The work suggests that Venus had relatively Earth-like conditions during its early history. It also suggests (as many scientists now believe) that Venus, Earth, and Mars formed from similar interstellar materials, and that all three originally had substantial amounts of water.
....
The wet greenhouse theory with its oceans far better explains Venus than
the "runaway greenhouse" model now used. It accounts for the first time for
the almost completely waterless state of the present-day planet. It
explains where the planet's missing oxygen is stored.
....
The old and new theories go as follows:
In the runaway greenhouse, Venus's huge primordial atmosphere would have
trapped much of the Sun's heat, preventing formation of any oceans at all,
and creating a dense water vapor-carbon dioxide atmosphere. An immense
amount of water vapor would have risen to the top of the atmosphere.
Solar ultraviolet radiation would have split (dissociated) the water
molecules into hydrogen and oxygen. The hydrogen (the lighest gas) would
have blown away to space by the super-fast hydrodynamic escape process -- and been lost forever -- destroying this water.
However, highly efficient hydrodynamic escape would have stopped when water was reduced to a minor constituent of the atmosphere (around 20 percent). But even 20 percent of an enormous atmosphere like Venus's is still a lot of water, and much of this water should be left on the planet today. However, the four atmosphere probes of NASA-Ames' Pioneer-Venus spacecraft didn't find it there.
This amount of leftover water would be enough to make an atmosphere (if
it consisted of nothing else but water vapor) ten to twenty times as dense
as the entire atmosphere of Earth.
Kasting's "wet greenhouse" theory answers this surplus water objection:
Venus formed with plenty of original water which condensed out and created Earth-scale oceans. The oceans were hot because of the planet's
closeness to the Sun and the trapping of incoming solar heat by atmospheric
carbon dioxide and water vapor. Because Venus's surface was so hot, much of the oceans evaporated until perhaps 50 percent of the atmosphere was water vapor. However, at this point the oceans won out. Water vapor pressure had built up so high that it prevented further evaporation from the oceans -- just as does the water vapor in a kitchen pressure cooker. The oceans were very hot, 200-300 degrees F (around boiling temperature on Earth). Yet because of the high vapor pressure of atmospheric water vapor, the bulk of the water on the planet remained liquid. Hydrogen would still have escaped rapidly from the top of the atmosphere by the super-efficient hydrodynamic escape mechanism. But the atmospheric water lost this way would have been steadily replenished by further evaporation from the underlying ocean. By remaining liquid over millions of years, the oceans were able to move the original huge mass of carbon dioxide gas out of the atmosphere. They did this by converting most of this gas into carbonate rocks in Venus's crust. (These are the same multi-step, ocean-planet interactions that have converted Earth's enormous mass of carbon dioxide into carbonate rock.) This process (generally described as "weathering") reduced the atmospheric density on Venus from perhaps 90 times Earth's atmosphere to perhaps only about the same density as Earth's atmosphere.
In both greenhouse theories, when water vapor dropped to around 20
percent of the atmosphere, hydrodynamic escape ceased to push hydrogen off the planet, and destroy its water. But 20 percent of a thin Earth-size atmosphere is about 100 times less water than 20 percent of an enormous, Venus-type, carbon dioxide atmosphere. This means that in the wet greenhouse model by the time hydrodynamic escape had ended, with its thin Venus atmosphere, nearly all of Venus's water was lost. Slower hydrogen escape processes in the several billion years since then have reduced the planet's water still further to today's tiny amount.
...
Venus has only a hundred thousandth as much water
as the Earth.
Kasting's "wet greenhouse" with oceans helps clear up another Venus mystery. If most of the planet's water split into hydrogen and oxygen, and all the hydrogen blew away to space, where is the oxygen that was left behind? The four Pioneer probe craft didn't find it in the atmosphere. Many scientist believe that much of the missing oxygen is locked up in the planet's crust. It has combined into such oxygen-rich minerals as hematite and magnetite.
However, to get the crust to lock up this much oxygen, an efficient
mechanism for remixing the planet's surface rocks is required. A few
hundred million years of precipitation, erosion, and weathering produced by
Venusian oceans may well be this mechanism.
.... If the hydrodynaamic escape was vigorous enough
-- due to enhanced ultraviolet heating from a magnetically active young Sun
-- much leftover oxygen may have been dragged into space with the hydrogen....
to replace the oxygen saturated soil. If the oxygen density gets too high in the process, the 20% rule would have precluded all the hydrogen outgassing.