Earth's outer core

The Earth has a special part deep inside it called the outer core. This outer core is a thick layer of liquid about 2,260 kilometers (1,400 miles) deep. It is made mostly of two metals: iron and nickel.

Right above the outer core is the solid inner core, and right below it is a thick part of the Earth called the mantle. The outer core starts about 2,889 kilometers (1,795 miles) under the ground at a place called the core-mantle boundary. It ends about 5,150 kilometers (3,200 miles) under the ground at another place called the inner core boundary.

This outer core is very important because it helps create Earth's magnetic field, which protects us from harmful space rays. It is one of the reasons our planet is special and able to support life.

Properties

The outer core of Earth is a liquid layer, unlike the solid inner core. We know it is liquid because some earthquake waves do not pass through it. It is made mostly of iron and nickel, like the inner core, but stays liquid because it is not under enough pressure to become solid.

Scientists think the outer core is very hot, with temperatures ranging from about 3,000 to 8,000 kelvins. Because of this heat, the outer core flows in a way that helps create Earth's magnetic field. As Earth cools slowly, the inner core grows a little each year by turning liquid from the outer core into solid.

Light elements

Composition

Earth's outer core is mostly made of iron and nickel. But it can't be just these metals because they are too heavy for what we measure in the outer core. Scientists think lighter elements, which have smaller building blocks, are also in the outer core. These make the outer core less heavy.

Even though we can't visit the outer core, scientists use experiments, earthquake measurements, and comparisons with meteorites to guess what it's made of. As of 2023, they think the outer core has small amounts of hydrogen, carbon, oxygen, silicon, sulfur, and nickel, along with iron. The temperatures at the edges of the core range from about 4,137 to 6,300 Kelvin.

Constraints

Accretion

The lighter elements in Earth's outer core depend on what was available when Earth formed. These elements needed to mix into liquid iron and not escape into space when Earth was young.

CI chondrites

Scientists study special meteorites called CI chondrites because they have similar elements to those in the early solar system. By comparing these meteorites to Earth's materials, scientists can learn about the lighter elements in Earth's outer core. For example, if silicon is less common in Earth's mantle compared to these meteorites, it might mean silicon is in the core.

Implications for Earth's accretion and core formation history

Knowing more about the lighter elements in Earth's outer core would help us understand how Earth formed and how its core came to be.

Consequences for Earth's accretion

Learning more about lighter elements in the outer core would help test ideas about how Earth grew. For example, it might show whether Earth formed from material missing certain elements or if it gathered material from farther away in the solar system.

Consequences for Earth's core formation

The lack of certain elements in Earth's mantle compared to meteorites is thought to happen because these elements reacted with metal when Earth's core formed. These reactions depend on oxygen, silicon, and sulfur, so learning more about these elements in the core would help us understand how the core formed.

For example, if hydrogen is in the outer core, it suggests that Earth may have gathered water earlier than we thought, possibly absorbing it into metals when the core formed.

Implications for Earth's magnetic field

Earth's magnetic field is created by movement in the outer core. This movement is driven by heat and by lighter elements moving upward while heavier elements sink. This process releases energy that powers the magnetic field.

Traditionally, it was thought that before the inner core formed, the magnetic field was mainly driven by heat movement. However, recent ideas suggest that iron might conduct heat better than we thought, meaning heat movement might not be as important. Instead, the magnetic field might have been kept going by the core being hot enough to dissolve lighter elements, which then moved out as the core cooled.

The magnetic field is important because it shields our planet from harmful space radiation and helps keep our atmosphere in place. One estimate suggests the core will not fully cool for about 91 billion years, long after the Sun is expected to end its life.