Exosphere

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Atmosphere diagram showing the exosphere and other layers. The layers are not to scale: from Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.

The exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule can escape to space (if it is moving fast enough) or be pulled back to Earth by gravity (if it isn’t) with little probability of colliding with another molecule. The main gases within the exosphere are the lightest gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the last layer before space. Since there is no clear boundary with space and the exosphere, the exosphere is sometimes used synonymously with outer space.

[edit] Lower Boundary

The altitude of its lower boundary, known as the thermopause or exobase, ranges from about 250 to 500 kilometres (160 to 310 mi) depending on solar activity.^{[citation needed]} Its lower boundary at the edge of the thermosphere has sometimes been estimated to be 500 to 1,000 km (310 to 620 mi) above the Earth's surface.^{[citation needed]} The exobase, is also called the critical level, the lowest altitude of the exosphere, and is formally defined in one of two ways:

1. The height above which there are the negligible atomic collisions between the particles and
2. The height above which constituent atoms are on purely ballistic trajectories.

At the exobase, the mean free path of a molecule is equal to one pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and since the Knudsen number is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where .

The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from orbit if no action is taken to maintain the orbit.

[edit] Upper Boundary

The upper boundary of the exosphere can be defined theoretically by the altitude (about 120,000 miles, 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. The exosphere observable from space as the geocorona is seen to extend to at least 60,000 miles from the surface of the Earth. The exosphere is a transitional zone between Earth’s atmosphere and interplanetary space. Its upper boundary has sometimes been estimated at about 10,000 km