If you accept my picture of how rocky planets form, we are now able to look at the volatiles. As before, this is my interpretation of what happened, and is not standard. Water is obvious, and was discussed in the previous post: water was what cemented the rocky fragments together, and the amount of water present in the rocks was proportional to the amount of cement present, and dependent on which type. Earth has the most water because it was in the optimal distance from the star; Venus had less because it was hotter, and because more of it came from basaltic rocks; Mars had less because it did not have many free aluminosilicates. The current geologies are consistent with this interpretation, although of course the evidence from Venus is a little on the thin side.
The accretion of carbon is more difficult to describe. At Earth’s distance from the star, essentially all carbon was in the form of carbon monoxide. However, there is another possible source, through what the chemist calls the reactive intermediate. The concept is that on dust there would be very low levels of methanol formed by reacting carbon monoxide and hydrogen, and that would immediately decay to form what is called “coke” when this process is carried out in a methanol-making plant. Such coke is a solid, and solids on the dust can be accreted. There should also be more chemistry, to make carbides, which are important for the origin of life, but not for the present discussion.
Similarly, at temperatures over 1000 degrees centigrade, nitrogen can react with a number of materials to form nitrides, which in turn are solids. Such solids will be accreted into the planet with the dust.
As the planet accretes, the interior heats up, simply through gravitational energy. I do not believe the standard theory is correct as far as major collisions are concerned, but irrespective the centre will heat, and together with radioactive decay, the centre and some way out becomes molten. At this point three things happen. Molten iron sinks to the core (thus generating more gravitational energy, and at the same time, the separated aluminosilicates can rise, forming the continents. The water becomes vapour, some of which dissolves in the hot silicate, and the rest either rises to form the oceans, or reacts chemically with something.
There are two important options regarding reaction, and it is important to note that these consume the number of moles of water that are reacted. The first is to oxidise something, like iron. That makes iron oxide, such as magnetite, which may also react with silica to make olivines and pyroxenes, together with some additional special minerals. Reaction of water with hot carbon makes carbon monoxide and hydrogen, the so-called synthesis gas. The second is to react with carbides and nitrides to make materials like methane, acetylene, hydrogen cyanide and ammonia, and at the same time, make oxides. Synthesis gas is in an equilibrium, and under pressure tends to end up as methane. Ammonia is in equilibrium with hydrogen, with pressure favouring ammonia. Geological pressures are far higher than anything we can achieve in a laboratory. So this theory predicts that the initial atmosphere will contain significant amounts of methane and ammonia, which will be degraded by UV radiation from the star to form, with water, carbon dioxide, nitrogen, and hydrogen, the latter being lost to space. So, is there any evidence for this, besides the prevalence of continents?
The first example comes from some rocks from Isua, Greenland, that are 3.8 billion years old. They contain primitive atmosphere in inclusions, as foamy magma congealed. The major gas inside is methane. The second comes from similar rock inclusions from 3.2 billion years ago from Barberton, in South Africa. This too involves magma that appears to have trapped seawater in pores. The salt levels are somewhat higher than modern seawater, presumably because the magma tended to boil off some of the water before it was trapped. What remains are ammonia levels approximately the same level as potassium ions. To me, that means the sea must have had high levels of ammonia in it, and assuming the ratio of nitrogen atoms to water was roughly the same as now, this would indicate that about ten per cent of the planet’s nitrogen was still in the form of ammonia then.
I also believe this mechanism better accounts for planetary deuterium/hydrogen ratios, which are always higher than the star’s. The standard explanation for that is that UV radiation from the star breaks down water, but water with deuterium in it is slightly heavier than ordinary water, so on density grounds it tends to rise less and a little hydrogen is preferentially lost to space. This enhancement is somewhat slight, but if it goes on for long enough, it will increase deuterium levels. From memory, the D/H ratio on Mars is about five times that of Earth, and we know that there are other isotope enhancements, due to the weaker Martian gravity, hence lighter molecules can be physically swept off to space. However, we now come to a problem with Venus, that has at least 100 times the enhancement. If this arose from such photolysis, what happened to the oxygen? This issue is serious because we then have to ask why does Earth not lose water the same way? The answer is that photolysis of water that would enhance deuterium also makes oxygen, and the oxygen makes ozone, and he ozone protects the water. It is also not a very rapid process, and if there was sufficient water on Venus, why did it not fix the carbon dioxide and make lime, like on Earth? The argument that it was hotter, and under pressure, is irrelevant: the hotter the water, the faster it reacts with rock.
My answer is simple. When the water reacts with solids as described above, there is something called the chemical isotope effect that is relevant. That means an O – H bond in water will react somewhere between 4 – 20 times preferentially than an O – D bond. Further, the water will not contain D2O, but rather H – O –D, which in turn will preferentially react the hydrogen atom. Unlike density differences, which is the cause of the vaporization and reaction in the upper atmosphere, this preference depends on the zero point vibrational energy of the bonds, and that makes very significant differences to the activation energy for the reactions. Thus for me, Venus never had significant water on its surface; the lesser amount of water it accreted was largely used up making the huge atmosphere, and whatever remains dissolved in the silicates of the lower mantle.
Next Monday post I should be able to explain the Martian fluvial systems.