Loss of water (and/or dissociated hydrogen) on rocky planets is an interesting area of study. A few weeks back I ran across a reference that the Earth is thought to have lost a quarter of its water to space over the past 4.5 billion years:
The determination was made by a similar H2O / D2O ratio analysis.
Given that the Earth has seven seas, that would be 1.75 oceans worth of water lost. The process is moderated by the presence of oxygen in the atmosphere (which recombines with dissociated hydrogen), so, curiously, not only is water necessary for life, but life may well help retain water on planets.
"The Earth has lost a quarter of its water" http://sciencenordic.com/earth-has-lost-quarter-its-water
The determination was made by a similar H2O / D2O ratio analysis.
Given that the Earth has seven seas, that would be 1.75 oceans worth of water lost. The process is moderated by the presence of oxygen in the atmosphere (which recombines with dissociated hydrogen), so, curiously, not only is water necessary for life, but life may well help retain water on planets.