How Earth endured a planet-wide inferno: The secret water vault under our feet
When Earth was a molten inferno, water may have been locked safely underground rather than lost to space. Researchers discovered that bridgmanite deep in the mantle can store far more water at high temperatures than previously believed. During Earth’s cooling, this hidden reservoir could have held water volumes comparable to today’s oceans. Over time, that buried water helped drive geology and rebuild the planet’s surface environment.
Around 4.6 billion years ago, Earth looked nothing like the calm, blue world we see today. Repeated and powerful impacts from space kept the planet's surface and interior in a turbulent, molten state. Much of Earth was covered by a global ocean of magma, with temperatures so extreme that liquid water could not survive. The young planet more closely resembled a blazing furnace than a place capable of supporting oceans or life.
Today, however, oceans cover about 70% of Earth's surface. How water managed to endure the transition from this molten early phase to a largely solid planet has long puzzled scientists and driven decades of research.
Water Hidden Deep Inside the Planet
A recent study led by Prof. Zhixue Du of the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences (GIGCAS) offers a new explanation. The team found that large amounts of water could have been stored deep within Earth's mantle as it cooled and crystallized from molten rock.
Their results, published in Science on December 11, are changing how scientists think about water storage deep inside the planet. The researchers showed that bridgmanite, the most abundant mineral in Earth's mantle, can function like a microscopic "water container." This ability may have allowed early Earth to trap significant amounts of water below the surface as the planet solidified.
According to the team, this early reservoir of water may have played a key role in Earth's transformation from a hostile, fiery world into one capable of supporting life.
Testing Water Storage Under Extreme Conditions
Earlier experiments suggested that bridgmanite could only hold small amounts of water. Those studies, however, were conducted at relatively low temperatures. To revisit the question, the researchers had to overcome two major hurdles. They needed to recreate the intense pressures and temperatures found more than 660 kilometers beneath Earth's surface, and they had to detect extremely small traces of water in mineral samples, some thinner than one tenth the width of a human hair and containing only a few hundred parts per million of water.
To meet these challenges, the team built a diamond anvil cell system combined with laser heating and high-temperature imaging. This custom-designed setup allowed them to push temperatures as high as ~4,100 °C. By reproducing deep mantle conditions and accurately measuring equilibrium temperatures, the researchers were able to explore how heat affects the way minerals absorb water.
Advanced Tools Reveal Hidden Water
Using the advanced analytical facilities at GIGCAS, the scientists applied techniques including cryogenic three-dimensional electron diffraction and NanoSIMS. Working with Prof. LONG Tao from the Institute of Geology of the Chinese Academy of Geological Sciences, they also incorporated atom probe tomography (APT).