Unexpected Astronomical Discoveries.

This week, three unexpected astronomical discoveries. The first relates to white dwarfs. A star like our sun is argued to eventually run out of hydrogen, at which point its core collapses somewhat and it starts to burn helium, which it converts to carbon and oxygen, and gives off a lot more energy. This is a much more energetic process than burning hydrogen to helium, so although the core contracts, the star itself expands and becomes a red giant. When it runs out of that, it has two choices. If it is big enough, the core contracts further and it burns carbon and oxygen, rather rapidly, and we get a supernova. If it does not have enough mass, it tends to shed its outer matter and the rest collapses to a white dwarf, which glows mainly due to residual heat. It is extremely dense, and if it had the mass of the sun, it would have a volume roughly that of Earth.

Because it does not run fusion reactions, it cannot generate heat, so it will gradually cool, getting dimmer and dimmer, until eventually it becomes a black dwarf. It gets old and it dies. Or at least that was the theory up until very recently. Notice anything wrong with what I have written above?

The key is “runs out”. The problem is that all these fusion reactions occur in the core, but what is going on outside. It takes light formed in the core about 100,000 years to get to the surface. Strictly speaking, that is calculated because nobody has gone to the core of a star to measure it, but the point is made. It takes that long because it keeps running into atoms on the way out, getting absorbed and re-emitted. But if light runs into that many obstacles getting out, why do you think all the hydrogen would work its way to the core? Hydrogen is light, and it would prefer to stay right where it is. So even when a star goes supernova, there is still hydrogen in it. Similarly, when a red giant sheds outer matter and collapses, it does not necessarily shed all its hydrogen.

The relevance? The Hubble space telescope has made another discovery, namely that it has found white dwarfs burning hydrogen on their surfaces. A slightly different version of “forever young”. They need not run out at all because interstellar space, and even intergalactic space, still has vast masses of hydrogen that, while thinly dispersed, can still be gravitationally acquired. The surface of the dwarf, having such mass and so little size, will have an intense gravity to make up for the lack of exterior pressure. It would be interesting to know if they could determine the mechanism of the fusion. I would suspect it mainly involves the CNO cycle. What happens here is that protons (hydrogen nuclei) in sequence enter a nucleus that starts out as ordinary carbon 12 to make the element with one additional proton, which then decays to produce a gamma photon, and sometimes a positron and a neutrino until it gets to nitrogen 15 (having been in oxygen 15) after which if it absorbs a proton it spits out helium 4 and returns to carbon 12. The gamma spectrum (if it is there) should give us a clue.

The second is the discovery of a new Atira asteroid, which orbits the sun every 115 days and has a semi-major axis of 0.46 A.U. The only known object in the solar system with a smaller semimajor axis is Mercury, which orbits the sun in 89 days. Another peculiarity of its orbit is that it can only be seen when it is away from the line of the sun, and as it happens, these times are very difficult to see it from the Northern Hemisphere. It would be interesting to know its composition. Standard theory has it that all the asteroids we see have been dislodged from the asteroid belt, because the planets would have cleaned out any such bodies that were there from the time of the accretion disk. And, of course, we can show that many asteroids were so dislodged, but many does not mean all. The question then is, how reliable is that proposed cleanout? I suspect, not very. The idea is that numerous collisions would give the asteroids an eccentricity that would lead them to eventually collide with a planet, so the fact they are there means they have to be resupplied, and the asteroid belt is the only source. However, I see no reason why some could not have avoided this fate. In my ebook “Planetary Formation and Biogenesis” I argue that the two possibilities would have clear compositional differences, hence my interest. Of course, getting compositional information is easier said than done.

The third “discovery” is awkward. Two posts ago I wrote how the question of the nature of dark energy might not be a question because it may not exist. Well, no sooner had I posted, than someone came up with a claim for a second type of dark energy. The problem is, if the standard model is correct, the Universe should be expanding 5 – 10% faster than it appears to be doing. (Now, some would say that indicates the standard model is not quite right, but that is apparently not an option when we can add in a new type of “dark energy”.) This only applied for the first 300 million years or so, and if true, the Universe has suddenly got younger. While it is usually thought to be 13.8 billion years old, this model has it at 12.4 billion years old. So while the model has “invented” a new dark energy, it has also lost 1.4 billion years in age. I tend to be suspicious of this, especially when even the proposers are not confident of their findings. I shall try to keep you posted.

Thorium as a Nuclear Fuel

Apparently, China is constructing a molten salt nuclear reactor to be powered by thorium, and it should be undergoing trials about now. Being the first of its kind, it is, naturally, a small reactor that will produce 2 megawatt of thermal energy. This is not much, but it is important when scaling up technology not to make too great of leaps because if something in the engineering has to be corrected it is a lot easier if the unit is smaller. Further, while smaller is cheaper, it is also more likely to create fluctuations, especially with temperature, and when smaller they are far easier to control. The problem with a very large reactor is if something is going wrong it takes a long time to find out, but then it also becomes increasingly difficult to do anything about it.

Thorium is a weakly radioactive metal that has little current use. It occurs naturally as thorium-232 and that cannot undergo fission. However, in a reactor it absorbs neutrons and forms thorium-233, which has a half-life of 22 minutes and β-decays to protactinium-233. That has a half-life of 27 days, and then β-decays to uranium-233, which can undergo fission. Uranium-233 has a half-life of 160,000 years so weapons could be made and stored.  

Unfortunately, 1.6 tonne of thorium exposed to neutrons and if appropriate chemical processing were available, is sufficient to make 8 kg of uranium-233, and that is enough to produce a weapon. So thorium itself is not necessarily a form of fuel that is free of weapons production. However, to separate Uranium-233 in a form to make a bomb, major chemical plant is needed, and the separation needs to be done remotely because apparently contamination with Uranium-232 is possible, and its decay products include a powerful gamma emitter. However, to make bomb material, the process has to be aimed directly at that. The reason is, the first step is to separate the protactinium-233 from the thorium, and because of the short half-life, only a small amount of the thorium gets converted. Because a power station will be operating more or less continuously, it should not be practical to use it to make fissile material for bombs.

The idea of a molten salt reactor is that the fissile material is dissolved in a liquid salt in the reactor core. The liquid salt also takes away the heat which, when the salt is cycles through heat exchangers, converts water to steam, and electricity is obtained in the same way as any other thermal station. Indeed, China says it intends to continue using its coal-fired generators by taking away the furnaces and replacing them with a molten salt reactor. Much of the infrastructure would remain. Further, compared with the usual nuclear power stations, the molten salt reactors operate at a higher temperature, which means electricity can be generated more efficiently.

One advantage of a molten salt reactor is it operates at lower pressures, which greatly reduces the potential for explosions. Further, because the fuel is dissolved in the salt you cannot get a meltdown. That does not mean there cannot be problems, but they should be much easier to manage. The great advantage of the molten salt reactor is it burns its reaction products, and an advantage of a thorium reactor is that most of the fission products have shorter half-lives, and since each fission produces about 2.5 neutrons, a molten salt reactor further burns larger isotopes that might be a problem, such as those of neptunium or plutonium formed from further neutron capture. Accordingly, the waste products do not comprise such a potential problem.

The reason we don’t directly engage and make lots of such reactors is there is a lot of development work required. A typical molten salt mix might include lithium fluoride, beryllium fluoride, the thorium tetrafluoride and some uranium tetrafluoride to act as a starter. Now, suppose the thorium or uranium splits and produces, say, a strontium atom and a xenon atom. At this point there are two fluorine atoms as surplus, and fluorine is an extraordinarily corrosive gas. As it happens, xenon is not totally unreactive and it will react with fluorine, but so will the interior of the reactor. Whatever happens in there, it is critical that pumps, etc keep working. Such problems can be solved, but it does take operating time to be sure such problems are solved. Let’s hope they are successful.

The Universe is Shrinking

Dark energy is one of the mysteries of modern science. It is supposed to amount to about 68% of the Universe, yet we have no idea what it is. Its discovery led to Nobel prizes, yet it is now considered possible that it does not even exist. To add or subtract 68% of the Universe seems a little excessive.

One of the early papers (Astrophys. J., 517, pp565-586) supported the concept. What they did was to assume type 1A supernovae always gave out the same light so by measuring the intensity of that light and comparing it with the red shift of the light, which indicates how fast it is going away, they could assess whether the rate of expansion of the universe was even over time. The standard theory at the time was that it was, and it was expanding at a rate given by the Hubble constant (named after Edwin Hubble, who first proposed this). What they did was to examine 42 type 1a supernovae with red shifts between 0.18 and 0.83, and compared their results on a graph with what they expected from the line drawn using the Hubble constant, which is what you expect with zero acceleration, i.e. uniform expansion. Their results at a distance were uniformly above the line, and while there were significant error bars, because instruments were being operated at their extremes, the result looked unambiguous. The far distant ones were going away faster than expected from the nearer ones, and that could only arise if the rate of expansion were accelerating.

For me, there was one fly in the ointment, so to speak. The value of the Hubble constant they used was 63 km/s/Mpc. The modern value is more like 68 or 72; there are two values, and they depend on how you measure them, but both are somewhat larger than this. Now it follows that if you have the speed wrong when you predict how far it travelled, it follows that the further away it is, the bigger the error, which means you think it has speeded up.

Over the last few years there have been questions as to exactly how accurate this determination of acceleration really is. There has been a question (arXiv:1912.04903) that the luminosity of these has evolved as the Universe ages, which has the effect that measuring the distance this way leads to overestimation of the distance. Different work (Milne et al. 2015.  Astrophys. J. 803: 20) showed that there are at least two classes of 1A supernovae, blue and red, and they have different ejecta velocities, and if the usual techniques are used the light intensity of the red ones will be underestimated, which makes them seem further away than they are.

My personal view is there could be a further problem. The type 1A occurs when a large star comes close to another star and begins stripping it of its mass until it gets big enough to ignite the supernova. That is why they are believed to have the same brightness: they ignite their explosion at the same mass so there are the same conditions, so there should be the same brightness. However, this is not necessarily the case because the outer layer, which generates the light we see, comes from the non-exploding star, and will absorb and re-emit energy from the explosion. Hydrogen and helium are poor radiators, but they will absorb energy. Nevertheless, the brightest light might be expected to come from the heavier elements, and the amount of them increases as the Universe ages and atoms are recycled. That too might lead to the appearance that the more distant ones are further away than expected, which in turn suggests the Universe is accelerating its expansion when it isn’t.

Now, to throw the spanner further into the works, Subir Sarkar has added his voice. He is unusual in that he is both an experimentalist and a theoretician, and he has noted that the 1A supernovae, while taken to be “standard candles”, do not all emit the same amount of light, and according to Sarkar, they vary by up to a factor of ten. Further, previously the fundamental data was not available, but in 1915 it became public. He did a statistical analysis and found that the data supported a cosmic acceleration but only with a statistical significance of three standard deviations, which, according to him, “is not worth getting out of bed for”.

There is a further problem. Apparently the Milky Way is heading off in some direction at 600 km/s, and this rather peculiar flow extends out to about a billion light years, and unfortunately most of the supernovae studied so far are in this region. This drops the statistical significance for cosmic expansion to two standard deviations. He then accuses the previous supporters of this cosmic expansion as confirmation bias: the initial workers chose an unfortunate direction to examine, but the subsequent ones “looked under the same lamppost”.

So, a little under 70% of what some claim is out there might not be. That is ugly. Worse, about 27% is supposed to be dark matter, and suppose that did not exist either, and the only reason we think it is there is because our understanding of gravity is wrong on a large scale? The Universe now shrinks to about 5% of what it was. That must be something of a record for the size of a loss.

Asteroid (16) Psyche – Again! Or Riches Evaporate, Again

Thanks to my latest novel “Spoliation”, I have had to take an interest in asteroid mining. I discussed this in a previous post (https://ianmillerblog.wordpress.com/2020/10/28/asteroid-mining/) in which I mentioned the asteroid (16) Psyche. As I wrote, there were statements saying the asteroid had almost unlimited mineral resources. Initially, it was estimated to have a density (g/cc) of about 7, which would make it more or less solid iron. It should be noted this might well be a consequence of extreme confirmation bias. The standard theory has it that certain asteroids differentiated and had iron cores, then collided and the rock was shattered off, leaving the iron cores. Iron meteorites are allegedly the result of collisions between such cores. If so, it has been estimated there have to be about 75 iron cores floating around out there, and since Psyche had a density so close to that of iron (about 7.87) it must be essentially solid iron. As I wrote in that post, “other papers have published values as low as 1.4 g/cm cubed, and the average value is about 3.5 g/cm cubed”. The latest value is 3.78 + 0.34.

These varied numbers show how difficult it is to make these observations. Density is mass per volume. We determine the volume by considering the size and we can measure the “diameter”, but the target is a very long way away, it is small, so it is difficult to get an accurate “diameter”. The next point is it is not a true sphere, so there are extra “bits” of volume with hills, or “bits missing” with craters. Further, the volume depends on a diameter cubed, so if you make a ten percent error in the “diameter” you have a 30% error overall. The mass has to be estimated from its gravitational effects on something else. That means you have to measure the distance to the asteroid, the distance to the other asteroid, and determine the difference from expected as they pass each other. This difference may be quite tiny. Astronomers are working at the very limit of their equipment.

A quick pause for some silicate chemistry. Apart from granitic/felsic rocks, which are aluminosilicates, most silicates come in two classes of general formula: A – olivines X2SiO4 or B – pyroxenes XSiO3, where X is some mix of divalent metals, usually mainly magnesium or iron (hence their name, mafic, the iron being ferrous). However, calcium is often present. Basically, these elements are the most common metals in the output of a supernova, with magnesium being the most. For olivines, if X is only magnesium, the density for A (forsterite) is 3.27 and for B (enstatite) 3.2. If X is only iron, the density for A (fayalite) is 4.39 and for B (ferrosilite) 4.00. Now we come to further confirmation bias: to maintain the iron content of Psyche, the density is compared to enstatite chondrites, and the difference made up with iron. Another way to maintain the concept of “free iron” is the proposition that the asteroid is made of “porous metal”. How do you make that? A porous rock, like pumice, is made by a volcano spitting out magma with water dissolved in it, and as the pressure drops the water turns to steam. However, you do not get any volatile to dissolve in molten iron.

Another reason to support the iron concept was that the reflectance spectrum was “essentially featureless”. The required features come from specific vibrations, and a metal does not have any. Neither does a rough surface that scatters light. The radar albedo (how bright it is with reflected light) is 0.34, which implies a surface density of 3.5, which is argued to indicate either metal with 50% porosity, or solid silicates (rock). It also means no core is predicted. The “featureless spectrum” was claimed to have an absorption at 3 μm, indicating hydroxyl, which indicates silicate. There is also a signal corresponding to an orthopyroxene. The emissivity indicates a metal content greater than 20% at the surface, but if this were metal, there should be a polarised emission, and that is completely absent. At this point, we should look more closely at what “metal” means. In many cases, while it is used to convey what we would consider as a metal, the actual use includes chemical compounds with a  metallic element. The iron levels may be as iron sulphide, the oxide, or, as what I believe the answer is, the silicate. I think we are looking at the iron content of average rock. Fortune does not await us there.

In short, the evidence is somewhat contradictory, in part because we are using spectroscopy at the limits of its usefulness. NASA intends to send a mission to evaluate the asteroid and we should wait for that data.

But what about iron cored asteroids? We know there are metallic iron meteorites so where did they come from? In my ebook “Planetary Formation and Biogenesis”, I note that the iron meteorites, from isotope dating, are amongst the oldest objects in the solar system, so I argue they were made before the planets, and there were a large number of them, most of which ended up in planetary cores. The meteorites we see, if that is correct, never got accreted, and finally struck a major body for the first time.

Food on Mars

Settlers on Mars will have needs, but the most obvious ones are breathing and eating, and both of these are likely to involve plants. Anyone thinking of going to Mars should think about these, and if you look at science fiction the answers vary. Most simply assume everything is taken care of, which is fair enough for a story. Then there is the occasional story with slightly more detail. Andy Weir’s “The Martian” is simple. He grows potatoes. Living on such a diet would be a little spartan, but his hero had no option, being essentially a Robinson Crusoe without a Man Friday. The oxygen seemed to be a given. The potatoes were grown in what seemed to be a pressurised plastic tent and to get water, he catalytically decomposed hydrazine to make hydrogen and then he burnt that. A plastic tent would not work. The UV radiation would first make the tent opaque so the necessary light would not get in very well, then the plastic would degrade. As for making water, burning hydrazine as it was is sufficient, but better still, would they not put their base where there was ice?

I also have a novel (“Red Gold”) where a settlement tries to get started. Its premise is there is a main settlement with fusion reactors and hence have the energy to make anything, but the main hero is “off on his own” and has to make do with less, but can bring things from the main settlement. He builds giant “glass houses” made with layers of zinc-rich glass that shield the inside from UV radiation. Stellar plasma ejections are diverted by a superconducting magnet at the L1 position between Mars and the sun (proposed years before NASA suggested it) and the hero lives in a cave. That would work well for everything except cosmic radiation, but is that going to be that bad? Initially everyone lives on hydroponically grown microalgae, but the domes permit ordinary crops. The plants grow in treated soil, but as another option a roof is put over a minor crater and water provided (with solar heating from space) in which macroalgae grow and marine microalgae, as well as fish and other species, like prawns. The atmosphere is nitrogen, separated from the Martian atmosphere, and some carbon dioxide, and the plants make oxygen. (There would have to be some oxygen to get started, but plants on Earth grew without oxygen initially.)

Since then there have been other quite dramatic proposals from more official sources that assume a lot of automation to begin with. One of the proposals involves constructing huge greenhouses by covering a crater or valley. (Hey, I suggested that!) but the roof is flat and made of plastic, the plastic being made from polyethylene 2,5-furandicarboxylate, a polyester made from carbohydrates grown by the plants. This is used as a bonding agent to make a concrete from Martian rock. (In my novel, I explained why a cement is very necessary, but there are limited uses.) The big greenhouse model has some limitations. In this, the roof is flat, and in essentially two layers, and in between are vertical stacks of algae growing in water. The extra value here is that water filters out the effect of cosmic rays, although you need several meters of it. Now we have a problem. The idea is that underneath this there is a huge habitat, and for every cubic meter of water, we have one tonne mass, and on Mars, about 0.4 tonne of force on the lower flat deck. If this bottom deck is the opaque concrete, then something bound by plastic adhesion will slip. (Our concrete on bridges is only inorganic, and the binding is chemical, not physical, and further there is steel reinforcing.) Below this there would need to be many weight-bearing pillars. And there would need to be light generation between the decks (to get the algae to grow) and down below. Nuclear power would make this easy. Food can be grown as algae in between decks, or in the ground down below.

As I see it, construction of this would take quite an effort and a huge amount of materials. The concept is the plants could be grown to make the cement to make the habitat, but hold on, where are the initial plants going to grow, and who/what does all the chemical processing? The plan is to have that in place from robots before anyone gets there but I think that is greatly overambitious. In “Red Gold” I had the glass made from regolith processed with the fusion energy. The advantage of glass over this new suggestion is weight; even on Mars with its lower gravity millions of tonnes remains a serious weight. The first people there will have to live somewhat more simply.

Another plan that I have seen involves finding a frozen lake in a crater, and excavating an “under-ice” habitat. No shortage of water, or screening from cosmic rays, but a problem as I see it is said ice will melt from the heat, erode the bottom of the sheet, and eventually it will collapse. Undesirable, that is.

All of these “official” options use artificial lighting. Assuming a nuclear reactor, that is not a problem in itself, although it would be for the settlement under the ice because heat control would be a problem. However, there is more to getting light than generating energy. What gives off the light, and what happens when its lifetime expires? Do you have to have a huge number of spares? Can they be made on Mars?

There is also the problem with heat. In my novel I solved this with mirrors in space focussing more sunlight on selected spots, and of course this provides light to help plants grow, but if you are going to heat from fission power a whole lot more electrical equipment is needed. Many more things to go wrong, and when it could take two years to get a replacement delivered, complicated is what you do not want. It is not going to be that easy.