- [Narrator] Millions of years after the birth of the planet, there's still no atmosphere as we know it.

(ethereal music) (debris whooshes) But that is going to change.

(ominous music) - Four-and-a-half billion years ago, the Earth would've been a very violent place.

- Such that it's called the Hadean, which comes from the god Hades, the Greek god of the underworld 'cause it was imagined to be a hellish kind of place.

(eerie music) (ground explodes) (eerie music) - [Narrator] Across the globe, molten magma surges up from within.

(eerie music) (lava whooshing) Rivers of fire unleash a cocktail of gases.

(explosion booms) (eerie music) Carbon dioxide, nitrogen, and methane explode out of volcanoes in thick plumes.

(eerie music) These are the building blocks of our early atmosphere.

(eerie music) And we can see echoes of this young planet still on Earth today.

(light electronic music) - The volcanoes that we have here in Iceland look very similar to those that we had four-and-a-half billion years ago.

The early Earth was dominated by volcanic activity, and this lasted for millions of years.

(light electronic music) Those volcanoes acted as pathways for the gasses that attract inside the magma to escape into our atmosphere.

- [Narrator] But how do scientists know what the atmosphere was like in the past?

Surprisingly, clues are hidden in some of the rocks we find today.

- Throughout Earth's history and evolution, there's been a dynamic interplay between the Earth's surface and the Earth's atmosphere.

And because of this interplay, there are chemical fingerprints that are trapped in rocks.

(light electronic music) - The rock I'm holding in my hand is 3.4 billion years old.

And there are tiny bubbles trapped in this rock that are samples of Earth's ancient atmosphere.

So we can measure the gases and piece together evidence of what the atmosphere was like in the past.

(light electronic music)