The environment of a planet is far from fixed. Gradually, over time, the surface and atmosphere of a planet will evolve and change. This mainly happens because light gases can be slowly lost to space. As explained in an article published in Proceedings of the Royal Society A, this includes water vapour. A process called atmospheric leakage slowly shapes a planet’s climate, as light gases slowly drift through space. A planet can slowly lose water this way, gradually drying out.

An article from American Scientist talks about it in detail, mentioning how in the upper atmosphere of a planet it is exhaust velocity. Much like a rocket traveling into orbit, if a molecule is moving fast enough with nothing in its path, it will simply escape from a planet’s atmosphere. This is even more likely if a molecule is split into its much lighter component atoms.

Atmospheric escape can occur on any planet with an atmosphere, although it is easier with lighter gases. As popular science notes that Earth’s gravity is not strong enough to hold helium, one of the lightest gases in existence, so any helium in Earth’s atmosphere will inevitably be lost to space. An article published in Annual journals of earth and planetary sciences explains how astronomers have also observed atmospheric leaks on exoplanets. This is particularly an important process on small planets, such as Earth or Mars, with a major effect on how a planet evolves over time.