exosphere The outer region of the upper atmosphere extending from a base of 500–750 km altitude. The zone has a very low concentration of gases (mostly molecules of oxygen, hydrogen, and helium, although some are ionized). Gases can escape from it into space, as molecular collisions are much reduced because of the low gas density. The exosphere and much of the underlying ionosphere form part of the magnetosphere.

exosphere Outer region of the upper atmosphere extending from a base of 500–750 km altitude. The zone has a very low concentration of gases (mostly molecules of oxygen, hydrogen, and helium, although some are ionized). Gases can escape from it into space, as molecular collisions are much reduced because of the low gas density. The exosphere and much of the underlying ionosphere form part of the magnetosphere.