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.