Has anyone ever done the calculation of just how much water has been lost from the Earth over the Phanerozoic? I mean through UV light slighting the water molecules up high and the hydrogen escaping because of its low mass and the Earth's warmth relative to gravity? I have heard discussions of this with respet to Venus, but not Earth...my google fu isn't so good today.
2 comments:
I thought the amount in question was kept small by the Earth's "cold trap" -- the fact that the troposphere/stratosphere boundary is colder than either the troposphere below it, or the stratosphere above. It's almost impossible for water vapor to get above the S/T cold trap, up to altitudes where dissociation could really take off.
Mind, the cold trap may have evolved over time; IMS, some model showed that Venus either never had one, or lost it.
Doug M.
Hm. Seems the paper I was thinking of was
Loss of water from Venus. I - Hydrodynamic escape of hydrogen
Authors: Kasting, J. F.; Pollack, J. B.
_Icarus_, March 1983
That's pretty old for planetary science; there's surely been better stuff since. But it does have this money quote:
"Hydrodynamic escape should have been the dominant loss process for hydrogen when the H2O mass mixing ratio in the lower atmosphere exceeded approximately 0.1. The escape rate would have depended upon the magnitude of the solar ultraviolet flux and the atmospheric EUV heating efficiency and, to a lesser extent, on the O2 content of the atmosphere. The time required for Venus to have lost the bulk of a terrestrial ocean of water is on the order of a billion years."
The H2O mass mixing ratio on Earth is about 0.4%, or 1/25 the 0.1 mentioned there. UV flux is about half. So assuming direct straight-line proportionality in both cases -- that's almost certainly wrong; we're making sweeping arbitrary assumptions based on a model that's 25 years old! but what the hell -- you'd expect the loss rate to be at most 1/50 that. Meaning a loss of 1/50 of an ocean every billion years.
But this is a super-crude high upper bound. I suspect the actual loss rate is a couple of orders of magnitude less. Earth's cold trap is wicked efficient; the UV-rich upper atmosphere is bone-dry. Meanwhile the wet lower atmosphere has greatly diminished UV flux /and/ so much O2 that free hydrogen gets snapped up pretty quickly.
Seems like a question someone would have looked into -- if you have academic access, I suspect you could find something with a few minutes of searching.
Doug M.
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