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.

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.

Asteroids

If you have been to more than the occasional science fiction movie, you will know that a staple is to have the trusty hero being pursued, but escaping by weaving in and out of an asteroid field. Looks like good cinema, they make it exciting, but it is not very realistic. If asteroids were that common, according to computer simulations their mutual gravity would bring them together to form a planet, and very quickly. In most cases, if you were standing on an asteroid, you would be hard pressed to see another one, other than maybe as a point like the other stars. One of the first things about the asteroid belt is it is mainly empty. If we combined all the mass of the asteroids we would get roughly 4% of the mass of the Moon. Why is that? The standard theory of planetary formation cannot really answer that, so they say there were a lot there, but Jupiter’s gravity drove them out, at the same time overlooking the fact their own theory says they should form a planet through their self-gravity if there were that amny of them. If that were true, why did it leave some? It is not as if Jupiter has disappeared. In my “Planetary formation and Biogenesis”, my answer is that while the major rocky planets formed by “stone” dust being cemented together by one other agent, the asteroid belt, being colder, could only manage dust being cemented together with two other agents, and getting all three components in the same place at the same time was more difficult.

There is a further reason why I do not believe Jupiter removed most of the asteroids. The distribution currently has gaps, called the Kirkwood gaps, where there are very few asteroids, and these occur at orbital resonances with Jupiter. Such a resonance is when the target body would orbit at some specific ratio to Jupiter’s orbital period, so frequently the perturbations are the same because in a given frame of reference, they occur in the same place. Thus the first such gap occurs at 2.06 A.U. from the sun, where any asteroid would go around the sun exactly four times while Jupiter went around once. That is called a 4:1 resonance, and the main gaps occur at 3:1, 5:2, 7:3 and 2:1 resonances. Now the fact that Jupiter can clear out these narrow zones but leave all the rest more or less unchanged strongly suggests to me there were never a huge population of asteroids and we are seeing a small residue.

The next odd thing about asteroids is that while there are not very many of them, they change their characteristics as they get further from the star (with some exceptions to be mentioned soon.) The asteroids closest to the sun are basically made of silicates, that is, they are essentially giant rocks. There appear to be small compositional variations as they get further from the star, then there is a significant difference. How can we tell? Well, we can observe their brightness, and in some cases we can correlate what we see with meteorites, which we can analyse. So, further out, they get significantly duller, and fragments that we call carbonaceous chondrites land on Earth. These contain a small amount of water, and organic compounds that include a variety of amino acids, purines and pyrimidines. This has led some to speculate that our life depended on these landing on Earth in large amounts when Earth was very young. In my ebook “Planetary Formation and Biogenesis”, I disagree. The reasons are that to get enough, a huge number of such asteroids would have to impact the Earth because they are still basically rock, BUT at the same time, hardly any of the silicate based asteroids would have to arrive, because if they did, the isotopes of certain elements on Earth would have to be different. Such isotope evidence also rules these out as a source of water, as does certain ratios such as carbon to chlorine. What these asteroid fragments do show, however, is that amino acids and other similar building blocks of life are reasonably easily formed. If they can form on a lump of rock in a vacuum, why cannot they form on Earth?

The asteroid belt also has the odd weird asteroid. The first is Ceres, the largest. What is weird about it is that it is half water. The rest are essentially dry or only very slightly wet. How did that happen, and more to the point, why did it not happen more frequently? The second is Vesta, the second largest. Vesta is rocky, although it almost certainly had water at some stage because there is evidence of quartz. It has also differentiated, and while the outer parts have olivine, deeper down we get members of the pyroxene class of rocks, and deeper down still there appears to be a nickel/iron core. Now there is evidence that there may be another one or two similar asteroids, but by and large it is totally different from anything else in the asteroid belt. So how did that get there?

I rather suspect that they started somewhere else and were moved there. What would move them is if they formed and came close to a planet, and instead of colliding with it, they were flung into a highly elliptical orbit, and then would circularise themselves where they ended up. Why would they do that? In the case of Vesta, at some stage it suffered a major collision because there is a crater near the south pole that is 25 km deep, and it is from this we know about the layered nature of the asteroid. Such a collision may have resulted in it remaining in orbit roughly near its present position, and the orbit would be circularised due to the gravity of Jupiter. Under this scenario, Vesta would have formed somewhere near Earth to get the iron core. Ceres, on the other hand, probably formed closer to Jupiter.

In my previous post, I wrote that I believed the planets and other bodies grew by Monarchic growth, but that does not mean there were no other bodies growing in a region. Monarchic growth means the major object grows by accreting things at least a hundred times smaller, but of course significant growth can occur for other objects. The most obvious place to grow would be at a Lagrange point of the biggest object and the sun. That is a position where the planet’s gravitational field and the sun’s cancel, and the body is in stable or metastable orbit there. Once it gets to a certain size, however, it is dislodged, and that is what I think was the source of the Moon, its generating body probably starting at L4, the position at the same distance from the sun as Earth, but sixty degrees in front of it. There are other metastable positions, and these may have also formed around Venus or Mercury, and these would also be unstable due to different rocky planets. The reason I think this is that for Vesta to have an iron core, it had to pick up bodies with a lot of iron, and such bodies would form in the hotter part of the disk while the star was accreting. This is also the reason why Earth has an iron core and Mars has a negligible one. However, as I understand it, the isotopes from rocks on Vesta are not equivalent to those of Earth, so it may well have started life nearer to Venus or Mercury. So far we have no samples to analyse that we know came from either of these two planets, and I am not expecting any such samples anytime soon.