The exobase is the level in the atmosphere above which the mean free path of particles is longer than a scale height. The region above the exobase is referred to as the exosphere and it is from this region that particles can easily escape from the planetís atmosphere. At levels below the exobase the atmosphere is dense enough that even if a particle does have kinetic energy greater than its gravitational potential energy, it will lose that energy in collisions with other particles before it can escape.
The density of gas at the exobase is given by ncQH=1, where Q is the collision cross section of the particles and H is an isothermal scale height. This value uses the plane parallel approximation for the atmosphere and is only valid as long as the losses cause small perturbations to the atmospheric content and structure.