The exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density.
The lithosphere is the solid, rocky layer covering the entire surface of the planet, composed of the crust and the hard uppermost mantle, and reacts to stresses as a brittle solid. The lithosphere ranges in thickness from 50 - 200 kmA and is fragmented into tectonic plates with boundaries where plates collide, diverge, or grind past each other.