Earth formed from dry clumps, suggesting water came later

Our blue planet is covered in oceans and teeming with life. But it wasn’t always like that – and new research suggests that Earth formed billions of years ago from dry, rocky clumps and that water arrived on our planet later.

We know from radiometric dating that Earth is about 4.543 billion years old. It was born soon after the Sun, 4.6 billion years old. All the planets in our solar system were born out of the giant disk of dust, gas and rocky material that orbited our central star in its youth.

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As gravity went to work, larger and larger bodies fused together to give rise to asteroids, planets and moons.

Scientists are still trying to work out the processes which led to the planets’ formation. In the case of Earth, researchers are able to chemically analyse magma which flows up to the surface from deep within the planet’s interior.

A new study published in the journal Science Advances, led by researchers at the California Institute of Technology (Caltech), shows that early Earth formed from hot, dry materials. This suggests that the water that now covers 70% of our planet’s surface came later in the history of Earth’s formation.

We cannot dig deep enough to reach the deeper layers of the Earth. The upper mantle begins 15 kilometres beneath the surface and extends to about 680 kilometres. The lower mantle spans 680 to 2,900 kilometres below the surface. Below that is the Earth’s core.

Scientists gain insights into the makeup of these different layers when the material which makes up the interior of the planet, rise to the surface in the form of lava.

Because the Earth formed over millions of years, accruing material over a long time, sampling from different layers gives scientists insight into what was happening at different points in Earth’s accretion history.

The new study of chemical signatures from deep within the planet shows that the early Earth was made up of hot, dry, rocky material that lacked so-called “volatiles” – life-essential materials like water and iodine. Samples from the upper mantle, corresponding to a later point in Earth’s history, showed a marked increase in volatiles – three times more than in the lower mantle.

Modelling suggests that most of these volatiles, including water, arrived on Earth in only the last 15% or less of the Earth’s formation history.

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Such information about how Earth formed can help us understand how the other terrestrial planets in our solar system, like Mercury and Venus, formed. But the implications could go far beyond our own cosmic backyard.

“Space exploration to the outer planets is really important because a water world is probably the best place to look for extraterrestrial life,” says author François Tissot, a professor at Caltech. “But the inner solar system shouldn’t be forgotten. There hasn’t been a mission that’s touched Venus’s surface for nearly 40 years, and there has never been a mission to the surface of Mercury. We need to be able to study those worlds to better understand how terrestrial planets such as Earth formed.”