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Why is it so hot inside Earth's core?


Narrator: Hi! Thanks for joining us as we ponder another weighty question at Space Place Musings. I'm Diane Fisher of the New Millennium Program. Dr. Marc Rayman, a scientist at the Jet Propulsion Laboratory, is here to ponder with us.

Marc, today our question is from the Southwest Florida Astronomical Society in Ft. Myers. They ask "Why is it so infernally hot in the center of Earth?"

Marc: Well, finally we get a question that's close to home. Although Earth's center is closer than most of the other objects we muse about at The Space Place, in some ways it is more remote. Its solid core is about the same size as Pluto, however instead of being billions of miles away through interplanetary space, it's only a few thousand miles away, but through dense rock.

Narrator: But, if we can't see it or probe it directly, can we really know anything for sure about Earth's core?

Marc: Scientists have several reliable ways to learn about Earth's deep interior. One of them is to study earthquakes. Earthquakes produce waves that travel through rock much the way sound waves travel through air. With seismometers around the world to detect the strength, timing, and other characteristics of the waves, geologists can figure out the structure of our planet. In some ways, this method is similar to using ultrasound to create a picture of the inside of a person's body.

Narrator: And what does that Earthquake wave picture show us?

Marc: One thing is that Earth's solid center is surrounded by an even larger layer of molten rock. So we have an inner core that is solid and an outer core that is liquid. Together they are over 2000 miles in radius, or around half the size of the entire planet. Above the core is the thick mantle, and on top of that is a relatively thin crust, which is just under our feet.

Narrator: And the deeper you go, the hotter it is, right?

Marc: That's right. Although it's very difficult to find out the temperature at great depths, the core may be between about 7,000 and 12,000 degrees Fahrenheit. To appreciate how hot that is, the surface of the Sun is about 10,000 degrees, so our planet's core might be hotter than the surface of a star!

Narrator: OK . . . well, how did it get that way?

Marc: Our planet formed from many smaller bits of rock that collided and stuck together when the solar system was developing more than four and a half billion years ago. As each piece of rock fell onto the forming planet, it added a little bit of energy, which caused the growing Earth to heat up. So our home had a hot beginning.

Narrator: But after billions of years, why hasn't it cooled off?

Marc: Good question. A brilliant 19th century physicist, William Thomson, who we know better as Lord Kelvin, asked a similar question. He assumed Earth had begun in a molten state and then calculated how long it would take to cool to its present conditions without any other source of internal energy. He returned to this problem many times over the decades, and his final estimate at the end of the century was that Earth must be only about 20 to 40 million years old. His conclusion disagreed with findings from geology and biology, both of which showed that Earth was much older than that.

Narrator: Why were his calculations so far off? Was there a problem with his method?

Marc: Kelvin's method was good, but 19th century scientists didn't know about radioactivity. At the beginning of the 20th century, when they recognized that decaying atoms released tremendous amounts of energy, scientists understood that Earth has not simply been cooling off since its formation. Now we know that radioactive elements that take billions of years to decay have kept Earth's interior hot.

Narrator: So with all this energy, it's almost like our planet is a giant engine.

Marc: That's right, and that engine does some powerful work. The heat flowing up from deep inside is what drives continental drift, the motion of the enormous plates that make up the crust. Earth's engine also powers volcanoes, bringing hot rock up from the depths through openings to the surface.

Narrator: Earth does have a lot in common with a very complex machine. Our listeners can test their knowledge of this planetary machine by playing The Space Place Quiz Show game. Just go to and type "quiz show" in the "Find it @ Space Place" field.

That's it for this time. Join us again soon when we muse on another hot topic about our universe.