# The Universe Made Simple/The Earth

| ← Introduction | The Universe Made Simple - The Earth | The Solar System → |

## Earth

The Earth is the third planet from the sun and the largest terrestrial planet. A terrestrial planet is one that has a solid surface and is not primarily gas. It is also the fifth largest planet. The Earth is 149,600,000 kilometers from the sun, which is equal to one astronomical unit or AU. Unlike the other planets, Earth has an English name that is not derived from Greek or Roman mythology. Some other interesting facts about the Earth are that it is the densest major body in the solar system and it is between 4.5 and 4.6 billion years old. Every 500,000,000 years or so, the Earth’s crust is completely destroyed and recreated by plate tectonics and erosion. Seventy-one percent of the Earth is covered in water and it is the only planet where water exists in liquid form. The Earth has over 13,000 artificial satellites orbiting it. These are random facts about the Earth to give you a better picture of what our planet is like in comparison to the other planets. The rest of this chapter will discuss the physical makeup, atmosphere, and magnetic field of the Earth. We will also talk about the moon and why the Earth is habitable.

## Physical Makeup

The Earth is 12,742 kilometers in diameter and is made up primarily of rock and iron. However, its chemical composition is a little more complicated. By mass, the Earth is 34.6% iron, 29.5% oxygen, 15.2% silicon, 12.7% magnesium, 2.4% nickel, 1.9% sulfur, and .05% titanium. This seems like a long confusing list, but it just means that the chemical composition of the Earth is diverse. The makeup of Earth’s mass is simpler than the chemical composition. The Earth weighs 5.9736 x 1024kg, .023% of which is water. The rest of the mass, which is the majority, is in the mantle and the core.
On the physical Earth, there are three main layers below the 1.4 x 1021kilogram ocean, the crust, the mantle, and the core. The crust is the outside of the Earth and is 0 to 40 kilometers thick. This is an average thickness, because the thickness varies around the Earth. For example, the crust is thinner under the ocean than under a continent. The crust is a solid layer of the Earth that weighs 2.6 x 1022kilograms. It made up of quartz and other silicates, feldspar for example. The mantle is below the crust and its total mass is 4.043 x 1024kilograms. The mantle is divided into four different layers: the upper mantle, transition region, lower mantle, and D layer. The upper mantle is obviously at the top of the mantle. It is 40 to 400 kilometers below the Earth’s surface and is semi-fluid. It is made up of olivine, pyroxene, calcium, and aluminum. Below the upper mantle is the transition region, which is 400 to 650 kilometers below the surface. Like the upper mantle, it is semi-fluid. Under the transition region is the lower mantle, which is begins 650 kilometers below the surface and goes to 2,700 kilometers below. Like the other two layers, it is semi-fluid. It is mostly silicon, magnesium, and oxygen with a little iron, calcium, and aluminum. Finally, the D layer is the last layer of the mantle and goes from 2,700 kilometers below the surface to 2,890 kilometers below. It is also semi-fluid.
The final layer of the Earth is the core, which is below the mantle and the crust. It is mostly iron with temperatures as high as 7,500 Kelvin. These temperatures are hotter than the surface of the sun. The core is divided into two parts, the outer core and the inner core. The outer core goes from 2,890 kilometers below the surface to 5,150 kilometers. It is partially plastic and partially semi-fluid. It weighs 1.835 x 1024kilograms. The inner core is 5,150 to 6,378 kilometers below the surface of the Earth. It is the center most part of the Earth. It is completely solid and weighs 9.675 x 1022kilograms.

## Atmosphere

Other than the physical Earth, there is also an atmosphere and magnetic field. The atmosphere weighs 5.1 x 1018kilograms and is 78% nitrogen and 21% oxygen. However, without photosynthesizing plants and algae to regenerate the oxygen, all of the oxygen would be gone within 5,000 years. Explain why. There are four main layers of the atmosphere, three of which we will discuss in detail. The four layers are the ionosphere, mesosphere, stratosphere, and troposphere. The troposphere is the region closest to the surface of the Earth and is where weather occurs. It is made up regions of rising and falling pockets of air. Air pressure at the top of this layer is only ten percent that of the pressure at sea level. Between the troposphere and the stratosphere is a buffer zone called the tropopause. The stratosphere is mostly made up of the ozone layer. In general, air flows horizontally in this area. In the ozone layer, there is a high concentration of ozone, hence its name. Ozone is a reactive form of oxygen. The ozone is responsible for absorbing the ultra-violet light radiation emitted by the sun. If this radiation reached the surface of the Earth, there would be devastating effects. The ozone layer could be depleting currently because of man-made carbon compounds. Above the stratosphere is the mesosphere. It is 50 to 85 kilometers above the surface of the Earth. Out of all of the layers of the atmosphere, the least is known about the mesosphere. Most meteors from space end up burning up in the mesosphere even though the top of the layer is the coldest part of Earth’s atmosphere. Satellites orbit above the mesosphere, while airplanes and weather balloons fly below it. Finally, there is the ionosphere also known as the thermosphere, which is very thin. In the ionosphere, atoms are ionized. This means that they gain or lose electrons so they no longer have an electric charge. The aurora also takes place in the ionosphere. The ionosphere is responsible for absorbing energetic photons from the sun and reflecting radio waves. Because this layer is so thin, its structure is influenced by charged particle wind from the sun also known as solar wind.

## Magnetic Field

Electric currents in the outer core produce the Earth’s magnetic field. Although we don’t completely understand the magnetic field, we do know that its origin is associated with electrical currents. These currents are produced by convective effects and the rotation called the dynamo effect in the spinning liquid metallic outer core. The magnetic field has a similar structure to that of a bar magnet with an axis that is tilted with respect to the rotation of the axis of the Earth. Currently, the geo-magnetic North Pole is located in northern Canada. The magnetic poles can move or reverse if there are irregularities. The magnetic field exerts forces on moving electrical charges. By doing this, it can trap charged particles like electrons and photons. The particles that are trapped are primarily from the solar wind. When they get trapped in the magnetic field, they create aurora. To trap the particles from the solar wind, the solar wind is deflected like water around the bow of the magnetic field.

## Why is the Earth habitable?

The Earth is a very unique planet because it has life. However, what makes the Earth habitable? There are four main reasons why the Earth is habitable. The first is the surface temperature and atmospheric pressure. Both of these things are just right so life can thrive and is protected without being frozen or burned. Secondly, the carbon cycle and photosynthesis are very important. In this process, plants remove carbon dioxide from the air and use it to form organic compounds. The compounds are then converted to oil, natural gases, or coal if the temperature and pressure conditions are correct. Much of the Earth’s carbon dioxide is below the ocean floor. It is released through heating processes like volcanism for example. The waste product of this entire system is oxygen. The third thing that makes Earth habitable is the oxygen. Oxygen is vital for life on Earth and its existence and creation by the carbon cycle are equally important. Finally, water is very important. We don’t only have ice, but also liquid water which is even more important.