Exploratour - Evolution of the Solar System
If a planet cannot bring new molecules into the atmosphere with volcanoes, then eventually the whole atmosphere can drift away, as if by evaporation. But for the atmosphere to drift away, the planet has to be small. The size of the planet matters. As the planets get larger, it is harder for the gases which are part of the atmosphere to "evaporate" and drift away. It is as if large planets have a "lid" on the atmosphere to keep the gases near the planet. Thus scientists say that the gases in Jupiter's atmosphere are probably the original gases collected by the forming Jupiter. Nothing can escape. On the other hand, the atmosphere of the planet Mercury, with no active volcanoes, has drifted away because the planet is so small. Small planets have nothing to help them form a "lid" so that they can keep their atmospheres.
This is page 31 of 60