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, we’ve 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.

We’re 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 they’re

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, that’ll 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 very “lifey” place.

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

solid, and we’re 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.