Escape Velocity of the Moon

At 2.38 km/s the Moon cannot hold an atmosphere. Gas molecules at lunar daytime temperatures reach a substantial fraction of that speed, so anything released (from outgassing, from comet impacts, from the Apollo landers themselves) escapes within weeks. What remains is an exosphere so thin, around 10⁵ particles per cubic centimetre, that atoms essentially never collide with one another.

The low escape speed is also what made Apollo feasible. The ascent stage carried a single small engine and no heat shield, since there was no atmosphere to slow it and none to fly through; and it is why lunar resources are so often discussed for propellant depots, where launching water or oxygen out of the Moon's shallow gravity well costs a small fraction of lifting it from Earth.