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NATURE

Ancient Earth Really Was a Serene Water World, New Evidence Confirms

Michelle Starr

11 March 2021

It's tricky to figure out what Earth might have looked like in the early years before life emerged. Geological detectives have now obtained more evidence that it was rather different to the planet we live on today.

According to a new analysis of the features of Earth's mantle over its long history, our whole world was once engulfed by a vast ocean, with very few or no land masses at all. It was an extremely soggy space rock.

So where the heck did all the water go? According to a team of researchers led by planetary scientist Junjie Dong of Harvard University, minerals deep inside the mantle slowly drunk up ancient Earth's oceans to leave what we have today.

"We calculated the water storage capacity in Earth's solid mantle as a function of mantle temperature," the researchers wrote in their paper.

"We find that water storage capacity in a hot, early mantle may have been smaller than the amount of water Earth's mantle currently holds, so the additional water in the mantle today would have resided on the surface of the early Earth and formed bigger oceans.

"Our results suggest that the long‐held assumption that the surface oceans' volume remained nearly constant through geologic time may need to be reassessed."

Deep underground, a great deal of water is thought to be stored in the form of hydroxy group compounds - made up of oxygen and hydrogen atoms. In particular, the water is stored in two high-pressure forms of the volcanic mineral olivine, hydrous wadsleyite and ringwoodite. Samples of wadsleyite deep underground could contain around 3 percent H2O by weight; ringwoodite around 1 percent.

Previous research on the two minerals subjected them to the high pressures and temperatures of the mantle of modern day Earth to figure out these storage capacities. Dong and his team saw another opportunity. They compiled all the available mineral physics data, and quantified the water storage capacity of wadsleyite and ringwoodite across a wider range of temperatures.

The results showed that the two minerals have lower storage capacities at higher temperatures. Because baby Earth, which formed 4.54 billion years ago, was much warmer internally than it is today (and its internal heat is still decreasing, which is very slow and also has absolutely nothing to do with its external climate), it means the water storage capacity of the mantle now is higher than it once was.

Moreover, as more olivine minerals are crystallizing out of Earth's magma over time, the water storage capacity of the mantle would increase that way, too.

In all, the difference in water storage capacity would be significant, even though the team was conservative with its calculations.

"The bulk water storage capacity of Earth's solid mantle was significantly affected by secular cooling due to its constituent minerals' temperature‐dependent storage capacities," the researchers wrote.

"The mantle's water storage capacity today is 1.86 to 4.41 times the modern surface ocean mass."

If the water stored in the mantle today is greater than its storage capacity in the Archean Eon, between 2.5 and 4 billion years ago, it's possible that the world was flooded and the continents swamped, the researchers found.

This finding is in agreement with a previous study that found, based on an abundance of certain isotopes of oxygen preserved in a geological record of the early ocean, that Earth 3.2 billion years ago had way less land than it does today.

If this is the case, it could help us answer burning questions about other aspects of Earth's history, such as where life emerged around 3.5 billion years ago. There's an ongoing debate over whether life first formed in saltwater oceans or freshwater ponds on land masses; if the entire planet was engulfed by oceans, that would solve that mystery.

Furthermore, the findings could also help us in the search for extraterrestrial life. Evidence suggests that ocean worlds are abundant in our Universe, so looking for signatures of these soggy planets could help us identify potentially hospitable worlds. And it could strengthen the case for looking for life on ocean worlds in our own Solar System, such as Europa and Enceladus.

Not least, it helps us better understand the delicate evolution of our planet, and the strange, often seemingly inhospitable turns along the way that eventually led to the emergence of humanity.

The research has been published in AGU Advances.