EARTH


  • Earth is the third planet from the Sun and the fifth largest planet.

  • Earth has one moon. Earth is the only planet whose English name does not come from Greek/Roman mythology. The name comes from Old English and Germanic. In Roman mythology, the goddess of Earth was Tellus - the fertile soil.

  • It was not until the 16th century that it was understood that Earth is just another planet.

  • Earth can be studied without the aid of spacecraft, but it was not until the 20th century that maps were made of the entire planet. Pictures of the planet taken from space are important because they help in weather prediction and in tracking and predicting hurricanes.

  • Earth is divided into several layers which have distinct chemical and seismic properties:
    • 0–40km – Crust
    • 40–400km – Upper Mantle
    • 400–650km – Transition Region
    • 650–2700km – Lower Mantle
    • 2700–2890km – D-Layer
    • 2890–5150km – Outer Core
    • 5150–6378km – Inner Core

  • Earth's crust changes in thickness. It is thinner under the oceans and thicker under the continents. The inner core and crust are solid and the outer core and mantle layers are fluid.

  • Most of the mass of Earth is in the mantle (94%.) The part we inhabit is a tiny fraction of the whole:
    • Atmosphere – 0.0000051%
    • Oceans – 0.001%
    • Crust – 0.026%
    • Mantle – 4.043%
    • Outer Core – 1.835%
    • inner Core – 1.835%

  • The core is composed mostly of iron, although it is possible that some lighter elements may be present. Temperatures at the center of the core may be as high as 7500 Kelvin, hotter than the surface of the Sun.

  • Taken as a whole, Earth's chemical composition (by mass) is:
    • Iron – 34.6%
    • Oxygen – 29.5%
    • Silicon – 15.2%
    • Magnesium – 12.7%
    • Nickel – 2.4%
    • Sulfur – 1.9%
    • Titanium – 0.05%

  • Earth is the most dense major body in the solar system. Unlike other planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. This is known as plate tectonics. Presently, there are eight major plates.

  • Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about four billion years old. Rocks older than three billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old.

  • Seventy-one percent of Earth's surface is covered with water. Earth is the only planet on which water can exist in a liquid form on the surface. Liquid water is essential for human life. The heat capacity of the oceans keeps Earth's temperature stable. Liquid water is also responsible for most of the erosion and weathering of Earth's continents.

  • Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. The tiny amount of carbon dioxide in the atmosphere maintains Earth's surface temperature because of the greenhouse effect. The greenhouse effect raises the temperature about 35 degrees above what it would normally be (from -1 F to +46 F). Without this, the oceans would freeze.

  • The interaction of Earth and the Moon slows Earth's rotation by about two milliseconds per century. Research shows that days were 18-hours long and a year lasted 481 days about 900 million years ago.

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