The outermost, or topmost, portion of the atmosphere. Its lower boundary is the critical level of escape, usually located at 500–1000 km above the earth's surface. 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. It is only from the exosphere that atmospheric gases can, to any appreciable extent, escape into outer space. See cone of escape, fringe region.