How does Mars lose water to space over time?

Mars is cold and has a thin atmosphere, so liquid water is unstable on the surface for long periods. Ice can sublimate directly into vapor, and ultraviolet sunlight can split water molecules high in the atmosphere into hydrogen and oxygen. The lightweight hydrogen escapes more easily into space, slowly drying the planet out over billions of years. NASA missions such as MAVEN have studied this atmospheric loss and shown that Mars’s climate changed dramatically from its wetter past.