ATMOSPHERE × EARTH

The First Air

~4.45 to 4.3 billion years ago

Hadean

Right after the collision that formed the Moon, the sky itself was made of rock: vaporised stone, which cooled and rained back out within a thousand years. Steam took its place, the whole future ocean held airborne, pressing down hundreds of times harder than the air we breathe, while the molten surface beneath took some two million years to freeze. Only then could all that steam fall as rain, and stay down. What stayed overhead was a sky of almost pure carbon dioxide, around a hundred atmospheres of it, above a young ocean close to 230 °C that only the weight of that sky kept liquid.

Why it matters

The ocean and the atmosphere came out of the same cooling: one fell, the other stayed aloft. Neither has been stable since, and every climate on this timeline is a later state of that first air.

Dating & uncertainty

None of this air survives to be sampled; it is reconstructed from models of a cooling magma ocean and from the gases the mantle still gives up. The open question is how fast the carbon dioxide was buried at sea, swallowed into rock. Buried quickly, it leaves a planet habitable ten or twenty million years after the impact; a wet, chemically patchy mantle could strip the greenhouse within 160 million years; and long afterwards, the air of the Archean still held ten to 2,500 times today's carbon dioxide.

Sources

1. Zahnle et al. (2007), Space Science Reviews · Emergence of a habitable planet
2. Miyazaki & Korenaga (2022), Nature · A wet heterogeneous mantle creates a habitable world in the Hadean
3. Catling & Zahnle (2020), Science Advances · The Archean atmosphere

Earth history as one day

00:28:3223 h 31 min before midnight

if Earth’s 4.54 billion years were compressed into a single day