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  Earth: Astronomy: Earth - A WATERY PLANET
Posted on Thursday, December 02 @ 14:07:25 CST by astronomy


Conditions on Earth's surface depend on the interaction of two spheres: the atmosphere and the hydrosphere.


Conditions on Earth's surface depend on the interaction of two spheres: the atmosphere and the hydrosphere.

The hydrosphere is the water on, or near, the surface. This water is found in oceans, rivers, and lakes; it is found below the surface as ground-water; it is locked up in frozen polar caps and glaciers; and it exists as vapor carried in the atmosphere. Earth is the only planet where temperatures allow surface water to exist in solid, liquid, and gaseous states.

Oceans make up 97 percent of the hydrosphere and contain enough water to cover the planet 2 miles (3 km) deep. The only reason we are not underwater is that the continental crust (some 30 percent of Earth's surface) has an average elevation high enough to keep it above sea level.

The sea appears so eternal that few people think about how it began. Its origin is, in fact, a scientific puzzle. Some water must have formed with the planet and then erupted from volcanoes. But Earth's birthplace in the solar nebula was too hot to allow much water to exist. Comets, however, are loaded with ice and organic elements, and many scientists think that most of the hydrosphere was deposited by catastrophic comet impacts early in Earth's history. Much smaller impacts occur to this day, continually adding water to the hydrosphere.




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