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(outer portion of atmosphere so thin that it is not gas-like)

An exosphere is the outer portion of an atmosphere, where the material is so sparse that collisions between particles are rare enough to be of little moment. The high frequency of collisions in a gas is part of what makes it "gas-like", yielding its relation between density, temperature and pressure. I imagine the exosphere particles have orbit-like trajectories, though they undoubtedly are often sufficiently eccentric that they drop below the exosphere. If a particle should achieve escape velocity through one of the rare collisions, the probability of actual escape is high. What's below the exosphere can be the gas of the atmosphere (e.g., Earth), or the entire atmosphere may be so thin that the exosphere reaches the surface (e.g., Mercury and the Moon). The exobase is the border between the exosphere and the normal, gas-like atmosphere below.

Further reading:

Referenced by pages:
atmospheric escape
transit spectroscopy