Origin of the Rocky Planet Water, Carbon and Nitrogen

The most basic requirement for life to start is a supply of the necessary chemicals, mainly water, reduced carbon and reduced nitrogen on a planet suitable for life. The word reduced means the elements are at least partly bound with hydrogen. Methane and ammonia are reduced, but so are hydrocarbons, and aminoacids are at least partly reduced. The standard theory of planetary formation has it (wrongly, in my opinion) that none of these are found on a rocky planet and have to come from either comets, or carbonaceous asteroids. So, why am I certain this is wrong? There are four requirements that must be met. The first is, the material delivered must be the same as the proposed source; the second is they must come in the same proportions, the third is the delivery method must leave the solar system as it is now, and the fourth is that other things that should have happened must have.

As it happens, oxygen, carbon, hydrogen and nitrogen are not the same through the solar system. Each exists in more than one isotope (different isotopes have different numbers of neutrons), and the mix of isotopes in an element varies in radial distance from the star. Thus comets from beyond Neptune have far too much deuterium compared with hydrogen. There are mechanisms by which you can enhance the D/H ratio, such as UV radiation breaking bonds involving hydrogen, and hydrogen escaping to space. The chemical bonds to deuterium tend to be several kJ/mol. stronger than bonds to hydrogen. The chemical bond strength is actually the same, but the lighter hydrogen has more zero point energy so it more easily breaks and gets lost to space. So while you can increase the deuterium to hydrogen ratio, there is no known way to decrease it by natural causes. The comets around Jupiter also have more deuterium than our water, so they cannot be the source. The chondrites have the same D/H ratio as our water, which has encouraged people to believe that is where our water came from, but the nitrogen in the chondrites has too much 15N, so it cannot be the source of our nitrogen. Further, the isotope ratios of certain heavy elements such as osmium do not match those on Earth. Interestingly, it has been argued that if the material was subducted and mixed in the mantle, it would be just possible. Given that the mantle mixes very poorly and the main sources of osmium now come from very ancient plutonic extrusions, I have doubts on that.

If we look at the proportions, if comets delivered the water or carbon, we should have five times more nitrogen, and twenty thousand times more argon. Comets from the Jupiter zone get around this excess by having no significant nitrogen or argon, and insufficient carbon. For chondrites, there should be four times as much carbon and nitrogen to account for the hydrogen and chlorine on Earth. If these volatiles did come from chondrites, Earth has to be struck by at least 10^23 kg of material (that is, ten followed by 23 zeros). Now, if we accept that these chondrites don’t have some steering system, based on area the Moon should have been struck by about 7×10^21 kg, which is approximately 9.5% of the Moon’s mass. The Moon does not subduct such material, and the moon rocks we have found have exactly the same isotope ratios as Earth. That mass of material is just not there. Further, the lunar anorthosite is magmatic in origin and hence primordial for the Moon, and would retain its original isotope ratios, which should give a set of isotopes that so not involve the late veneer, if it occurred at all.

The third problem is that we are asked to believe that there was a narrow zone in the asteroid belt that showered a deluge of asteroids onto the rocky planets, but for no good reason they did not accrete into anything there, and while this was going on, they did not disturb the asteroids that remain, nor did they disturb or collide with asteroids closer to the star, which now is most of them. The hypothesis requires a huge amount of asteroids formed in a narrow region for no good reason. Some argue the gravitational effect of Jupiter dislodged them, but the orbits of such asteroids ARE stable. Gravitational acceleration is independent of the body’s mass, and the remaining asteroids are quite untroubled. (The Equivalence Principle – all bodies fall at the same rate, other than when air resistance applies.)

Associated with this problem is there is a number of elements like tungsten that dissolve in liquid iron. The justification for this huge barrage of asteroids (called the late veneer) is that when Earth differentiated, the iron would have dissolved these elements and taken them to the core. However, they, and iron, are here, so it is argued something must have brought them later. But wait. For the isotope ratios this asteroid material has to be subducted; for them to be on the continents, they must not be subducted. We need to be self-consistent.

Finally, what should have happened? If all the volatiles came from these carbonaceous chondrites, the various planets should have the same ratio of volatiles, should they not? However, the water/carbon ratio of Earth appears to be more than 2 orders of magnitude greater than that originally on Venus, while the original water/carbon ratio of Mars is unclear, as neither are fully accounted for. The N/C ratio of Earth and Venus is 1% and 3.5% respectively. The N/C ratio of Mars is two orders of magnitude lower than 1-2%. Thus if the atmospheres came from carbonaceous chondrites:

Only the Earth is struck by large wet planetesimals,

Venus is struck by asteroidal bodies or chondrites that are rich in C and especially rich in N and are approximately 3 orders of magnitude drier than the large wet planetesimals,

Either Earth is struck by a low proportion of relatively dry asteroidal bodies or chondrites that are rich in C and especially rich in N and by the large wet planetesimals having moderate levels of C and essentially no N, or the very large wet planetesimals have moderate amounts of carbon and lower amounts of nitrogen as the dry asteroidal bodies or chondrites, and Earth is not struck by the bodies that struck Venus,

Mars is struck only infrequently by a third type of asteroidal body or chondrite that is relatively wet but is very nitrogen deficient, and this does not strike the other bodies in significant amounts,

The Moon is struck by nothing,

See why I find this hard to swallow? Of course, these elements had to come from somewhere, so where? That is for a later post. In the meantime, see why I think science has at times lost hold of its methodology? It is almost as if people are too afraid to go against the establishment.

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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.