How many oceans did the Hadean eon have?


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Zero. A bit fat 0. The reason it's called the "Hadean eon" is after Hades, which is the Greek word for the underworld---it's meant to invoke fire and brimstone, because that's what the world was like.

The Hadean was the very first period of the Earth's existence, during which the Earth was still being constantly hit by asteroids and planetesimals (basically chunks of planet that haven't finished becoming planets yet). These endless collisions at incredibly high energies (we're talking billions of megatons; one of them created the Moon) heated the whole Earth to temperatures high enough to melt most metals, and thus far too hot to sustain liquid water of any kind. Comets did hit, bringing water, but that water immediately flash-boiled away.

This epoch went on for about 500 million years, until finally the Earth began to cool to a more moderate temperature that solid rocks could form on the crust. (The core remained molten, of course, and still does today.) Not long (geologically) after that, it was even cool enough that water didn't boil away, so when water continued to arrive in comets it began to form what we now know as oceans.

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