Photodissociation and Surface Water
The process of splitting water (H2O) is called Photodissociation. H2O is split by ultraviolet (UV) light into Hydrogen (H+) and Hydroxyl (OH-) radicals, in the upper atmosphere. In a high-gravity environment H+ is held in the atmosphere and recombines with OH- to reform H2O. In a low-gravity environment the light-weight H+ escapes into space, leaving OH- behind. OH- is then split by UV light into a Hydrogen (H+) and Oxygen (O2-) radicals. Again, H+ escapes into space. The heavier O2- remains in the atmosphere and combines with another O2- to form Oxygen gas (O2), releasing heat. The heavier and cooler O2 then falls towards the surface, increasing the partial pressure of Oxygen. The O2 then combines with Carbon at the surface, releasing heat and increasing the partial pressure of Carbon Dioxide (CO2) in the atmosphere. The higher levels of CO2 create a greenhouse effect, warming the planet. The warmer conditions increase the rate of evapouration of H2O from the surface, driving more H2O into the upper atmosphere faster. There the H2O is split by UV light, creating a positive feedback loop. The warmer conditions also increase the rate of H+ evapouration into space. This positive feedback will quickly remove H2O from the surface of a low-gravity planet. So, a planet must have enough gravity to hold H+ in it's upper atmosphere for there to be H2O at the surface.