Exosphere
The exosphere (from the Greek έξο exo, outside, outside ' and σφαίρα sphaira, ball ' ) is the outermost layer of the earth's atmosphere; they liked the smooth transition to the interplanetary space, however, is by definition of NASA and Fédération Aéronautique Internationale already its part.
It is a part of the hetero- sphere, ie that region of the atmosphere from 120 km altitude at which the gases segregate according to their atomic masses and layers. From a height of 1000 km only hydrogen occurs as the lightest gas, this area is called geocorona.
The exosphere connects to the thermosphere, and begins ( according to different sources) about at an altitude between 400 km and 1000 km. Its outer boundary is indicated with about 10,000 km. However, the boundary is not well defined, because the gas density decreases continuously and theoretically never reaches zero. All particles contained in it are largely ionized. The exosphere is the only layer of the atmosphere from the gas molecules can leave the gravitational field of the earth because of their own speed, since so few particles are present that their braking effect can be neglected. The exosphere is therefore also known as Dissipationssphäre.
The high temperature of over 1000 ° C, prevailing in the exosphere results from the velocity of the particles ( fast particles correspond to high temperatures ). An ordinary thermometer would, however, show temperatures near absolute zero, since the gas density in this amount is far too small to cause a measurable heat transport.
Also other planets or moons have exosphere, such as the planet Mercury, but also the Earth's moon.