Earth is the blue planet of the universe and third planet from the Sun. It is fifth largest planet in the solar system and just slightly larger than nearby Venus, Earth is the biggest of the terrestrial planets.

The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman Gods and Goddesses. However, the name Earth is an English or German word, which simply means the Ground.

Formation of the Earth:

When the solar system settled into its current layout about 4.5 billion years ago, Earth formed when gravity pulled swirling gas and dust in to become the third planet from the Sun. Like its fellow terrestrial planets, Earth has a central core, a rocky mantle and a solid crust.

Structure of the Earth:

Structure of the Earth

Earth is composed of four main layers.  They are:

• Inner core
• Outer Core
• Mantle
• Crust

The inner core is a solid sphere made of iron and nickel metals about 1,221km in radius. There the temperature is as high as 9,800 degrees Fahrenheit (5,400 degrees Celsius). Surrounding the inner core is the outer core. This layer is about 2,300 km thick, made of iron and nickel fluids.

In between the outer core and crust is the mantle, the thickest layer. This hot, viscous mixture of molten rock is about 2,900 km thick and has the consistency of caramel. The outermost layer of Earth is crust and it is about 30 km deep on average on land.

Note: – At the bottom of the ocean, the crust is thinner and extends about 5km from the sea floor to the top of the mantle.

Atmosphere of the Earth:

The atmosphere of the Earth consists of 78% of Nitrogen, 21% of Oxygen, and 1% of other gases such as Argon, Carbon dioxide and Neon. The atmosphere affects Earth’s long-term climate and short-term local weather that shields us from much of the harmful radiation coming from the Sun. It also protects us from meteoroids, most of which burn up in the atmosphere, seen as meteors in the night sky, before they can strike the surface as meteorites.

Interior feature of the Earth:

Now, we will discuss the interior feature of the Earth. But, we know little directly about the interior of the earth. Therefore, I elaborated the two important concepts to deal with the interior of the Earth below. They are:

1. Seismic Waves:

The seismic waves, which are vibrations in the body of the earth, provided the information regarding the interior features of the earth. Most of our information about the surface and interior of the Earth has come from the seismic waves.

There are two general categories of seismic waves. They are:

1. P-Waves: These Waves are longitudinal pressure waves and can propagate in both solids and liquids.
2. S-Waves: These Waves are transverse waves that can propagate in solids but not in liquids.

Here is an illustration of the difference between P-Waves and S-Waves. These seismic waves, which are generated naturally by earthquakes, by volcanoes, and by impacts, and may be produced artificially by explosions and mechanical devices, tell us about the interior in several general ways. The figure on the right illustrates for a planet with varying interior density and a liquid core.

P and S Waves

First, seismic waves have their direction of motion changed (refracted) by variations in the interior density. Thus, by studying the way such waves propagate in the Earth we can learn something about density variations. Second, the fact that P-Waves propagate in liquids but S-Waves do not allows us to determine if portions of the interior are liquid.

2. Geological Differentiation:

According to this concept, the Earth, which is seen as its present interior structure is made through the geological process which is called as differentiation. It is illustrated in the following figure.

The Process of Geological Differentiation

The process of geological differentiation:

Within about 1 billion years of its formation, the Earth was melted by heat arising from a combination of sources:

• Gravitational energy left from the formation of the planet
• Meteor bombardment
• Decay of radioactive material trapped in the body of the Earth

While the Earth was molten, gravity acted to concentrate more dense material near the center and less dense material nearer the surface. When the Earth solidified again (except for the liquid outer core) it was left with a layered structure with more dense material like iron and nickel near the center and less dense rocks nearer the surface. As the outer layers cooled and solidified, large cracks developed because of thermal stress, leaving the lithosphere broken up into large blocks or plates.

Hydrosphere of the Earth:

The abundance of water on Earth’s surface is a unique feature that distinguishes the Blue Planet from other planets in the Solar System. Earth’s hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. About 97.5% of the water is saline and the remaining 2.5% is freshwater. Most fresh water is present as ice in ice caps and glaciers, which are about 68.7%.

Moon:

Moon

The natural satellite of the Earth is referred to Moon, and it is the fifth largest moon in the solar system. It is brightest and largest object in our night sky, the moon makes Earth a more livable planet by moderating our home planet’s wobble on its axis, leading to a relatively stable climate. It also causes tides, creating a rhythm that has guided humans for thousands of years. The moon was likely formed after a Mars-sized body collided with Earth.

The most widely accepted theory of the Moon’s origin is the Giant-Impact Hypothesis. According to the hypothesis, it formed from the collision of a Mars-size proto-planet called Theia with the early Earth. This hypothesis explains the Moon’s relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of Earth’s crust.

Size and Distance of the Moon:

With a radius of 1,737.5 km, the moon is less than a third the width of Earth. The moon is an average of 384,400 km away from the Earth. The moon is slowly moving away from Earth, getting about an inch farther away each year.