Deep within the hot interior of the planet, ice lurks. Now, a form of super-compact ice, found embedded in diamonds, offers the first direct clue that there is abundant water more than 610 kilometers deep in the mantle.
This ice, identified by its crystal structure and called ice-VII, doesn’t exist at Earth’s surface. It forms only at pressures greater than about 24 gigapascals — corresponding to depths between 610 and 800 kilometers, researchers report March 8 in Science. Its presence in diamonds suggests that there is water-rich fluid in the transition zone between the upper and lower mantle, and even into the top of the lower mantle.
“This is really the first time that we see water at such depths,” says Oded Navon, a mantle petrologist at the Hebrew University of Jerusalem who was not involved in the new study.