The simple fact is that H2 is more likely to escape the planets gravity than H20, since the water molecule is much heavier. If the Earth is being bombarded by these snowballs, then Mars should be too. Of course, there isn't much water on mars. Well, the gravity of Mars is less and Mars is being bombarded by the solar wind, since it has no magnetic field (a solid core). The implication would be that the solar wind removes the water from Mars as least as fast as the planet gains water. I would be more concerned about losing free hydrogen from this planet than about losing water. Do they really know what happens in the upper atmosphere?
