The reason water exists on earth, and not on mars, may be that life on earth saved the water by discovering photosynthesis.
The discovery of photosynthesis caused the "great oxygenation event" which pumped oxygen into the atmosphere. Oxygen reacts with atmospheric hydrogen to form water. Without the oxygen, the very light hydrogen molecules would float to the top of the atmosphere and are easily blown away by solar winds, which is what happened on Mars. But with high oxygen concentrations on Eath, hydrogen molecules react to form heavier water molecules before they have a chance to be blown away, and thus hydrogen and water are retained.
I read about this in the book "Oxygen" by Nick Lane.
The discovery of photosynthesis caused the "great oxygenation event" which pumped oxygen into the atmosphere. Oxygen reacts with atmospheric hydrogen to form water. Without the oxygen, the very light hydrogen molecules would float to the top of the atmosphere and are easily blown away by solar winds, which is what happened on Mars. But with high oxygen concentrations on Eath, hydrogen molecules react to form heavier water molecules before they have a chance to be blown away, and thus hydrogen and water are retained.
I read about this in the book "Oxygen" by Nick Lane.