Earth formed from the Sun’s protoplanetary disk about 4.6 billion years ago. In the beginning, it was a molten spheroid with scorching temperatures. Over time, it cooled, and a solid crust formed. Eventually, the atmosphere cooled, and life became a possibility.

But how did all of that happen? The atmosphere was rich in carbon, and that carbon had to be removed before the temperature could drop and Earth could become habitable.

Where did all the carbon go?

The first 500 million years of Earth’s existence are informally called the Hadean eon. The name comes from Hades, the Greek God of the Underworld. Hades is also an informal name for Hell itself.

The Hadean eon is aptly named. Even after it began to cool and solidify, Earth was still scorching hot. The atmosphere contained 100,000 times the current level of atmospheric carbon. Early Earth was similar to Venus, where a thick atmosphere traps heat and keeps temperatures high. During the Hadean, Earth’s surface temperature would’ve exceeded 200 Celsius (400 F.)

Before Earth could cool, it had to scrub a lot of carbon from its atmosphere. But scientists have found it challenging to piece together events on the very young Earth. For one thing, the geological evidence is scant.

But a pair of researchers think they have a new explanation for removing all that atmospheric carbon, and it involves a type of rock that no longer exists.

An artistic conception of the early Earth, showing a surface pummeled by large impacts. Credit: Simone Marchi.
An artistic conception of the early Earth, showing a surface pummeled by large impacts. Credit: Simone Marchi.

A new research article titled “A wet heterogeneous mantle creates a habitable world in the Hadean” presents the team’s findings. The first author is Yoshinori Miyazaki, a Stanback Postdoctoral Fellow at Caltech. Jun Korenaga, a professor of Earth and planetary sciences at Yale, is the other author. The journal Nature published the study.

“This period is the most enigmatic time in Earth history,” said Korenaga in a press release. “We’re presenting the most complete theory, by far, for Earth’s first 500 million years.”

The Hadean was not only enigmatic but dynamic. The planet underwent a lot of changes during those 600 million years. But geological evidence shows that Earth’s surface environment was similar to present-day Earth’s by the middle of the Hadean. “Under what conditions a harsh surface environment could turn into a habitable one remains uncertain,” the authors write in their paper.

A few things had to happen before Earth could be habitable. Oceans had to form, plate tectonics had to start, and greenhouse gases had to be removed quickly from the atmosphere. “Somehow, a massive amount of atmospheric carbon had to be removed,” Miyazaki said. “Because there is no rock record preserved from the early Earth, we set out to build a theoretical model for the very early Earth from scratch.”

In Earth’s early days, it was a magma ocean, a sphere of molten rock and nothing else. During this phase, the planet was forming via accretion. In the accretion process, materials agglomerate together creating a large object over time. Earth took between 70 million and 100 million years to assemble. During that time, planetesimals slammed into the Earth-to-be, generating heat and keeping Earth in a molten state.

Artist's impression of magma ocean planet. Credit: Mark Garlick
Artist’s impression of magma ocean planet. Credit: Mark Garlick

The magma ocean is an essential stage in the life of rocky planets. The liquid state allows heavier elements to sink to the core and lighter elements to “float” on top. This is how planets like Earth become differentiated into a core, a mantle, and a crust. The outer core is still molten to this day, and

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