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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Evidence that the Standard Theory of Planetary Formation is Wrong.

Every now and again, something happens that makes you feel both good and depressed at the same time. For me it was last week, when I looked up the then latest edition of Nature. There were two papers (Nature, vol 541 (Dauphas, pp 521 – 524; Fischer-Gödde and Kleine, pp 525 – 527) that falsified two of the most important propositions in the standard theory of planetary formation. What we actually know is that stars accrete from a disk of gas and dust, the disk lingers on for between a million years and 30 million years, depending on the star, then the star’s solar winds clear out the dust and gas. Somewhere in there, planets form. We can see evidence of gas giants growing, where the gas is falling into the giant planet, but the process by which smaller planets or the cores of giants form is unobservable because the bodies are too small, and the dust too opaque. Accordingly, we can only form theories to fill in the intermediate process. The standard theory, also called oligarchic growth, explains planetary formation in terms of dust accreting to planetesimals by some unknown mechanism, then these collide to form embryos, which in turn formed oligarchs or protoplanets (Mars sized objects) and these collided to form planets. If this happened, they would do a lot of bouncing around and everything would get well-mixed. Standard computer simulations argue that Earth would have formed from a distribution of matter from further out than Mars to inside Mercury’s orbit. Earth the gets its water from a “late veneer” from carbonaceous chondrites from the far side of the asteroid belt.

It is also well known that certain elements in bodies in the solar system have isotopes that vary their ratio depending on the distance from the star. Thus meteorites from Mars have different isotope ratios from meteorites from the asteroid belt, and again both are different from rocks from Earth and Moon. The cause of this isotope difference is unclear, but it is an established fact. This is where those two papers come in.

Dauphas showed that Earth accreted from a reasonably narrow zone throughout its entire accretion time. Furthermore, that zone was the same as that which formed enstatite chondrites, which appear to have originated from a region that was much hotter than the material that, say, formed Mars. Thus enstatite chondrites are reduced. What that means is that their chemistry was such that there was less oxygen. Mars has only a small iron core, and most of its iron is as iron oxide. Enstatite chondrites have free iron as iron, and, of course, Earth has a very large iron core. Enstatite chondrites also contain silicates with less magnesium, which will occur when the temperatures were too hot to crystallize out forsterite. (Forsterite melts at 1890 degrees C, but it will also dissolve to some extent in silica melts at lower temperatures.) Enstatite chondrites also are amongst the driest, so they did not provide Earth’s water.

Fischer-Gödde and Kleine showed that most of Earth’s water did not come from carbonaceous chondrites, the reason being, if it did, the non-water part would have added about 5% to the mass of Earth, and the last 5% is supposed to be from where the bulk of elements that dissolve in hot iron would have come from. The amounts arriving earlier would have dissolved in the iron and gone to the core. One of those elements is ruthenium, and the isotope ratios of Earth’s ruthenium rule out an origin from the asteroid belt.

Accordingly, this evidence rules out oligarchic growth. There used to be an alternative theory of planetary accretion called monarchic growth, but this was soon abandoned because it cannot explain first why we have the number of planets we have where they are, and second where our water came from. Calculations show it is possible to have three to five planets in stable orbit between Earth and Mars, assuming none are larger than Earth, and more out to the asteroid belt. But they are not there, so the question is, if planets only grow from a narrow zone, why are these zones empty?

This is where I felt good. A few years ago I published an ebook called “Planetary Formation and Biogenesis” and it required monarchic growth. It also required the planets in our solar system to be roughly where they are, at least until they get big enough to play gravitational billiards. The mechanism is that the planets accreted in zones where the chemistry of the matter permitted accretion, and that in turn was temperature dependent, so specific sorts of planets form in zones at specific distances from the star. Earth formed by accretion of rocks formed during the hot stage, and being in a zone near that which formed enstatite chondrites, the iron was present as a metal, which is why Earth has an iron core. The reason Earth has so much water is that accretion occurred from rocks that had been heat treated to about 1550 degrees Centigrade, in which case certain aluminosilicates phase separated out. These, when they take up water, form cement that binds other rocks to form a concrete. As far as I am aware, my theory is the only current one that requires these results.

So, why do I feel depressed? My ebook contained a review of over 600 references from journals until a few months before the ebook was published. The problem is, these references, if properly analysed, provided papers with plenty of evidence that these two standard theories were wrong, but each of the papers’ conclusions were ignored. In particular, there was a more convincing paper back in 2002 (Drake and Righter, Nature 416: 39-44) that came to exactly the same conclusions. As an example, to eliminate carbonaceous chondrites as the source of water, instead of ruthenium isotopes, it used osmium isotopes and other compositional data, but you see the point. So why was this earlier work ignored? I firmly believe that scientists prefer to ignore evidence that falsifies their cherished beliefs rather than change their minds. What I find worse is that neither of these papers cited the Drake and Righter paper. Either they did not want to admit they were confirming a previous conclusion, or they were not interested in looking thoroughly at past work other than that which supported their procedures.

So, I doubt these two papers will change much either. I might be wrong, but I am not holding my breath waiting for someone with enough prestige to come out and say enough to change the paradigm.