As the search for Earth-like exoplanets gathers pace, in order to understand them, we need comprehensive theories for how planetary atmospheres form and evolve. Written by two well-known planetary scientists, this text explains the physical and chemical principles of atmospheric evolution and planetary atmospheres, in the context of how atmospheric composition and climate determine a planet's habitability. The authors survey our current understanding of the atmospheric evolution and climate on Earth, on other rocky planets within our Solar System, and on planets far beyond. Incorporating a rigorous mathematical treatment, they cover the concepts and equations governing a range of topics, including atmospheric chemistry, thermodynamics, radiative transfer, and atmospheric dynamics, and provide an integrated view of planetary atmospheres and their evolution. This interdisciplinary text is an invaluable one-stop resource for graduate-level students and researchers working across the fields of atmospheric science, geochemistry, planetary science, astrobiology, and astronomy.

Atmospheric Evolution on Inhabited and Lifeless Worlds

The atmosphere of the Archean eon-one-third of Earth's history-is important for understanding the evolution of our planet and Earth-like exoplanets. New geological proxies combined with models constrain atmospheric composition. They imply surface O2 levels <10-6 times present, N2 levels that were similar to today or possibly a few times lower, and CO2 and CH4 levels ranging ~10 to 2500 and 102 to 104 times modern amounts, respectively. The greenhouse gas concentrations were sufficient to offset a fainter Sun. Climate moderation by the carbon cycle suggests average surface temperatures between 0° and 40°C, consistent with occasional glaciations. Isotopic mass fractionation of atmospheric xenon through the Archean until atmospheric oxygenation is best explained by drag of xenon ions by hydrogen escaping rapidly into space. These data imply that substantial loss of hydrogen oxidized the Earth. Despite these advances, detailed understanding of the coevolving solid Earth, biosphere, and atmosphere remains elusive, however.

The Archean atmosphere

The early Earth’s environment is controversial. Climatic estimates range from hot to glacial, and inferred marine pH spans strongly alkaline to acidic. Better understanding of early climate and ocean chemistry would improve our knowledge of the origin of life and its coevolution with the environment. Here, we use a geological carbon cycle model with ocean chemistry to calculate self-consistent histories of climate and ocean pH. Our carbon cycle model includes an empirically justified temperature and pH dependence of seafloor weathering, allowing the relative importance of continental and seafloor weathering to be evaluated. We find that the Archean climate was likely temperate (0–50 °C) due to the combined negative feedbacks of continental and seafloor weathering. Ocean pH evolves monotonically from 6.6 − 0.4 + 0.6 (2σ) at 4.0 Ga to 7.0 − 0.5 + 0.7 (2σ) at the Archean–Proterozoic boundary, and to 7.9 − 0.2 + 0.1 (2σ) at the Proterozoic–Phanerozoic boundary. This evolution is driven by the secular decline of pCO2, which in turn is a consequence of increasing solar luminosity, but is moderated by carbonate alkalinity delivered from continental and seafloor weathering. Archean seafloor weathering may have been a comparable carbon sink to continental weathering, but is less dominant than previously assumed, and would not have induced global glaciation. We show how these conclusions are robust to a wide range of scenarios for continental growth, internal heat flow evolution and outgassing history, greenhouse gas abundances, and changes in the biotic enhancement of weathering.

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Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model.

Stellar evolution models predict that the solar luminosity was lower in the past, typically 20-25% lower during the Archean (3.8-2.5 Ga). Despite the fainter Sun, there is strong evidence for the presence of liquid water on Earth's surface at that time. This "faint young Sun problem" is a fundamental question in paleoclimatology, with important implications for the habitability of the early Earth, early Mars and exoplanets. Many solutions have been proposed based on the effects of greenhouse gases, atmospheric pressure, clouds, land distribution and Earth's rotation rate. Here we review the faint young Sun problem for Earth, highlighting the latest geological and geochemical constraints on the early Earth's atmosphere, and recent results from 3D global climate models and carbon cycle models. Based on these works, we argue that the faint young Sun problem for Earth has essentially been solved. Unfrozen Archean oceans were likely maintained by higher concentrations of CO 2 , consistent with the latest geological proxies, potentially helped by additional warming processes. This reinforces the expected key role of the carbon cycle for maintaining the habitability of terrestrial planets. Additional constraints on the Archean atmosphere and 3D fully coupled atmosphere-ocean models are required to validate this conclusion.

https://hal.archives-ouvertes.fr/hal-03103930/document

Is the Faint Young Sun Problem for Earth Solved

Tectonostratigraphy of the Late Archean Dharwar Supergroup, Dharwar Craton, India: Defining a tectonic history from spatially linked but temporally distinct intracontinental and arc-related basins

The ∼2700 Ma Talya Conglomerate is Comprised of 15 Diamictites (i.e. Matrix-supported Conglomerates) Interbedded with Mudstone and Sandstone Units, and is Interpreted as a Glaciomarine Deposit. the Entire Thickness of This Conglomeratic Member within the Vanivilas formation, the Lowest formation of the Chitradurga Group of the Neoarchean Dharwar Supe Rgroup, is Exposed in a 543 M-thick Measured Section. it is in a Sub-vertical Attitude, is Highly Sheared, and has Undergone Greenschist Facies Metamorphism. the Diamictites had an Original Matrix of Laminated Mud/Silt and Fine Sand. while Including Diamictites Throughout, the Talya is a Fining-upward Sequence with Intercalated Sandstones Dominant in its Lower Portion and Mudstones Dominant in its Upper Portion. we Interpret that the Talya Conglomerate was Deposited in a Marine Environment, with Diamictites Composed of Ice-rafted Detritus (ird) Deposited from Icebergs Calved from Tidewater Glacier Tongues And/or possibly from Ice Shelves. in these 'rainout Diamictites' the Larger Clasts were Dropped into Finegrained Bottom Sediment Deposited by Sediment Plumes and Currents. the Source Ice Sheet was Located to the West and Southwest on a Land Mass that Included the Older than 2720 Ma Bababudan Group of Quartzites and Mafic Volcanics and Older than 3000 Ma Basement of Granitic/gneissic Rocks. Application of Walther's Law Indicates that the Mudstonebearing Portion of the Talya was Deposited upon the Sandstone-bearing Portion as the Sea further Inundated the Land Mass Due to Glacial Retreat and a Decrease in Glacial Mass, thereby Resulting in the Fining- upward Nature of the Talya Conglome Rate. we also Interpret the Lower Portion of the Kaldurga Conglomerate, Located 50-75 Km to the Southwest of the Talya, to be Equivalent with the Talya. the Kaldurga Contains mostly Granitic Basement Detritus, perhaps Exposed Due to Basement Uplift Related to Isostatic Rebound Caused by Glacial Melting or Due to Tectonism Related to Westward Subduction.

S. M. Chandrakant

Papers

The Talya Conglomerate: an Archean (∼2.7 Ga) Glaciomarine formation, Western Dharwar Craton, Southern India