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How did Earth’s earliest atmosphere form?
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evolution of the atmosphere
A “best guess” reconstruction of the abundance of O2 in Earth's atmosphere as a function of time. The O2 abundance axis is logarithmic.
Earth’s earliest atmosphere formed through a combination of processes, though the exact timing and composition remain subjects of scientific debate. Three primary mechanisms are proposed for its origin:1
- Direct Capture: As Earth accreted from the solar nebula, its gravitational field may have captured primordial gases—primarily hydrogen and helium—from the surrounding gas cloud. Because hydrogen and helium are light, much of this initial atmosphere likely escaped into space.21
- Volcanic Outgassing: As the planet’s interior differentiated and the crust stabilized, volcanic activity released "excess volatiles" from the interior, including water vapour, nitrogen, and carbon dioxide. This secondary atmosphere was likely reducing, containing compounds like methane and ammonia, before evolving as the crust cooled and gases equilibrated.3124
- Importation: Some scientists propose that volatile-rich bodies, such as comets, delivered water and other gases to Earth late in its accretionary history through impacts.15
Following these events, water vapour condensed to form the oceans, and carbon dioxide was gradually consumed by weathering reactions with crustal minerals. Free oxygen was notably absent until the later emergence of photosynthetic organisms.34