In a recent edition of Science magazine (372, p1136-7) there is an outline of two NASA probes to determine whether Venus had water. One argument is that Venus and Earth formed from the same material, so they should have started off very much the same, in which case Venus should have had about the same amount of water as Earth. That logic is false because it omits the issue of how planets get water. However, it argued that Venus would have had a serious climatic difference. A computer model showed that when planets rotate very slowly the near absence of a Coriolis force would mean that winds would flow uniformly from equator to pole. On Earth, the Coriolis effect leads to the lower atmosphere air splitting into three cells on each side of the equator: tropical, subtropical and polar circulations. Venus would have had a more uniform wind pattern.
A further model then argued that massive water clouds would form, blocking half the sunlight, then “in the perpetual twilight, liquid water could have survived for billions of years.” Since Venus gets about twice the light intensity as Earth does, Venusian “perpetual twilight” would be a good sunny day here. The next part of the argument was that since water is considered to lubricate plates, the then Venus could have had plate tectonics. Thus NASA has a mission to map the surface in much greater detail. That, of course, is a legitimate mission irrespective of the issue of water.
A second aim of these missions is to search for reflectance spectra consistent with granite. Granite is thought to be accompanied by water, although that correlation could be suspect because it is based on Earth, the only planet where granite is known.
So what happened to the “vast oceans”? Their argument is that massive volcanism liberate huge amounts of CO2 into the atmosphere “causing a runaway greenhouse effect that boiled the planet dry.” Ultraviolet light now broke down the water, which would lead to the production of hydrogen, which gets lost to space. This is the conventional explanation for the very high ratio of deuterium to hydrogen in the atmosphere. The concept is the water with deuterium is heavier, and has a slightly higher boiling point, so it would be the least “boiled off”. The effect is real but it is a very small one, which is why a lot of water has to be postulated. The problem with this explanation is that while hydrogen easily gets lost to space there should be massive amounts of oxygen retained. Where is it? Their answer: the oxygen would be “purged” by more ash. No mention of how.
In my ebook “Planetary Formation and Biogenesis” I proposed that Venus probably never had any liquid water on its surface. The rocky planets accreted their water by binding to silicates, and in doing so helped cement aggregate together and get the planet growing. Earth accreted at a place that was hot enough during stellar accretion to form calcium aluminosilicates that make very good cements and would have absorbed their water from the gas disk. Mars got less water because the material that formed Mars had been too cool to separate out aluminosilicates so it had to settle for simple calcium silicate, which does not bind anywhere near as much water. Venus probably had the same aluminosilicates as Earth, but being closer to the star meant it was hotter and less water bonded, and consequently less aluminosilicates.
What about the deuterium enhancement? Surely that is evidence of a lot of water? Not necessarily. How did the gases accrete? My argument is they would accrete as solids such as carbides, nitrides, etc. and the gases would be liberated by reaction with water. Thus on the road to making ammonia from a metal nitride
M – N + H2O → M – OH + N-H ; then M(OH)2 → MO + H2O and this is repeated until ammonia is made. An important point is one hydrogen atom is transferred from each molecule of water while one is retained by the oxygen attached to the metal. Now the bond between deuterium and oxygen is stronger than that from hydrogen, the reason being that the hydrogen atom, being lighter, has its bond vibrate more strongly. Therefore the deuterium is more likely to remain on the oxygen atom and end up in further water. This is known as the chemical isotope effect, and it is much more effective at concentrating deuterium. Thus as I see it, too much of the water was used up making gas, and eventually also making carbon dioxide. Venus may never have had much surface water.