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.

Monarchic Growth of Giant Planets

In the previous post, I outlined the basic mechanism of how I thought the giant planets formed, and how their mechanism of formation put them at certain distances from the sun. Given that, like everyone else, I assign Jupiter to the snow point, in which case the other planets are where they ought to be. But that raises the question, why one planet in a zone? Let’s take a closer look at this mechanism.

In the standard mechanism, dust accretes into objects by some unknown mechanism, and does this essentially based on collision probability, and so the disk progresses with a distribution of roughly equal sized objects that collide under the same rules, and eventually become what is called planetesimals, which are about the size of the classical asteroid. (I say classical because as we get better at this, we are discovering a huge number of much smaller “asteroids”, and we have the problem of what does the word asteroid mean?) This process continues, and eventually we get Mars-sized objects called oligarchs, or embryos, then these collide to get planets. The size of the planet depends on how many oligarchs collide, thus fewer collided to make Venus than Earth, and Mars is just one oligarch. I believe this is wrong for four reasons: the first is, giants cannot grow fast enough; second, the dust is still there in 30 My old disks; the collision energies should break up the bodies at any given size because collisions form craters, not hills; the system should be totally mixed up, but isotope evidence shows that bodies seem to have accreted solely from the material at roughly their own distance from the sun.

There is an alternative called monarchic growth, in which, if one body can get a hundred times bigger than any of the others, it alone grows by devouring the others. For this to work, we need initial accretion to be possible, but not extremely probable from dust collisions. Given that we see disks by their dust that are estimated to be up to 30 My old, that seems a reasonable condition. Then, once it starts, we need a mechanism that makes further accretion inevitable, that is, when dust collides, it sticks. The mechanism I consider to be most likely (caveat – I developed it so I am biased) is as follows.

As dust comes into an appropriate temperature zone, then collisions transfer their kinetic energy into heat that melts an ice at the point of contact, and when it quickly refreezes, the dust particles are fused to the larger body. So accretion occurs a little below the melting temperature, and the probability of sticking falls off as the distance from that appropriate zone increases, but there is no sharp boundary. The biggest body will be in the appropriate zone because most collisions will lead to sticking, and once the body gets to be of an appropriate size, maybe as little as a meter sized, it goes into a Keplerian orbit. The gas and dust is going slower, due to gas drag (which is why the star is accreting) so the body in the optimal zone accretes all the dust and larger objects it collides with. Until the body gets sufficiently large gravitationally, collisions have low relative velocity, so the impact energy is modest.

Once it gets gravitationally bigger, it will accrete the other bodies that are at similar radial distance. The reason is that if everything is in circular orbits, orbits slightly further from the star have longer periodic times, in part because they move slightly slower, and in part because they have slightly further to go, so the larger body catches up with them and its gravity pulls the smaller body in. Unless it has exactly the same radial distance from the star, they will pass very closely and if one has enough gravity to attract the other, they will collide. Suppose there are two bodies at the same radial distance. That too is gravitationally unstable once they get sufficiently large. All interactions do not lead to collisions, and it is possible that one can be thrown inwards while the other goes outwards, and the one going in may circularise somewhere else closer to the star. In this instance, Ceres has a density very similar to the moons of Jupiter, and it is possible that it started life in the Jovian region, came inwards, and then finished accreting material from its new zone.

The net result of this is that a major body grows, while smaller bodies form further away, trailing off with distance, then there is a zone where nothing accretes, until further out there is the next accretion zone. Such zones get further away as you get further from the star because the temperature gradient decreases. That is partly why Neptune has a Kuiper Belt outside it. The inner planets do not because with a giant on each side, the gravity causes them to be cleaned out. This means that after the system becomes settled, a lot of residues start bombing the planet. This requires what could be called a “Great Bombardment”, but it means each system gets a bombardment mainly of its own composition, and there could be no significant bombardment with bodies from another system. This means the bombardment would have the same chemical composition as the planet itself.

Accordingly, we have a prediction. Is it right? It is hard to tell on Earth because while Earth almost certainly had such a bombardment, plate tectonics has altered the surface so much. Nevertheless, the fact the Moon has the same isotopes as Earth, and Earth has been churned but the Moon has not, is at least minor support. There is, of course, a second prediction. There seem to be many who assume the interior of the Jovian satellites will have much nitrogen. I predict very little. There will be some through adsorption of ammonia onto dust, and since ammonia binds more strongly than neon, then perhaps there will be very modest levels, but the absence of such material in the atmosphere convinces me it will be very modest.

Star and Planetary Formation: Where and When?

Two posts ago, as a result of questions, I promised to write a post outlining the concept of planetary accretion, based on the current evidence. Before starting that, I should explain something about the terms used. When I say something is observed, I do not mean necessarily with direct eyesight. The large telescopes record the light signals electronically, similarly to how a digital camera works. An observation in physics means that a signal is received that can be interpreted in one only certain way, assuming the laws of physics hold. Thus in the famous two-slit experiment, if you fire one electron through the slits, you get one point impact, which is of too low an energy for the human eye to see. Photomultipliers, however, can record this as a pixel. We have to assume that the “observer” uses laws of physics competently.

The accretion of a star almost certainly starts with the collapse of a cloud of gas. What starts that is unknown, but it is probably some sort of shock wave, such as a cloud of debris from a nearby supernova. Another cause appears to be the collision of galaxies, since we can see the remains of such collisions that are accompanied by a large number of new stars forming. The gas then collapses and forms an accretion disk, and these have been observed many times. The gas has a centre of mass, and this acts as the centre of a gravitational field, and as such, the gas tries to circulate at an orbital velocity, which is where the rate of falling into the star is countered by the material moving sideways, at a rate that takes it away from the star so that the distance from the centre remains the same. If they do this, angular momentum is also conserved, which is a fundamental requirement of physics. (Conservation of angular momentum is why the ice skater spins slowly with arms outstretched; when she tucks her arms in, she spins faster.

The closer to the centre, the strnger gravity requires faster orbital velocity. Thus a stream of gas is moving faster than the stream just further from the centre, and slower than the stream just closer. That leads to turbulence and friction. Friction slows the gas, meaning it starts to fall starwards, while the friction converts kinetic energy to heat. Thus gas drifts towards the centre, getting hotter and hotter, where it forms a star. This has been observed many times, and the rate of stellar accretion is such that a star takes about a million years to form. When it has finished growing, there remains a dust-filled gas cloud of much lower gas density around it that is circulating in roughly orbital velocities. Gas still falls into the star, but the rate of gas falling into the star is at least a thousand times less than during primary stellar accretion. This stage lasts between 1 to 30 million years, at which point the star sends out extreme solar winds, which blow the gas and dust away.

However, the new star cannot spin fast enough to conserve angular momentum. The usual explanation is that gas is thrown out from near the centre, and there is evidence in favour of this in that comets appear to have small grains of silicates that could only be formed in very hot regions. The stellar outburst noted above will also take away some of the star’s angular momentum. However, in our system, the bulk of the angular momentum actually resides in the planets, while the bulk of the mass is in the star. It would seem that somehow, some angular momentum must have been transferred from the gas to the planets.

Planets are usually considered to form by what is called oligarchic growth, which occurs after primary stellar accretion. This involves the dust aggregating into lumps that stick together by some undisclosed mechanism, to make first, stone-sized objects, then these collide to form larger masses, until eventually you get planetesimals (asteroid-sized objects) that are spread throughout the solar system. These then collide to form larger bodies, and so on, at each stage collisions getting bigger until eventually Mars-sized bodies collide to form planets. If the planet gets big enough, it then starts accreting gas from the disk, and provided the heat can be taken away, if left long enough you get a gas giant.

In my opinion, there are a number of things wrong with this. The first is, the angular momentum of the planets should correspond roughly to the angular momentum of the dust, which had velocity of the gas around it, but there is at least a hundred thousand times more gas than dust, so why did the planets end up with so much more angular momentum than the star? Then there is timing. Calculations indicate that to get the core of Jupiter, it would take something approaching 10 million years, and that assumes a fairly generous amount of solids, bearing in mind the solid to gas ratio. At that point, it probably accretes gas very quickly. Get twice as far away from the star, and collisions are much slower. Now obviously this depends on how many planetesimals there are, but on any reasonable estimate, something like Neptune should not have formed. Within current theory, this is answered by having Neptune and Uranus being formed somewhere near Saturn, and then moved out. To do that, while conserving angular momentum, they had to throw similar masses back towards the star. I suppose it is possible, but where are the signs of the residues? Further, if every planet is made from the same material, the same sort of planet should have the same composition, but they don’t. The Neptune is about the same size as Uranus, but it is about 70% denser. Of the rocky planets, Earth alone has massive granitic/feldsic continents.

Stronger evidence comes from the star called LkCa 15 that apparently has a gas giant forming that is already about five times bigger than Jupiter and about three times further away. The star is only 3 million years old. There is no time for that to have formed by this current theory, particularly since any solid body forming during the primary stellar accretion is supposed to be swept into the star very quickly.

Is there any way around this? In my opinion, yes. I shall put up my answer in a later post, although for those who cannot wait, it is there in my ebook, “Planetary Formation and Biogenesis”.

Evidence that the Standard Theory of Planetary Formation is Wrong.

Every now and again, something happens that makes you feel both good and depressed at the same time. For me it was last week, when I looked up the then latest edition of Nature. There were two papers (Nature, vol 541 (Dauphas, pp 521 – 524; Fischer-Gödde and Kleine, pp 525 – 527) that falsified two of the most important propositions in the standard theory of planetary formation. What we actually know is that stars accrete from a disk of gas and dust, the disk lingers on for between a million years and 30 million years, depending on the star, then the star’s solar winds clear out the dust and gas. Somewhere in there, planets form. We can see evidence of gas giants growing, where the gas is falling into the giant planet, but the process by which smaller planets or the cores of giants form is unobservable because the bodies are too small, and the dust too opaque. Accordingly, we can only form theories to fill in the intermediate process. The standard theory, also called oligarchic growth, explains planetary formation in terms of dust accreting to planetesimals by some unknown mechanism, then these collide to form embryos, which in turn formed oligarchs or protoplanets (Mars sized objects) and these collided to form planets. If this happened, they would do a lot of bouncing around and everything would get well-mixed. Standard computer simulations argue that Earth would have formed from a distribution of matter from further out than Mars to inside Mercury’s orbit. Earth the gets its water from a “late veneer” from carbonaceous chondrites from the far side of the asteroid belt.

It is also well known that certain elements in bodies in the solar system have isotopes that vary their ratio depending on the distance from the star. Thus meteorites from Mars have different isotope ratios from meteorites from the asteroid belt, and again both are different from rocks from Earth and Moon. The cause of this isotope difference is unclear, but it is an established fact. This is where those two papers come in.

Dauphas showed that Earth accreted from a reasonably narrow zone throughout its entire accretion time. Furthermore, that zone was the same as that which formed enstatite chondrites, which appear to have originated from a region that was much hotter than the material that, say, formed Mars. Thus enstatite chondrites are reduced. What that means is that their chemistry was such that there was less oxygen. Mars has only a small iron core, and most of its iron is as iron oxide. Enstatite chondrites have free iron as iron, and, of course, Earth has a very large iron core. Enstatite chondrites also contain silicates with less magnesium, which will occur when the temperatures were too hot to crystallize out forsterite. (Forsterite melts at 1890 degrees C, but it will also dissolve to some extent in silica melts at lower temperatures.) Enstatite chondrites also are amongst the driest, so they did not provide Earth’s water.

Fischer-Gödde and Kleine showed that most of Earth’s water did not come from carbonaceous chondrites, the reason being, if it did, the non-water part would have added about 5% to the mass of Earth, and the last 5% is supposed to be from where the bulk of elements that dissolve in hot iron would have come from. The amounts arriving earlier would have dissolved in the iron and gone to the core. One of those elements is ruthenium, and the isotope ratios of Earth’s ruthenium rule out an origin from the asteroid belt.

Accordingly, this evidence rules out oligarchic growth. There used to be an alternative theory of planetary accretion called monarchic growth, but this was soon abandoned because it cannot explain first why we have the number of planets we have where they are, and second where our water came from. Calculations show it is possible to have three to five planets in stable orbit between Earth and Mars, assuming none are larger than Earth, and more out to the asteroid belt. But they are not there, so the question is, if planets only grow from a narrow zone, why are these zones empty?

This is where I felt good. A few years ago I published an ebook called “Planetary Formation and Biogenesis” and it required monarchic growth. It also required the planets in our solar system to be roughly where they are, at least until they get big enough to play gravitational billiards. The mechanism is that the planets accreted in zones where the chemistry of the matter permitted accretion, and that in turn was temperature dependent, so specific sorts of planets form in zones at specific distances from the star. Earth formed by accretion of rocks formed during the hot stage, and being in a zone near that which formed enstatite chondrites, the iron was present as a metal, which is why Earth has an iron core. The reason Earth has so much water is that accretion occurred from rocks that had been heat treated to about 1550 degrees Centigrade, in which case certain aluminosilicates phase separated out. These, when they take up water, form cement that binds other rocks to form a concrete. As far as I am aware, my theory is the only current one that requires these results.

So, why do I feel depressed? My ebook contained a review of over 600 references from journals until a few months before the ebook was published. The problem is, these references, if properly analysed, provided papers with plenty of evidence that these two standard theories were wrong, but each of the papers’ conclusions were ignored. In particular, there was a more convincing paper back in 2002 (Drake and Righter, Nature 416: 39-44) that came to exactly the same conclusions. As an example, to eliminate carbonaceous chondrites as the source of water, instead of ruthenium isotopes, it used osmium isotopes and other compositional data, but you see the point. So why was this earlier work ignored? I firmly believe that scientists prefer to ignore evidence that falsifies their cherished beliefs rather than change their minds. What I find worse is that neither of these papers cited the Drake and Righter paper. Either they did not want to admit they were confirming a previous conclusion, or they were not interested in looking thoroughly at past work other than that which supported their procedures.

So, I doubt these two papers will change much either. I might be wrong, but I am not holding my breath waiting for someone with enough prestige to come out and say enough to change the paradigm.

Our closest planetary system?

One of the interesting things about science is how things can change. When I wrote my ebook Planetary Formation and Biogenesis, finishing in 2011, it was generally accepted that the star Tau ceti had no planets, and all the star had orbiting it was a collection of rocks or lumps of ice, in short, debris that had never accreted. Now, it appears, five planets have been claimed to orbit it, and most have very low eccentricities. (The eccentricity measures the difference between closest and farthest distance from the star in an elliptical orbit. If the eccentricity is zero, the orbit is circular.) Orbits close to zero indicate that there have been no major disruptions to the planetary system, which can occur if the planets get too big. Once they get to a size where their gravitational pull acts on each other, the planets may play a sort of game of planetary billiards, often ejecting one from the system, and leaving the rest with highly elliptical orbits, and sometime planets very close to the star.

The question then is, how does my theory perform? The theory suggests that planets form due to chemical interactions, at least to begin, although once they reach a certain size, gravity is the driving force. This has a rather odd consequence in that while the planets are small, their differences of composition are marked, but once they get big enough to accrete everything, they become much more similar, until they become giants, in which case they appear more or less the same. The chemical interactions depend on temperature, and for the rocky planets, on a sequence of temperatures. The first important temperature is during stellar accretion, when temperatures become rather high in the rocky planet zone. For example, the material that led to the start of Earth had to get to at least 1538 degrees Centigrade, so that iron would melt. All the iron bearing meteorites almost certainly reached this temperature, as there is no other obvious way to melt the iron that forms them. At the same time, a number or silicates melt and phase separate. (That is forming two layers, like oil and water.) There is then a second important temperature. When the star has finished forming, which occurs when most of the available gas has reached it, there remains a much lower density gas disk, which cools.

The initial high temperatures are caused by large amounts of gas falling towards the star, and it gets hot due to friction as it loses potential energy. Accordingly, the potential energy depends on the gravitational field of the star, which is proportional to the mass of the star. The heat also depends on the rate of gas falling in, i.e. how much is falling, and very approximately that depends on the square of the mass of the star. Unfortunately, it also depends on how efficient the disk was at radiating heat, and that is unknowable. Accordingly, if all systems have the same pattern of disk cooling, then very very roughly, the same sort of planet will be at a distance proportional to the cube of the stellar mass, at least on this theory.

There are only two solitary stars within 12 light years from Earth that are sufficiently similar to our star that they might be considered to be of interest as supporting life, and only one, Tau ceti is old enough to be of interest as potentially having life. Tau ceti has a mass of approximately 0.78 times our sun’s mass, so on my theory the prediction of the location of the earth equivalent based on our system being a standard (which it may well not be, but with a sample of one, a statistical analysis is not possible) would be at approximately 0.48 AU, an AU (astronomical unit) being the distance from Earth to the sun. The planets present are at 0.105 AU, 0.195 AU, 0.374 AU, 0.552 A.U. and 1.35 AU. If the 0.552 planet is an Earth equivalent, all the others are somewhat further from the planet than expected, or alternatively, if the 0.374 AU planet is the earth equivalent, they are much closer than expected. Which it is within the theory depends on how fast the star formed or how transparent was the disk, both of which are unknowable. Alternatively, the Earth equivalent would be defined by its composition, which again is currently unknowable. The Jupiter equivalent should be at about 2.5 AU on my theory. If it were to be the 1.35 AU planet that was the Jupiter equivalent (mainly water ice) then the 0.552 planet would be the Mars equivalent, and while it would be just in the habitable zone (0.55 – 1.16 AU estimate) the core would have the chemistry of Mars, plus whatever it accreted gravitationally.

Tau ceti is thus the closest star where we have seen a planet in the habitable zone. The planet in the habitable zone is about 4.3 times as massive as earth, so it would be expected to have a stronger gravitational acceleration at its surface, but possibly not that much more because Earth’s gravity is enhanced by its reasonably massive iron core. Planets that accrete much of their mass through simple gravity probably also accrete a lot more water towards the end because water is more common than rock in the disk, apart from the initial stones and iron concentrated through melting. So, with a planet possibly in the habitable zone, but of unknown water content, and unknown nature, if we had the technology would you vote to send a probe to find out what it is like? The expense would make the basic NASA probes look like chickenfeed, and of course, we would never get an answer in our lifetimes, unless someone develops a motor capable of reaching relativistic speeds.

The Fermi paradox, raised by Enrico Fermi, posed the question, if alien life is possible on other planets, given that there are so many stars older than our sun, why haven’t we been visited (assuming we have not)? For all those who say there are better things to spend their money on, they answer that question. The sheer expense of getting started may mean that all civilizations prefer to stay in their own system. Not, of course, that that stops me and others from writing science fictional stories about them.