How Hot Is Earth's Core?
The short answer
About 5,400 °C (9,800 °F) in the solid inner core, by NASA's figure. That's roughly the temperature of the Sun's surface, about 5,500 °C (10,000 °F). Estimates for the core run from about 5,000 to more than 6,000 °C, a range that takes in the Sun's surface temperature.
Nobody has been anywhere near it. The deepest hole ever drilled, the Kola Superdeep Borehole, reached 12,262 m (about 7.5 miles). The center is about 6,371 km (3,959 miles) down, so the hole got about 0.2% of the way.
What's between us and the core
Under the crust is the mantle. It's solid rock, not lava. Over very long stretches of time it slowly flows, about as fast as your fingernails grow.
About 2,890 km (1,800 miles) down, the outer core begins. It's liquid, mostly iron with some nickel, and about 2,300 km (1,400 miles) thick.
At the center sits the inner core, a solid ball of iron and nickel about 1,221 km (759 miles) in radius. That makes it about 2,442 km (1,517 miles) across, roughly 3% wider than Pluto, which is 2,377 km (1,477 miles) wide.
Why the hottest part is solid
At the surface, heat like that would melt iron. At the center, the weight of all the rock and metal above presses down at nearly 3.6 million atmospheres. The pressure is too great for the iron atoms to move into a liquid state.
Nobody measured this with a thermometer. Earthquake waves travel through the whole planet, and one kind, S waves, can't pass through liquid. They vanish at the outer core. In 1936 the Danish seismologist Inge Lehmann found faint waves bouncing off a solid ball in the middle. Temperature estimates come from lab work. In 2013 a team squeezed iron between diamonds and heated it with lasers. They put the temperature at the edge of the solid core at about 6,000 °C, plus or minus 500.
Why it matters: the magnetic field
The liquid outer core never stops churning, driven by radioactive heating and chemical differentiation. Moving iron carries electric currents, and those currents make Earth's magnetic field. It works like a self sustaining generator, not a bar magnet.
That field deflects most of the solar wind. A little gets in and spirals toward the poles, where it helps light up the auroras.
Mars lost its global magnetic field about 3.7 billion years ago. NASA's MAVEN mission found that the solar wind and radiation stripped away most of its air over billions of years. Mars still loses about 100 grams (a quarter pound) of atmosphere to the solar wind every second.
Key numbers
| What | Number | Source |
|---|---|---|
| Temperature of the inner core | about 5,400 °C (9,800 °F) | NASA, Earth facts |
| Temperature of the Sun's surface | about 5,500 °C (10,000 °F) | NASA, Sun facts |
| Temperature at the edge of the solid inner core, from lab work | about 6,000 °C, plus or minus 500 | ESRF, 2013 |
| Deepest hole ever drilled, the Kola Superdeep Borehole | 12,262 m (about 7.5 miles) | Scientific American |
| Distance to Earth's center (mean radius) | 6,371 km (3,959 miles) | NASA Earth Fact Sheet |
| How far the Kola hole got toward the center | about 0.2% | Our arithmetic from Scientific American and NASA Earth Fact Sheet |
| Depth where the liquid outer core begins | about 2,890 km (1,800 miles) | Wikipedia, Earth's outer core |
| Radius of the inner core | 1,221 km (759 miles) | NASA, Earth facts |
| Width of Pluto | 2,377 km (1,477 miles) | NASA, Pluto facts |
| Pressure at Earth's center | nearly 3.6 million atmospheres | National Geographic Education, Core |
| When Mars lost its global magnetic field | about 3.7 billion years ago | NASA, MAVEN video release |
| Air Mars loses to the solar wind today | about 100 grams (a quarter pound) per second | NASA, MAVEN solar wind release, 2015 |
What the film simplifies
- The film opens with "as hot as the surface of the Sun" and later rounds the core to "about ten thousand degrees Fahrenheit". NASA's figures are 5,400 °C (9,800 °F) for the core and 5,500 °C (10,000 °F) for the Sun's surface, so "roughly as hot" is the honest version, never "hotter than".
- "A ball of iron" is the short version. Both core layers are mostly iron, with some nickel and a few lighter elements.
- "Rock so hot, it flows" can sound like lava. The mantle is solid rock that creeps over very long times, and the film draws it as solid rock.
- "A giant magnet" and "bends the solar wind around us" are shorthand. Earth works like a generator driven by moving liquid iron, not a bar magnet, and the field deflects most of the solar wind, not all of it.
- NASA blames the solar wind and radiation for Mars losing its air. Scientists still debate how much a magnetic field protects an atmosphere: Venus has no field of its own, and Earth, Mars and Venus lose gas at similar rates.
- The rocket's dive into the core and the talking Gravity are cartoon. Gravity's "weight of the whole planet" is shorthand for the weight of all the rock and metal above the core.
For teachers
Runtime 1 minute 23 seconds · Topics: Earth's interior, Heat and pressure, Magnetic field, Solar wind · Full script below
Ask your class
The inner core is hot enough to melt iron at the surface. Why is it solid?
Answer
Pressure. The weight of everything above squeezes it at nearly 3.6 million atmospheres, too hard for the iron atoms to move into a liquid state.
The deepest hole reached 12,262 m. Earth's center is about 6,371 km down. What fraction of the way did the hole get?
Answer
12.262 km divided by 6,371 km is about 0.002, so about 0.2% of the way.
Which part of the core makes Earth's magnetic field, the solid inner core or the liquid outer core, and why does it matter for us?
Answer
The liquid outer core. Its churning iron carries electric currents that make the field, and the field deflects most of the solar wind around Earth.
The script
Host: Under your feet is a ball of iron as hot as the surface of the Sun.
Rocket: Tour stop one: the center of the Earth! Going down!
Host: Tiny problem. Nobody's ever been down there.
Host: Our deepest hole ever? Seven and a half miles. The center is four thousand miles down.
Rocket: Hold my fuel!
Host: First, the mantle. Rock so hot, it flows. About as fast as your fingernails grow.
Host: Eighteen hundred miles down: an ocean of liquid iron.
Rocket: Cool, cool! I can't swim!
Host: And at the very center, solid iron. A ball the size of Pluto.
Host: It's about ten thousand degrees Fahrenheit. Roughly the surface of the Sun.
Host: Hot enough to melt iron. So why is it solid?
Gravity: That would be me. Squeezing it with the weight of the whole planet.
Host: Now the twist. That ocean of liquid iron never stops churning.
Host: And churning liquid iron turns the whole planet into a giant magnet.
Lil Sat: Incoming! Solar wind! Solar wind!
Host: That magnetic field bends the solar wind around us.
Host: Mars lost its magnetic field. And over billions of years, the solar wind stripped away most of its air.
Host: So the hottest place on the planet is also its bodyguard.
Host: Next stop: a star so dense, a teaspoon weighs a billion tons. Follow!
Sources
- Earth's inner core reaches about 5,400 °C (9,800 °F).NASA, Earth facts
- The Sun's surface is about 5,500 °C (10,000 °F).NASA, Sun facts
- The deepest hole ever dug reached about 7.5 miles.Scientific American
- Earth's center is about 4,000 miles down.NASA Earth Fact Sheet
- Mantle rock flows about as fast as fingernails grow.USGS
- Pluto is 2,377 km wide, close to the inner core's width.NASA, Pluto facts
- Churning liquid iron in the outer core makes Earth's magnetic field.USGS
- The solar wind stripped away most of Mars's atmosphere.NASA, MAVEN
- Lab work puts the temperature at the inner core's edge at 6,000 °C, plus or minus 500, at 3.3 million atmospheres; iron squeezed between diamonds and heated with lasers.ESRF, 2013
- Pressure at the center is nearly 3.6 million atmospheres, too great for iron atoms to move into a liquid state; S waves can't pass through fluids.National Geographic Education, Core
- The outer core begins about 2,889 km (1,795 miles) below the surface and is mostly iron and nickel.Wikipedia, Earth's outer core
- Most of the solar wind is deflected by Earth's magnetosphere; particles that get in spiral toward the poles and help make auroras.NASA, What Is the Solar Wind?
- Mars lost its global magnetic field about 3.7 billion years ago.NASA, MAVEN video release
- The solar wind strips about 100 grams (roughly a quarter pound) of Mars's atmosphere every second.NASA, MAVEN solar wind release, 2015
- S waves can't pass through the liquid outer core, which shows the outer core is liquid.Wikipedia, Shadow zone
- Inge Lehmann discovered the solid inner core in 1936.Wikipedia, Inge Lehmann
- How much a magnetic field protects an atmosphere is debated; Earth, Mars and Venus lose gas at similar rates, though Mars and Venus have no field of their own.Gunell et al., 2018