Exosphere

The exosphere (Ancient Greek: ἔξω éxō "outside, external, beyond", Ancient Greek: σφαῖρα sphaĩra "sphere") is the uppermost layer of Earth's atmosphere. In the exosphere the density is so low that particles collide only rarely. That makes it possible for energetic particles to escape Earth's gravity altogether. Here, the Earth's atmosphere gradually thins out and merges with interplanetary space.

Several moons, such as Earth's moon and the Galilean satellites, have exospheres without denser atmospheres. These consist of atoms and molecules which are ejected from surface rocks and follow a parabolic trajectory until the collide with the surface. Generally each of these atoms and molecules has an independent trajectory. Authors differ as to whether such moons are considered to have atmospheres or not. Smaller bodies such as asteroids, in which the atoms emitted from the surface escape to space, are not considered to have exospheres.

Read more about Exosphere:  Earth's Exosphere