The thermosphere is the thickest
layer in the atmosphere. Only the lightest gases—mostly oxygen, helium,
and hydrogen—are found here.
The thermosphere extends from the mesopause
(the upper boundary of the mesosphere) to 690 kilometers (429 miles)
above the surface of the Earth. Here, thinly scattered molecules of gas absorb
x-rays and ultraviolet radiation. This absorption process propels the
molecules in the thermosphere to great speeds and high temperatures.
Temperatures in the thermosphere can rise to 1,500 degrees Celsius
(2,732 degrees Fahrenheit, or 1,773 kelvin).
Though the
temperature is very high, there is not much heat. How is that possible?
Heat is created when molecules get excited and transfer energy from one molecule
to another. Heat happens in an area of high pressure (think of water
boiling in a pot). Since there is very little pressure in the
thermosphere, there is little heat transfer.
The Hubble Space Telescope and the International Space Station (ISS) orbit
the Earth in the thermosphere. Even though the thermosphere is the
second-highest layer of Earth’s atmosphere, satellites that operate here
are in “low-Earth orbit.”
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