EARTH

The Earth is very special. It is the only planet that we know about that has life! It is the only planet with life for several reasons. One reason is that it is just the right distance from the Sun. Another reason is that it is just the right size to hold the right atmosphere.
Space travel has also taught us how special Earth is to us. From space, the Earth looks like a blue and white marble.
The total surface area of Earth is about 199 million square miles. What is unique about the surface area, is that over 70 percent of it is covered with water.
Astronomers think that Earth was born about 4.6 billion years ago. It formed with the rest of the solar system by a contracting cloud of gas and dust.
The Earth has three layers. The surface layer is called the crust. The crust is made mostly of lightweight rocks such as granite and basalt. The crust is about 22 miles thick.
The next layer is called the mantle. The mantle is made mostly of dense silicate rock. The mantle is about 1,800 miles thick!
The central layer is called the core. The core is about 2,170 miles thick. The core is made of dense iron and nickel. The temperature at the core is at least 7000 degress!
However, the Earth has not always looked the way it does now. Geologic studies show that the Earth has evolved dramatically since it formed 4.5 billion years ago. Some of the changes have been rapid, some have been slow. One of the ways Earth was different, was that Earth's atmosphere had much less oxygen for the first billion years of its existence. We know this from oxygen-poor sediments deposited during that period. The amount of oxygen increased after plant life had evolved.
If you could go back in time to the first half of Earth's history, you would find little life. You would also not notice the continents. The Earth has always been changing.
One big change happend about 65 million years ago. An extraordinary 75 percent of plant and animal species, including dinosaurs, became extinct in only a few million years or less! Maybe it is a good thing, because this alowed mammals to emerge.
What was it that changed Earth so suddenly? We think that we found the answer in 1990. Geologists found aevidence that a meterorite that was 6 miles wide, crashed into the Earth at that time!
The impact had awesome effects. The blast ejected debris into space. This debris fell back into the atmosphere, lighting the sky with the glow of a billion meteors. The heat from this glow iginited forest fires around the world killing many animals. The glow was not the only thing the impact caused. The impact also caused dust that obsured the sunlight for months. This killed off many plants. Without the plants the food chain was disrupted. This killed off more life.
Earth has a complex history. Species have evolved in a way that is best suited to the environment. If the environment is changed suddenly, it could have devestating effects.
Our planet is always changing. We must take care of it. It is the only place we have!




FACTS ABOUT THE EARTH Average distance fron the sun: 149,600,000 km Length of year: 365 days Length of day: 23 hours 56 minutes Diameter: 12,756 km Surface gravity: (Earth = 1) 1 Composition: nickel-iron, silicates; nitrogen-oxygen atmosphere.	EARTH The Earth is very special. It is the only planet that we know about that has life! It is the only planet with life for several reasons. One reason is that it is just the right distance from the Sun. Another reason is that it is just the right size to hold the right atmosphere. Space travel has also taught us how special Earth is to us. From space, the Earth looks like a blue and white marble. The total surface area of Earth is about 199 million square miles. What is unique about the surface area, is that over 70 percent of it is covered with water. Astronomers think that Earth was born about 4.6 billion years ago. It formed with the rest of the solar system by a contracting cloud of gas and dust. The Earth has three layers. The surface layer is called the crust. The crust is made mostly of lightweight rocks such as granite and basalt. The crust is about 22 miles thick. The next layer is called the mantle. The mantle is made mostly of dense silicate rock. The mantle is about 1,800 miles thick! The central layer is called the core. The core is about 2,170 miles thick. The core is made of dense iron and nickel. The temperature at the core is at least 7000 degress! However, the Earth has not always looked the way it does now. Geologic studies show that the Earth has evolved dramatically since it formed 4.5 billion years ago. Some of the changes have been rapid, some have been slow. One of the ways Earth was different, was that Earth's atmosphere had much less oxygen for the first billion years of its existence. We know this from oxygen-poor sediments deposited during that period. The amount of oxygen increased after plant life had evolved. If you could go back in time to the first half of Earth's history, you would find little life. You would also not notice the continents. The Earth has always been changing. One big change happend about 65 million years ago. An extraordinary 75 percent of plant and animal species, including dinosaurs, became extinct in only a few million years or less! Maybe it is a good thing, because this alowed mammals to emerge. What was it that changed Earth so suddenly? We think that we found the answer in 1990. Geologists found aevidence that a meterorite that was 6 miles wide, crashed into the Earth at that time! The impact had awesome effects. The blast ejected debris into space. This debris fell back into the atmosphere, lighting the sky with the glow of a billion meteors. The heat from this glow iginited forest fires around the world killing many animals. The glow was not the only thing the impact caused. The impact also caused dust that obsured the sunlight for months. This killed off many plants. Without the plants the food chain was disrupted. This killed off more life. Earth has a complex history. Species have evolved in a way that is best suited to the environment. If the environment is changed suddenly, it could have devestating effects. Our planet is always changing. We must take care of it. It is the only place we have!


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