Science

For close to a century, geoscientists have pondered a mystery: Where did Earth’s lighter elements go? Compared to amounts in the Sun and in some meteorites, Earth has less hydrogen, carbon, nitrogen, and sulfur, as well as noble gases like helium—in some cases, more than 99 percent less.

Some of the disparity is explained by losses to the solar system as our planet formed. But researchers have long suspected that something else was going on too.

Recently, a team of scientists reported a possible explanation—that the elements are hiding deep in the solid inner core of Earth. At its super-high pressure—360 gigapascals, 3.6 million times atmospheric pressure—the iron there behaves strangely, becoming an electride: a little-known form of the metal that can suck up lighter elements.

Study coauthor Duck Young Kim, a solid-state physicist at the Center for High Pressure Science & Technology Advanced Research in Shanghai, says the absorption of these light elements may have happened gradually over a couple of billion years—and may still be going on today. It would explain why the movement of seismic waves traveling through Earth suggests an inner core density that is 5 percent to 8 percent lower than expected were it metal alone.

Electrides, in more ways than one, are having their moment. Not only might they help solve a planetary mystery, they can now be made at room temperature and pressure from an array of elements. And since all electrides contain a source of reactive electrons that are easily donated to other molecules, they make ideal catalysts and other sorts of agents that help to propel challenging reactions.