At the center of the Earth lies a two-part core. The Earth's rotation makes this ocean flow and swirl, and the moving metal generates the planet's magnetic field.
Most of Earth's heat is stored in the mantle, Marone says, and there are four sources that keep it hot. First, there's the heat left over from when gravity first condensed a planet from the cloud of hot gases and particles in pre-Earth space. As the molten ball cooled, some 4 billion years ago, the outside hardened and formed a crust.
The mantle is still cooling down. It only contributes 5 to 10 percent of the total, "about the same amount as gravitational heat. To explain gravitational heat, Marone again evokes the image of the hot, freshly formed Earth, which was not of a consistent density. In a gravitational sorting process called differentiation, the denser, heavier parts were drawn to the center, and the less dense areas were displaced outwards.
The friction created by this process generated considerable heat, which, like the original heat, still has not fully dissipated. Then there's latent heat, Marone says. This type arises from the core's expanding as the Earth cools from the inside out. Just as freezing water turns to ice, that liquid metal is turning solid—and adding volume in the process. The heat released by this expansion is seeping into the mantle. For all this, however, Marone says, the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium , , and Thorium contained within the mantle.
These isotopes radiate heat as they shed excess energy and move toward stability. Radioactivity is present not only in the mantle, but in the rocks of Earth's crust. For example, Marone explains, a 1-kilogram block of granite on the surface emanates a tiny but measurable amount of heat about as much as a.
That may not seem like much. But considering the vastness of the mantle, it adds up, Marone says. Sometime billions of years in the future, he predicts, the core and mantle could cool and solidify enough to meet the crust. If that happens, Earth will become a cold, dead planet like the moon. Long before such an occurrence, however, the Sun will likely have evolved into a red-giant star, and grown large enough to engulf our fair planet.
At that point, whatever heat is left in the mantle will hardly matter. You might think that this would be good for people — especially those living in places like Tokyo — but volcanic eruptions also produce fertile soil for farming, and gases that make up the air that we breathe. After all this, Earth could look a bit like Mars. On the surface of Mars, scientists have seen features that are related to volcanoes and moving plates. But they are not moving any more, and there is no magnetic field and only a thin atmosphere left.
We do not know whether the core of Mars is still molten or not, but a robot called InSight recently landed on Mars that will help us to find out!
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