Atmospheric changes over Earth's long history
Chemistry of the atmosphere • Evolution of the Earth's atmosphere
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Formation and loss of the primordial atmosphere
Cause: The planet forms within a protoplanetary disc and initially acquires light gases such as hydrogen and helium from the solar nebula. Effect: These light gases escape to space because Earth's gravity and the young Sun's high-energy radiation cannot retain them. Limiting factors: Planetary mass and solar radiation intensity determine the efficiency of gas loss, so smaller bodies lose primordial atmospheres faster.
Secondary atmosphere by volcanic outgassing
Cause: Intense volcanic activity releases gases trapped in the planet's interior, primarily water vapour, carbon dioxide, nitrogen, methane and ammonia. Effect: The atmosphere becomes dominated by water vapour and carbon dioxide with increasing nitrogen. Limiting factors: Rate of volcanism and the composition of the mantle control relative gas proportions.
Cooling, condensation and ocean formation
Cause: Planetary cooling reduces atmospheric temperatures and allows water vapour to condense. Effect: Formation of oceans removes large amounts of water from the atmosphere and dissolves carbon dioxide, lowering atmospheric CO2. Limiting factors: Surface temperature, atmospheric pressure and the availability of condensation nuclei govern how rapidly oceans form and how much CO2 dissolves.
Chemical weathering and carbonate formation
Cause: Rainwater and dissolved CO2 form weak acids that react with silicate rocks. Effect: Weathering transports dissolved ions to oceans where carbonate minerals form and lock away CO2 in sediments and rocks. Limiting factors: Rock exposure area, rainfall amounts and tectonic uplift influence the weathering rate and long-term CO2 drawdown.
Rise of oxygen through photosynthesis
Cause: Photosynthetic microorganisms use carbon dioxide and water to produce organic matter and free oxygen as a by-product. Effect: Atmospheric oxygen concentration increases gradually, leading to oxidative chemical conditions, the appearance of an ozone layer, and new metabolic pathways in life. Limiting factors: Availability of nutrients, light, and sinks for oxygen (such as reduced minerals) control the timing and extent of oxygen accumulation.
Great Oxidation Event and its consequences
Cause: Continued oxygen production eventually overwhelms geological sinks such as dissolved iron and reduced volcanic gases. Effect: A sustained rise in atmospheric oxygen changes surface chemistry, leads to oxidized mineral deposits, permits evolution of aerobic organisms and allows an ozone layer to form that reduces harmful UV. Limiting factors: Balance between oxygen sources (photosynthesis) and sinks (oxidation reactions, volcanic emissions) determines the magnitude of oxygenation.
Long-term atmospheric evolution since oxygenation
Cause: Ongoing interactions among biological processes, tectonics and climate systems drive smaller-scale changes in gas levels. Effect: Carbon dioxide fluctuates with volcanic activity and weathering, methane levels vary with biological and geological sources, and oxygen remains at levels set by biosphere and geosphere balance. Limiting factors: Plate tectonics, biological productivity and human activities affect the short- and long-term variability of atmospheric composition.
Key notes
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