A Planet Destroyer

Probably everyone now knows that there are planets around other stars, and planet formation may very well be normal around developing stars. This, at least, takes such alien planets out of science fiction and into reality. In the standard theory of planetary formation, the assumption is that dust from the accretion disk somehow turns into planetesimals, which are objects of about asteroid size and then mutual gravity brings these together to form planets. A small industry has sprung up in the scientific community to do computerised simulations of this sort of thing, with the output of a very large number of scientific papers, which results in a number of grants to keep the industry going, lots of conferences to attend, and a strong “academic reputation”. The mere fact that nobody knows how to get to their initial position appears to be irrelevant and this is one of the things I believe is wrong with modern science. Because those who award prizes, grants, promotions, etc have no idea whether the work is right or wrong, they look for productivity. Lots of garbage usually easily defeats something novel that the establishment does not easily understand, or is prepared to give the time to try.

Initially, these simulations predicted solar systems similar to ours in that there were planets in circular orbits around their stars, although most simulations actually showed a different number of planets, usually more in the rocky planet zone. The outer zone has been strangely ignored, in part because simulations indicate that because of the greater separation of planetesimals, everything is extremely slow. The Grand Tack simulations indicate that planets cannot form further than about 10 A.U. from the star. That is actually demonstrably wrong, because giants larger than Jupiter and very much further out are observed. What some simulations have argued for is that there is planetary formation activity limited to around the ice point, where the disk was cold enough for water to form ice, and this led to Jupiter and Saturn. The idea behind the NICE model, or Grand Tack model (which is very close to being the same thing) is that Uranus and Neptune formed in this zone and moved out by throwing planetesimals inwards through gravity. However, all the models ended up with planets being in near circular motion around the star because whatever happened was more or less happening equally at all angles to some fixed background. The gas was spiralling into the star so there were models where the planets moved slightly inwards, and sometimes outwards, but with one exception there was never a directional preference. That one exception was when a star came by too close – a rather uncommon occurrence. 

Then, we started to see exoplanets, and there were three immediate problems. The first was the presence of “star-burners”; planets incredibly close to their star; so close they could not have formed there. Further, many of them were giants, and bigger than Jupiter. Models soon came out to accommodate this through density waves in the gas. On a personal level, I always found these difficult to swallow because the very earliest such models calculated the effects as minor and there were two such waves that tended to cancel out each other’s effects. That calculation was made to show why Jupiter did not move, which, for me, raises the problem, if it did not, why did others?

The next major problem was that giants started to appear in the middle of where you might expect the rocky planets to be. The obvious answer to that was, they moved in and stopped, but that begs the question, why did they stop? If we go back to the Grand Tack model, Jupiter was argued to migrate in towards Mars, and while doing so, throw a whole lot of planetesimals out, then Saturn did much the same, then for some reason Saturn turned around and began throwing planetesimals inwards, which Jupiter continued the act and moved out. One answer to our question might be that Jupiter ran out of planetesimals to throw out and stopped, although it is hard to see why. The reason Saturn began throwing planetesimals in was that Uranus and Neptune started life just beyond Saturn and moved out to where they are now by throwing planetesimals in, which fed Saturn’s and Jupiter’s outwards movement. Note that this does depend on a particular starting point, and it is not clear to me  that since planetesimals are supposed to collide and form planets, if there was an equivalent to the masses of Jupiter and Saturn, why did they not form a planet?

The final major problem was that we discovered that the great bulk of exoplanets, apart from those very close to the star, had quite significant elliptical orbits. If you draw a line through the major axis, on one side of the star the planet moves faster and closer to it than the other side. There is a directional preference. How did that come about? The answer appears to be simple. The circular orbit arises from a large number of small interactions that have no particular directional preference. Thus the planet might form from collecting a huge number of planetesimals, or a large amount of gas, and these occur more or less continuously as the planet orbits the star. The elliptical orbit occurs if there is on very big impact or interaction. What is believed to happen is when planets grow, if they get big enough their gravity alters their orbits and if they come quite close to another planet, they exchange energy and one goes outwards, usually leaving the system altogether, and the other moves towards the star, or even into the star. If it comes close enough to the star, the star’s tidal forces circularise the orbit and the planet remains close to the star, and if it is moving prograde, like our moon the tidal forces will push the planet out. Equally, if the orbit is highly elliptical, the planet might “flip”, and become circularised with a retrograde orbit. If so, eventually it is doomed because the tidal forces cause it to fall into the star.

All of which may seem somewhat speculative, but the more interesting point is we have now found evidence this happens, namely evidence that the star M67 Y2235 has ingested a “superearth”. The technique goes by the name “differential stellar spectroscopy”, and what happens is that provided you can realistically estimate what the composition should be, which can be done with reasonable confidence if stars have been formed in a cluster and can reasonably be assigned as having started from the same gas. M67 is a cluster with over 1200 known members and it is close enough that reasonable details can be obtained. Further, the stars have a metallicity (the amount of heavy elements) similar to the sun. A careful study has shown that when the stars are separated into subgroups, they all behave according to expectations, except for Y2235, which has far too high a metallicity. This enhancement corresponds to an amount of rocky planet 5.2 times the mass of the earth in the outer convective envelope. If a star swallows a planet, the impact will usually be tangential because the ingestion is a consequence of an elliptical orbit decaying through tidal interactions with the star such that the planet grazes the external region of the star a few times before its orbital energy is reduced enough for ingestion. If so, the planet should dissolve in the stellar medium and increase the metallicity of the outer envelope of the star. So, to the extent that these observations are correctly interpreted, we have the evidence that stars do ingest planets, at least sometimes.

For those who wish to go deeper, being biased I recommend my ebook “Planetary Formation and Biogenesis.” Besides showing what I think happened, it analyses over 600 scientific papers, most of which are about different aspects.

Some Unanswered Questions from the Lunar Rocks

In the previous post I hinted that some of what we found from our study of moon rocks raises issues of self-consistency when viewed in terms of the standard paradigm. To summarize the relevant points of that paradigm, the argument goes that the dust in the accretion disk that was left behind after the star formed accreted into Mars-sized bodies that we shall call embryos, and these moved around in highly elliptical orbits and eventually collided to form planets. While these were all mixed up – simulations suggest what made Earth included bodies from outside Mars’ current orbit, and closer to the star than Mercury’s current orbit. These collisions were extraordinarily violent, and the Earth formed from a cloud of silicate vapours that condensed to a ball of boiling silicates at a little under 3000 degrees C. Metallic iron boils at 2862 degrees C, so it was effectively refluxing, and under these conditions it would extract elements such as tungsten and gold that dissolve in iron and take them with it to the core. About sixty million years after Earth formed, one remaining embryo struck Earth, a huge amount of silicates were sent into space, and the Moon condensed from this. The core of this embryo was supposedly iron, and it migrated into the Earth to join our core, leaving the Moon a ball of silicate vapour that had originated from Earth and condensed from something like 10,000 degrees C. You may now see a minor problem for Earth: if this iron took out all the gold, tungsten, etc, how come we can find it? One possibility is the metals formed chemical compounds. That is unlikely because at those temperatures elements that form only moderate-strength chemical bonds would not survive, and since gold is remarkably unreactive, that explanation won’t work. Another problem is that the Moon has very little water and no nitrogen. This easily explained through their being lost to space from the silicate vapours, but where did the Earth get its volatiles? And if the Moon did condense from such high temperatures, the last silicate to condense would be fayalite, but that was not included in the Apollo rocks, or if it were, nothing was made of that. This alone is not necessarily indicative, though, because fayalite is denser than the other olivines, and if there were liquid silicates for long enough it would presumably sink.

The standard paradigm invokes what is called “the late veneer”; after everything was over, Earth got bombarded with carbonaceous asteroids, which contain water, nitrogen, and some of these otherwise awkward metals. It is now that we enter one of the less endearing aspects of modern science: everything tends to be compartmentalised, and the little sub-disciplines all adhere to the paradigm and add small findings that support their view, even if they do not do so particularly well, and there is a reluctance to look at the overall picture. The net result is that while many of the findings can be made to seemingly provide answers to their isolated problems, there is an overall problem with self-consistency. Further, clues that the fundamental proposition might be wrong are carefully shelved.

The first problem was noted at the beginning of the century: the isotope ratios of metals like osmium from such chondrites are different from our osmium. There are various hand-waving argument to the extent that it could just manage if it were mixed with enough of our mantle, but leaving whether the maths are right aside, nobody seems to have noticed the only reason we are postulating this late veneer is that originally the iron stripped all the osmium from the mantle. You cannot dilute A with B if B is not there. There are a number of other reasons, one of which is the nitrogen of such chondrites has more 15N than our nitrogen. Another is to get the amounts of material here we need a huge amount of carbonaceous asteroids, but they have to come through the ordinary asteroids without perturbing them. That takes some believing.

But there is worse. All the rocks found by the Apollo program have none of the required materials and none of the asteroidal isotope signatures. The argument seems to be, they “bounced off” the Moon. But the Moon also has some fairly ferocious craters, so why did the impactors that caused them not bounce off? Let’s suppose they did bounce off, but they did not bounce off the Earth (because the only reason we argue for this is that we need them, so it is said, to account for our supply of certain metals). Now the isotope ratios of the oxygen atoms on the Moon have a value, and that value is constant over rocks that come from deep within the Moon, thanks to volcanism, and for the rocks from the highlands, so that is a lunar value. How can that be the same as Earth’s if Earth subsequently got heavily bombarded with asteroids that we know have different values? My answer, in my ebook “Planetary Formation and Biogenesis” is simple: there were no embryo impacts in forming Earth therefore the iron vapours did not extract out the heavy elements, and there were no significant number asteroid impacts. Almost everything came here when Earth accreted, and while there have been impacts, they made a trivial contribution to Earth’s supply of matter.

The Apollo Program – More Memories from Fifty Years Ago.

As most will know, it is fifty years ago since the first Moon landing. I was doing a post-doc in Australia at the time, and instead of doing any work that morning, when the word got around on that fateful day we all downed tools and headed to anyone with a TV set. The Parkes radio telescope had allowed what they received to be live-streamed to Australian TV stations. This was genuine reality TV. Leaving aside the set picture resolution, we were seeing what Houston was seeing, at exactly the same time. There was the Moon, in brilliant grey, and we could watch the terrain get better defined as the lander approached, then at some point it seemed as if the on-board computer crashed. (As computers go, it was primitive. A few years later I purchased a handheld calculator that would leave that computer for dead in processing power.) Anyway, Armstrong took control, and there was real tension amongst the viewers in that room because we all knew if anything else went wrong, those guys would be dead. There was no possible rescue. The ground got closer, Armstrong could not fix on a landing site, the fuel supply was getting lower, then, with little choice because of the fuel, the ground got closer faster, the velocity dropped, and to everyone’s relief the Eagle landed and stayed upright. Armstrong was clearly an excellent pilot with excellent nerves. Fortunately, the lander’s legs did not drop into a hole, and as far as we could tell, Armstrong chose a good site. Light relief somewhat later in the day to watch them bounce around on the lunar surface. (I think they were ordered to take a 4-hour rest. Why they hadn’t rested before trying to land I don’t know. I don’t know about you, but if I had just successfully landed on the Moon, and would be there for not very long, a four-hour rest would not seem desirable.)

In some ways that was one of America’s finest moments. The average person probably has no idea how much difficult engineering went into that, and how everything had to go right. This was followed up by six further successful landings, and the ill-fated Apollo 13, which nevertheless was a triumph in a different way in that despite a near-catastrophic situation, the astronauts returned to Earth.

According to the NASA website, the objectives of the Apollo program were:

  • Establishing the technology to meet other national interests in space.
  • Achieving preeminence in space for the United States.
  • Carrying out a program of scientific exploration of the Moon.
  • Developing human capability to work in the lunar environment.

The first two appear to have been met, but obviously there is an element of opinion there. It is debatable that the last one achieved much because there has been no effort to return to the Moon or to use it in any way, although that may well change now. Charles Duke turns 84 this year and he still claims the title of “youngest person to walk on the Moon”.

So how successful was the scientific program? In some ways, remarkably, yet in others there is a surprising reluctance to notice the significance of what was found. The astronauts brought back a large amount of lunar rocks, but there were some difficulties here in that until Apollo 17, the samples were collected by astronauts with no particular geological training. Apollo 17 changed that, but it was still one site, albeit with a remarkably varied geological variety. Of course, they did their best and selected for variety, but we do not know what was overlooked.

Perhaps the most fundamental discovery was that the isotopes from lunar rocks are essentially equivalent to earth rocks, and that means they came from the same place. To put this in context, the ratio of isotopes of oxygen, 16O/17O/18O varies in bodies seemingly according to distance from the star, although this cannot easily be represented as a function. The usual interpretation is that the Moon was formed when a small planet, maybe up to the size of Mars, called Theia crashed into Earth and sent a deluge of matter into space at a temperature well over ten thousand degrees Centigrade, and some of this eventually aggregated into the Moon. Mathematical modelling has some success at showing how this happened, but I for one am far from convinced. One of the big advantages of this scenario is that it shows why the Moon has no significant water, no atmosphere, and never had any, apart from some water and other volatiles frozen in deep craters at the South Pole that almost certainly arrived from comets and condensed there thanks to the cold. As an aside, you will often read that the lunar gravity is too weak to hold air. That is not exactly true; it cannot hold it indefinitely, but if it started with carbon dioxide proportional in mass, or even better in cross-sectional area, to what Earth has, it would still have an atmosphere.

One of the biggest disadvantages of this scenario is where did Theia come from? The models show that if the collision, which happened about 60 million years after the Earth formed, occurred from Theia having a velocity much above the escape velocity from Earth, the Moon cannot form. It gets the escape velocity from falling down the Earth’s gravitational field, but if it started far enough further out that would have permitted Theia to have lasted 60 million years, then its velocity would be increased by falling down the solar gravitational field, and that would be enhanced by the eccentricity of its trajectory (needed to collide). Then there is the question of why are the isotopes the same as on Earth when the models show that most of the Moon came from Theia. There has been one neat alternative: Theia accreted at the Earth-Sun fourth or fifth Lagrange point, which gives it indefinite stability as long as it is small. That Theia might have grown just too big to stay there explains why it took so long and starting at the same radial distance as Earth explains why the isotope ratios are the same.

So why did the missions stop? In part, the cost, but that is not a primary reason because most of the costs were already paid: the rockets had already been manufactured, the infrastructure was there and the astronauts had been trained. In my opinion, it was two-fold. First, the public no longer cared, and second, as far as science was concerned, all the easy stuff had been done. They had brought back rocks, and they had done some other experiments. There was nothing further to do that was original. This program had been a politically inspired race, the race was run, let’s find something more exciting. That eventually led to the shuttle program, which was supposed to be cheap but ended up being hideously expensive. There were also the deep space probes, and they were remarkably successful.

So overall? In my opinion, the Apollo program was an incredible technological program, bearing in mind from where it started. It established the US as firmly the leading scientific and engineering centre on Earth, at least at the time. Also, it got where it did because of a huge budget dedicated to one task. As for the science, more on that later.

Earth’s Twin: Venus

Leaving aside the Moon and the Sun, Venus is the brightest object in the sky, and at times the closest. Further, Venus is the only planet that is comparable to Earth; its mass is about 81.5%, its size is about 95%, and its gravity is about 90.5% that of Earth. The orbit of Venus has only 1/3 of Earth’s eccentricity, and while Earth has an axial tilt of 23.5 degrees (which results in right now I am embedded in winter and many of the readers will be enjoying a pleasant summer, or maybe a heat wave) Venus has a tilt of only 2.6 degrees. That means that Venus has a more or less uniform temperature and no seasons. At first sight, that would make it an attractive target for space probes, but while NASA has sent eleven orbiters and eight landers to Mars, it has only sent two orbiters to Venus. Why the lack? Not for lack of interest since from 1990 NASA has considered nearly thirty proposals, but it approved none. The dead hand of the committee strikes again. The reason is that Venus, up close, is strangely unattractive.

The first problem is atmospheric pressure, which is about 90 bar over most of the planet, and it has an average surface temperature of about 460 degrees C but this can vary by +160 degrees C. The second problem is the nature of the atmosphere. Most of it is carbon dioxide. Venus also has about four times the amount of nitrogen than Earth has, and all of that is relatively harmless. What is less harmless is the atmosphere has clouds of sulphuric acid, together with hydrogen chloride and hydrogen fluoride. Hydrogen fluoride is particularly nasty, because it reacts with glass, and while the sulphuric acid will attack all the basic electronics, etc, the hydrogen fluoride will attack lenses. Very shortly, photography, or seeing where a rover is going, will no longer be possible. And, of course, if it can survive, the heat soon kills it. The first lander to return data was Venera 7, a 1970 Soviet lander that survived for 23 minutes. In 1975, Venera 9 sent back the first pictures from the surface, but it too did not last very long. Funding committees do not encourage very expensive rovers with a very short life.

This may change. NASA is designing a “station” that should last at least sixty days, and operate at the ambient temperature. The electronics would be made of silicon carbide, a substance that conducts electricity and melts somewhere above 2,800 degrees C. No danger of that melting, although all the metals in the craft would have to be resistant to the ambient heat and the corrosion. Titanium would probably manage reasonably well. So maybe we shall get to know more about the planet.

There have apparently been proposals to “colonise” Venus through “settlements” floating above the cloud levels, i.e.presumably some ship-like structure supported by gigantic balloons. Personally, I feel this is unreal. The total weight must displace an equal weight of gas, and the idea is to get above the clouds. Up there, the gas is nowhere near as dense (the pressure is only about half that 90 bar at the top of the highest mountain) and to go higher the pressure really drops away. So to support sufficient mass you would need very large balloons, made of what? Any fabric or rubber would be broken down by the solar UV at that height. Metals would corrode. And what would the gas be? The obvious ones would be hydrogen and helium (no danger of fire because there is no air) but these gases leak like crazy. You may think you can hold it, but for centuries? Then there is another minor problem: at the top of the atmosphere winds can reach several hundred kilometres per hour.

So what is “wrong” with Venus, from our point of view? There are two things. The first is the very slow rotation, which happens to be retrograde. The direction is not so much a problem, but the slowness is. However, the main one is, no significant water. If Venus had the amount of water Earth has, it would have fixed all that carbon dioxide as limestone or dolomite, in which case the atmospheric pressure would be about 3 times our atmosphere (because it has four times the amount of nitrogen). If we wanted to have breathable air, we would have to add another atmosphere of oxygen.

So in theory we could terraform Venus. At the expense of much energy what we would have to do is bring in a number of Kuiper Belt objects, or maybe cometary material from around Jupiter would be better because they contain much less additional nitrogen and carbon monoxide, and make them hit Venus, preferably on the side in a way that the angular momentum of the incoming object was added to the current Venusian rotation, in other words, spin it up. Give it water, and chemistry would do the rest, although it would probably also be preferable to cool it by shading it from the sun at least to some extent. Yes, the temperatures would still be high, but as long as it can cool to 300 degrees C, the pressure will ensure there is some liquid, and the fixing of the gas will start, and initiate positive feedback

Suppose we could give Venus as much water as Earth, then the planet would be more like a water world. It is an interesting question whether Venus has any felsic/granitic material. This is the stuff that makes continents. The great bulk of the material on any rocky planet is basaltic, which in turn is because the oxides of silicon, magnesium and iron are the most commonly available rock-forming materials. Aluminium, as an element, is over an order of magnitude less common than silicon, which it replaces in aluminosilicates. Being less dense than basalt, granite floats on the basalt, provided it can separate itself from the basalt. In my ebook “Planetary Formation and Biogenesis”, I propose that the separation essentially has to take place prior to and during planetary formation. Venus does have two minicontinents: Ishtar and Aphrodite Terrae.

The actual differentiation of the planet, when the granite moves from the deep and comes out on the surface occurs slowly (the small amounts of plagioclase on Mars apparently took about two billion years.) and the rate probably depends on the amount actually accreted. The evidence is that on Earth very large amounts erupted in massive pulses. In the absence of such granite, a large planet will be rather flat, apart from some volcanic peaks.

There would still be a problem in that Venus has no plate tectonics. They are needed to provide the recycling of carbon dioxide, as eventually if the lot were fixed, any life would presumably die. We don’t know what starts plate tectonics. One possibility is the presence of granitic continents, another is the forces arising from rotational motion.  It is just possible they could start if there were more rotational motion, but we don’t know. All in all, not an attractive planet in detail, so maybe we should look after our own better.

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.

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!