Volatiles on Rocky Planets

If we accept the mechanism I posted before is how the rocky planets formed, we still do not have the chemicals for life. So far, all we have is water and rocks with some planets having an iron core. The mechanism means that until the planet gets gravitationally big enough to attract gas it only accretes solids, together with the water that bonded to the silicates. There re two issues: how the carbon and nitrogen arrived, and if these arrived as solids, which is the only available mechanism, what happened next?

In the outer parts of the solar system the carbon occurs as carbon monoxide, methanol, some carbon dioxide, and “carbon”, which essentially many forms but looks like tar, is partially graphite, and there are even mini diamonds. There are also polyaromatic hydrocarbons, and even alkanes, and some other miscellaneous organic chemicals. Nitrogen occurs as nitrogen gas, ammonia, and some cyanide. As this comes closer to the star, and in the region of the carbonaceous chondrites, it starts getting hot enough for some of this to condense and react on the silicates, which is why these have the aminoacids, etc. However, as you get closer to the star, it gets too hot and seemingly the inner asteroids are mainly just silicates. At this point, the carbon is largely converted to carbon monoxide, and the nitrogenous compounds to nitrogen. However, on some metal oxides or metals, carbon forms carbides, nitrogen nitrides, and some other materials, such as cyanamides are also formed. These are solids, and accordingly these too will be accreted with the dust and be incorporated within the planet.

As the interior of the planet gets hotter, the water gets released from the silicates and they lose their amorphous structure and become rocks. The water reacts with these chemicals and to a first approximation initially produces carbon monoxide, methane and ammonia. Carbon monoxide reacts with water on certain metals and silicates to make hydrocarbons, formaldehyde, which in turn condenses to other aldehydes (on the path to making sugars) ammonia (on the path to make aminoacids) and so on. The chemistry is fairly involved, but basically given the initial mix, temperature and pressure, both in ready supply below the Earth’s surface, what we need for life emerges and will make its way to the surface. Assuming this mechanism is correct, then provided everything is present in an adequate mix, then life should evolve. That leaves open the question, how broad is the “right mix” zone?

Before considering that, it is obvious this mechanism relies on the temperature being correct on at least two times during the planetary evolution. Initially it has to get hot enough to make the cements, and the nitrides and carbides. Superficially, that applies to all rocky planets, but maybe not for the nitrides. The problem here is Mars has very little nitrogen, so either it has gone somewhere, or it was never there. If Mars had ammonia, since it dissolves in ice down to minus 80 degrees C, ammonia on Mars would solve the problem of how could water flow there when it is so cold. However, if that is the case, the nitrogen has to be in some solid form buried below the surface. In my opinion, it was carried there as urea dissolved in water, which is why I would love to see some deep digging there.

The second requirement is that later the temperature has to be cool enough that water can set the cements. The problem with Venus is argued that it was hotter and it only just managed to absorb some water, but not enough. One counter to that is that the hydrogen on Venus has an extremely high deuterium content. The usual explanation for this is that if water gets to the top of the atmosphere, it may be hit with UV which may knock off a hydrogen atom, which is lost to space, and solar wind may take the whole molecule, however water with deuterium is less likely to get there because the heavier molecules are enhanced in the lower atmosphere, or the oceans. If this were true, for Venus to have the deuterium levels it must have started with a huge amount of water, and the mechanism above would be wrong. An embarrassing problem is where is the oxygen from that massive amount of water.

However, the proposed mechanism also predicts a very large deuterium enhancement. The carbon and nitrogen in the atmosphere and in living things has to be liberated from rocks by reaction with water, and what happens is as the water transfers hydrogen to either carbon or nitrogen it also leaves a hydroxyl attached to any metal. Two hydroxyls liberate water and leave an oxide. At this point we recall that chemical bond to deuterium is stronger than that to hydrogen, the reason being that although in theory the two are identical from the electromagnetic interactions, quantum mechanics requires there to be a zero point energy, and somewhat oversimplifying, the amount of such energy is inversely proportional to the square root of the mass of the light atom. Since deuterium is twice the mass of hydrogen, the zero point energy is less, and being less, its bond is stronger. That means there is a preference for the hydrogen to be the one that transfers, and the deuterium eventually turns up in the water. This preferential retaining of deuterium is called the chemical isotope effect. The resultant gases, methane and ammonia as examples, break down with UV radiation and make molecular nitrogen and carbon dioxide, with the hydrogen going to space. The net result of this is the rocky planet’s hydrogen gradually becomes richer in deuterium.

The effects of the two mechanisms are different. For Venus, the first one requires huge oceans; the second one little more than enough water to liberate the gases. If we look at the rocky planets, Earth should have a modest deuterium enhancement with both mechanisms because we know it has retained a very large amount of water. Mars is more tricky, because it started with less water under the proposed accretion of water mechanism, and it has less gravity and we know that all gases there, including carbon dioxide and nitrogen have enhanced heavier isotopes. That its deuterium is enhanced is simply expected from the other enhancements. Venus has about half as much CO2 again as Earth, and three times the amount of nitrogen, little water, and a very high deuterium enhancement. In my mechanism, Venus never had much water in the first place because it was too hot. Most of what it had was used up forming the atmosphere, and then providing the oxygen for the CO2. There was never much on the surface. To start with Venus was only a bit warmer than Earth, but as the CO2 began to build, whereas on Earth much of this would be dissolved in the ocean, where it would react with calcium silicate and also begin weathering the rocks that were more susceptible to weathering, such as dunite and peridotite. (I have discussed this previously: https://wordpress.com/post/ianmillerblog.wordpress.com/833 ), on Venus there were no oceans, and liquid water is needed to form these carbonates.

So, where will life be found? The answer is around any star where rocky planets formed with the two favourable temperature profiles, and ended up in the habitable zone. If more details as found in my ebook “Planetary Formation and Biogenesis” are correct, then this is most likely to occur around a G type star, like our sun, or a heavy K type star. The star also has to be one of the few that ejects it accretion disk remains early. Accordingly life should be fairly well spaced out, which may be why we have yet to run into other life forms.

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Was there an Initial Atmosphere from Accretion?

One of the problems with modern science is that once a paradigm has been selected, a layer of “authorities” is set up, and unless the scientist adopts the paradigm, little notice is taken of him or her. This is where conferences become important, because there is an audience that is more or less required to listen. The problem then for the person who has a different view is to show why that view is important enough to be considered. The barrier is rightly high. A new theory MUST do something the old one did not do, and it must not be contradicted by known facts. As I said, a high barrier.

In the previous post, I argued that the chemicals required for life did not come from carbonaceous chondrites or comets, and that is against standard thought. Part of the reason this view is held is that the gases had to come from somewhere, so from where? There are two obvious possible answers. The first is the gases were accreted with the planet as an atmosphere. In this hypothesis, the Earth formed while the disk gases were still there and simple gravity held them. Once the accretion disk was removed by the star, the hydrogen and helium were lost to space because Earth’s gravity was not strong enough, but other gases were retained. This possibility is usually rejected, and in this case the rejection is sound.

The first part of the proposition was almost certainly correct. Gases would have been accreted from the stellar disk, even on rocky planets, and these gases were largely hydrogen and helium. The next part is also correct. Once the disk gases were removed, that hydrogen and helium would be lost to space because Earth’s gravity was not strong enough to hold it. However, the question then is, how was it lost? As it happens, insufficient gravity was not the primary cause, and the loss was much faster than simply seeping off into space. Early in the life of a new star there are vicious solar winds and extreme UV radiation. It is generally accepted that such radiation would boil off the hydrogen and helium, and these would be lost so quickly that the other gases would be removed by hydrodynamic drag, and only some of the very heavier gases, such as krypton and xenon could remain. There is evidence to support this proposal, in that for krypton and xenon higher levels of heavier isotopes are observed. This would happen if most of these gases were removed from the top of the atmosphere, and since the lighter isotopes would preferentially find their way there, they would be removed preferentially. Since this is not observed for neon or argon isotopes, the argument is that all neon and argon in the atmosphere was lost this way, and if so, all nitrogen and carbon oxides, together with all water in the atmosphere would be lost. Basically, apart from the amount of krypton and xenon currently in the atmosphere, there would be no other gases. The standard theory of planetary formation has it that the Earth was a ball of magma, and if so, all water on the surface would be in the gas phase, so for quite some time Earth would be a dry lump of rock with an atmosphere that had a pressure that would be so low only the best vacuum pumps today could match it.

There could be the objection that maybe the star was not that active and we did retain some gases. After all, we weren’t around to check. Can you see why not? I’ll give the reason shortly. However, if we accept that the gases could not have come from the accretion disk, the other alternative is they came from below the ground, i.e. they were emitted by volacanic activity. How does that stand up?

One possibility might be that gases, including water, were adsorbed on the dust, then subsequently emitted by volcanoes. You might protest that if the Earth was a magma ocean, all that water would be immediately ejected from the silicates as a gas, but it turns out that while water is insoluble in silica at surface pressures, at pressures of 5000 atmospheres, granitic magma can dissolve up to 10% water at 1100 degrees C, at least according to Wikipedia. Irrespective of the accuracy of the figures, high temperature silicates under pressure most certainly dissolve water, and it probably hydrolyses the silicate structure and makes it far less viscous. It has been estimated that the water remaining in the mantle is 100 times greater than the current oceans so there is no problem in expecting that the oceans were initially emitted by volcanic activity. As an aside, deep in the mantle the pressures are far greater than 5000 atmospheres. This water is also likely to be very important for another reason, namely reducing the viscosity and lowering the magma density. This assists pull subduction, where the dry, or drier, basalt from the surface is denser than the other material around it and hence descends into the mantle. If the water were not there, we would not have plate tectonics, and if there were no plate tectonics, there would be no recycling of carbon dioxide, so eventually all the carbon dioxide on the surface would be converted to lime and there would be nothing for plants to use. End of life!

However, we know that our atmospheric gases were not primarily adsorbed as dust. How do we know that? In the accretion disk the number of nitrogen atoms is roughly the same as the number of neon atoms, and their heats of adsorption on dust are roughly the same. The only plausible physical means of separating them in the accretion disk is selective sublimation from ice, but ice simply could not survive where Earth formed. So, if our nitrogen came from the disk by simple physical means, then we would have roughly the same amount of neon in our atmosphere as nitrogen. We don’t, and the amount of neon we have is a measure of the amount of gas we have from such adsorption. Neon is present at 0.0018%, which is not very much.

So, in answer to the initial question, for a period there was effectively no atmosphere. To go any further we have to consider how the planets formed, and as some may suspect, I do not accept the standard theory for reasons that will become apparent in the next post.

Meanwhile, may I remind readers that my ebooks on Smashwords are on discount through July. Links to novels:

Puppeteer: http://www.smashwords.com/books/view/69696

‘Bot War: https://www.smashwords.com/books/view/677836

Troubles: https://www.smashwords.com/books/view/174203

Meanwhile, if you want to know scientifically about biofuels:

Biofuels: https://www.smashwords.com/books/view/454344

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.