Rocky Planets and their Atmospheres

The previous post outlined how I consider the rocky planets formed. The most important point was that Earth formed a little inside the zone where calcium aluminosilicates could melt and phase separate while the star was accreting, as when the disk cooled down this would create a dust that, when reacted with the water vapour in the disk, would act as a cement. The concept is that this would bind basaltic rocks together, especially if the dust was formed in the collision between the rocks. The collisions were, by and large, gentle at first, driven by the gas sweeping smaller material closer to bigger material. Within this proposed mechanism, because the planet grows by collisions with objects at low relative velocities, the planet starts with a rather porous structure. It gradually heats up due to gravitational potential energy being converted to heat as more material lands, and eventually, if it gets to 1550 degrees, iron melts and runs down the pores towards the centre, while aluminosilicates, with densities about 0.4 – 1.2 g/cm3less than basalt, move upwards. The water is driven from the cements and also rises through the porous rock to eventually form the sea. The aluminosilicates form the granitic/felsic continents upon which we live.

Earth had the best setting of aluminosilicates because after the accretion disk cooled, it was at a temperature where these absorbed water best. Venus is smaller because it was harder to get started, as the cement was sufficiently warm that water had trouble reacting, but once it got going the density of basaltic and iron-bearing rocks was greater. This predicts Venus will have small granitic/felsic cratons on its surface; we have yet to find them. Mercury probably formed simply by accreting silicates and iron during the stellar accretion stage. Mars did not have a good supply of separated aluminium oxides, so it is very short of granite/felsic rock, although the surface of Syrtis Major appears to have a thin sheet of plagioclase. Because the iron did not melt at Mars, its outer rock would have contained a lot of iron dust or iron oxide. Reaction with water would have oxidised it subsequently. Most Martian rocks have roughly the same levels of calcium as Earth, about half the aluminium content, and about half as much again of iron oxide, which as an aside, may be why Mars does not have plate tectonics: because of the iron levels it cannot make eclogite which is necessary for pull subduction.

However, there is also a lot of chemistry going on in the stage 1 accretion disk in addition to what I have used to make the planets. In the vapour phase, carbon is mainly in the form of carbon monoxide in the rocky planet zone, but this can react catalytically with hydrogen to make methanol and hydrocarbons. These will have a very short lifetime and would be what chemists call reactive intermediates, but they would condense on silicates to make carbonaceous material, and they will react with oxides and metal vapour to make carbides. At the temperatures of at least the inner rocky planet zone, nitrogen reacts with oxides to make nitrides, and with carbides to make cyanamides, and some other materials.

Returning to the planet while it is heating up, the water coming off the cement should be quite reactive. If it meets iron dust it will oxidise it. If it meets a carbide there will be options, although the metal will invariably become an oxide. If the carbide was of the structure of calcium carbide it will make acetylene. If it oxidises anything, it will make hydrogen and the oxide. For many carbides it may make methane and metal oxide, or carbon monoxide, and invariably some hydrogen. Carbon monoxide can be oxidised by water to carbon dioxide, making more hydrogen, but carbon monoxide and hydrogen make synthesis gas, and a considerable variety of chemicals can be made, most of which are obvious contenders to help make life. Nitrides react with water largely to make ammonia, but ammonia is also reactive, and hydrogen cyanide and cyanoacetylene should be made. In the very early stages of biogenesis, hydrogen cyanide is an essential material, even though now it is poisonous.

This explains a little more of what we see in terms of the per centage composition. Mars, as noted above, has extremely little felsic/granitic material, and has a much higher proportion of iron oxide. It has less carbon dioxide than expected, even after allowing for some having escaped to space, and that is because since Mars was cooler, the high temperature carbide formation was slower. It has less water because the calcium silicates absorb less, although there is an issue here of how much is buried under the surface. The nitrogen is a puzzle. Mars has extremely little nitrogen, and the question is, why not. One possibility is that the temperatures were too low for significant nitride production. The other possibility, which I proposed in my novel Red Gold, is that at least some nitrogen was there and was emitted as ammonia. If so, it solves another puzzle: Mars has clear signs of ancient river flows, but all evidence is it was too cold for ice to melt. However, ammonia dissolves in ice and melts it down to minus eighty degrees Centigrade. So, in my opinion, the river flows were ammonia/water solutions. The carbon would have been emitted as methane, but that oxidises to carbon dioxide in the presence of water vapour and UV light.  Ammonia reacts with carbon dioxide first to form ammonium carbonate (which will also lower the melting point of ice) then urea. If I am right, there will be buried deposits of urea, or whatever it converts to after billions of years, in selected places on Mars.

The experts argue that methane and ammonia would only survive for a few years due to the UV radiation. However, smog would tend to protect them, and Titan still has methane. Liquid water also tends to protect ammonia. There are two samples from early Earth. One is of the atmosphere encased in rock at Isua, Greenland. It contains methane (as well as some hydrogen). The other is from Barberton (South Africa) which contains samples of seawater trapped in rock. The concentration of ammonia in seawater at 3.2 Gy BP was such that about 10% of the planet’s nitrogen currently in the atmosphere was in the sea in the form of ammonia.

We finally get to the initial question: why is Venus so different? The answer is simple. It will have had a lower per centage of cement and a high per centage of basalt simply because it formed at a hotter place. Accordingly, it would have much less water than Earth. However, it would have had more carbides and nitrides, and that valuable water got used up making the atmosphere, and in oxidising sulphur to sulphates. Accordingly, I expect Venus to have relatively small deposits of granite on the surface.

There is also the question of the deuterium to hydrogen ratio, which is at least a hundred times higher than solar. If the above mechanism is right, all the oxygen in the oxides, and all the nitrogen in the atmosphere, came from water reacting. My answer is that just about all the water was used up making the atmosphere, sulphates, and whatever. The initial reaction is of the sort:

R – X  + H2O  ->  R –OH + H – X

In this, one hydrogen atom has to transfer from the water to the X (where it will later be dislodged and lost to space). If there is a choice, the atom that is most weakly bonded will move, and deuterium is bonded quite more strongly than hydrogen. The electronic binding is the same, but there are zero point vibrations, and hydrogen, being lighter uses more of this as vibrational energy. In general chemistry, the chemical isotope effect, as it is called, can make the hydrogen between four and twenty-five times more likely to move, depending on the activation energy. Venus did not need to lose the supply of water equivalent to Earth’s oceans to get its high deuterium content; the chemical isotope effect is far more effective.

Further details can be found in my ebook “Planetary Formation and Biogenesis”

A Further Example of Theory Development.

In the previous post I discussed some of what is required to form a theory, and I proposed a theory at odds with everyone else as to how the Martian rivers flowed. One advantage of that theory is that provided the conditions hold, it at least explains what it set out to do. However, the real test of a theory is that it then either predicts something, or at least explains something else it was not designed to do.

Currently there is no real theory that explains Martian river flow if you accept the standard assumption that the initial atmosphere was full of carbon dioxide. To explore possible explanations, the obvious next step is to discard that assumption. The concept is that whenever forming theories, you should look at the premises and ask, if not, what?

The reason everyone thinks that the original gases were mainly carbon dioxide appears to be because volcanoes on Earth largely give off carbon dioxide. There can be two reasons for that. The first is that most volcanoes actually reprocess subducted material, which includes carbonates such as lime. The few that do not may be as they are because the crust has used up its ability to turn CO2 into hydrocarbons. That reaction depends on Fe (II) also converting to Fe (III), and it can only do that once. Further, there are many silicates with Fe (II) that cannot do it because the structure is too tightly bound, and the water and CO2 cannot get at the iron atoms. Then, if that did not happen, would methane be detected? Any methane present mixed with the red hot lava would burn on contact with air. Samples are never taken that close to the origin. (As an aside, hydrocarbon have been found, especially where the eruptions are under water.)

Also, on the early planet, iron dust will have accreted, as will other reducing agents, but the point of such agents is, they can also only be used once. What happens now will be very different from what happened then. Finally, according to my theory, the materials were already reduced. In this context we know that there are samples of meteorites that have serious reduced matter, such as phosphides, nitrides and carbides (both of which I argue should have been present), and even silicides.

There is also a practical point. We have one sample of Earth’s sea/ocean from over three billion years ago. There were quite high levels of ammonia in it. Interestingly, when that was found, the information ended up as an aside in a scientific paper. Because it was inexplicable to the authors, it appears they said the least they could.

Now if this seems too much, bear with me, because I am shortly going to get to the point of this. But first, a little chemistry, where I look at the mechanism of making these reduced gases. For simplicity, consider the single bond between a metal M and, say, a nitrogen atom N in a nitride. Call that M – N. Now, let it be attacked by water. (The diagram I tried to include refused to cooperate. Sorry) Anyway, the water attacks the metal and because the number of bonds around the metal stays the same, a hydrogen atom has to get attached to N, thus we get M-OH  + NH. Do this three times and we have ammonia, and three hydroxide groups on a metal ion. Eventually, two hydroxides will convert to one oxide and one molecule of water will be regenerated. The hydroxides do not have to be on the same metal to form water.

Now, the important thing is, only one hydrogen gets transferred per water molecule attack. Now suppose we have one hydrogen atom and one deuterium atom. Now, the one that is preferentially transferred is the one that it is easier to transfer, in which case the deuterium will preferentially stay on the oxygen because the ease of transfer depends on the bond strength. While the strength of a chemical bond starts out depending only on the electromagnetic forces, which will be the same for hydrogen and deuterium, that strength is reduced by the zero point vibrational energy, which is required by quantum mechanics. There is something called the Uncertainty Principle that says that two objects at the quantum level cannot be an exact distance from each other, because then they would have exact position, and exact momentum (zero). Accordingly, the bonds have to vibrate, and the energy of the vibration happens to depend on the mass of the atoms. The bond to hydrogen vibrates the fastest, so less energy is subtracted for deuterium. That means that deuterium is more likely to remain on the regenerated water molecule. This is an example of the chemical isotope effect.

There are other ways of enriching deuterium from water. The one usually considered for planetary bodies is that as water vapour rises, solar winds will blow off some water or UV radiation will break a oxygen – hydrogen bond, and knock the hydroden atom to space. Since deuterium is heavier, it is slightly less likely to get to the top. The problem with this is that the evidence does not back up the solar wind concept (it does happen, but not enough) and if the UV splitting of water is the reason, then there should be an excess of oxygen on the planet. That could work for Earth, but Earth has the least deuterium enrichment of the rocky planets. If it were the way Venus got its huge deuterium enhancement, there had to be a huge ocean initially, and if that is used to explain why there is so much deuterium, then where is the oxygen?

Suppose the deuterium levels in a planet’s hydrogen supply is primarily due to the chemical isotope effect, what would you expect? If the model of atmospheric formation noted in the previous post is correct, the enrichment would depend on the gas to water ratio. The planet with the lowest ratio, i.e. minimal gas/water would have the least enrichment, and vice versa. Earth has the least enrichment. The planet with the highest ratio, i.e. the least water to make gas, would have the greatest enrichment, and here we see that Venus has a huge deuterium enrichment, and very little water (that little is bound up in sulphuric acid in the atmosphere). It is quite comforting when a theory predicts something that was not intended. If this is correct, Venus never had much water on the surface because what it accreted in this hotter zone was used to make the greater atmosphere.