Earth



Earth
Credit: NASA


The Earth is an amazing place. We know it better than anywhere else in the Universe, but it is also the home of one of the greatest mysteries in space. It is the only place we have found so far that has life. Life has taken on thousands of shapes, sizes and amazing abilities on the Earth. It has stretched from the bottom of the ocean to the top of mountains, from acid lakes to stormy shores, from bacteria that can eat radioactivity to animals the size of small buildings.

It is our home, and it is the third planet away from the Sun. This means that when you look up at the Sun during the day, there are only two other planets orbiting it closer than us. Most of the solar system is much, much further away, and is much, much colder than we are. This, it turns out, is a very important thing. We are in just the right part of the Sun’s atmosphere to enjoy a pleasant day at the beach. If we were closer, the water all around us could boil away in the Sun’s heat and there wouldn’t be any cool streams to swim in. And if we were farther away, we could be in a world covered in ice and rock.

In fact, some scientists believe that at certain points in the past, the Earth was much colder than it was today, and that it was completely covered in snow and ice. They call this idea “Snowball Earth” and as you can see, it would have been a very different place than the one we know today.



What “Snowball Earth” may have looked like
Earth
Image rendered in Celestia

Think About It

Why do the seasons change? Why does it get hotter in summer and colder in winter? The Earth doesn’t go around the Sun in a perfect circle, but it is still at more or less the same distance. It is not because the Earth gets closer or farther away from the Sun that the seasons change.

Seasons change because the Earth leans on its side as it goes around the Sun. We say that the Earth is “tilted on its axis” or has an “axial tilt.” While the Earth is on one side of the Sun, the part leaning away from it gets slightly less sunlight. And when the Earth is on the other side of the Sun this side now points towards the Sun and gets more sunlight.


Illustration of the Seasons in Northern countries, with summer on the left (the North is leaning towards the Sun) and winter on the right (when North is leaning away from the Sun)
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Earth

This tiny difference is enough to change the freezing snowstorms of winter into a scorching hot summer day. It is also why people in northern countries have summer (they are on the part pointing towards the Sun) while people in southern countries are in the middle of winter (they are in the part pointing away from the Sun).


Atmosphere

Earth
Credit: NASA

The Earth, like many planets, has an atmosphere. For a planet like the Earth, this means that there is a layer of gas trapped around its surface. In our case, that gas is what we call air. The gases in the air are mostly nitrogen (78.09%), oxygen (20.95%), argon (0.95%), and carbon dioxide (0.038%). Since we breathe oxygen and plants breathe carbon dioxide, this atmosphere is perfect for us to survive. The air also contains a lot of water vapor. This is because Earth’s surface is mostly covered with water (about 71%). This water from all the oceans, seas, rivers, lakes, ponds, puddles, and everything else evaporates into the air when it gets hot enough, filling the air with water vapor.

More and more, scientists realize that having life on Earth has also had an effect on the planet. There are so many plants, animals and bacteria on Earth that just by breathing we have actually changed what is in the atmosphere!

In fact, scientists believe that some gases would not be in our atmosphere at all if there wasn’t life on the planet. So astronomers are now using this information to look for atmospheres like the Earth on other planets. Maybe if we find an atmosphere like ours somewhere else, we might find life there too!

When we step outside, or even sit at home or at school, we are in the Earth’s atmosphere. When it is windy or when clouds drift overhead, that is because of things happening in the Earth’s atmosphere, making the air move (wind), or making water stick to gas and particles in the air (clouds). So the next time it is windy or that there is a big thunderstorm outside (or even if you just see some clouds drifting overhead) remember that these are just small bits and pieces of a giant atmosphere stretching all the way around the Earth!

The atmosphere is so big that it has many different parts. Just like the air might feel dry in the desert or cold when it’s foggy, the atmosphere is different in different places.

As you get higher up, the atmosphere also changes. Some gases are lighter, and some are heavier. They rise and fall in the atmosphere depending on their weight. It is just like when bubbles float to the top of your bathtub when you use bubble bath. This is because the soap in the bubble bath is lighter than the water in the tub! Some gases also mix differently at different temperatures and pressures, so there will be more or less of them at different places in the atmosphere.

All of this means that gases build up at different points in the atmosphere, creating “layers” of gas.

One example is the ozone layer, which got its name because it is where ozone gas builds up in the atmosphere. This layer is famous because ozone absorbs harmful radiation coming from the Sun to the Earth. It is also famous because we now know that some kinds of pollution are destroying the ozone layer at different places over the Earth. Many people are afraid that this will increase the Sun’s dangerous radiation on the surface. So what we put into the atmosphere can have a real effect on what is happening right here on the surface! This is why scientists today are trying to learn as much as possible about this thin layer of gas sitting on our planet.

There are layers of atmosphere that are named according to the gases in them, like the ozone layer, but scientists also divide the atmosphere into layers based on the temperature and pressure at a certain altitude, or height.

The ozone layer, for example, is in the layer of the atmosphere we call the stratosphere. The stratosphere is much higher than the part of the atmosphere that we live in here on the ground. In our everyday lives, even when we are flying in airplanes high above the clouds, we still live in the lowest layer of the atmosphere, called the “troposphere.”

But even all of these parts of the atmosphere are only a few miles thick at any point on the Earth. In fact, even though there is no fixed line between the atmosphere and space, the atmosphere starts to fade away pretty close to the surface of the Earth, only about 60 miles up. If this sounds like a lot, then think that this is only about as far as you would drive in an hour on the highway. But it takes much, much longer to drive to around the planet, or even across the United States!

So the atmosphere is actually very thin. In fact, if the Earth was the size of a bowling ball, the atmosphere would be thinner than a coat of paint on the surface of the ball! So every now and then look up and remember how precious the atmosphere is. Without it the surface of the Earth would be like walking on the Moon.

Notice how thin the atmosphere is on the Earth. It is the thin blue line on the edge of the Earth at the back of the picture. Every city on Earth, airplane, and person you know is in this thin layer of air on the planet (unless they are an astronaut of course)

Earth
Astronaut in Space. Credit: NASA

Having an atmosphere not only means we can breathe air wherever we go, but it acts like a shield against all sorts of things that are flying towards us from space. The atmosphere stops dangerous radiation coming from space by absorbing or scattering it. But it also stops much more solid objects. Every day tons of rock flies towards the surface of the Earth only to be stopped by our atmosphere. We call these rocks meteorites. If you don’t think air can stop a rock from space, then imagine flying a kite in a tornado. Meteorites are flying so fast in space that by the time they hit the atmosphere this isn’t even close to the energy they get from the air around them. Most of these rocks get blown up or broken up into dust miles before they ever reach the ground! If you’re lucky, you can see these rocks burning up as they shoot towards the Earth. We call them shooting stars!



EarthMagnetism and the Earth
The Earth is surrounded by a gigantic bubble that stretches up from inside the Earth all the way out into space. Luckily for us, this bubble is invisible, otherwise it would fill up the sky everywhere we looked. But it is there, even as you’re reading this. It is a magnetic field many times bigger than the Earth that comes from deep within the Earth’s core. It might be hard to imagine, but look up and this invisible bubble is acting like a powerful force field at this very moment, bouncing off particles and rays that are coming at us from outer space.

Even though the Earth is a comfortable island for us in space, we are also living in a much, much bigger world: the world of our galaxy and of the Universe. This world might be very far away, but without the protection of the Earth’s magnetic field, even these very faraway places would have a huge effect on us here on Earth!

Mostly, though, the particles and rays that our force field is fighting right now are coming from our Sun. Because the Sun is the closest Star to us, there is much, much more energy that hits us from the Sun than from the rest of the Universe combined. Want to see the proof? Just look outside when it’s daytime! We get so much energy (light) from the Sun that we can’t see any other stars in the sky when it is daytime. Of course, when we face away from the Sun at night, you get a better idea of all the other stars that are out there. There are many more than our Sun, but they are all very far away, while our Sun is relatively close by.

Illustration of the Earth’s Magnetic Field deflecting charged particles from the Sun

Earth

Without this magnetic field, the Earth would be constantly hit by harmful particles flying off of the Sun. Have you ever heard of the auroras? People living near the North Pole and South Pole have. These are beautiful curtains of light that shine in the sky. They are caused by particles giving off light when they hit gases in the high atmosphere. These particles ride along this giant magnetic field around the Earth, which leads them crashing towards the Earth’s North and South Poles.

Earth



Think About It

Every day we experience the power of invisible rays on the Earth. Your cell phone picks up invisible signals from towers and satellites that can be many miles away. Your microwave oven uses invisible rays to heat up food quickly, and you and your parents might listen to the radio, which transforms invisible radio waves into electric pulses and then sound. Magnetism is a form of invisible energy that is very powerful and can disrupt these signals or even shut them down completely. That is why this force field is so effective at stopping harmful rays that come towards us from space. So while we may not notice it because it is invisible, it acts like an umbrella in space, stopping the flow of harmful particles that come at us from space.


Inside the Earth

Earth

The Earth is made up of different layers of rock. It has a very thin top layer called the crust, a much deeper layer called the mantle divided into the upper mantle and mantle, then a layer called the core, divided into the outer and inner core. Each layer is different and acts differently. The crust on the Earth is like the hard shell of an egg. This is where we live. It includes the oceans and the land that we walk around on. It is only around 22 miles thick on average on dry land. We call this the “continental crust.” Under the ocean, it is even thinner, only around 4 miles thick on average. We call this the “oceanic crust.” This might seem like a lot, but it is very, very thin when compared to the mantle, which is over 1796 miles thick! The mantle is the thickest part of the Earth, with the outer core being a bit thinner at around 1410 miles thick, and the inner core (and center of the Earth) being a giant ball around 776 miles wide.

If you could imagine going outside right now and starting to dig, you would notice that the Earth has some amazing places right under our feet. Not only have we found ancient cities, and even caves made out of crystal within the crust, but we also know that the further you go the weirder it gets. The mantle of the Earth, for example, seems to be less solid than the hard rock that is under our feet. In fact, it seems to slip and flow around very, very slowly, probably because the material in it is starting to melt. This is even though it is made up of solid rock! Since this layer moves, the crust that is sitting on top of it sometimes cracks and slips around, exactly like the shell of an egg.

Unfortunately we hear a lot about earthquakes and tsunamis, but these terrible events only happen because of this cracking and slipping of the Earth’s crust because of the mantle moving underneath!

We divide up the big pieces of the crust into areas we call plates, and call the movement of the crust on top of the fluid mantle “plate tectonics.”

If you could imagine continuing into the mantle, you would enter a world that is nothing like the one we know up here. It would be incredibly hot, and you would start to feel your body getting crushed by the pressure around you.

In fact, this is exactly what happens to the rock deep inside the Earth. It gets so hot that it starts to melt into lava. If you thought a volcano was bad, it is nothing compared to the huge ball of molten rock in the outer core. This is like being inside of the huge engine that powers the Earth. Imagine how big and powerful this must be to power an entire planet! Right now, as you’re reading this, this enormous world of lava, fire and bubbling hot rock bigger than all the continents combined is flowing around miles below our feet! It is because it is so big that it can power the huge force field that surrounds the entire Earth and even way out into space. So even though we would never want to be in the outer core, we can be thankful that it is making life so nice for us here on the surface with our giant magnetic field!

If you could somehow continue through this ocean of lava, you would hit the solid core of our planet. We think that it is made up of solid iron, over 770 miles across! This is the metallic center of the rotating ball of lava. It is thanks to the way that this core interacts with the material in the outer core that the magnetic field really gets going.


Think About It

Did you know that no one has ever been more than a few miles down into the Earth? In fact, even though it is so thin compared to the rest of the planet, we have never even made it all the way through the Earth’s crust, even in the deepest mines, and even using special instruments to dig deep below the bottom of the ocean into the heart of the Earth. We use special instruments that can listen to vibrations deep underground to measure what we think is down there. Different materials vibrate in different ways, so we can make good guesses for what we think is down there. This is why we are pretty sure that there is a molten outer core and solid inner core of the Earth, but also that if this is true then it must be hotter than the surface of the Sun! So luckily there is a lot of rock between us!


The Big Picture

While we play sports or videogames, watch TV, or do our chores on any given day, it is easy to forget that the Earth is a planet flying through an incredibly huge galaxy and Universe. Some of these places seem strange compared to the Earth, since we are so used to living here. But there is also a lot more to learn about the Earth, and there may be more strange things to discover right here. Did you know that people have explored less than 10% of the ocean? We have found animals that we thought were completely extinct, and we keep discovering new species that we never even knew existed. Did you know that more people have been into space than have ever visited the deepest parts of the ocean? So while we think we know a lot about Earth, there is a lot more to explore.

Imagine if we found out that right now, a dinosaur was living at the bottom of the ocean. Think this is impossible? If no one has been there, then no one really knows. If you had the whole country to hide in, it would be easy to play hide and seek, wouldn’t it? Well there are places bigger than the whole country under the ocean that no one has ever visited. Think about it!

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