When did Earth get its water? Much later than expected, study suggests

Looking back into the origins of our world billions of years ago, celestial bodies were beginning to form in the gargantuan, swirling disk of dust, gas, and rocky debris that orbited our sun. 

This process, known as accretion, gradually gave rise to the planets, moons, and asteroids that populate our cosmic landscape. Scientists continue to grapple with understanding the complex procedures that gave birth to planets, including Earth.

Magma holds clues

One of the primary methods that researchers use to understand Earth’s formation is the examination of magma, the molten or semi-molten natural rock, which emerges from deep within the Earth’s core. 

Magma samples contain chemical signatures that chronicle our planet’s birth – detailing the timing and nature of the constituent materials that contributed to the formation of Earth. This is similar to how fossils offer us clues about the biological past of Earth.

In a groundbreaking new study from the California Institute of Technology, evidence suggests that early Earth was created from hot, parched materials. This revelation suggests that water arrived relatively late in the history of Earth’s formation. 

The study fundamentally alters our understanding of our planet’s creation story, shedding light on a crucial chapter in Earth’s development. The research was conducted in the laboratories of Professor Francois Tissot and Yigang Zhang from the University of Chinese Academy of Sciences. Weiyi Liu, a graduate student at Caltech, is the first author of the paper published in the journal Science Advances.

Samples from Earths mantle 

Even though humans have not yet developed a way to physically penetrate into the heart of our planet, the interior is not entirely inaccessible to us. Deep within Earth’s mantle, rocks can make their way to the surface through volcanic eruptions, bringing important samples with them.

These parental magmas can originate from different strata within the Earth, including the upper mantle, which starts roughly 15 kilometers beneath the surface and stretches to around 680 kilometers; or the lower mantle, extending from 680 kilometers down to the core – mantle boundary approximately 2,900 kilometers beneath our feet. 

Chemical signatures 

By analyzing magmas from varying depths, scientists are able to infer information about the distinct chemical composition of Earth’s layers.

The formation of Earth was not an overnight phenomenon, but was a protracted process involving materials gradually coming together over a vast span of time. Through the study, the team was able to discern that the early Earth was primarily composed of desiccated, rocky substances. 

The chemical signatures derived from the planet’s depths indicated an absence of so-called volatiles, which are substances that readily evaporate, like water and iodine. By contrast, the upper mantle samples contained a higher proportion of volatiles, three times that found in the lower mantle. 

Liu developed a model based on these chemical ratios, demonstrating that Earth initially formed from arid, rocky materials and that significant infusion of life-essential volatiles, including water, transpired during the last 15 percent (or less) of Earth’s formation.

Planetary formation theories

The research significantly contributes to the ongoing dialogue around planetary formation theories – a field that has witnessed multiple paradigm shifts in the past decades and continues to be the locus of vibrant scientific debate. It also provides predictions for the nature of other terrestrial planets like Mercury and Venus, which are presumed to have been formed from similar dry materials.

Tissot emphasizes the importance of space exploration in this context: “Space exploration to the outer planets is really important because a water world is probably the best place to look for extraterrestrial life.” 

“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.” 

More about Earth’s water

Earth’s water constitutes a significant part of the planet’s unique identity, playing a crucial role in sustaining a wide array of life forms. Until recently, it was widely accepted that water was a key part of Earth’s formation. However, the Caltech study has introduced a significant paradigm shift in this understanding.

According to the new study, the early Earth, contrary to previous assumptions, was a barren, arid place, consisting mainly of hot, dry materials. Magma evidence strongly suggests that Earth’s abundant water – essential for life as we know it – made a rather late appearance in the planet’s formative history.

The late addition of water to Earth’s composition has far-reaching implications for our understanding of life’s evolution. The late addition of water could mean that life also started to evolve relatively late in Earth’s history.

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