Why has the earth’s interior been as hot as the sun’s surface for billions of years?

Why has the earth's interior been as hot as the sun's surface for billions of years?
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How does the interior of the earth stay boiling hot for billions of years? Henry, age 11, Somerville, Massachusetts

Our earth is built like an onion – one layer at a time.

Starting from top to bottom there is the crust, which encompasses the surface you are walking on; then further down the mantle, mostly solid rock; then still deeper the outer core of liquid iron; and finally the inner core of solid iron with a radius 70% the size of the moon. The deeper you dive, the hotter it gets—parts of the core are as hot as the Sun’s surface.

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Journey to the Center of the Earth

As a Professor of Earth and Planetary Sciences, I study the inside of our world. Just like a doctor can use a technique called sonography To take pictures of the structures inside your body using ultrasound waves, scientists use a similar technique to image the Earth’s internal structures. But instead of using ultrasound, geoscientists use it seismic waves – Sound waves generated by earthquakes.

On the surface of the earth you can of course see earth, sand, grass and pavement. Seismic tremors reveal what lies beneath: Rocks, large and small. This is all part of the crust, which can be as deep as 30 kilometers; it floats on the layer called the mantle.

The upper part of the mantle typically moves with the crust. Together they are called the lithospherewhich averages about 60 miles (100 kilometers) thick, although it can be thicker in places.

The lithosphere is divided into several large blocks called slabs. For example, the Pacific Plate underlies all of the Pacific Ocean, and the North American Plate covers most of North America. Plates are a kind of jigsaw puzzle pieces that roughly fit together and cover the earth’s surface.

The panels are not static; instead they move. Sometimes it’s the tiniest fraction of a centimeter over a period of years. Other times there is more movement and it is more sudden. It is this type of movement that causes earthquakes and volcanic eruptions.

In addition, plate motion is a critical and likely essential factor driving the evolution of life on Earth, as the moving plates change the environment and force life to adapt to new conditions.

The heat is on

Plate motion requires a hot coat. And indeed, the deeper you go into the earth, the temperature rises.

At the bottom of the plates, about 60 miles (100 kilometers) deep, the temperature is about 2,400 degrees Fahrenheit (1,300 degrees Celsius).

When you reach the boundary between the mantle and the outer core, which is at a depth of 2,900 kilometers, the temperature is almost 2,700 °C (5,000 F).

Then, at the boundary between the outer and inner cores, the temperature doubles to almost 10,800 F (over 6,000 C). That’s the part that is as hot as the surface of the sun. At this temperature, practically everything – metals, diamonds, people – vaporizes into gas. But because the core is under such high pressure deep inside the planet, the iron that makes it up remains either a liquid or a solid.

collisions in space

Where is all the heat coming from?

It’s not from the sun. While it warms us and all plants and animals on Earth’s surface, sunlight cannot penetrate miles into the planet’s interior.

Instead, there are two sources. One of them is the heat that the earth inherited during its formation 4.5 billion years ago. The earth was made from the solar nebulaa gigantic gaseous cloud, amidst endless collisions and mergers between boulders and debris called planetesimals. This process took tens of millions of years.

A tremendous amount of heat was generated in these collisions, enough to melt the entire earth. Although some of that heat was lost in space, the rest was trapped inside the Earth, where much of it still exists today.

The other source of heat: the decay of radioactive isotopes that are scattered all over the planet.

To understand this, first imagine an element as a family with isotopes as its members. Each atom of a given element has the same number of protons, but different isotopic relatives have different numbers of neutrons.

Radioactive isotopes are not stable. They release a steady stream of energy that turns into heat. Potassium-40, Thorium-232, Uranium-235, and Uranium-238 are four of the radioactive isotopes that keep Earth’s interior hot.

Some of these names may sound familiar to you. For example, uranium-235 is used as a fuel in nuclear power plants. The earth is not in danger of running out of these heat sources: although most original uranium-235 and potassium-40 are gonethere is enough thorium-232 and uranium-238 to last for another billion years.

Together with the hot core and mantle, these energy-releasing isotopes provide the heat to drive the movement of the plates.

No heat, no plate motion, no life

Even now, the moving plates are changing the surface of the earth and are constantly doing new lands and new oceans over millions and billions of years. The plates also affect the atmosphere over similarly long timescales.

But without the Earth’s internal heat, the plates would not have moved. The earth would have cooled. Our world would probably have been uninhabitable. you wouldn’t be here

Keep that in mind the next time you feel the earth beneath your feet.

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This article is republished by The conversation, an independent non-profit news site dedicated to sharing ideas from academic experts. The conversation has a variety of fascinating free newsletters.

It was written by: Shichun Huang, University of Tennessee.

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Shichun Huang does not work for, advise, own any interest in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations other than her academic appointment.

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