The Ice Giants’ Magnetism

One interesting measurement made from NASA’S sole flyby of Uranus and Neptune is that they have complicated magnetic fields, and seemingly not the simple dipolar field as found on Earth. The puzzle then is, what causes this? One possible answer is ice.

You will probably consider ice as not particularly magnetic nor particularly good at conducting electric current, and you would be right with the ice you usually see. However, there is more than one form of ice. As far back as 1912, the American physicist Percy Bridgman discovered five solid phases of water, which were obtained by applying pressure to the ice. One of the unusual properties of ice is that as you add pressure, the ice melts because the triple point (the temperature where solid, liquid and gas are in equilibrium) is at a lower temperature than the melting point of ice at room pressure (which is 0.1 MPa. A pascal is a rather small unit of pressure; the M mean million, G would mean billion). So add pressure and it melts, which is why ice skates work. Ices II, III and V need 200 to 600 MPa of pressure to form. Interestingly, as you increase the pressure, Ice III forms at about 200 Mpa, and at about -22 degrees C, but then the melting point rises with extra pressure, and at 350 MPa, it switches to Ice V, which melts at – 18 degrees C, and if the pressure is increased to 632.4 MPa, the melting point is 0.16 degrees C. At 2,100 MPa, ice VI melts at just under 82 degrees C. Skates don’t work on these higher ices. As an aside, Ice II does not exist in the presence of liquid, and I have no idea what happened to Ice IV, but my guess is it was a mistake.

As you increase the pressure on ice VI the melting point increases, and sooner or later you expect perhaps another phase, or even more. Well, there are more, so let me jump to the latest: ice XVIII. The Lawrence Livermore National Laboratory has produced this by compressing water to 100 to 400 GPa (1 to 4 million times atmospheric pressure) at temperatures of 2,000 to 3,000 degrees K (0 degrees centigrade is about 273 degrees K, and the scale is the same) to produce what they call superionic ice. What happens is the protons from the hydroxyl groups of water become free and they can diffuse through the empty sites of the oxygen lattice, with the result that the ice starts to conduct electricity almost as well as a metal, but instead of moving electrons around, as happens in metals, it is assumed that it is the protons that move.

These temperatures and pressures were reached by placing a very thin layer of water between two diamond disks, following which six very high power lasers generated a sequence of shock waves that heated and pressurised the water. They deduced what they got by firing 16 additional high powered lasers that delivered 8 kJ of energy in a  one-nanosecond burst on a tiny spot on a small piece of iron foil two centimeters away from the water a few billionths of a second after the shock waves. This generated Xrays, and from the way they diffracted off the water sample they could work out what they generated. This in itself is difficult enough because they would also get a pattern from the diamond, which they would have to subtract.

The important point is that this ice conducts electricity, and is a possible source of the magnetic fields of Uranus and Neptune, which are rather odd. For Earth, Jupiter and Saturn, the magnetic poles are reasonably close to the rotational poles, and we think the magnetism arises from electrically conducting liquids rotating with the planet’s rotation. But Uranus and Neptune have quite odd magnetic fields. The field for Uranus is aligned at 60 degrees to the rotational axis, while that for Neptune is aligned at 46 degrees to the rotational axis. But even odder, the axes of the magnetic fields of each do not go through the centre of the planet, and are displaced quite significantly from it.

The structure of these planets is believed to be, from outside inwards, first an atmosphere of hydrogen and helium, then a mantle of water, ammonia and methane ices, then interior to that a core of rock. My personal view is that there will also be carbon monoxide and nitrogen ices in the mantle, at least of Neptune. The usual explanation for the magnetism has been that magnetic fields are generated by local events in the icy mantles, and you see comments that the fields may be due to high concentrations of ammonia, which readily forms charged species. Such charges would produce magnetic fields due to the rapid rotation of the planets. This new ice is an additional possibility, and it is not beyond the realms of possibility that it might contribute to the other giants.

Jupiter is found from our spectroscopic analyses to be rather deficient in oxygen, and this is explained as being due to the water condensing out as ice. The fact that these ices form at such high temperatures is a good reason to believe there may be such layers of ice. This superionic ice is stable as a solid at 3000 degrees K, and that upper figure simply represents the highest temperature the equipment could stand. (Since water reacts with carbon, I am surprised it got that high.) So if there were a layer of such ice around Jupiter’s core, it too might contribute to the magnetism. Whatever else Jupiter lacks down there, pressure is not one of them.

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Marsquakes

One of the more interesting aspects of the latest NASA landing on Mars is that the rover has dug into the surface, inserted a seismometer, and is looking for marsquakes. On Earth, earthquakes are fairly common, especially where I live, and they are generated through the fact that our continents are gigantic lumps of rock moving around over the mantle. They can slide past each other or pull themselves down under another plate, to disappear deep into the mantle, while at other places, new rock emerges to take their place, such as at the mid-Atlantic ridge. Apparently the edges of these plates move about 5 – 10 cm each year. You probably do not notice this because the topsoil, by and large, does not move with the underlying crust. However, every now and again these plates lock and stop moving there. The problem is, the rest of the rock is moving, so considerable strain energy is built up, the lock gives way, very large amounts of energy are released, and the rock moves, sometimes be several meters. The energy is given out as waves, similar in many ways as sound waves, through the rock. If you see waves in the sea, you will note that while the water itself stays more or less in the same place on average, in detail something on the surface, like a surfer, goes up and down, and in fact describes what is essentially a circle if far enough out. Earthquake waves do the same thing. The rock moves, and the shaking can be quite violent. Of course, the rock moves where the actual event occurred, and sometimes the waves trigger a further shift somewhere else.

Such waves travel out in all directions through the rock. Now another feature of all waves is that when they strike a medium through which they will travel with a different velocity, they undergo partial reflection and refraction. There is an angle of incidence when only reflection occurs, and of course, on a curved surface, the reflected waves start spreading as the angles of incidence vary. A second point is that the bigger the difference in wave speed between the two media, the more reflection there is. On Earth, this has permitted us to gather information on what is going on inside the Earth. Of course Earth has some big advantages. We can record seismic events from a number of different places, and even then the results are difficult to interpret.

The problem for Mars is there will be one seismometer that will measure wave frequency, amplitude, and the timing. The timing will give a good picture of the route taken by various waves. Thus the wave that is reflected off the core will come back much sooner than the wave that travels light through and is reflected off the other side, but it will have the same frequency pattern on arrival, so from such patterns and timing you can sort out, at least in principle, what route they took and from the reflection/refraction intensities, what different materials they passed through. It is like a CT scan of the planet. There are further complications because wave interference can spoil patterns, but waves are interesting that they only create that effect at the site where they interfere. Otherwise, they pass right through other waves and are unchanged when they emerge, apart from intensity changes if energy was absorbed by the medium. There is an obvious problem in that with only one seismometer it is much harder to work out where the source was but the scientists believe over the lifetime of the rover they will detect at least a couple of dozen quakes.

Which gets to the question, why do we expect quakes? Mars does not have plate tectonics, possibly because its high level of iron oxide means eclogite cannot form, and it is thought that the unusually high density of eclogite leads to pull subduction. Accordingly the absence of plate tectonics means we expect marsquakes to be of rather low amplitude. However, minor amplitude quakes are expected. One reason is that as the planet cools, there is contraction in volume. Accordingly, the crust becomes less well supported and tends to slip. A second cause could be magma moving below the surface. We know that Mars has a hot interior, thanks to nuclear decay going on inside, and while Mars will be cooler than Earth, the centre is thought to be only about 200 Centigrade degrees cooler than Earth’s centre. While Earth generates more heat, it also loses more through geothermal emissions. Finally, when meteors strike, they also generate shockwaves. Of course the amplitude of these waves is tiny compared with that of even modest earthquakes.

It is hard to know what we shall learn. The reliance on only one seismometer means the loss of directional analysis, and the origin of the quake will be unknown, unless it is possible to time reflections from various places. Thus if you get one isolated event, every wave that comes must have originated from that source, so from the various delays, paths can be assigned. The problem with this is that low energy events might not generate enough reflections of sufficient amplitude to be detected. The ideal method, of course, is to set off some very large explosions at known sites, but it is rather difficult to do that from here.

What do we expect? This is a bit of guesswork, but for me we believe the crust is fairly thick, so we would expect about 60 km of solid basalt. If we get significantly different amounts, this would mean we would have to adjust our thoughts on the Martian thermonuclear reactions. I expect a rather tiny (for a planet) iron core, the clue here being the overall density of Mars is 3.8, its surface is made of basalt, and basalt has a density of 3.1 – 3.8. There just is not room for a lot of iron in the form of the metal. It is what is in between that is of interest. Comments from some of the scientists say they think they will get clues on planetary formation, which could come from deep structures. Thus if planets really formed from the combination of planetesimals, which are objects of asteroid, size, then maybe we shall see the remains in the form of large objects of different sonic impedance. On the other hand, the major shocks to the system by events such as the Hellas impactor may mean that asymmetries were introduced by such shock waves melting parts. My guess is the observations will not be unambiguous in terms of their meaning, and it will be interesting to see how many different scenarios are considered.

More on M 87

In my post regarding the M 87 black hole, I stated that you do not necessarily know what the black hole and its environs look like from that image. The popular image that seems to have taken over the web is just plain misleading because it focuses on radio waves very close to the event horizon. Katherine Bouma, who developed the algorithm that made the picture possible, explained that there were “an infinite number of possible images” that could explain the data, in part due to atmospheric disturbances of different kinds at widely dispersed sites, while the difficulty in getting good data intensity meant that there was an inherent uncertainty. On the other hand, the hole and the ring were valid. However, there is more that the black hole effects.

There is another image obtained from the Chandra X-ray telescope of the environs of the M 87 black hole You will see somewhere near the middle a small black dot that on closer investigation appears to be shaped like a cross. X marks the spot! This is exactly the case. The black hole, the radius of which is almost five times the distance from the sun to Pluto, is too small to register on this image. And whereas the first image was based on radio waves, this is recording x-rays.

Chandra2

As you can see, the effect covers a monstrous volume.

Why Xrays? This is because of the huge amount of energy being lost during a radiation event. To illustrate, a red dwarf star largely emits infrared radiation, and only a relatively low fraction is in the visible wavelength electromagnetic spectrum. Thus Proxima Centauri, which has a surface temperature of about 3,000 degrees K, puts out 85% of its emissions in the infrared spectrum (although flares do contribute some xrays) and its visible emissions are only 0.0056% as luminous as the sun (which has a surface temperature of about 5772 degrees K. Now, consider that five times the distance from the sun to Pluto does not really register on that image, this will give some indication of the power of the black hole. Those xrays are generated from the energy  given off by dust heating as they fall into the regions of the black hole and give off the extraordinary energy of an xray in the spectrum.

The dust accelerates, gets hotter than the surface of most stars as a consequance of collisions, and eventually becomes a plasma that is orbiting the black hole at relativistic speeds.  As an aside, this shows that the science fiction plots of a space ship suddenly coming across a lurking black hole is somewhat improbable. Observation should give good clues well in advance as the size of the Xray emissions around this one is comparable to the distance of most of the farthest stars we can see with the naked eye.

One of the most bizarre things about black holes is that sometimes you can see a jet of material travelling at relativistic speeds away from the polar regions of the black hole. The jets show only a few degrees of dispersion and these can travel up to millions of parsecs. One parsec is about 3.26 light years, so the nearest star to us, Proxima Centauri, is about 1.3 parsecs (4.2 light years) from the sun. The jets can leave a galaxy, and there are images from Hubble where the length of the jet dwarfs the galaxy from which it came. On the other hand, the black hole at the centre of our galaxy does not currently generate such a jet. In the Chandra X-ray image you can see the jet coming out of the M 87 system. (There will be another on exactly the other side of the system.)

This is one of the most powerful events that occur in the Universe, and while we have a qualitative idea of some of what might be powering it, the details are still to be worked out. One theory is that as the plasma spirals into the black hole, the magnetic field gets compressed and rotates, and this magnetic field then exerts a force on the plasma, with the net result that matter is ejected in a jet from the inner regions around the black hole. There is at least one alternative theory. Roger Penrose has proposed that the rotating black hole causes “frame dragging”, which can extract relativistic energies and momentum, essentially from the spin of the black hole. Of course the material being ejected did not come from the black hole, but rather some of the material falling into the black hole is caught before it reaches it and from whatever the cause is ejected at these fantastic energies. The net result is that energy and angular momentum are removed, which also aids the black hole to accrete more mass.

Which raises the question, why does the black hole at the centre of our galaxy not do this? The simple answer may well be that there is no matter close enough to fall in, and what is nearby is in stars that are stable and in a stable orbit. There is a lot we do not know about black holes, and we are unlikely to in the near future.

The M 87 Black Hole

By now, unless you have been living under a flat rock somewhere, you have probably seen an image of a black hole. This image seems to be in just about as many media outlets as possible, so you know what the black hole and its environs look like, right? Not necessarily. But, you say, you have seen a photograph. Well, actually, no you haven’t. That system is so far away that to get the necessary resolution you need to gather light over a very wide array, so the image was obtained from a very large number of radio telescopes and the image was reconstructed by a sequence of mathematical processes. Nevertheless, the black sphere and the ring will represent fairly accurately part of what is there.

No radiation can escape from a black hole so the black bit in the middle is fair, however the image as presented gives no idea of its size. Its radius is about 19 billion km, which is a little under five times the distance from the sun to Pluto. This is really a monster. Ever wondered what happens to photons that are emitted at right angles to the gravitational field? Well, at 28 billion km or thereabouts they go into orbit around the black hole and would do that for an infinite time unless they get absorbed by dust falling in. The bright stuff you see is outside the rotating photons, and is travelling clockwise at about half light speed.

The light is obviously not orange and the signals were received as radio waves, but when emitted they would be extremely high energy photons. We see them as radio waves because they have lost that much energy climbing out of the black hole’s gravitational field. One way of looking at this is to think of light as a wave. The more energy the light has, the greater the frequency of wave crests passing by. As the energy lowers due to gravity lowering the energy of the light, the wave gets “stretched” and the number of crests passing by lowers. At the edge of the black hole the wave is so stretched it takes an infinite amount of time for a second crest to appear, which means no light can escape. Just outside the event horizon the gravity is not quite strong enough to stop it, but a gamma ray wave might take 100,000 of our years for the next crest to pass when it gets to us. The wave is moving but it is so red-shifted we could not see it. Further away from the event horizon the light is a little less stretched, so we see it as radio waves, which is what we were looking at in this image, even if it still started as gamma rays or Xrays.

It has amused me to see the hagiolatry bestowed upon Einstein regarding this image. One quote: “Albert Einstein’s towering genius is on display yet again.” As a comment, I am NOT trying to run down Einstein, but let us be consistent here. You may note Newton also predicted a mass at which light could not escape. In Newtonian mechanics the energy of the light would be given by E =mc^2/2, while the gravitational potential energy would be GMm/R. This permits us to calculate a radius where light cannot escape as R = 2GM/c^2, which happens to be exactly the same as the Schwarzchild radius from General Relativity.

Then we see statements such as “General relativity describes gravity as a consequence of the warping of space-time.” Yes, but that implies something that should not be there. General Relativity is a geometric theory, and describes the dynamics of particles in geometric terms. The phrase “as a consequence of” should be replaced with “in terms of”. The use of “consequence” implies cause, and this leads to statements involving cosmic fabric being bent, and you get images of something like a trampoline sheet, which is at best misleading. Here is another quote that annoys me: “Massive objects create a sort of dent or well in the cosmic fabric, which passing bodies fall into because they’re following curved contours (not as a result of some mysterious force at a distance, which had been the prevailing view before Einstein came along)”. No! Both theories are done a great disservice. Einstein gave a geometric description of how bodies move, but there is no physical cause, and it has the same problem, only deeper, than the Newtonian description had, because you must then ask, how does one piece of space-time know exactly how much to distort? Meanwhile, Newton gave a description of the dynamics of particles essentially in terms of calculus. Whereas Einstein describes effects in terms of a number of tensors, which most people do not understand, Newton invented the term “force”.

Now you will often see the argument that light is bent around the sun and that “proves” General Relativity is correct. Actually, Newtonian physics predicted  the same effect, but general Relativity bends it twice as much as predicted by Newtonian physics, so yes, in that sense General relativity is correct if the bend is correctly found to be twice that of Newton. You will then see statements along the lines this proves the bent path is “due to the warping of spacetime”. That is, of course, nonsense. The reason is that in Einstein’s relativity E = mc^2, which is twice that of the Newtonian energy, as you can see from the above. The reason for the difference appears to be the cosmic speed limit of light speed, which Newton may or may not have considered, but had no reason to go further. Why do I say Newton might have considered it? Because as a postulate, the fundamental nature of the speed of light goes all the way back to Empedocles. Of course, he did not make much of it.

Finally, I saw one statement that “the circular nature of the black hole again confirms the correctness of Einstein’s theory of General Relativity. Actually, Aristotle provided one of the first recorded reasons why gravity leads to a sphere. Newton would certainly have predicted a basic sphere, and of course the algorithm used to make the image would not have led to any other result unless there were something really dramatically non spherical. The above is not intended to downplay Einstein, but I am not a fan of the hagiolatry that accompanies him either.

Asteroid (101955) Bennu

The results of the OSIRIS-REx probe have now started to be made public, and while this probe was launched to answer questions about carbonaceous asteroids, and while some information has been obtained that is most certainly interesting, what it has mainly done, in my opinion, is to raise more questions. As is often the case with scientific experiments and observations.

Bennu is a carbonaceous asteroid with a semimajor axis of about 1.26 AU, where 1 AU is the Earth-Sun distance. Its eccentricity is 0.2, which means it is Earth-crossing and could collide with Earth. According to Wikipedia, it has a 1 in 2700 chance of impacting Earth between 2175 – 2199. I guess I shall never know, but it would be a threat. It has a diameter of approximately 500 meters, and a mass of somewhere in the vicinity of 7 x 10^10 kg, which means an impact would be extremely damaging near where it struck, but it would not be an extinction event. (The Chicxulub impactor would have been between five to seven orders of magnitude bigger.) So, what do we know about it?

It is described as a rubble pile, although what that means varies in terms of who says it. It is generally not considered to be an original accretion, and it is usually assumed to have formed inside a much larger planetoid which provided heat and pressure to form more complex minerals. Exactly why they are so sure of this is a puzzle to me, because we do not know what the minerals are, and how they are bound into the asteroid. Carbonaceous asteroids usually are found in the outer asteroid belt, and the assumption is this was dislodged inwards as a result of the collision that formed it. Standard theory assumes there were such collisions, but it also assumes such collisions led to planetary formation, and the rather awkward fact that there are no planets in the asteroid belt tends to be overlooked. These collisions are doing a lot of work, first making protoplanets then planets, and second, smashing up protoplanets to make asteroids, with no explanation why two different results arise other than “we need two different results”. Note that the collision velocities in the asteroid belt would be much milder than for the rocky planets, so smashing is more likely the closer to the star. Its relevance to planetary formation may be low since it did not form a planet, and there are no planets that have compositions that could realistically be considered to have come from such a chemical composition.

It is often said that Earth was bombarded with carbonaceous chondrites early on, and that is where the reduced carbon and nitrogen came from to sustain life, as well as the amino acids and nucleobases used to create life. Additionally, it is asserted that the iron and a number of other metals that dissolve in iron that we have on the surface must have come from asteroids, the reason being that in the early formation of Earth, the whole was a mass of boiling silicates in which such metals would dissolve in iron and go to the core. That we have them means something else must have brought them later. This shows one of the major faults of science, in my opinion. Rather than take the observation as a reason to go back and question whether the boiling silicates might be wrong, they introduce a further variable. Unfortunately, this “late veneer”  is misleading because the advocates have refused to accept that we have fragments of asteroids as meteorites. Their isotopes show they could only have contributed the right amount of metals, etc, if they were emulsified in all of Earth’s silicates. But wait. Why would these be emulsified and not go to the core while the original metals were not emulsified and did go to the core?

These asteroids are also believed by many to be the origin of life. They have very small amounts of amino acids and nucleobases, but they have a much wider range of amino acids than are used by our life. If they were the source, why did we not use them? Even more convincing, the nitrogen in the meteorite fragments has more 15N than Earth’s nitrogen. Ours is of solar composition; the asteroids apparently processed it. There is no way to reduce the level of heavy isotopes so these asteroids cannot be the source.

Now, what does a rubble pile conjure up in your mind? I originally considered it to be, well, a pile of rubble, loosely adhering, but Bennu cannot be that. First, consider the escape velocity, which is more than 20 cm/sec in the polar regions but reduces down to 10 cm/sec at the equator, due to the centrifugal force of its rotation. That is not much, and anything loose would be lost in any impact. Yet the surface is littered with boulders, three more than 40 m long. Any significant shock would seemingly dislodge such boulders, especially smaller ones, but there they are, some half buried. There are also impact craters, some up to 150 meters in diameter. Whatever hit it to create that and excavate a hole 150 m in diameter must have delivered a shock wave that should impart more than 10 cm s−1 to a loosely lying boulder, although there is one possible exception, which is when the whole structure was sufficiently flexible to give without fragmenting and absorb the energy by converting it to heat while adding to the kinetic energy of the whole.

Which brings us back to the rubble pile. Bennu’s relative density is 1.19, so if placed in water it would not float, but it would not sink very quickly either. For comparison, it is less than half that of granite and about a third of many basalts. CI asteroidal material has a bulk density of 1.57, while CM asteroidal material has a bulk density of 2.2.  Accordingly, it is concluded that Bennu has a lot of voids in it, which is where the concept of the rubble pile comes to bear. On the other hand, there is considerable stiffness, so something is restricting movement.

So what do we not know about this asteroid? First, we have only a modest idea of what it is made of, although a sample return might be possible. It may well be made entirely of large boulders plus the obvious voids put together with something sticking the boulders together, but what is the something? If made of boulders, what are the boulders made of? It never got hot enough out there to melt silicates, so whatever they are must b held together by some agent, but what? How resilient is that something, and how many times can it be used before it fails? This is important in case we decide it would be desirable to alter its orbit to avoid a collision with Earth. What holds the boulders together? This is important if we want to know how planets form, and whether such an asteroid will be useful in any way. (If, for example, we were to build a giant space station, the nitrogen, organic material and water in such an asteroid would be invaluable.) More to do to unravel this mystery.

Processing Minerals in Space

I have seen some recent items on the web that state that asteroids are full of minerals and fortunes await. My warning is, look deeper. The reason is, most asteroids have impact craters, and from basic physics but some rather difficult calculations you can show these were formed from very energetic collisions. That the asteroid did not fly to bits indicates it is a solid with considerable mechanical strength. That implies the original dust either melted to form a solid, or a significant chemical reaction took place. For those who have read my “Planetary Formation and Biogenesis” you will know why they melted, assuming I am right. So what has that got to do with things? Quite simply, leaving aside metals like gold, the metal oxides in molten silica form the olivine or pyroxene families, or aluminosilicates. That is they form rocks. To give an example of the issue, I recently read a paper where various chondrites were analysed, and the method of analysis recorded the elements separately. The authors were making much of the fact that the chondrites contained 19% iron. Yikes! But wait. Fayalite contains almost 55% iron by weight, but it is useless as an ore. The olivine and pyroxene structures have tetrahedral silicon oxides (the pyroxene as a strand polymer) where the other valence of the oxygen is bound to a divalent cation, mostly magnesium because magnesium is the most common divalent element in the supernova dust. What these authors had done was to analyse rock.

If you read my previous post you will see that I have uncovered yet another problem with science: the authors were very specialized but they went outside their sphere of competency, quite accidentally. They cited numbers because so much in science depends on numbers. But it is also imperative to know what the numbers mean.

On Earth, most of the metals we obtain come from ores, which have formed through various forms of geochemical processing. Thus to get iron, we usually process haematite, which is an iron oxide, but the iron almost certainly started as an average piece of basalt that got weathered. It is most unlikely that good deposits of haematite will be found on asteroids, although it is possible on Mars where small amounts have been found. If Mars is to be settled, processing rocks will be mandatory for survival but the problems are different from those of asteroids. For this post, I wish to restrict myself to discussing asteroids as a source of metals. Let us suppose an asteroid is collected and brought to a processing site, the question is, what next?

The first problem is size-reduction, i.e.breaking it down to more manageable pieces. How do you do that? If you hit it with something, you immediately separate, following Newton’s third law. If you want to see the difficulties, stand on a small raft and try to keep on hitting something. Ah, you say, anchor yourself. How? You have to put something like a piton into solid rock, and how do you do that without some sort of impact? Of course it can be done, but it is not easy. Now you start smashing it. What happens next is bits of asteroid fly off into space. Can you collect all of the pieces? If not, you are a menace because the asteroid’s velocity v, which will be in the vicinity of 30 km/s if near Earth, has to be added to whatever is given to the fragments. Worse, they take on the asteroid’s eccentricity ε(how much difference there is between closest and farthest distance from the sun) and whatever eccentricity has been added by the fragmentation. This is important because the relative velocity of impact assuming the target is on a circular orbit is proportional to εv. Getting hit by a rock at these sort of velocities is no joke.

However, suppose you collect all the rock, you have two choices: you can process the rock as is, or you can try to refine it. If you adopt the latter idea, how do you do it? On Earth, such processing arises through millions of years of action with fluids, or through superheated fluids passing through high temperature rock. That does not sound attractive. Now some asteroids are argued to have iron cores so the geochemical processing has been done for you. Of course you still have to work your way through the rock, and then you have to size reduce the iron, which again raises the question, how? There is also a little less good news awaiting you: iron cores are almost certainly not pure iron. The most likely composition is iron with iron silicide, iron phosphide, iron carbide and a lot of iron sulphide. There will also be some nickel, together with corresponding compounds, and (at last joy?) certain high value metals that dissolve in iron. So what do you do with this mess?

Then, supposing you separate out a pure chemical compound, how do you get the metal out? The energy input required can be very large. Currently, there is a lot of effort being put into removing CO2from the atmosphere. The reason we do not pull it apart and dump the carbon is that all the energy liberated from burning it has to be replaced, i.e.a little under 400 kJ/mol. and that is such a lot of energy. Consider that as a reference unit. It takes roughly two such units to get iron from iron oxide, although you do get two iron atoms. It takes about five units to break forsterite into two magnesium atoms and one silicon. It takes ten such units to break down kaolinite to get two aluminium atoms and two silicon atoms. Breaking down rock is very energy intensive.

People say, electrolysis. The problem with electrolysis is the material has to dissolve in some sort of solvent and then be separated into ions. Thus when making aluminium, bauxite, an aluminium oxide is used. Clays, which are aluminosilicates such as kaolinite or montmorillinite, are not used, despite being much cheaper and more easily obtained. In asteroids any aluminium will almost certainly be in far more complicated aluminosilicates. Then there is the problem of finding a solvent for electrolysis. For the least active metals, such as copper, water is fine, but that will not work for the more active ones, such as aluminium. Titanium would be even more difficult to make, as it is made from the reduction of titanium tetrachloride with magnesium. You have to make all the starting materials!

On Earth, many oxides are reduced to metal by heating with carbon (usually very pure coal) and allow the carbon to take the oxygen and disappear as a gas. The problem with that, in space, is there is no readily available source of suitable carbon. Carbonaceous chondrites have quite complicated molecules. The ancients used charcoal, and while this is NOT pure carbon, it is satisfactory because the only other element there in volume tends to be oxygen. (Most charcoal is about 35% oxygen.) The iron in meteors could certainly be useful, but for some other valuable elements, such as platinum, while it may be there as the element, it will probably be scattered through the matrix and be very dilute.

Undoubtedly there will be ways to isolate such elements, but such methods will probably be somewhat different from what we use. In some of my novels I have had fusion power tear the molecules to atoms, ionise them, and separate out the elements in a similar way to how a mass spectrometer works, that is they are accelerated and then bent with powerful electromagnetic fields. The “bend” in the subsequent trajectory depends on the mass of the ions, so each isotope is separated. Yes, that is fiction, but whatever is used would probably seem like fiction now. Care should be taken with any investment!

Space Mining

Most readers will have heard that there are a number of proposals to go mine asteroids, or maybe Mars. The implication is that Earth will become short of resources, so we can mine things in space. However, if we mine there for the benefit here, how would we get such resources here, and in what form. If the resources are refined elsewhere, then there is the “simple” cost of getting them here. If we bring them down in a shuttle, we have to get the shuttle back up there, and the cost is huge. If on the other hand, we drop them (and gravity is cheap) we have to stop whatever we send from burning up in the atmosphere, so to control the system we have to build some sort of spacecraft out there to bring them down. Overall, this is unlikely to be profitable. On the other hand if we build structures in space, such as space stations, or on Mars for settlers, then obviously it is very much cheaper to use local resources, if we can refine them there.

So, what are the local resources? The answer is it depends on the history. All the solid elements are expelled in novae (light elements only) or supernovae (all). The very light elements lithium, beryllium and boron are rather rare because they tend to be destroyed in the star before the explosion. The elements vary in relative amounts made, and basically the heavier the element the less is made, and elements with an even number of protons are more common than elements with odd numbers. Iron, and to some extent nickel, are more common than those around them because the nuclei are particularly stable. The most common elements are magnesium, silicon and with iron about 10% less. Sulphur is about half as common, calcium and aluminium are about 6 – 8% as common as silicon, while the metals such as copper and zinc are about 100,000 times less common than aluminium. The message from all that is that unless there is some process that has sorted the various elements, an object in space is likely to have the composition of dust, which are mainly silicates, i.e. rock. There may well be metal sulphides as well, as there is a lot of sulphur there.

So what sorting could there be? The most obvious is that if the body formed close enough to the star during primary accretion, the heat in the accretion disk could be sufficient to melt the element, if it were there as an element. It appears that iron was, because we get iron meteorites and iron-cored meteorites. The accretion disk, of course, was primarily hydrogen, and at the melting point of iron, hydrogen will reduce iron oxides to iron, also making water. So we could expect asteroids to have iron cores? Well, we are sure most members of the asteroid belt do not, and the reason why not is presumably it did not get hot enough to melt iron where they formed. However, since the regolith (fine “soil”) on the Moon has iron dust in it, perhaps there was iron dust where the asteroids formed. However, the problem is what caused them to solidify. If they melted, steam would be created, and that would oxidise iron dust, so the iron then would be as an oxide, or a silicate.

The ores we have on Earth are there due to geochemical processing. For example, in the mantle, water forms a supercritical fluid that dissolves all sorts of things, including silica and gold. When this comes to the surface, it cools and deposits its solids, which is why gold is found in some quartz veins. The big iron oxide deposits we have were formed through carbon dioxide weathering iron-containing silicates (such as olivine and pyroxene) to make ferrous and magnesium solutions in the oceans. When oxygen came along, the ferrous precipitated to form goethite and haematite, which we now mine. All the ore deposits on Earth are there because of geochemical processing.

There will be limited such processing on Mars, and on the Moon. Thus on the Moon, as it cooled some materials crystallised out before others. The last to crystallise on the Moon was what we call KREEP, which stands for potassium, rare earths and phosphate, which is what it largely comprises. There is also anorthite, a calcium aluminosilicate on the Moon. As for Mars, it seems to be mainly basaltic, which means it is mainly iron magnesium silicate. The other elements will be there, of course, mixed up, but how do you get them out? Then there is the problem of chemical compatibility. Suppose you want rare earths? The rare earths are not that rare, actually, and are about as common as copper. But copper occurs in nice separate ores, at least on Earth, but rare earths have chemical properties somewhat similar to aluminium. For every rare earth atom, there are 100,000 aluminium atoms, all behaving similarly, although not exactly the same. So it is far from easy to separate them from the aluminium, then there is the problem of separating them from each other.

There is what I consider a lot of nonsense spoken about asteroids. Thus one was reported to be “mainly diamond”. On close questioning, it had an infrared signature typical of carbon. That would be typically amorphous graphitic carbon, and no, they did not know specifically it was diamond. Another proposal was to mine asteroids for iron. There may well be some with an iron core, and Vesta probably does have such a core, but most do not. I have heard some say there will be lots of platinum there. Define lots, because unless there has been some form of sorting, it will be there proportionately to its dust concentration, and while there is more than in most bits of basalt, there will still be very little. In my opinion, beware of investment opportunities to get rich quickly through space mining.