Earth |
| Tuesday, 19 December 2006 20:44 | |||
Third planet from the Sun and the largest of the four ‘inner,’ or ‘terrestrial planets’; surface is 70% covered in H2O, water…DATA
The Planet Earth
The solid inner core spins at a different speed to the molten outer layers and, along with currents in the outer core, produces Earth’s magnetic field. The crust and the uppermost mantle together form the lithosphere, which is made up of tightly fitting slabs called plates. The plates, which float on a semi-molten layer of mantle known as the asthenosphere, move around in accordance with each other in interactions called ‘plate tectonics’. In the beginning, Earth was so different from our world today that it was like another planet. It was a desert without seas or shores, without soil or green leaves. Imagine Earth as a sphere of red-hot rock from horizon to horizon. So how did the oceans form? After the birth of the solar system, debris littered outer space. Scientists believe that Earth's surface must have been constantly battered by these asteroid collisions. If a thin crust formed over the surface of our still-molten planet, it couldn't have lasted. Churning seas of lava on Earth's surface probably broke the crust and melted it back down. Scientists think that the Moon was much closer then, and that it revolved around the Earth about every six hours. Its pocked face would have filled a large part of the sky. Earth's first atmosphere was composed of hydrogen and helium, the most common gases in the universe. As the infant sun flared to full strength, however, these gases were absorbed. Thereafter, Earth's only atmosphere was what it created for itself. It was composed of gases that seeped up through cracks, fissures, and volcanic craters. These gases were probably methane, carbon dioxide, ammonia, and water vapor. There was very little oxygen. Shooting stars constantly flamed through this atmosphere as meteorites and comets fell to Earth. If it is true that comets are rocks encased in many tons of ice, then each comet brought water into the atmosphere. But the planet was still much too hot for water to exist in any form but steam. If a single drop of rain happened to fall, it vaporized instantly. Between the water vapor that rose from cracks in the earth, and the ice that fell with the comets, Earth acquired a vast water supply, but none of it stayed on the ground. Millions of years passed. The bombardment of asteroids slowed, and some of the heat created by Earth's formation radiated away into space. The planet's primitive atmosphere cooled to a temperature below the boiling point of water. Then, from the clouds of water vapor, rain began to fall. And it continued to fall. Huge floods of water flowed across the surface of the planet. Rain fell on rivers of lava and steamed back up into the atmosphere to form thunderclouds. Beneath this global ocean, Earth's crust continued to buckle and heave. Heat from the planet's molten core tried to escape to the surface. Eventually, volcanoes would rise above the surface of the ocean, then islands, and finally continents appeared. Earth's surface would be built and rebuilt. These processes were set in motion when our planet was born and are described by a theory called plate tectonics.
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Earth’s average surface temperature of 13°C (55°F) and its huge amounts of water make it ideal for supporting life. Continental land mass makes up 30% of the surface, it has one natural satellite, the Moon, and a dense core rich in iron and nickel, surrounded by a mantle of silicate rocks. The crust is a thin outer layer of lighter rocks that varies in depth from 50km (30 miles), the thickest continental crust, to 5km (3 miles), the thinnest oceanic crust. The boundary between the crust and the mantle is known as the Moho (Mohorovicic) discontinuity.