This article was originally published at The Conversation. (opens in new tab) The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.
Shichun Huang (opens in new tab), 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 (opens in new tab), I study the insides of our world. Just as a doctor can use a technique called sonography (opens in new tab) 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 (opens in new tab): 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 (opens in new tab), 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 (opens in new tab). 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 (opens in new tab).
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.
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 (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.
To understand this, first imagine an element as a family with isotopes as its members (opens in new tab). Every atom of a given element has the same number of protons, but different isotope cousins have varying numbers of neutrons.
Radioactive isotopes (opens in new tab) 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 (opens in new tab). 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 (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).
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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.
Didn't think so!
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.
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.
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 . . . . .
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.
There are some model answers in the link but IMO, still not very good and lacking details.