Galactic Collisions

As some may know, the Milky Way galaxy and the Andromeda galaxy are closing together and will “collide” in something like 4 – 5 billion years. If you are a good distance away, say in a Magellenic Cloud, this would look really spectacular, but what about if you were on a planet like Earth, right in the middle of it, so to speak? Probably not a lot of difference from what we see. There could be a lot more stars in the sky (and there should be if you use a good telescope) and there may be enhanced light from a dust cloud, but basically, a galaxy is a lot of empty space. As an example, light takes 8 minutes and twenty seconds to get from the sun to Earth. Light from the nearest star takes 4.23 years to get here. Stars are well-spaced.

As we understand it, stars orbit the galactic centre. The orbital velocity of our sun is about 828,000 km/hr, a velocity that makes our rockets look like snails, but it takes something like 230,000,000 years to make an orbit, and we are only about half-way out. As I said, galaxies are rather large. So when the galaxies merge, there will be stars going in a lot of different directions until things settle down. There is a NASA simulation in which, over billions of years, the two pass through each other, throwing “stuff” out into interstellar space, then they turn around and repeat the process, except this time the centres merge, and a lot more “stuff” is thrown out into space. The meaning of “stuff” here is clusters of stars. Hundreds of millions of stars get thrown out into space, many of which turn around and come back, eventually to join the new galaxy. 

Because of the distance between stars the chances of stars colliding comes pretty close to zero, however, it is possible that a star might pass by close enough to perturb planetary orbits. It would have to come quite close to affect Earth, as, for example, if it came as close as Saturn, it would only make a minor perturbation to Earth’s orbit. On the other hand, if that close it could easily rain down a storm of comets, etc, from further out, and seriously disrupt the Kuiper Belt, which could lead to extinction-type collisions. As for the giant planets, it would depend on where they were in their orbit. If a star came that close, it could be travelling at such a speed that if Saturn were on the other side of the star it could know little of the passage.

One interesting point is that such a galactic merger has already happened for the Milky Way. In the Milky Way, the sun and the majority of stars are all in orderly near-circular orbits around the centre, but in the outer zones of the galaxy there is what is called a halo, in which many of the stellar orbits are orbiting in the opposite direction. A study was made of the stars in the halo directly out from the sun, where it was found that there are a number of the stars that have strong similarities in composition, suggesting they formed in the same environment, and this was not expected. (Apparently how active star formation is alters their composition slightly. These stars are roughly similar to those in the Large Magellenic Cloud.)  This suggests they formed from different gas clouds, and the ages of these different stars run from 13 to 10 billion years ago. Further, it turned out that the majority of the stars in this part of the halo appeared to have come from a single source, and it was proposed that this part of the halo of our galaxy largely comprises stars from a smaller galaxy, about the size of the Large Magellenic Cloud that collided with the Milky Way about ten billion years ago. There were no comments on other parts of the halo, presumably because parts on the other side of the galactic centre are difficult to see.

It is likely, in my opinion, that such stars are not restricted to the halo. One example might be Kapteyn’s star. This is a red dwarf about eleven light years away and receding. It, too, is going “the wrong way”, and is about eleven billion years old. It is reputed to have two planets in the so-called habitable zone (reputed because they have not been confirmed) and is of interest in that since the star is going the wrong way, presumably as a consequence of a galactic merger, this shows the probability running into another system sufficiently closely to disrupt the planetary system is of reasonably low probability.

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.

Space News

There were two pieces of news relating to space recently. Thirty years ago we knew there were stars. Now we know there are exoplanets and over 4,000 of them have been found. Many of these are much larger than Jupiter, but that may be because the bigger they are, the easier it is to find them. There are a number of planets very close to small stars for the same reason. Around one giant planet there are claims for an exomoon, that is a satellite of a giant planet, and since the moon is about the size of Neptune, i.e.the Moon is a small giant in its own right, it too might have its satellite: an exomoonmoon. However, one piece of news is going to the other extreme: we are to be visited by an exocomet. Comet Borisov will pass by within 2 A.U. of Earth in December. It is travelling well over the escape velocity of the sun, so if you miss it in December, you miss it. This is of some interest to me because in my ebook “Planetary Formation and Biogenesis” I outlined the major means I believe were involved in the formation of our solar system, but also listed some that did not leave clear evidence in our system. One was exo-seeding, where something come in from space. As this comet will be the second “visitor” we have recorded recently, perhaps they are more common than I suspected.

What will we see? So far it is not clear because it is still too far away but it appears to be developing a coma. 2 A.U. is still not particularly close (twice the distance from the sun), so it may be difficult to see anyway, at least without a telescope. Since it is its first visit, we have no real idea how active it will be. It may be that comets become better for viewing after they have had a couple of closer encounters because from our space probes to comets in recent times it appears that most of the gas and dust that forms the tail comes from below the surface, through the equivalent of fumaroles. This comet may not have had time to form these. On the other hand, there may be a lot of relatively active material quite loosely bound to the surface. We shall have to wait and see.

The second piece of news was the discovery of water vapour in the atmosphere of K2-18b, a super-Earth that is orbiting an M3 class red dwarf that is a little under half the size of our sun. The planet is about eight times the mass of earth, and has about 2.7 times the radius. There is much speculation about whether this could mean life. If it has, with the additional gravity, it is unlikely that, if it did develop technology, it would be that interested in space exploration. So far, we know there is probably another planet in the system, but that is a star-burner. K2-18b orbits its star in 33 days, so birthdays would come round frequently, and it would receive about five per cent more solar radiation than Earth does, although coming from a red dwarf, there will be a higher fraction of infra-red light and less visible.

The determination of the water could be made because first, the star is reasonably bright so good signals can be received, second, the planet transits across the star, and third, the planet is not shrouded with clouds. What has to happen is that as the planet transits, electromagnetic radiation from the star is absorbed by any molecule at the frequency determined by the bond stretching or bending energies. The size of the planet compared with its mass is suggestive of a large atmosphere, i.e.it has probably retained some of the hydrogen and helium of the accretion disk. This conclusion does have risks because if it were primarily a water or ice world (water under sufficient pressure forms ice stable at quite high temperatures) then it would be expected to have an even greater size for the mass.

The signal was not strong, in part, from what I can make out, it was recorded in the overtone region of the water stretching frequency, which is of low intensity. Accordingly, it was not possible to look for other gases, but the hope is, when the James Webb telescope becomes available and we can look for signals in the primary thermal infrared spectrum, this planet will be a good candidate.So, what does this mean for the possibilities of life? At this stage, it is too early to tell. The mechanism for forming life as outlined in my ebook, “Planetary Formation and Biogenesis” suggests that the chances of forming life do not depend on planetary size, as long as there is sufficient size to maintain conditions suitable for life, such as an adequate atmospheric pressure, liquid water, and the right components, and it is expected that there will be an upper size, but we do not know what that will be, except again, water must be liquid at temperatures similar to ours. That would eliminate giants. However, more precise limits are more a matter of guess-work. The composition of the planet may be more important. It must be able to support fumaroles and I suspect it should have pre-separated felsic material so that it can rapidly form continents, with silica-rich water emitted, i.e.the type of water that forms silica terraces. That is because the silica acts as a template to make ribose. Ribose is important for biogenesis because something has to link the nucleobases to the phosphate chain. The nucleobases are required because they alone are the materials that form with the chemicals likely to be around, and they alone form multiple hydrogen bonds that can form selectively and add as a template for copying, which is necessary for retaining useful information. Phosphate is important because it alone has three functional sites – two to form a polymer, and one to convey solubility. Only the furanose form of the sugar seems to manage the linkage, at least under conditions likely to have been around at the time and ribose is the only sugar with significant amounts of the furanose form. I believe the absence of ribose means the absence of reproduction, which means the absence of life. But whether these necessary components are there is more difficult to answer.

Gravitational Waves, or Not??

On February 11, 2016 LIGO reported that on September 14, 2015, they had verified the existence of gravitational waves, the “ripples in spacetime” predicted by General Relativity. In 2017, LIGO/Virgo laboratories announced the detection of a gravitational wave signal from merging neutron stars, which was verified by optical telescopes, and which led to the award of the Nobel Prize to three physicists. This was science in action and while I suspect most people had no real idea what this means, the items were big news. The detectors were then shut down for an upgrade to make them more sensitive and when they started up again it was apparently predicted that dozens of events would be observed by 2020, and with automated detection, information could be immediately relayed to optical telescopes. Lots of scientific papers were expected. So, with the program having been running for three months, or essentially half the time of the prediction, what have we found?

Er, despite a number of alerts, nothing has been confirmed by optical telescopes. This has led to some questions as to whether any gravitational waves have actually been detected and led to a group at the Neils Bohr Institute at Copenhagen to review the data so far. The detectors at LIGO correspond to two “arms” at right angles to each other running four kilometers from a central building. Lasers are beamed down each arm and reflected from a mirror and the use of wave interference effects lets the laboratory measure these distances to within (according to the LIGO website) 1/10,000 the width of a proton! Gravitational waves will change these lengths on this scale. So, of course, will local vibrations, so there are two laboratories 3,002 km apart, such that if both detect the same event, it should not be local. The first sign that something might be wrong was that besides the desired signals, a lot of additional vibrations are present, which we shall call noise. That is expected, but what was suspicious was that there seemed to be inexplicable correlations in the noise signals. Two labs that far apart should not have the “same” noise.

Then came a bit of embarrassment: it turned out that the figure published in Physical Review Letters that claimed the detection (and led to Nobel prize awards) was not actually the original data, but rather the figure was prepared for “illustrative purposes”, details added “by eye”.  Another piece of “trickery” claimed by that institute is that the data are analysed by comparison with a large database of theoretically expected signals, called templates. If so, for me there is a problem. If there is a large number of such templates, then the chances of fitting any data to one of them is starting to get uncomfortably large. I recall the comment attributed to the mathematician John von Neumann: “Give me four constants and I shall map your data to an elephant. Give me five and I shall make it wave its trunk.” When they start adjusting their best fitting template to fit the data better, I have real problems.

So apparently those at the Neils Bohr Institute made a statistical analysis of data allegedly seen by the two laboratories, and found no signal was verified by both, except the first. However, even the LIGO researchers were reported to be unhappy about that one. The problem: their signal was too perfect. In this context, when the system was set up, there was a procedure to deliver artificially produced dummy signals, just to check that the procedure following signal detection at both sites was working properly. In principle, this perfect signal could have been the accidental delivery of such an artifical signal, or even the deliberate insertion by someone. Now I am not saying that did happen, but it is uncomfortable that we have only one signal, and it is in “perfect” agreement with theory.

A further problem lies in the fact that the collision of two neutron stars as required by that one discovery and as a source of the gamma ray signals detected along with the gravitational waves is apparently unlikely in an old galaxy where star formation has long since ceased. One group of researchers claim the gamma ray signal is more consistent with the merging of white dwarfs and these should not produce gravitational waves of the right strength.

Suppose by the end of the year, no further gravitational waves are observed. Now what? There are three possibilities: there are no gravitational waves; there are such waves, but the detectors cannot detect them for some reason; there are such waves, but they are much less common than models predict. Apparently there have been attempts to find gravitational waves for the last sixty years, and with every failure it has been argued that they are weaker than predicted. The question then is, when do we stop spending increasingly large amounts of money on seeking something that may not be there? One issue that must be addressed, not only in this matter but in any scientific exercise, is how to get rid of the confirmation bias, that is, when looking for something we shall call A, and a signal is received that more or less fits the target, it is only so easy to say you have found it. In this case, when a very weak signal is received amidst a lot of noise and there is a very large number of templates to fit the data to, it is only too easy to assume that what is actually just unusually reinforced noise is the signal you seek. Modern science seems to have descended into a situation where exceptional evidence is required to persuade anyone that a standard theory might be wrong, but only quite a low standard of evidence to support an existing theory.

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.

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.