Exosphere

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Exosphere

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The outermost, or topmost, portion of the atmosphere. Its lower boundary is the critical level of escape, variously estimated at 500 to 1000 kilometers above the earth's surface. Also called region of escape. See atmospheric shell. </dd>
In the exosphere, the air density is so low that the mean free path of individual particles depends upon their direction with respect to the local vertical, being greatest for upward moving particles (see cone of escape, fringe region). It is only from the exosphere that atmospheric gases can, to any appreciable extent, escape into outer space. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use