Why is there virtually no atmosphere on the Moon?
Gas molecules have kinetic energy derived from heat sources, like the Sun. This means they are moving with some velocity. Although the average velocity of gas molecules is typically low; some of them will have high velocity. These extraordinary velocities can be great enough to break away from the mass producing the gravity that most of the time keeps the gases bound to it. When that happens, we call the velocities escape velocities because the molecules escape from the attracting mass and into space where they are lost forever. When the attracting mass is relatively small, like the Moon, the escape velocity is also relatively small. So it doesn’t take much kintetic energy and heat source to cause the molecules to fly out into space from small mass planetoids like the Moon. Earth’s gravity pull is about 6 times more pull than the Moon, and that’s why it keeps the gases better than the Moon does.
