...of years ago, for the same reason that while helium is the second most common element in the universe, it's relatively rare on Earth.
All of the helium on Earth comes from the decay of uranium and thorium. When it leaches from rock into natural gas deposits it can be isolated. However once it's released into the air, it ultimately boils off into space, since if one utilizes the Maxwell Boltzman distribution for the gas, one can observe that a significant portion of the gas molecules exceed the escape velocity of the earth at higher altitudes at terrestrial temperatures. (The velocity of a gas at a given temperature is inversely proportional to its mass)
1H2 gas with has half the mass of helium, and thus will have a higher fraction of molecules exceeding the escape velocity.
Effects like this are thought to have participated in the dehydration of Mars. Since that planet lacks the capacity to shield itself from radiation, lacking oxygen, atmospheric water would have been subject to radiolytic water splitting, with the resulting oxygen reacting with surface iron (giving Mars its red color) and the hydrogen boiling off into space.
It is probable that Earth's higher mass and escape velocity (as well as it's significant magnetic field) prevented all of its water from similarly boiling off into space before oxygen appeared in the atmosphere when photosynthesis began.
Since we are happily destroying the ocean life while we wait for all these "solar breakthroughs" to pan out - even if they never actually do so - and because we are working as hard as we can to destroy Earth's ozone layer, we might expect that oxygen will become depleted on this planet again, helping us to hope for the boiling off of our planetary water sooner rather than later.
Happy Friday.
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