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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Planets being formed?

This has been an interesting period for planetary science. In the last post, I mentioned the landing of Philae on a comet. As an update, unfortunately all has not gone well. The comet landed well, it bounced, and ended up in a shady spot, do most of what it has managed has relied on batteries. We do not know yet what data it has sent back, so we have no real idea on how successful the venture was, but from my point of view, the news is less good. In my last post, I mentioned that I would like to see what was encased in the ice. What happened was that Philae left it to the last to drill down (because they were afraid that the action of the drill might launch Philae back off the comet, as its gravity is very weak) and they wanted to do as much as they could before that risk was taken. They drilled, but apparently the drill hit something very hard, and when they withdrew the drill and tried to analyze its core, it appeared that there was no sample inside the drill. This is one of the curses of this sort of work. When designing some form of robot, you have to guess exactly what conditions you will meet.

However, a most interesting image has also been released by the European Space Agency. The star, HL Tauri has been found with an accretion disk around it. The star is about 1 million years old, and the disk has rings in it, with dark gaps between them. The most obvious cause for such rings would be the formation of planets, although that does not mean there is a planet in every gap, because while a planet will clear out dust on its path, gravitational resonance will also clear out material. Gravitational resonance is a term for when the orbital period at a given position is an exact multiple/fraction of another. Thus if the planet had a period, say, 12 years (roughly Jupiter’s “year”) there would be 2:1 resonances at a distance where the orbital period was 24 years, or at a distance where the orbital period was 6 years. Where this happens, over a period of time the various gravitational effects, instead of cancelling and circularizing, tend to reinforce and the bigger object causes the very much smaller one to change orbit.

So, are there planets there? One answer is, we don’t know because we have not seen them. Up to a point, this is a bit of a negative in this case. At first sight it may seem obvious that we would not see planets because they are too dull, but that is not the case with very newly formed giants. Thus there is a star HR 8799 and we can see four giant planets around it. The reason we can see them is that they are newly-formed giants, and when they take up the gas, the gravitational energy of the gas falling onto the planet heats it to a yellow-white heat, and the planets glow relatively brightly. Given we cannot see planets here, but we can see the disk, what does that mean?

One obvious thing that it can mean is that planets have yet to get big enough to glow brightly. In my theory of planetary formation (Planetary Formation and Biogenesis) our star had to have formed its planets by about 1 million years. The reason for this assessment is that there is a star LkCa 15 that is 3 million years old, and it has a planet much bigger than Jupiter, and significantly further from the star. Planetary growth should be faster, the closer to the star, at least for the same sort of planet, because the density of matter increases as it falls into the star. (The circumferences of the orbits decrease, and if the same amount of matter is presence, there much be more per unit volume.) Incidentally, we know about the planet around LkCa 15 because we “see” it, at least in images obtained by powerful telescopes, so it is glowing. Since we only see one giant, my theory requires there to be three other giants we cannot see, presumably because they are yet of insufficient size to glow sufficiently brightly for us to image them. So, if I am right, 1 My gets you giants of the size we have, and the longer the disk lasts, the bigger the giants get.

All of which shows there is still a lot of interest in planetary research

Philae, the comet, news reports, and the possibility of another planet.

The big news on the science front this week was the landing of Philae on a comet, and I found it interesting to note how the news media reported it. One of the most fascinating things for me was the announcement of the comet’s velocity. Yes, it is travelling very fast, but what is important when it came to landing on the comet is not how fast the comet is going, because that is meaningless. The important thing to consider is the relative velocity of Philae and the comet, and that was rather small.

There is no absolute velocity; velocity only has meaning when you consider it in the frame of reference of something else, in which case it is called the relative velocity. It is from this sort of thinking that we get relativity, the first version of which came from Galileo when he noted that if you are inside a ship you cannot tell how fast you are going – and even outside the issue is questionable. Outside, you can see how fast you are travelling relative to the water, but the water may be travelling relative to the land. In the case of trying to land a vehicle on a comet, the trick was to match the mother ship’s velocity as near as possible to that of the comet, and then let Philae approach and land at a relatively small change of velocity. In fact the hardest part of this was not to get Philae safely down, but rather to keep it down. The danger was always that it would bounce off, as the comet has only an extremely small gravitational force. In fact it appears that Philae did do some bouncing, and unfortunately it landed in a shady spot from where it cannot easily recharge its batteries through its solar cells. That means the information we get back will be dependent on what it could do for the day or so the original charge in the batteries lasted.

So, what will Philae tell us, assuming all goes well with its experimental equipment. We probably will not get an analysis of gas coming off the comet, because gas emissions probably do not get underway sufficiently well until the comet gets closer to the sun. However, we should get a good account of the composition of the very top surface of the comet. Unfortunately, that may not be very informative because the interesting more volatile material has probably been given off on previous encounters with the star, and any organic material there will have been subjected to considerable UV radiation. Comets spend their lives in very cold places, and the colder the site, the slower chemical reactions are (although this does not apply to photochemistry, because the energy to get these started comes from the light). The problem is, the comets would have formed when the solar system formed, which was 4.5 billion years ago, and while such reactions might be slow, 4.5 billion years remains a long time.

I have seen some reports saying that it will show that comets brought water to Earth. It will show nothing of the sort. At best, it might show comets are a possible source, but the evidence that we have so far is that cometary water has too much deuterium in it, so that is not where our planet’s water came from. It might have organic molecules in it, such as amino acids. It might, but that does not mean that is where the chemicals that permitted life to form came from. These issues are very complicated, and for those interested, I outlined the issues in my ebook Planetary Formation and Biogenesis.

Finally, what I would like to see is evidence of neon secreted into the ices. My mechanism for how planets accreted involves the melt fusion of bodies with a particular ice encased in water ice acting as the fusing agent. The mechanism is a little like that of forming snowballs, and the outer giants in this solar system are predicted to total four, based on four different temperature ranges of known ices. However, there is a possible fifth: neon. Was neon encased in ice? For neon to act as such an agent, the ice probably had to be below fifteen degrees above absolute zero as it started to approach the star. Would it? If it did, the comet might contain neon, but whether we can detect it is another matter because any such neon would have been lost from the surface of the comet in previous stellar encounters. Degassing from the centre might show it, but Philae probably won’t have the power to tell. For me it is important because if neon was encased in the ices that were in our accretion disk, then there is a possibility of another planet out there, probably in the order of 3-4 times further from the sun than Neptune. It would be a lot smaller than Neptune (because growth there is slower because everything is more dilute) so it would not be easy to find, but it would be nice to know if there were a reason to look for it.