Expert Voices

How has Earth's core stayed as hot as the sun's surface for billions of years?

A cutaway illustration of the Earth showing Earth's core.
A cutaway illustration of the Earth showing Earth's core. (Image credit: Rost-9D/Getty Images)

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Shichun Huang, Associate Professor of Earth and Planetary Sciences, University of Tennessee.

Our Earth is structured sort of like an onion – it's one layer after another.

Starting from the top down, there's the crust, which includes the surface you walk on; then farther down, the mantle, mostly solid rock; then even deeper, the outer core, made of liquid iron; and finally, the inner core, made of solid iron, and with a radius that's 70% the size of the moon's. The deeper you dive, the hotter it gets – parts of the core are as hot as the surface of the sun.

Related: Earth's layers: Exploring our planet inside and out

Journey to the center of the Earth

As a professor of earth and planetary sciences, I study the insides of our world. Just as a doctor can use a technique called sonography to make pictures of the structures inside your body with ultrasound waves, scientists use a similar technique to image the Earth's internal structures. But instead of ultrasound, geoscientists use seismic waves – sound waves produced by earthquakes.

At the Earth's surface, you see dirt, sand, grass and pavement, of course. Seismic vibrations reveal what's below that: rocks, large and small. This is all part of the crust, which may go down as far as 20 miles (30 kilometers); it floats on top of the layer called the mantle.

The upper part of the mantle typically moves together with the crust. Together, they are called the lithosphere, which is about 60 miles (100 kilometers) thick on average, although it can be thicker at some locations.

The lithosphere is divided into several large blocks called plates. For example, the Pacific plate is beneath the whole Pacific Ocean, and the North American plate covers most of North America. Plates are kind of like puzzle pieces that fit roughly together and cover the surface of the Earth.

The plates are not static; instead, they move. Sometimes it's the tiniest fraction of inches over a period of years. Other times, there's more movement, and it's more sudden. This sort of movement is what triggers earthquakes and volcanic eruptions.

What's more, plate movement is a critical, and probably essential, factor driving the evolution of life on Earth, because the moving plates change the environment and force life to adapt to new conditions.

An illustration depicting the different layers of the Earth. (Image credit: eliflamra/Getty Images)

The heat is on

Plate motion requires a hot mantle. And indeed, as you go deeper into the Earth, the temperature increases.

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

By the time you get to the boundary between the mantle and the outer core, which is 1,800 miles (2,900 kilometers) down, the temperature is nearly 5,000 F (2,700 C).

Then, at the boundary between outer and inner cores, the temperature doubles, to nearly 10,800 F (over 6,000 C). That's the part that's as hot as the surface of the sun. At that temperature, virtually everything – metals, diamonds, human beings – vaporizes into gas. But because the core is at such high pressure deep within the planet, the iron it's made up of remains liquid or solid.

An illustration depicting the layers of Earth's internal structure.. (Image credit: forplayday/Getty Images)

Collisions in outer space

Where does all that heat come from?

It is not from the sun. While it warms us and all the plants and animals on Earth's surface, sunlight can't penetrate through miles of the planet’s interior.

Instead, there are two sources. One is the heat that Earth inherited during its formation 4.5 billion years ago. The Earth was made from the solar nebula, a gigantic gaseous cloud, amid endless collisions and mergers between bits of rock and debris called planetesimals. This process took tens of millions of years.

An enormous amount of heat was produced during those collisions, enough to melt the whole Earth. Although some of that heat was lost in space, the rest of it was locked away inside the Earth, where much of it remains even today.

The other heat source: the decay of radioactive isotopes, distributed everywhere in the Earth.

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

Radioactive isotopes are not stable. They release a steady stream of energy that converts to heat. Potassium-40, thorium-232, uranium-235 and uranium-238 are four of the radioactive isotopes keeping Earth's interior hot.

Some of those names may sound familiar to you. Uranium-235, for example, is used as a fuel in nuclear power plants. Earth is in no danger of running out of these sources of heat: Although most of the original uranium-235 and potassium-40 are gone, there's enough thorium-232 and uranium-238 to last for billions more years.

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

No heat, no plate movement, no life

Even now, the moving plates keep changing the surface of the Earth, constantly making new lands and new oceans over millions and billions of years. The plates also affect the atmosphere over similarly lengthy time scales.

But without the Earth's internal heat, the plates would not have been moving. The Earth would have cooled down. Our world would likely have been uninhabitable. You wouldn’t be here.

Think about that, the next time you feel the Earth under your feet.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Shichun Huang
Associate Professor of Earth and Planetary Sciences, University of Tennessee

Shichun Huang uses elemental and isotopic tracers, together with petrology and mineralogy, to study the Earth's mantle and the early solar system.

  • orsobubu
    these are all stupid and unsupported theories; no physical mechanism to trap all that heat during 5 billion years, no physical mechanism to explain the starting incredibly high temperature the core should have had due to the merger of planetesimals, no physical mechanism to explain the high temperature of the original dust disk around the sun, no physical mechanism to explain an hypotetical gravitational collapse of the new condensed planet earth, no physical mechanism to explain the amount of energy producing the heat by radioactivity decay, no data to show the huge loss of temperature from the core during past earth's life, no data to show the necessary production of many types of radioactive isotopes as a result of the fission process, no physical mechanism to explain why the same heat creation process is not ongoing in all the other solar system solid bodies
    Reply
  • Classical Motion
    The earth remains a hard nut to crack. Our deepest attempts are only small pricks. We found it saturated with super-heated high pressure water. This water should be highly conductive. Our crust might have an electrical character to it. There might even be electrical layers in the earth, like there is above the earth.

    Some studies show the gravity reaches a maximum at a shell, with a decrease in gravity inside that shell in the center. The gravity gradient under the shell might be opposite the gravity gradient above that shell. A pulling up force at the center. Wouldn't that be wild.

    This water would easily flash at the surface and give a huge source of power. But it would not be free. Like other geo-hydro sites, there are contaminates in the steam and water. High maintenance and replacement repair.
    Reply
  • Jan Steinman
    orsobubu said:
    these are all stupid and unsupported theories…
    Uhm… would you care to present your credentials, and documentation for your beliefs?

    Didn't think so!
    Reply
  • rod
    "Where does all that heat come from? It is not from the sun. While it warms us and all the plants and animals on Earth's surface, sunlight can't penetrate through miles of the planet’s interior. Instead, there are two sources. One is the heat that Earth inherited during its formation 4.5 billion years ago. The Earth was made from the solar nebula (opens in new tab), a gigantic gaseous cloud, amid endless collisions and mergers between bits of rock and debris called planetesimals. This process took tens of millions of years. An enormous amount of heat was produced during those collisions, enough to melt the whole Earth. Although some of that heat was lost in space, the rest of it was locked away inside the Earth, where much of it remains even today. The other heat source: the decay of radioactive isotopes, distributed everywhere in the Earth."

    This is view is interesting in the article. The giant impact for the origin of our Moon does not feature a fully formed Earth as we see today but a proto-earth and a proto-moon that evolves after the giant impact, thus both earth and moon must continue to grow in size and mass until what we see today. Explaining how Earth evolved in the solar nebula and Venus evolved so very differently, from the same nebula and postulated protoplanetary disc, remains very challenging. This model interpretation presented explaining the heat today, could have some holes in it.

    https://forums.space.com/threads/nasa-scientist-explains-why-venus-is-earths-evil-twin-video.59691/
    Reply
  • WisdomLost
    I've heard the isotope/radiation theory, and thought it was an interesting guess. Personally, I always thought the core was active due to the tidal effect of our relatively large moon.

    With the constant churning of the moon's pull, the mantle stays relatively fluid, and the core oscillates within that "fluid". Resulting friction would produce a lot of heat.

    I don't know a lot about the inner workings of Mars, but also assumed the lack of a large moon accounted for the dead core (though recent seismic readings suggest it's not as dead as we thought).

    Venus is just plain hot. Solar accounts for the extreme temperature at the surface, but I don't know if the core is active. Same for Mercury. There are no other "rock" planets in our solar system to compare to, so my guess probably couldn't be "proven" by comparison. However, we do see evidence of active cores on moons orbiting large gas giants.
    Reply
  • rod
    Perhaps there is heat/cooling problems in the solar system, whether Earth or some small moons or other planets. I have read over the years different reports that indicate heat/cooling issues come up when showing how something remains hot over a 4.5 Gyr solar system model.
    Reply
  • theglassisempty
    orsobubu said:
    these are all stupid and unsupported theories; no physical mechanism to trap all that heat during 5 billion years, no physical mechanism to explain the starting incredibly high temperature the core should have had due to the merger of planetesimals, no physical mechanism to explain the high temperature of the original dust disk around the sun, no physical mechanism to explain an hypotetical gravitational collapse of the new condensed planet earth, no physical mechanism to explain the amount of energy producing the heat by radioactivity decay, no data to show the huge loss of temperature from the core during past earth's life, no data to show the necessary production of many types of radioactive isotopes as a result of the fission process, no physical mechanism to explain why the same heat creation process is not ongoing in all the other solar system solid bodies

    It does take that long to cool off, do the math. And, there is residual radioactivity keeping up the temperature. Your ramblings are the only unsupported "theories" around here . . . . .
    Reply
  • rod
    Some interesting comments here in various posts. Who, what, when, where, how, and why are good investigative questions to ask.

    For example, what was the original core temperature of Earth when it was a proto-earth before the giant impact with Theia - creating the Moon? The proto-earth is not the same size or mass as we live on today but smaller in size and mass. Initial conditions must be defined, heat loss rates, heat sources for adding heat, etc., accretion growth rates, etc.
    Reply
  • rod
    https://www.scientificamerican.com/article/why-is-the-earths-core-so/, Why is the earth's core so hot? And how do scientists measure its temperature?

    There are some model answers in the link but IMO, still not very good and lacking details.
    Reply
  • ScientistMan
    Given the difference in half life between U235 and U238, it is common knowledge that the natural uranium enrichment was much higher early in the earth's life. Just like there were natural reactors in 'operation' at that time (like Oklo in Gabon), perhaps the entire core (pardon the pun) was one massive yet self regulated nuclear fission reactor producing an insane amount of heat over that time. Is there evidence to suggest any significant quantities of uranium in the core?
    Reply