Pronunciation key

( eksō-sfir )



The outermost region of Earth's atmosphere, with its base located at altitudes exceeding 400-600 kilometers (280-370 miles) above the surface of the earth. The outer boundary is defined as that level where the molecular density falls off to that typical of the interplanetary medium of our solar system, approximately 5000 kilometers or 3000 miles above the earth's surface. At this altitude, molecules are so low in numbers that atoms of atmospheric gas readily escape into space after impacting each other. The numbers of such molecules is so low that the probability of collision between particles is very small. These particles are acted only upon by the gravitational field of the Earth and follow what are called ballistic trajectories. Due to this molecules in this atmospheric region having a greater chance of escaping into outer space, the base of the exosphere is called the critical level of escape; but because most molecules have speeds considerably lower than the escape velocity, the rate of escape is relatively low.

One element, Helium, is the most abundant gas in the exosphere. Atomic hydrogen is more common than atomic oxygen above 850 kilometers (530 miles) and molecular nitrogen (N2), molecular oxygen (O2) and argon become quite rare with greater altitude.

Temperatures in the exosphere is about 700°C (1,300°F), but may vary from only 300°C (570°F) during decreased sunspot activity. Temperatures may rise as high as 1,700°C (3,090°F) during maximums in sunspot activity.

Image Credit: Scholastic Children's Encyclopedia ©2004
Layers of Earth's Atmosphere, between the surface and outer space.

See AIR (The Four Layers).

ex′o•spher′ic adj.


  • Funk and Wagnalls Standard Dictionary, Comprehensive International Edition ©1976
  • World Book Encyclopedia ©1981
  • Encyclopedia Britannica Micropedia ©1984
  • The American Heritage Dictionary, Second College Edition ©1985
  • Grolier Encyclopedia of Knowledge ©1991
  • Scholastic Children's Encyclopedia ©2004
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