"Earth" Page 2
The Earth is a solid mass, with a
dense core of magnetic, metallic material.
Around the "Core" is a thick mantle of heavy crystalline rock covered by a layer of solid granite
and basalt which form the base of the continents and the ocean floor.
Atmospheric scientists commonly divide the atmosphere into five layers, or regions, on the basis of the vertical distribution of temperature.
1. The troposphere is the first layer above the surface and contains half of the Earth's atmosphere. Weather occurs in this layer and the temperature of the atmosphere drops with increased height until the "Tropopause" [the upper boundary between the troposphere and the stratosphere] is reached, at 6 to 8 miles or 25,000 to 60,000 feet.
Many jet aircrafts fly in
the stratosphere because
it is very stable. Also, the ozone layer absorbs harmful rays from the Sun.
In the stratosphere, the temperature rises with increasing altitude to about 50 km [30 miles]. The troposphere and the stratosphere both have distinct circulation systems.
3. Meteors, rock fragments and space junk usually burn up in the mesosphere.
The mesosphere, is characterized by rapid decreases of temperature.
Like the troposphere, this region is subject to strong seasonal variations of temperature at high latitudes.
4. The thermosphere is
the layer with the auroras. It is also where the space shuttle orbits.
In the thermosphere, where temperatures can rapidly rise with solar radiation, temperatures of more than 1,000° C [1,832° F] at about 400 km [240 miles]
have been known to occur.
5. The upper limit of our
atmosphere merges into space in the extremely thin exosphere.
The density of the atmosphere is so low in this layer that molecular collisions rarely occur. In the exosphere, light atoms, such as those of hydrogen
and helium, may acquire sufficient velocity to escape the Earth's gravitational pull.
The heat now escaping from the Earth's surface mainly comes in the form of radioactivity throughout the mantle. Some heat may still come from the dense core at the Earth's center, and some may be original heat. The question of whether the Earth began hot or cold has been definitely settled, although majority opinion favors an intense origin with early heating through radioactivity and the upheaval of the metallic core. Ages determined by the analysis of radioactive isotopes and their daughter products provide a clue to early history. The Earth as a distinct body is known to have an age of 4.6 Billion years, and samples of lunar material show a similar age. The oldest continental rocks currently found, however, in Canada, have ages [since the time they solidified] of approximately 3.9 Billion years. Presumably the record at the Earth's surface of the first 700 million years was erased by the elevated temperatures that prevailed in those times.