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”http://www.amazon.com/dp/B007T0QE6I.

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How do Rocky Planets Form?

A question in my last post raised the question of how do rocky planets form, and why is Venus so different from Earth? This will take two posts; the first covers how the planets form and why, and the second how they evolve immediately after formation and get their atmospheres.

First, a quick picture of accretion. At first, the gas cloud collapses and falls into the star, and in this stage the star the size of the sun accretes something like 2.5 x 10^20 kg per second. Call that stage 1. When the star has gobbled up most of the material, such accretion slows down, and in what I shall call stage 2 it accretes gas at least four orders of magnitude slower. The gas heats due to loss of potential energy as it falls into the star, although it also radiates heat from the dust that gets hot. (Hydrogen and helium do not radiate in the infrared easily.) In stage 1, the gas reached something like 1600 degrees C at 1 A.U. (the distance from Earth to the sun). In stage 2, because far less gas was falling in, the disk had temperatures roughly what bodies have now. Even in stage 2, standard theory has it that boulder-sized objects will fall into the star within about a hundred years due to friction with the gas.

So how did planets form? The standard explanation is that after the star had finished accreting, the dust very rapidly accreted to planetesimals (bodies about 500 km across) and these collided to form oligarchs, and in turn these collided to form planets. I have many objections to this. The reasons include the fact there is no mechanism to form the planetesimals that we assume to begin with. The calculations originally required one hundred million years (100 My) to form Earth, but we know that it had to be essentially formed well before that because the collision that formed the Moon occurred at about 50 My after formation started. Calculations solved the Moon-forming problem by saying it only took 30 My, but without clues why this time changed. Worse, there are reasons to believe Earth had to form within about 1 My of stage 2 because it has xenon and krypton that had to come from the accretion disk. Finally, in the asteroid belt there is evidence of some previous collisions between asteroids. What happens is they make families of much smaller objects. In short, the asteroids shatter into many pieces upon such collisions. There is no reason to believe that similar collisions much earlier would be any different.

The oldest objects in the solar system are either calcium aluminium inclusions or iron meteorites. Their ages can be determined by various isotope decays and both had to be formed in very hot regions. The CAIs are found in chondrites originating from the asteroid belt, but they needed much greater heat to form than was there in stage 2. Similarly, iron meteorites had to form at a temperature sufficient to melt iron. So, how did they get that hot and not fall into the sun? The only time the accretion disk got sufficiently hot at a reasonable distance from the sun was when the star was accreting in stage 1. In my opinion, this shows the calculations were wrong, presumably because they missed something. Worse, to have enough material to make the giants, about a third of the stellar mass has to be in the disk, but observation of other disks in stage 2 shows there is simply not enough mass to make the giants.

The basic argument I make is that whatever was formed in the late stages of stellar accretion stayed more or less where it was. One of the puzzles of the solar system is that most of the mass is in the star, but most of the angular momentum resides in the planets, and since angular momentum has to be conserved and since most of that was with the gas initially, my argument is any growing solids took angular momentum from the gas, which sends then mass further from the star, and it had to be taken before the star stopped accreting. (I suggest a mechanism in my ebook.)

Now to how the rocky planets formed. During primary stellar accretion, temperatures reached about 1300 degrees C where Mars would form and 1550 degrees C a little beyond where Earth would grow. This gives a possible mechanism for accretion of dust. At about 800 degrees C silicates start to get sticky, so dust can accrete into small stones there, and larger ones closer to the star. There are a number of different silicates, all of which have long polymers, but some, especially aluminosilicates are a little more mobile than others. At about 1300 degrees C, calcium silicate starts to phase separate out, and about 1500 degrees C various aluminosilicates phase separate. This happens because the longer the polymer, the more immiscible it is in another polymer melt (a consequence of the first two laws of thermodynamics, and which makes plastics recycling so difficult.) If this were the only mechanism for forming rocky planets, the size of the finished planet would diminish significantly with distance from the star. Earth, Venus and Mercury are in the wrong order. Mercury may have accreted this way, but further out, stones or boulders would be the biggest objects.

Once primary stellar accretion ends, temperatures were similar to what they are now. Stones collide, but with temperatures like now, they initially only make dust. There is no means of binding silicates through heat. However, if stones can come together, dust can fill the spaces. The key to rocky planet formation is that calcium silicate and calcium aluminosilicates could absorb water vapour from the disk gases, and when they do that, they act as cements that bind the stones together to form a concrete. The zone where the aluminosilicates start to get formed is particularly promising for absorbing water and setting cement, and because iron starts to form bodies here, lumps of iron are also accreted. This is why Earth has an iron core and plenty of water. Mars has less water because calcium silicate absorbs much less water, and its iron is mainly accreted as fine dust.

Finally, Mars is smaller because the solids density is less, and the disk is cleared before it has time to fully grow. The evidence for the short-lived disk is from the relatively small size of Jupiter compared with corresponding planets around similar sized stars that our sun cleared out the accretion disk sooner than most. This is why we have rocky planets, and not planets like the Neptune-sized planets in the so-called habitable zone around a number of stars. Venus is smaller than Earth because it was harder to get going, through the difficulty of water setting the cement, which is partly why it has very little water on its surface. However, once started it grows faster since the density of basaltic rocks is greater. Mercury is probably smaller still because it formed a slightly different way, through excessively mobile silicates in the first stage of the accretion disk, and by later being bombed by very large rocky bodies that were more likely to erode it. That is somewhat similar to the standard explanation of why Mercury is small but has a large iron core. The planets grow very quickly, and soon gravity binds all dust and small stones, then as it grows, gravity attracts objects that have grown further away, which perforce are large, but still significantly smaller than the main body in the zone.

Next post: how these rocky planets started to evolve to where they are now.