A hidden world of water exists far beneath Earth’s surface. This deep reservoir is not a free-flowing body of liquid, but rather a complex system where water is integrated within the very structure of rocks. The presence of this internal water suggests a more intricate global water cycle than previously imagined, influencing the dynamic processes that shape our planet.
Earth’s Vast Internal Ocean
Earth’s deep “ocean” refers to water chemically locked within the crystalline structures of minerals found in the mantle. This water exists as hydroxyl groups, rather than liquid H2O. A significant portion of this bound water resides within the “transition zone,” a region of the mantle located between approximately 410 and 660 kilometers deep.
Minerals like wadsleyite and ringwoodite, which are high-pressure forms of olivine, are particularly capable of absorbing and storing substantial amounts of water within their crystal lattices. Experiments show that these minerals can hold 1 to 3 percent of their weight in water under the extreme conditions of the transition zone. Considering the vastness of this layer, the total volume of water stored here could be several times greater than all the surface oceans combined.
How Scientists Detect Deep Water
Scientists employ various sophisticated methods to infer the presence of water deep within Earth’s interior. One primary approach involves seismology, the study of seismic waves from earthquakes. Changes in the speed and behavior of these waves, such as slowing down or reflecting, indicate variations in rock properties, including their water content. Researchers have observed such seismic anomalies beneath continents, suggesting the widespread presence of water-bearing minerals.
Laboratory experiments are also used by simulating the extreme pressures and temperatures found in Earth’s mantle. Scientists use specialized equipment, like diamond-anvil cells, to recreate these conditions and observe how minerals like ringwoodite and wadsleyite absorb and retain water. Analysis of rare deep-earth minerals, found in diamonds, provides direct evidence. For example, a piece of ringwoodite found in a diamond from Brazil contained measurable amounts of water, confirming theoretical predictions.
The Role of Internal Water on Earth
The water hidden within Earth’s mantle influences the planet’s geological processes. Even small amounts of water can reduce the melting point of mantle rocks, making them more prone to partial melting. Water also lowers the viscosity of these rocks, allowing them to flow more easily. This effect facilitates mantle convection, the slow churning of Earth’s interior, which drives the movement of tectonic plates.
Water in subduction zones, where oceanic plates dive beneath continental plates, can “lubricate” these zones, easing the descent of the oceanic crust into the mantle. As water-rich minerals descend, they can release water, leading to dehydration melting that contributes to volcanism. This deep water is part of a larger, whole-Earth water cycle, where water moves between the surface and the deep interior over vast geological timescales, impacting the long-term habitability of our planet.
Deep Water Beyond Our Planet
The concept of internal water extends far beyond Earth, with confirmed or suspected subsurface oceans on several icy moons in our solar system. Jupiter’s moon Europa and Saturn’s moon Enceladus are prime examples, believed to harbor vast liquid water oceans beneath their thick ice shells. These oceans are thought to be kept warm and liquid by tidal heating, caused by the gravitational pull of their massive parent planets.
These subsurface oceans have implications for astrobiology, as they represent environments where extraterrestrial life could potentially exist. Scientists are actively exploring these moons for signs of biological activity. Beyond our solar system, exoplanet research suggests that many water-rich planets may exist. On these distant worlds, water could be found in various states, including supercritical fluid or exotic high-pressure ice phases, deep within their interiors.