Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean

Enigmatic evolution of microbial nitrogen fixation: insights from Earth's past

The evolution of nitrogen fixation undoubtedly altered nearly all corners of the biosphere, given the essential role of nitrogen in the synthesis of biomass. To date, there is no unified view on what planetary conditions gave rise to nitrogen fixation or how these conditions have sustained it evolutionarily. Intriguingly, the concentrations of metals that nitrogenases require to function have changed throughout Earth's history. In this review, we describe the interconnection of the metal and nitrogen cycles with nitrogenase evolution and the importance of ancient ecology in the formation of the modern nitrogen cycle. We argue that exploration of the nitrogen cycle's deep past will provide insights into humanity's immediate environmental challenges centered on nitrogen availability.

Evolution of iron and oxygen biogeochemical cycles during the Precambrian

Iron (Fe) is an essential element for life, and its geochemical cycle is intimately linked to the coupled history of life and Earth's environment. The accumulated geologic records indicate that ferruginous waters existed in the Precambrian oceans not only before the first major rise of atmospheric O2 levels (Great Oxidation Event; GOE) during the Paleoproterozoic, but also during the rest of the Proterozoic. However, the interactive evolution of the biogeochemical cycles of O2 and Fe during the Archean-Proterozoic remains ambiguous. Here, we develop a biogeochemical model to investigate the coupled biogeochemical evolution of Fe-O2 -P-C cycles across the GOE. Our model demonstrates that the marine Fe cycle was less sensitive to changes in the production rate of O2 before the GOE (atmospheric pO2  < 10-6 PAL; present atmospheric level). When the P supply rate to the ocean exceeds a certain threshold, the GOE occurs and atmospheric pO2 rises to ~10-3 -10-1 PAL. After the GOE, the marine Fe(II) concentration is highly sensitive to atmospheric pO2 , suggesting that the marine redox landscape during the Proterozoic may have fluctuated between ferruginous conditions and anoxic non-ferruginous conditions with sulfidic water masses around continental margins. At a certain threshold value of atmospheric pO2 of ~0.3% PAL, the primary oxidation pathway of Fe(II) shifts from the activity of Fe(II)-utilizing anoxygenic photoautotrophs in sunlit surface waters to abiotic process in the deep ocean. This is accompanied by a shift in the primary deposition site of Fe(III) hydroxides from the surface ocean to the deep sea, providing a plausible mechanistic explanation for the observed cessation of iron formations during the Proterozoic.

Why the day is 24 hours long: The history of Earth's atmospheric thermal tide, composition, and mean temperature

The Sun drives a semidiurnal (12-hour) thermal tide in Earth's atmosphere. Zahnle and Walker suggested that an atmospheric oscillation with period P_res ≈ 10.5 hours resonated with the Solar driving ≈600 million years ago (Ma), when the length of day (lod) was ≈21 hours. They argued that the enhanced torque balanced the Lunar tidal torque, fixing the lod. We explore this hypothesis using two different global circulation models (GCMs), finding P_res = 11.4 and 11.5 hours today, in excellent agreement with a recent measurement. We quantify the relation between P_res, mean surface temperature [Formula: see text], composition, and Solar luminosity. We use geologic data, a dynamical model, and a Monte Carlo sampler to find possible histories for the Earth-Moon system. In the most likely model, the lod was fixed at ≈19.5 hours between 2200 and 600 Ma ago, with sustained high [Formula: see text] and an increase in the angular momentum L_EM of the Earth-Moon system of ≈5%.

Magnetic Circular Dichroism in Archean Stratospheric Oxygen: Enantiomeric Excess of Amino Acids Produced in Volcanic Plumes

While there is consensus that Archean atmosphere was anoxic with O2 pressure, p(O2) <10-6 PAL (present atmospheric level) at sea-level, evidence suggests that p(O2) at stratospheric altitudes of 10-50 km was orders of magnitude higher, a result of photodissociation of CO2 by UVC sunlight and incomplete mixing of O2 with other gases. Molecular O2 is paramagnetic due to triplet ground state. Magnetic circular dichroism (MCD) by stratospheric O2 is examined in earth's magnetic field and shows maximum circular polarization │(I+ - I-)│ at altitude of 15-30 km (I+/I- is intensity of left/right circularly polarized light). While (I+ - I-)/(I+ + I-) is small (~10-10), it is an unexplored source of enantiomeric excess (EE) by asymmetric photolysis of amino acid precursors produced in volcanic eruptions. The precursors reside in stratosphere for periods of over a year due to relative absence of vertical transport. Due to negligible thermal gradient across equator, they are trapped in the hemisphere where they are produced, with interhemispheric exchange time of over a year. The precursors diffuse through altitudes of maximum circular polarization before getting hydrolyzed on ground to amino acids. Enantiomeric excess of ~10-12 is calculated for precursors and amino acids. While small, this EE is orders of magnitude higher than predicted (~10-18) by parity violating energy differences (PVED) and could be the seed for growth of biological homochirality. Preferential crystallization (PC) is described as a plausible mechanism for amplification of solution EE of some amino acids from 10-12 to 10-2, for period of several days.

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.

Of fiery sparks and glittering spots: melting-resolidification and spherical particle formation in abrasion

The curious occurrence of perfectly spherical particle debris when a steel substrate is slid against a hard abrasive was first documented by Robert Hooke in the seventeenth century. Similar observations now abound in other abrasion-type processes, from industrial grinding to sliding rock faults. The prevalent hypothesis, originally proposed by Hooke, is that these particles form due to high local temperatures, resulting in particle ejection, melting and resolidification. In this work, we revisit this hypothesis, using a model steel-abrasive contact, a combination of in situ and post-process investigations, and complementary analytical calculations. Our results reveal two primary findings—firstly, the temperature of particles ejected from the contact zone is far from the melting point, and secondly, exothermic surface oxidation plays a critical role in actually melting the particle. Melting is either complete or partial, leading to spherical particles or ‘slivers’, as described originally by Hooke. Finally, we confirm that resulting particle surface patterns are typical of rapid solidification from the melt. Apart from throwing light on a centuries’ old curiosity, our results precisely quantify the melting-resolidification process, with implications for a variety of applications, ranging from abrasion and powder production to the formation of micrometeorite dust.

A revised lower estimate of ozone columns during Earth's oxygenated history

The history of molecular oxygen (O₂) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O₂ increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O₂ concentrations have likely fluctuated (between 10^-3 and 1.5 times the present atmospheric level) since the GOE ∼2.4 Gyr ago, resulting in a time-varying ozone (O₃) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O₃ in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O₃ columns (reduced by up to 4.68 times for a given O₂ level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O₃ leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH₄). We show that a CH₄ supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O₃ columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of three-dimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.

High pCO2 Reduces Sensitivity to CO2 Perturbations on Temperate, Earth-like Planets Throughout Most of Habitable Zone

The nearly logarithmic radiative impact of CO₂ means that planets near the outer edge of the liquid water habitable zone (HZ) require ∼10⁶ × more CO₂ to maintain temperatures that are conducive to standing liquid water on the planetary surface than their counterparts near the inner edge. This logarithmic radiative response also means that atmospheric CO₂ changes of a given mass will have smaller temperature effects on higher pCO₂ planets. Ocean pH is linked to atmospheric pCO₂ through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response of pH to changes in pCO₂ also decreases at higher initial pCO₂. Here, we use idealized climate and ocean chemistry models to demonstrate that CO₂ perturbations large enough to cause catastrophic changes to surface temperature and ocean pH on temperate, low-pCO₂ planets in the innermost region of the HZ are likely to have much smaller effects on planets with higher pCO₂, as may be the case for terrestrial planets with active carbonate-silicate cycles receiving less instellation than the Earth. Major bouts of extraterrestrial fossil fuel combustion or volcanic CO₂ outgassing on high-pCO₂ planets in the mid-to-outer HZ should have mild or negligible impacts on surface temperature and ocean pH. Owing to low pCO₂, Phanerozoic Earth's surface environment may be unusually volatile compared with similar planets receiving lower instellation.

Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions

Ultraviolet (UV) light has long been invoked as a source of energy for prebiotic chemical synthesis, but experimental support does not involve sources of UV light that look like the young Sun. Here we experimentally investigate whether the UV flux available on the surface of early Earth, given a favorable atmosphere, can facilitate a variety of prebiotic chemical syntheses. We construct a solar simulator for the UV light of the faint young Sun on the surface of early Earth, called StarLab. We then attempt a series of reactions testing different aspects of a prebiotic chemical scenario involving hydrogen cyanide (HCN), sulfites, and sulfides under the UV light of StarLab, including hypophosphite oxidation by UV light and hydrogen sulfide, photoreduction of HCN with bisulfite, the photoanomerization of α-thiocytidine, the production of a chemical precursor of a potentially prebiotic activating agent (nitroprusside), the photoreduction of thioanhydrouridine and thioanhydroadenosine, and the oxidation of ethanol (EtOH) by photochemically generated hydroxyl radicals. We compare the output of StarLab to the light of the faint young Sun to constrain the timescales over which these reactions would occur on the surface of early Earth. We predict that hypophosphite oxidation, HCN reduction, and photoproduction of nitroprusside would all operate on the surface of early Earth in a matter of days to weeks. The photoanomerization of α-thiocytidine would take months to complete, and the production of oxidation products from hydroxyl radicals would take years. The photoreduction of thioanhydrouridine with hydrogen sulfide did not succeed even after a long period of irradiation, providing a lower limit on the timescale of several years. The photoreduction of thioanhydroadenosine with bisulfite produced 2'-deoxyriboadenosine (dA) on the timescale of days. This suggests the plausibility of the photoproduction of purine deoxyribonucleotides, such as the photoproduction of simple sugars, proceeds more efficiently in the presence of bisulfite.

Atmospheric Nitrogen When Life Evolved on Earth

The amount of nitrogen (N₂) present in the atmosphere when life evolved on our planet is central for understanding the production of prebiotic molecules and, hence, is a fundamental quantity to constrain. Estimates of atmospheric molecular nitrogen partial surface pressures during the Archean, however, widely vary in the literature. In this study, we apply a model that combines newly gained insights into atmospheric escape, magma ocean duration, and outgassing evolution. Results suggest <420 mbar surface molecular nitrogen at the time when life originated, which is much lower compared with estimates in previous works and hence could impact our understanding of the production rate of prebiotic molecules such as hydrogen cyanide. Our revised values provide new input for atmospheric chamber experiments that simulate prebiotic chemistry on the early Earth. Our results that assume negligible nitrogen escape rates are in agreement with research based on solidified gas bubbles and the oxidation of iron in micrometeorites at 2.7 Gyr ago, which suggest that the atmospheric pressure was probably less than half the present-day value. Our results contradict previous studies that assume N₂ partial surface pressures during the Archean were higher than those observed today and suggest that, if the N₂ partial pressure were low in the Archean, it would likely be low in the Hadean as well. Furthermore, our results imply a biogenic nitrogen fixation rate from 9 to 14 Teragram N₂ per year (Tg N₂/year), which is consistent with modern marine biofixation rates and, hence, indicate an oceanic origin of this fixation process.