Water equivalent to two Pacific Oceans could be stashed 2,900 km inside Earth

Liquid water makes life on Earth possible. Water is what facilitates the mantle layers sliding past each other, which is needed for the tectonic cycle and, in turn, to regulate the climate on the planet over its lifespan. But scientists have long grappled with the answer to the location of this water. A new study has now suggested that this water could be present 2,900 kilometres beneath the surface. The research is published in Nature Geoscience.

This region falls near the boundary between the mantle and its liquid outer core. "Ultralow velocity zones" have been detected here in the past with little explanation. Two minerals in the lower mantle - bridgmanite and ferropericlase - are largely dry. Assuming other minerals could hold water in the deeper reaches would also have a problem. The extremely high temperatures in the deepest mantle would lead to these minerals breaking down, or would require unusual compositions to stay stable.

New minerals discovered in the hunt for water locations inside Earth

Researchers started hunting for what else might make this water possible. They recreated high temperatures and pressures in the laboratory by squeezing tiny samples between two diamond tips and blasting them with lasers. Two candidates emerged - both iron oxyhydroxides (Fe5O12Hx and Fe7O12Hx) that had not been known about before. They noted that this mineral had the capability to stash away huge amounts of water.

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Conditions inside Earth could be similar

These precise conditions could exist under deep-mantle conditions, experiments have shown. However, there is no proof that the phases are present inside Earth. "Identifying these iron oxyhydroxides is important because they are seemingly stable, dense phases that capture and retain water across a wide range of lower-mantle conditions," geoscientist Alfred Wilson from the University of Leeds wrote in the commentary alongside the new study.

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Any water present inside Earth would exist inside minerals and not like the vast oceans we see. Both these minerals emerged as the best candidates for the job. They were found to be stable in extreme conditions. The researchers believe that when a molten "basal magma ocean" cooled and crystallised early in Earth's history, the newly formed minerals were likely born and seeped into the core-mantle boundary.

Wilson said the finding is a crucial piece in the puzzle about how Earth obtained and retained its water.