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.

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.

Science that does not make sense

Occasionally in science we see reports that do not make sense. The first to be mentioned here relates to Oumuamua, the “interstellar asteroid” mentioned in my previous post. In a paper (arXiv:1901.08704v3 [astro-ph.EP] 30 Jan 2019) Sekanina suggests the object was the debris of a dwarf interstellar comet that disintegrated before perihelion. One fact that Sekanina thought to be important was that no intrinsically faint long-period comet with a perihelion distance less than about 0.25 AU, which means it comes as close or closer than about two-thirds the distance from the sun as Mercury, have ever been observed after perihelion. The reason is that if the comet gets that close to the star, the heat just disintegrates it. Sekanina proposed that such an interstellar comet entered our system and disintegrated, leaving “a monstrous fluffy dust aggregate released in the recent explosive event, ‘Oumuamua should be of strongly irregular shape, tumbling, not outgassing, and subjected to effects of solar radiation pressure, consistent with observation.” Convinced? My problem: just because comets cannot survive close encounters with the sun does not mean a rock emerging from near the sun started as a comet. This is an unfortunately common logic problem. A statement of the form “if A, then B” simply means what it says. It does NOT mean, there is B therefor there must have been A.

At this point it is of interest to consider what comets are comprised of. The usual explanation is they are formed by ices and dust accreting. The comets are formed in the very outer solar system (e.g.the Oort cloud) by the ices sticking together. The ices include gases such as nitrogen and carbon monoxide, which are easily lost once they get hot. Here, “hot” is still very cold. When the gases volatalise, they tend to blow off a lot of dust, and that dust is what we see as the tail, which is directed away from the star due to radiation pressure and solar wind. The problem with Sekanina’s interpretation is, the ice holds everything together. The paper conceded this when it said it was a monstrous fluffy aggregate, but for me as the ice vaporizes, it will push the dust apart. Further, even going around a star, it will still happen progressively. The dust should spread out, as a comet tail. It did not for Oumuamua.

The second report was from Bonomo, in Nature Astronomy(doi.org/10.1038/s41550-018-0648-9). They claimed the Kepler 107 system provided evidence of giant collisions, as described in my previous post, and the sort of thing that might make an Oumuamua. What the paper claims is there are two planets with radii about fifty per cent bigger than Earth, and the outer planet is twice as dense (relative density ~ 12.6 g/cm^3) than the inner one (relative density ~ 5.3 g/cm^3). The authors argue that this provides evidence for a giant collision that would have stripped off much of the silicates from the outer planet, thus leaving more of an iron core. In this context, that is what some people think is the reason for Mercury having a density almost approaching that of Earth so the authors are simply tagging on to a common theme.

So why do I think this does not make sense? Basically because the relative density of iron is 7.87 g/cm^3. Even if this planet is pure iron, it could not have a density significantly greater than 7.8. (There is an increase in density due to compressibility under gravity, but iron is not particularly compressible so any gain will be small.) Even solid lead would not do. Silicates and gold would be OK, so maybe we should start a rumour? Raise money for an interstellar expedition to get rich quick (at least from the raised money!) However, from the point of view of the composition of dust that forms planets, that is impossible so maybe investors will see through this scam. Maybe.

So what do I think has happened? In two words, experimental error. The mass has to be determined by the orbital interactions with something else. What the Kepler mehod does is determine the orbital characteristics by measuring the periodic times, i.e.the times between various occultations. The size is measured from the width of the occultation signal and the slope of the signal at the beginning and the end. All of these have possible errors, and they include the size of the star and the assumed position re the equator of the star, so the question now is, how big are these errors? I am starting to suspect, very big.

This is of interest to me since I wrote an ebook, “Planetary Formation and Biogenesis”. In this, I surveyed all the knowedge I could find up to the time of writing, and argued the standard theory was wrong. Why? It took several chapters to nail this, but the essence is that standard theory starts with a distribution of planetesimals and lets gravitational interactions lead to their joining up into planets. The basic problems I see with this are that collisions will lead to fragmentation, and the throwing into deep space, or the star, bits of planet. The second problem is nobody has any idea how such planetesimals form. I start by considering chemical interactions, and when I do that, after noting that what happens will depend on the temperatures around where it happens (what happens in chemistry is often highly temperature dependent) you get very selective zoes that differ from each other quite significantly. Our planets are in such zones (if you assume Jupiter formed at the “snow zone”) and have the required properties. Since I wrote that, I have been following the papers on the topic and nothing has been found that contradicts it, except, arguably things like the Kepler 107 “extremely dense planet”. I argue it is impossible, and therefore the results are in error.

Should anyone be interested in this ebook, see http://www.amazon.com/dp/B007T0QE6I

Oumuamua (1I) and Vega

Oumuamua is a small asteroidal object somewhere between 100 – 1000 meters long and is considerably longer than it is broad. Basically, it looks like a slab of rock, and is currently passing through the solar system on its way to wherever. It is our first observation of an interstellar object hence the bracketed formal name: 1 for first, I for interstellar. How do we know it came from interstellar space? Its orbit has been mapped, and its eccentricity determined. The eccentricity of a circular orbit is zero; an eccentricity greater than zero but less than one means the object is in an elliptical orbit, and the larger the eccentricity, the bigger the difference between closest and furthest approach to the sun. Oumuamua was found to have an eccentricity of 1.1995, which means, being greater than 1, it is on a hyperbolic orbit. It started somewhere where the sun’s gravity is irrelevant, and it will continue on and permanently leave the sun’s gravitational field. We shall never see it again, so the observation of it could qualify it for entry in “The Journal of Irreproducible Results”.

Its velocity in interstellar space (i.e.without the sun’s gravitational effects) was 26.3 km/s. We have no means of knowing where it came from, although if is trajectory is extrapolated backwards, it came from the direction of Vega. Of course it did not come from Vega, because when it passed through the space that Vega now occupies, Vega was somewhere else. Given there is no sign of ice on Oumuamua, which would form something like a cometary tail, it presumably came from the rocky zone closer to its system’s star, and this presumably has given rise to the web speculation that Oumuamua was some sort of alien space ship. Sorry, but no, it is not, and it does not need motors to enter interstellar space.

The way a body like Oumuamua could be thrown into interstellar space goes like this. There has to be a collision between two rocky bodies that are big enough to form fragments of the required size and the collision has to be violent enough to give the fragment a good velocity. That will also make a lot of dust. The fragments would be assumed to then go into elliptical orbits, but if there are both rocky planets and giants, the body could be ejected in the same way the Voyager space craft have left our solar system, namely through gravity assists. If the object is on the right trajectory it could get a gravity assist from an earth-like rocky planet, then another one from a giant that could give it enough impetus to leave the system. This presumably happened a long time ago, so we have no idea where the object came from.

Notwithstanding that, Oumuamua brought Vega to my attention, and it is, at least for me, an interesting star. That, of course, is because I have published a theory of planetary formation that is at odds with the generally accepted one. Vega has about twice the mass of the sun, and because it is bigger, it burns faster, and will have a life of about a billion years. It is roughly half-way through that, so it won’t have had time for planets to evolve intelligent life. The concentration of elements heavier than helium in Vega is about a third that of the sun. Vega also has an abnormally fast rate of rotation, so much so that it is about 88% of what would be required to start the star breaking up. This is significant because one of the oddities of our solar system is that the bulk of the angular momentum resides in the planets, while by far the bulk of the mass lies in the star. The implication might be that the lower level of heavier elements meant that Vega did not form cores fast enough and hence it does not have the giant planets of sufficient size to have taken up sufficient angular momentum. The situation could be like an ice skater who spins very fast, but slows the rotation by extending her arms. If the arms are very short, the spin cannot be slowed as much.

The infra-red emissions from Vega are consistent with a dust disk from about 70 – 100 A.U. out to 330 A.U. from the star (an A.U. is the distance from the sun to the Earth). This is assumed to have arisen from recent collisions of objects comparable to those in the Kuiper Belt here. There is apparently another dusty zone at 8 A.U., which would have to have originated from collisions between rocky objects. So far there is no evidence of planets around Vega, but equally there is no evidence there are none. We view Vega almost aligned with its axis of rotation, so most of the usual techniques for finding planets will not work. The transiting technique of the Kepler program requires us to be aligned with the ecliptic (which should be aligned with the equator) and the Doppler technique has similar limitations, although it has more tolerance for deviation. The Doppler technique detects the gravitational wobble of the star and if you could detect such a wobble directly, you could see it from along the polar axis. Unfortunately, we can’t, at least not yet, and worse, detecting such wobbles works best with very large planets around small stars. Here, if you follow my theory and accept the low metallicity, we expect small planets around a very large star. Direct observation has so far only worked for the first few million years of the star, where giant planets are radiating yellow to white light from their surface temperature that is so hot because of the gravitational accretion energy. These cool down reasonably quickly.

What grabbed my attention about Vega was the 8 A.U. dust zone. That can only be generated by a number of collisions because such dust zones have to be replenished. That is because solar radiation slows dust down, and it gradually falls into the star. So to have a good number of frequent collisions, you need a very large number of objects that could collide, which effectively requires a belt of boulders. So why have they not collided and formed a planet, when the standard theory of planetary formation says planets are formed by the collision of boulders to form planetesimals, and these collide to form embryos, which collide to form planets. In my ebook, “Planetary Formation and Biogenesis” I provide an answer, which is basically that to form rocky planets, the collisions have to happen in the accretion disk, and they happen very fast, and they happen because water vapour in the disk helps set cement. Once the accretion disk is removed, further accretion is impossible, other than from objects colliding with a big enough object for gravity to hold all the debris. Accordingly, collisions of boulder-sized objects or asteroids will make dust, and that would create a dust belt that would not last all that long. The equivalent of the Kuiper Belt around Vega appears to be between 3 – 6 times further out. In my theory, if the planet accreted in the same as the sun, it would be approximately 8 times further out. However, lower dust content may make it harder to radiate energy, hence accretion may be slower. If this second belt scales accordingly, it could correspond to our asteroid belt.  We know occasional collisions did occur in our asteroid belt because we see families of smaller fragments whose trajectories extrapolate back to a singe event. So maybe dust belts are tolerably common for short periods in the life of a star. It would not be a great coincidence we see one around Vega; there are a huge number of stars, we see a very large number of accretion disks, so dust belts should turn up sooner or later.

Finally, why does the star spin faster? Again, in my theory, the planets accrete from the solid and take their angular momentum, but then they also take angular momentum from the disk gas through a mechanism similar to the classical Magnus force. Vega has less dust to make planets, hence less angular momentum is taken that way, and because the planets should be smaller there is less gravity to take angular momentum from the gas, and more gas anyway. So the star retains a higher fraction of its angular momentum. All of this does not prove that my theory is right, but it is comforting that it at least has some sort of plausible support. If interested further, check out http://www.amazon.com/dp/B007T0QE6I.

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.

Rocky Planet Formation

In the previous posts I have argued that the evidence strongly supports the concept that the sun eliminated its accretion disk within about 1 My after the star formed. During this 1 My, the disk would be very much cooler than while the sun was accreting, and the temperatures were probably not much different from those now at any given distance from the star in the rocky planet zone. Gas was still falling into the star, but at least ten thousand times slower. We also know (see previous posts) that small solid objects such as CAIs and iron bearing meteorites are much older than the planets and asteroids. If the heavier isotope distributions of xenon and krypton are caused by the hydrodynamic loss to space, which is the most obvious reason, then Earth had to have formed before the disk cleanout, which means Earth was more or less formed within about 1 My after the formation of the sun.

The basic problem for forming rocky planets is how does the rocky material stick together? If you are on the beach, you may note that sand does not turn into a solid mass. A further problem is the collisions of large objects involve huge energies. Glancing collisions lead to significant erosion of both objects, and even direct hits lead to local pulverization and intense heat, together with a shock wave going through the bodies. When the shock wave returns, the pulverized material is sent into space. Basically craters are formed, and a crater is a hole. Adding holes does not build up mass. Finally, if the two are large enough and about equal sized, they each tend to shatter as a consequence of the shock waves. This is why I believe the Monarchic growth makes more sense, where what collides with the major body is much smaller. Once the forming object is big enough, it accretes all small objects it collides with, due to gravity, but the problem is, how do small bodies stick together?

The mechanism I developed goes like this. While the star is accreting, we get very high temperatures and anything over 1000 degrees will lead to silicates softening and becoming sticky. This generates pebbles, stones and boulders that get increasingly big as we get closer to the star, because more of the silicates get more like liquids. At 1550 degrees C, iron melts, and the iron liquids coalesce. That is where the iron meteorites come from. By about 1750 – 1800 degrees silicates get quite soft, and it may be that Mercury formed by a whole lot of “liquids” forming a sticky mass. Behind that would be a distribution of ever decreasingly sized silicate masses, with iron cores where temperatures got over 1550. This would be the origin of the cores for Earth, Venus and Mercury. Mars has no significant iron core because the iron there was still in the very small particulate size.

The standard theory says the cores separated out with heavier liquids sinking, but what most people do not realize is that the core of the Earth does not comprise liquid silicates, at least not the mobile sort. You have no doubt heard that heat rises by convection at hot spots, but it is not a sort of kettle down there. The rate of movement has been estimated at 1 mm per year, which would mean the silicates would rise 1000 km every billion years. We are still well short of one complete turnover. Further an experiment where two different silicates were heated to 2000 degrees C under pressure of 26 Gpa showed that the silicates would only diffuse contents a few meters over the life of the Earth. They may be “liquid” but the perovskite silicates are so viscous nothing moves far in them. So how did the core form so quickly? In my opinion, the reason is the iron has already separated from the silicates, and the collision of a whole lot of small spherical objects do not pack well; there will be channels, and molten iron that already exists in larger masses will flow down them. Less-viscous aluminosilicates will flow up and form the continents.

The next part unfortunately involves some physical chemistry, and there is no way around it. I am going to argue that the silicates that formed the boulders separated into phases. An example is oil and water. Molecules tend to have an energy of association, that is all the water molecules have an energy that tends to hold them all together as a liquid as opposed to a gas, and that tends to keep phases separate because one such energy between like molecules is invariably stronger than the energy between different ones. There is also something called entropy, which favours things being mixed. Now the heat of association of polymers is proportional to the number of mers, while the entropy is (to a first approximation) proportional to the number of molecules. Accordingly, the longer the polymers, the less likely they are to blend, and the more likely to phase separate. That is one of the reasons that recycling plastics is such a problem: you cannot blend them because if the polymers are long, they tend to separate in processing, and your objects have “faults” running through them.

The reason this is important, from my point of view, is that at about 1300 degrees C, calcium silicate tends to phase separate from the rest, and about 1500 degrees C, a number of calcium aluminosilicates start to phase separate. These are good hydraulic cements, and my argument is that after cool down, collisions between boulders makes dust, and the cements are particularly brittle. Then if significant boulders come together gently, e.g. as in the postulated “rubble piles”, the cement dust works it way through them, and water vapour from the disk will set the cement. This works up to about 500 degrees C, but there are catches. Once it gets significantly over 300 degrees C, less water is absorbed, and the harder it is to set it. Calcium silicate only absorbs one molecule of water, but some aluminosilicates can absorb up to twenty molecules per mer. This lets us see why the rocky planets look like they do. Mars is smaller because only the calcium silicate cement can form at that distance, and because iron never melted it does not have an iron core. It has less water because calcium silicate can only set one molecule of water per cement molecule, and it does not have easily separable aluminosilicates so it has very little felsic material. Earth is near the optimum position. It is where the iron core material starts, and because it is further from the sun than the inner planets, there is more iron to sweep up. The separated aluminosilicates rise to the surface and form the felsic continents we walk on, and provided more water when setting the cement. Venus formed where it was a little hot, so it was a slow starter, but once going, it will have had bigger boulders to grow with. It has plenty of iron core, but less felsic material, and it started with less water than Earth. This is conditional on the Earth largely forming before the disk gases were ejected. If we accept that, we have a platform for why Earth has life, but of course that is for later.

From Whence Star-burning Planets?

This series started out with the objective of showing how life could have started, and some may be wondering why I have spent so much time talking about the cold giant planets. The answer is simple. To find the answer to a scientific problem we seldom go directly to it. The reason is that when you go directly to what you are trying to explain you will get an explanation, however for any given observation there will be many possible explanations. The real explanation will also explain every connected phenomenon, whereas the false explanations will only explain some. The ones that are seemingly not directed at the specific question you are trying to answer will nevertheless put constraints on what the eventual answer must include. I am trying to make things easier in the understanding department by considering a number of associated things. So, one more post before getting on to rocky planets.

In the previous two posts, I have outlined how I believe planets form, and why the outer parts of our solar system look like they do. An immediate objection might be, most other systems do not look like ours. Why not? One reason is I have outlined so far how the giants form, but these giants are a considerable distance from the star. We actually have rather little information about planets in other systems at these distances. However, some systems have giants very close to the star, with orbits (years) that take days and we do not. How can that be?

It becomes immediately obvious that planets cannot accrete from solids colliding that close to the star because the accretion disk get to over 10,000 degrees C that close, and there are no solids at those temperatures. The possibilities are that either there is some mechanism that so far has not been considered, which raises the question, why did it not operate here, or that the giants started somewhere else and moved there. Neither are very attractive, but the fact these star-burning giants only occur near a few stars suggests that there is no special mechanism. Physical laws are supposedly general, and it is hard to see why these rare exceptions occur. Further, we can see how they might move.

There is one immediate observation that suggests our solar system is expected to be different from many others and that is, if we look again at LkCa 15b, that planet is three times further from the star than Jupiter is from our star, which means the gas and dust there would have more than three times less concentrated, and collisions between dust over nine times rarer, yet it is five times bigger. That star is only 2 – 3 My old, and is about the same size as our star. So the question is, why did Jupiter stop growing so much earlier when it is in a more favourable spot through having denser gas? The obvious answer is Jupiter ran out of gas to accrete much sooner, and it would do that through the loss of the accretion disk. Stars blow away their accretion disks some time between 1 and 30 My after the star essentially finishes accreting. The inevitable conclusion is that our star blew out its disk of gases in the earliest part of the range, hence all the planets in our system will be, on average, somewhat smaller than their counterparts around most other stars of comparable size. Planets around small stars may also be small simply because the system ran out of material.

Given that giants keep growing as long as gas keeps being supplied, we might expect many bigger planets throughout the Universe. There is one system, around the star HR 8799 which has four giants arrayed in a similar pattern to ours, albeit the distances are proportionately scaled up and the four planets are between five and nine times bigger than Jupiter. The main reason we know about them is because they are further from the star and so much larger, hence we an see them. It is also because we do not observe then from reflected light. They are very young planets, and are yellow-white hot from gravitational accretion energy. Thus we can see how planets can get very big: they just have to keep growing, and there are planets that are up to 18 times bigger than Jupiter. If they were bigger, we would probably call them brown dwarfs, i.e. failed stars.

There are some planets that have highly elliptical orbits, so how did that situation arise? As planets grow, they get gravitationally stronger, and if they keep growing, eventually they start tugging on other planets. If they can keep this up, the orbits get more and more elliptical until eventually they start orbiting very close to each other. They do not need to collide, but if they are big enough and come close enough they exchange energy, in which case one gets thrown outwards, possibly completely out of its solar system, and one gets thrown inwards, usually with a highly elliptical orbit. There are a number of systems where planets have elliptical orbits, and it may be that most do, and if they do, they will exchange energy gravitationally with anything else they come close to. This may lead to a sort of gravitational billiards, where the system gets progressively smaller, and of course rocky planets, being smaller are more likely to get thrown out of the system, or to the outer regions, or into the star.

Planets being thrown into the star may seem excessive, nevertheless in the last week it was announced that a relatively new star, RW Aur A, over the preceding year had a 30 fold increase in the amount of iron in its spectrum. The spectrum of a star comes from whatever is on its surface, so the assumption is that something containing a lot of iron, which would be something the size of a reasonably sized asteroid at least, fell into the star. That means something else knocked it out of its orbit, and usually that means the something else was big.

If the orbit is sufficiently elliptical to bring it very close to the star one of two things happen. The first is it has its orbit circularized close to the star by tidal interactions, and you get one of the so-called star-burners, where they can orbit their star in days, and their temperatures are hideously hot. Since their orbit is prograde, they continue to orbit, and now tidal interactions with the star will actually slowly push the planet further from the star, in the same way our moon is getting further from us. The alternative is that the orbit can flip, and become retrograde. The same thing happens as with the prograde planets, except that now tidal interactions lead to the planet slowly falling into the star.

The relevance of all this is to the question, how common is life in the Universe? If we want a rocky planet in a circular orbit in the habitable zone, then we can eliminate all systems with giants on highly elliptical orbits, or in systems with star burners. However, there is a further possibility that is not advantageous to life. Suppose there are rocky planets formed but the star has yet to elimiinate its accretion disk. The rocky planet will also keep growing and in principle could also become a giant. This could be the reason why some systems have Neptune-sized planets or “superEarths” in the habitable zone. They probably do not have life, so now we have to limit the number of possible star systems to those that eliminate their accretion disk very early. That probably elimimates about 90% of them. Life on a planet like ours might be rarer than some like to think.

Monarchic Growth of Giant Planets

In the previous post, I outlined the basic mechanism of how I thought the giant planets formed, and how their mechanism of formation put them at certain distances from the sun. Given that, like everyone else, I assign Jupiter to the snow point, in which case the other planets are where they ought to be. But that raises the question, why one planet in a zone? Let’s take a closer look at this mechanism.

In the standard mechanism, dust accretes into objects by some unknown mechanism, and does this essentially based on collision probability, and so the disk progresses with a distribution of roughly equal sized objects that collide under the same rules, and eventually become what is called planetesimals, which are about the size of the classical asteroid. (I say classical because as we get better at this, we are discovering a huge number of much smaller “asteroids”, and we have the problem of what does the word asteroid mean?) This process continues, and eventually we get Mars-sized objects called oligarchs, or embryos, then these collide to get planets. The size of the planet depends on how many oligarchs collide, thus fewer collided to make Venus than Earth, and Mars is just one oligarch. I believe this is wrong for four reasons: the first is, giants cannot grow fast enough; second, the dust is still there in 30 My old disks; the collision energies should break up the bodies at any given size because collisions form craters, not hills; the system should be totally mixed up, but isotope evidence shows that bodies seem to have accreted solely from the material at roughly their own distance from the sun.

There is an alternative called monarchic growth, in which, if one body can get a hundred times bigger than any of the others, it alone grows by devouring the others. For this to work, we need initial accretion to be possible, but not extremely probable from dust collisions. Given that we see disks by their dust that are estimated to be up to 30 My old, that seems a reasonable condition. Then, once it starts, we need a mechanism that makes further accretion inevitable, that is, when dust collides, it sticks. The mechanism I consider to be most likely (caveat – I developed it so I am biased) is as follows.

As dust comes into an appropriate temperature zone, then collisions transfer their kinetic energy into heat that melts an ice at the point of contact, and when it quickly refreezes, the dust particles are fused to the larger body. So accretion occurs a little below the melting temperature, and the probability of sticking falls off as the distance from that appropriate zone increases, but there is no sharp boundary. The biggest body will be in the appropriate zone because most collisions will lead to sticking, and once the body gets to be of an appropriate size, maybe as little as a meter sized, it goes into a Keplerian orbit. The gas and dust is going slower, due to gas drag (which is why the star is accreting) so the body in the optimal zone accretes all the dust and larger objects it collides with. Until the body gets sufficiently large gravitationally, collisions have low relative velocity, so the impact energy is modest.

Once it gets gravitationally bigger, it will accrete the other bodies that are at similar radial distance. The reason is that if everything is in circular orbits, orbits slightly further from the star have longer periodic times, in part because they move slightly slower, and in part because they have slightly further to go, so the larger body catches up with them and its gravity pulls the smaller body in. Unless it has exactly the same radial distance from the star, they will pass very closely and if one has enough gravity to attract the other, they will collide. Suppose there are two bodies at the same radial distance. That too is gravitationally unstable once they get sufficiently large. All interactions do not lead to collisions, and it is possible that one can be thrown inwards while the other goes outwards, and the one going in may circularise somewhere else closer to the star. In this instance, Ceres has a density very similar to the moons of Jupiter, and it is possible that it started life in the Jovian region, came inwards, and then finished accreting material from its new zone.

The net result of this is that a major body grows, while smaller bodies form further away, trailing off with distance, then there is a zone where nothing accretes, until further out there is the next accretion zone. Such zones get further away as you get further from the star because the temperature gradient decreases. That is partly why Neptune has a Kuiper Belt outside it. The inner planets do not because with a giant on each side, the gravity causes them to be cleaned out. This means that after the system becomes settled, a lot of residues start bombing the planet. This requires what could be called a “Great Bombardment”, but it means each system gets a bombardment mainly of its own composition, and there could be no significant bombardment with bodies from another system. This means the bombardment would have the same chemical composition as the planet itself.

Accordingly, we have a prediction. Is it right? It is hard to tell on Earth because while Earth almost certainly had such a bombardment, plate tectonics has altered the surface so much. Nevertheless, the fact the Moon has the same isotopes as Earth, and Earth has been churned but the Moon has not, is at least minor support. There is, of course, a second prediction. There seem to be many who assume the interior of the Jovian satellites will have much nitrogen. I predict very little. There will be some through adsorption of ammonia onto dust, and since ammonia binds more strongly than neon, then perhaps there will be very modest levels, but the absence of such material in the atmosphere convinces me it will be very modest.

The Formation of the Giant Planets

Before I can discuss how we got the elements required for life delivered to Earth, it is necessary to work out how the planets formed, and why we have what we have. While the giant planets are almost certainly not going to have life, at least not as we would recognise it, they are important because what we have actually gives some important clues as to how planets form, and hence how common life will be, and why so many exoplanetary systems are so different from ours. The standard theory says a core accreted, then when it got sufficiently big, which calculations have at about 10 to 12 times the Earth mass it starts accreting gas in substantial amounts and it grows very slowly, the problem restricting growth being how it can compress its volume and get rid of the heat so generated. After a number of million years, its mass gets big enough, and it accretes everything that comes into range. Apart from the rather slow time the calculations give, that general description is almost certainly essentially correct. The reason we believe the core has to get to about 10 – 12 Earth masses before disk gases get accreted in serious amounts is because the evidence is the Neptune and Uranus have about 2 Earth masses of hydrogen and helium. So far, reasonably good. The fact that there is evidence the calculations are wrong is not damning; the fact that if the mechanism is not properly understood in close detail then the calculations will inevitably be wrong. The original calculations had these stages taking about 10 My to get to a planet the size of Jupiter. The very first calculations had it taking about a billion years to get to Neptune, but that obviously cannot be right because the disk gases had long gone before that. The real problem is how to get to the cores.

The standard theory says they started by the accretion of a distribution of planetesimals that were formed by the accretion of dust, and therefore were distributed according to the dust concentrations through the disk. There are two problems with this for me. First, we see these disks, and we see them because their dust scatters light. Some such disks are 30 My old and still dusty, so the dust itself is not rapidly accreting, altough often there are bands where there seems to be little dust. The second problem is that there is no recognized mechanism by which the dust can accrete and stick together strongly enough not to be disrupted by any other dustball that collides with it. Mathematics indicate that such dustballs, if they reach about 2 cm size, erode from gas motion relative to them.

So, how did the cores form? I think we have evidence from the fact their systems all have different compositions. The theory I outlined in my ebook Planetary Formation and Biogenesis goes like this. The dust that comes from deep space has a lot of very fluffy ice around it, and this has many pores. Within those pores, and around the ice, are the ices of other volatiles. (Such compound ices have been made in the lab, and their behaviour verified.) As the ices come in, the more volatile ones start subliming away at temperatures a little above their melting point, and hydrogen has even been maintained as an ice enclosed in water ice pores up to a little under 15 degrees K, which is well above its boiling point. So, as the ices come in and the disk gets gradually hotter, the ices selectively boil away. The relevant temperatures are: neon (~25 K); nitrogen and carbon monoxide (~65 K); argon and methane (~ 85); ammonia and methanol (~170 K); water (273 K).

What I suggest happened is the same mechanism that forms snowballs started planetary accretion. When snow is squeezed at a temperature a little below its triple point, the pressure causes localised melt fusion, and the particles stick together. In this case we have several ices entrained in the ice/dust, and I suggest the same happens for each ice. This has consequences. The temperature profile in these disks is observed to be where the temperature T is proportional to r^-0.75, r the distance from the star, with a significant variation, which is expected because the faster the gas comes in, or the warmer it was to start with, the further out a specific temperature will be found, while the denser the gas flow, the greater the temperature gradient. Now, because Jupiter is the biggest planet, and water ice is the most common single material, assume (like everyone else) that Jupiter is more or less where the water ice so fuses. If we assume the average disk temperature profile (actually r^-0.82 is better for what follows for our solar system) then the remaining giants are quite close to where they are supposed to be. So the mechanism is that ices come together, they hit, the collisional energy melts an ice in the impact zone such that they rapidly refreeze, and the particles stick together. To predict where the planets should be I put Jupiter at 5.2 A.U. as a water-ice core sets the constant of proportionality. The next ice is ammonia/methanol/water, which could melt between 164 – 195 oK, which suggests that Saturn should be between 7.8 – 9.6 A.U. Saturn has a semimajor axis of 9.5 A.U. The next ice out is methane/argon, with melting between 84 – 90 oK. The calculated position of Uranus is between 20-21.7 A.U., while the observed position is 19.2 A.U. The next ice, carbon monoxide/nitrogen melts between 63 – 68 oK, which predicts Neptune to be between 28.1 – 30.7 A.U., and Neptune has a semimajor axis of 30 A.U. Note that as they form, we excpect some movement through gravitational interactions and the effects of the gas.

This means the Jovian system is both nitrogen and carbon deficient, apart from Jupiter itself which accreted gas from the disk, and the very tenuous atmosphere of Europa is reported to actually have more sodium in it than nitrogen. Sorry, but no life under the ice at Europa because there is nothing much with which to have organic chemistry. The reason for the lack of atmosphere is the satellites have nothing in them that could form a gas at those temperatures. The major component is hydroxyl, from the photochemiclo deomposition of water, but this is extremely reactive and does not build up.

The Saturnian system has water, plus methanol and ammonia. The ammonia has been seen at Enceladus, and its decay product during UV radiation, nitrogen, is the main gas of Titan. The methane there will come from reactions of methanol and rocks. The Uranian system has methane and argon. Unfortunately the satellites are too small to have atmospheres, and Neptune’s satellites are similar, as while Triton has nitrogen volcanoes, it is probably a captured Kuiper Belt object, as it orbits Neptune the wrong way. However the atmosphere of Neptune has more nitrogen than expected from the accretion disk, whereas Uranus does not. More specific details are in the ebook, but in my opinion, the above describes reasonably well how these systems formed, and why they have the chemical composition we see. The Kuiper belt objects are the same as the core of Neptune, and are essentially a “tail” of the accretion process.

Finally, the perceptive will notice the possibility of two further zones of accretion. Further out, there will be a zone where neon trapped in ice might accrete, and even further out, because hydrogen can be trapped in ice even up to about 15 K, a hydrogen accretion zone where the liquid hydrogen dissolves neon, which then refreezes. The latter is not impossible. There are exoplanets a few hundred A.U. from the star, or, say, over ten times further than Neptune. On the other hand, there is a further mechanism that could form them, namely collapse of the disk, which presumably starts the star. So I should be able to predict where this planet 9 is? That is not so easy because the temperature of the disk follows the above relationship only approximately, and when we get down to these low temperatures, any deviation, including the initial temperature of the gas (which in the above relation is taken as zero, but it isn’t) suddenly becomes important. My published estimate for a neon-based planet is at a hundred A.U., with a possible minus 30 and a plus fifty A.U. Not exactly helpful. If I knew thie initial temperature, and the rate of heat loss from the disk by radiation at those distances I could be far more precise.

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