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”