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  • [MUSIC]

  • Hey! I’m just making a planet. Well, this is how planets like Earth get their structure,

  • anyway. Because of different layers of density!

  • Even today, with all of our modern technology, weve only been able to drill about a third

  • of the way through Earth’s crust, so how do we really know if it’s solid, liquid,

  • hollow? Luckily, Earth has this tendency to violently shake and occasionally burp up some

  • of its insides, and that’s taught us a lot about our planet’s guts without having to

  • go down there.

  • Were used to seeing density at work.

  • That’s the same reason that the atmosphere, the least dense part of our planet is on the

  • outside, and the crust, the second least dense part of Earth, is beneath our feet.

  • Because I’m less dense than the dirt, I don’t sink into the ground. And even though

  • there’s about one ton of atmosphere above my head, it’s not dense enough to send me

  • floating.

  • The main layers of Earth are organized in the same way. Depending on whether theyre

  • divided up by how they squish around or what they are made of, geologists give different

  • names to the different layers of the Earth.

  • So that’s how it is now. But to really understand why Earth is organized the way it is, we need

  • to go back to before our planet even existed.

  • In the very young universe, hydrogen and helium were pretty much the only elements around.

  • They condensed into stars, began the process of nuclear fusion, and eventually died, spitting

  • heavier elements, from carbon and oxygen to things like nickel and gold, back out into

  • the universe. One of those heavy elements, iron, is the most stable element produced

  • outside of a supernova. The early universe produced a lot of iron, thatll be important

  • in a second.

  • Fresh hydrogen and helium went on to form new stars like our sun, and the heavier elements

  • collided to form the dust and debris that would become our solar system’s planets,

  • moons, asteroids and everything else.

  • High temperatures in the early inner solar system meant that light, volatile elements

  • could only condense further out, which is why the four inner planets of our solar system

  • are dense and rocky, while outer gas giants like Saturn could hypothetically float in

  • a really, really, really big swimming pool.

  • After proto-Earth grew larger, radioactivity, gravity and violent boom booms melted the

  • messy mixture of rocks and minerals. And this is where things started to get organized.

  • Just like that tower of density, the heaviest materials like iron and nickel worked their

  • way to the core, and the lighter materials like aluminum and silicon stayed near the

  • surface.

  • The inner core experiences pressures more than three million times what we do on Earth’s

  • surface, which means that despite being as hot as the surface of the sun, the iron in

  • our planets inner sphere is likely solid, not liquid.

  • The outer core is most likely liquid because it’s hot, but not under as much pressure

  • as the inner core. We know that’s the case because of earthquakes up here on the surface.

  • As certain kinds of seismic waves travel through the Earth, the liquid outer core either refracts

  • them, or blocks them altogether, creating seismic shadows on the opposite side of the

  • planet.

  • If you want to use pressure to melt metal at home, just stack up 16 million pennies.

  • The one on the bottom should liquify in no time!

  • Mercury is so close to the sun that its atmosphere has long since blown away, but luckily for

  • you and me, our liquid metal outer core lets us have an atmosphere. Deep metallic convection

  • currents create a magnetic field that shields Earth from the solar wind. Otherwise we’d

  • be pounded with deadly radiation, our atmosphere would be blown away, and Earth wouldn’t

  • be a verylifeyplace.

  • Over time, Earth continues to cool, so more and more of its liquid outer core is turning

  • solid, and were shrinking little by little. Every earthquake that we feel is Earth taking

  • one step closer to cooling off, and becoming a real third rock from the sun.

  • So there you have it. Like Carl Sagan said: “We are star stuff who has taken its density

  • into its own hands.”

  • It's density, right?

  • "I'm your density!"

  • Stay curious.

[MUSIC]

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