[ek-soh-sfeer] 
| the highest region of the atmosphere, where the air density is so low that a fast-moving air molecule is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules. |
| ex·o·sphere (ěk'sō-sfîr') n.
|
exosphere
outermost region of a planet's atmosphere, where molecular densities are low and the probability of collisions between molecules is very small. The base of the exosphere is called the critical level of escape because, in the absence of collisions, lighter, faster-moving atoms such as hydrogen and helium may attain velocities that allow them to escape the planet's gravitational field. Most molecules, however, have velocities considerably lower than the escape velocity, so their rate of escape to outer space is quite low.
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soʊˌsfɪər
