The exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density.
The altitude of its lower boundary, known as the thermopause and exobase, ranges from about 250 to 500 kilometres (160 to 310 mi) depending on solar activity
The upper boundary of the exosphere can be defined theoretically by the altitude about 190,000 kilometres (120,000 mi), half the distance to the Moon, at which the influence of solar radiation pressure on atomic hydrogen velocities exceeds that of the Earth’s gravitational pull