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.

Asteroid Mining

One thing you see often in the media is the concept that perhaps in the future we can solve our resources problem by mining asteroids. Hopefully, that is fine for science fiction, and I use that word “hopefully” because my next piece of science fiction, currently in the editing mode, includes collecting asteroids for minerals extraction. However, what is the reality?

We know we have a resource problem. An unfortunately large and growing number of elements are becoming scarcer and harder to obtain. As a consequence, ores are getting less concentrated, and so much material has to be thrown away. As an example, the earliest use of copper at around 7,000 BC used native copper. All the people had to do was take a piece and hammer it into some desirable shape. Some time later someone found that if something like malachite was accidentally in a fireplace, it got reduced to copper, and metallurgy was founded. Malachite is 57.7% copper, while if you were lucky enough to find cuprite you got a yield of almost 89% copper. Now the average yield of copper from a copper ore is 0.6% and falling. The rest is usually useless silicates. So, you may think, if we have worked through all the easily available stuff here, nobody has worked through the asteroids. There we could get “the good stuff”.

At this point it is worth contemplating what an ore is and where it came from? All the elements heavier than lithium were made in supernovae or through collisions of neutron stars. Either way, if we think of the supernova, the elements are made at an extremely high temperature, and they are flying away from the stellar core at a very high velocity. The net result is they end up as particles that make the particles in smoke look big. This “smoke” gets mixed in with gas clouds that end up making stars and planets. To get some perspective on concentrations, for every million silicon atoms you will get, on average, about 900,000 iron atoms, almost 24,000,000 oxygen atoms, 5420 chlorine atoms, 52,700 sodium atoms, 522 copper atoms, almost half a silver atom, 0.187 gold atoms, 1.34 platinum atoms and about 0.009 uranium atoms.

So what happens depends on whether the elements react in the accretion disk, so that molecules form. For example, all the sodium atoms will either form a chloride or a hydroxide, but the gold atoms will by and large not react. About half the iron atoms form an oxide or stay as the element, and the oxides will end up as silicates (basalt). What happens next depends on how the objects accrete. That is not agreed. Most scientists say they simply don’t know. I believe the bodies are accreted through chemistry. If the former, we have to assume the elements end up as a mix that have those elements in proportion, except for those that make gases. If the latter, then some will be more concentrated than others.

On earth, elements are concentrated into ores by geochemistry. The heat and water processes some elements, and heat and volcanism concentrates others. Thus gold is concentrated by it dissolving in supercritical water, together with silica, which is why you often find gold in quartz veins. The relevance to asteroids is that processing does not happen in most because they are not big enough to generate the required heat. The relevance now is that the elements you want will either be bound up with silicates, or be scattered randomly through the bulk. To get the metals out, you have to get rid of the silicates, and if you look at the figures, the copper content is actually less than in our ores on earth. Now look at the mining wastes on Earth, and ask yourself what would you do with that in space? (There is an answer – build space stations with rocky shells.)

So why do we think of mining asteroids. One reason comes from asteroid Psyche. One scientific paper once claimed asteroid had a density as high as 7.6 g/cm cubed. That would clearly be worth mining, because the iron would also dissolve nickel, cobalt, platinum, gold, etc. You will various news items that wax on about how this asteroid alone would solve our problems and make everyon extremely rich. However, other papers have published values as low as 1.4 g/cm cubed, and the average value is about 3.5 g/cm cubed (which is what it would be if it were solid basalt). 

Why the differences? Basically because density depends on the mass (determined by gravitational interactions) and volume.  The uncertainty in the volume, thanks to observational uncertainty due to the asteroid being so far away and the fact it is not round, can give an error of up to 50%. The mass requires very accurate measurements when near something else and again huge errors are possible.

So the question then is, if someone wants to get metals out of asteroids, how will they do it? If the elements are there as oxides or sulphides, what do you do about that? On Earth you heat with coal and air, followed by coal. You cannot do that in space. On Earth, minerals can be concentrated by various means that use liquids, such as froth flotation, but you cannot do that easily in space because first liquids like water are scarce, and second, if you have them, unless they are totally enclosed they boil off into space. Flotation requires “gravity”, which requires a centrifuge. Possible, but very expensive,If you were building a giant space station, yes, asteroids would be valuable because the cost of getting components from Earth is huge, but we still need technology to refine them. Otherwise the cost of getting the materials to Earth would be horrifying. Be careful if you see an investment offering.