What Happens Inside Ice Giants?

Uranus and Neptune are a bit weird, although in fairness that may be because we don’t really know much about them. Our information is restricted to what we can see in telescopes (not a lot) and the Voyager fly-bys, which, of course, also devoted a lot of attention to the Moons, since a lot of effort was devoted to images. The planets are rather large featureless balls of gas and cloud and you can only do so much on a “zoom-past”. One of the odd things is the magnetic fields. On Earth, the magnetic field axis corresponds with the axis of rotation, more or less, but not so much there. Earth’s magnetic field is believed to be due to a molten iron core, but that could not occur there. That probably needs explaining. The iron in the dust that is accreted to form planets is a fine powder; the particles are in the micron size. The Earth’s core arises because the iron formed lumps, melted, and flowed to the core because it is denser. In my ebook “Planetary Formation and Biogenesis” I argue that the iron actually formed lumps in the accretion disk. While the star was accreting, the region around where Earth is reached something like 1600 degrees C, above the melting point of iron, so it formed globs. We see the residues of that in the iron-cored meteorites that sometimes fall to Earth. However, Mars does not appear to have an iron core. Within that model, the explanation is simple. While on Earth the large lumps of iron flowed towards the centre, on Mars, since the disk temperature falls off with distance from the star, at 1.5 AU the large lumps did not form. As a consequence, the fine iron particles could not move through the highly viscous silicates, and instead reacted with water and oxidised, or, if you prefer, rusted.

If the lumps that formed for Earth could not form at Mars because it was too far away from the star, the situation was worse for Uranus. As with Mars, the iron would be accreted as a fine dust and as the ice giants started to warm up from gravitational collapse, the iron, once it got to about 500 degrees Centigrade, would rapidly react with the water and oxidise to form iron oxides and hydrogen. Why did that not happen in the accretion disk? Maybe it did, and maybe at Mars it was always accreted as iron oxides, but by the time it got to where Earth is, there would be at least ten thousand times more hydrogen than iron, and hot hydrogen reduces iron oxide to iron. Anyway, Uranus and Neptune will not have an iron core, so what could generate the magnetic fields? Basically, you need moving electric charge. The planets are moving (rotating) so where does the charge come from?

The answer recently proposed is superionic ice. You will think that ice melts at 0 degrees Centigrade, and yes, it does, but only at atmospheric pressure. Increase the pressure and it melts at a lower temperature, which is how you make snowballs. But ice is weird. You may think ice is ice, but that is not exactly correct. There appear to be about twenty ices possible from water, although there are controversial aspects because high pressure work is very difficult and while you get information, it is not always clear about what it refers to. You may think that irrespective of that, ice will be liquid at the centre of these planets because it will be too hot for a solid. Maybe.

In a recent publication (Nature Physics, 17, 1233-1238 November 2021) authors studied ice in a diamond anvil cell at pressures up to 150 GPa (which is about 1.5 million times greater than our atmospheric pressure) and about 6,500 degrees K (near enough to Centigrade at this temperature). They interpret their observations as there being superionic ice there. The use of “about” is because there will be uncertainty due to the laser heating, and the relatively short times up there. (Recall diamond will also melt.)

A superionic ice is proposed wherein because of the pressure, the hydrogen nuclei can move about the lattice of oxygen atoms, and they are the cause of the electrical conduction. These conditions are what are expected deep in the interior but not at the centre of these two planets. There will presumably be zones where there is an equilibrium between the ice and liquid, and convection of the liquid coupled with the rotation will generate the movement of charge necessary to make the magnetism. At least, that is one theory. It may or may not be correct.

Is There a Planet 9?

Before I start, I should remind everyone of the solar system yardstick: the unit of measurement called the Astronomical Unit, or AU, which is the distance from Earth to the Sun. I am also going to define a mass unit, the emu, which is the mass of the Earth, or Earth mass unit.

As you know, there are eight planets, with the furthest out being Neptune, which is 30 AU from the Sun. Now the odd thing is, Neptune is a giant of 17 emu, Uranus is only about 14.5 emu, so there is more to Neptune than Uranus, even though it is about 12 AU further out. So, the obvious question is, why do the planets stop at Neptune, and that question can be coupled with, “Do they?” The first person to be convinced there had to be at least one more was Percival Lowell, he of Martian canal fame, and he built himself a telescope and searched but failed to find it. The justification was that Neptune’s orbit appeared to be perturbed by something. That was quite reasonable as Neptune had been found by perturbations in Uranus’ orbit that were explained by Neptune. So the search was on. Lowell calculated the approximate position of the ninth planet, and using Lowell’s telescope, Clyde Tombaugh discovered what he thought was planet 9.  Oddly, this was announced on the anniversary of Lowell’s birthday, Lowell now being dead. As it happened, this was an accidental coincidence. Pluto is far too small to affect Neptune, and it turns out Neptune’s orbit did not have the errors everyone thought it did – another mistake. Further, Neptune, as with the other planets has an almost circular obit but Pluto’s is highly elliptical, spending some time inside Neptune’s orbit and sometimes as far away as 49 AU from the Sun. Pluto is not the only modest object out there: besides a lot of smaller objects there is Quaoar (about half Pluto’s size) and Eris (about Pluto’s size). There is also Sedna, (about 40% Pluto’s size) that has an elliptical orbit that varies the distance to the sun from 76 AU to 900 AU.

This raises a number of questions. Why did planets stop at 30 AU here? Why is there no planet between Uranus and Neptune? We know HR 8977 has four giants like ours, and the Neptune equivalent is about 68 AU from the star, and that Neptune-equivalent is about 6 times the mass of Jupiter. The “Grand Tack” mechanism explains our system by arguing that cores can only grow by major bodies accreting what are called planetesimals, which are bodies about the size of asteroids, and cores cannot grow further out than Saturn. In this mechanism, Neptune and Uranus formed near Saturn and were thrown outwards and lifted by throwing a mass of planetesimals inwards, the “throwing”: being due to gravitational interactions. To do this there had to be a sufficient mass of planetesimals, which gets back to the question, why did they stop at 30 AU?

One of the advocates for Planet 9 argued that Planet 9, which was supposed to have a highly elliptical orbit itself, caused the elliptical orbits of Sedna and some other objects. However, this has also been argued to be due to an accidental selection of a small number of objects, and there are others that don’t fit. One possible cause of an elliptical orbit could be a close encounter with another star. This does happen. In 1.4 million years Gliese 710, which is about half the mass of the Sun, will be about 10,000 AU from the Sun, and being that close, it could well perturb orbits of bodies like Sedna.

Is there any reason to believe a planet 9 could be there? As it happens, the exoplanets encylopaedia lists several at distances greater that 100 AU, and in some case several thousand AU. That we see them is because they are much larger than Jupiter, and they have either been in a good configuration for gravitational lensing or they are very young. If they are very young, the release of gravitational energy raises them to temperatures where they emit yellow-white light. When they get older, they will fade away and if there were such a planet in our system, by now it would have to be seen by reflected light. Since objects at such great distances move relatively slowly they might be seen but not recognized as planets, and, of course, studies that are looking for something else usually encompass a wide sky, which is not suitable for planet searching.For me, there is another reason why there might be such a planet. In my ebook, “Planetary Formation and Biogenesis” I outline a mechanism by which the giants form, which is similar to that of forming a snowball: if you press ices/snow together and it is suitably close to its melting point, it melt-fuses, so I predict the cores will form from ices known to be in space: Jupiter – water; Saturn – methanol/ammonia/water; Uranus – methane/argon; Neptune – carbon monoxide/nitrogen. If you assume Jupiter formed at the water ice temperature, the other giants are in the correct place to within an AU or so. However, there is one further ice not mentioned: neon. If it accreted a core then it would be somewhere greater than 100 AU.  I cannot be specific because the melting point of neon is so low that a number of other minor and ignorable effects are now significant, and cannot be ignored. So I am hoping there is such a planet there.

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.

The Formation of the Giant Planets

Before I can discuss how we got the elements required for life delivered to Earth, it is necessary to work out how the planets formed, and why we have what we have. While the giant planets are almost certainly not going to have life, at least not as we would recognise it, they are important because what we have actually gives some important clues as to how planets form, and hence how common life will be, and why so many exoplanetary systems are so different from ours. The standard theory says a core accreted, then when it got sufficiently big, which calculations have at about 10 to 12 times the Earth mass it starts accreting gas in substantial amounts and it grows very slowly, the problem restricting growth being how it can compress its volume and get rid of the heat so generated. After a number of million years, its mass gets big enough, and it accretes everything that comes into range. Apart from the rather slow time the calculations give, that general description is almost certainly essentially correct. The reason we believe the core has to get to about 10 – 12 Earth masses before disk gases get accreted in serious amounts is because the evidence is the Neptune and Uranus have about 2 Earth masses of hydrogen and helium. So far, reasonably good. The fact that there is evidence the calculations are wrong is not damning; the fact that if the mechanism is not properly understood in close detail then the calculations will inevitably be wrong. The original calculations had these stages taking about 10 My to get to a planet the size of Jupiter. The very first calculations had it taking about a billion years to get to Neptune, but that obviously cannot be right because the disk gases had long gone before that. The real problem is how to get to the cores.

The standard theory says they started by the accretion of a distribution of planetesimals that were formed by the accretion of dust, and therefore were distributed according to the dust concentrations through the disk. There are two problems with this for me. First, we see these disks, and we see them because their dust scatters light. Some such disks are 30 My old and still dusty, so the dust itself is not rapidly accreting, altough often there are bands where there seems to be little dust. The second problem is that there is no recognized mechanism by which the dust can accrete and stick together strongly enough not to be disrupted by any other dustball that collides with it. Mathematics indicate that such dustballs, if they reach about 2 cm size, erode from gas motion relative to them.

So, how did the cores form? I think we have evidence from the fact their systems all have different compositions. The theory I outlined in my ebook Planetary Formation and Biogenesis goes like this. The dust that comes from deep space has a lot of very fluffy ice around it, and this has many pores. Within those pores, and around the ice, are the ices of other volatiles. (Such compound ices have been made in the lab, and their behaviour verified.) As the ices come in, the more volatile ones start subliming away at temperatures a little above their melting point, and hydrogen has even been maintained as an ice enclosed in water ice pores up to a little under 15 degrees K, which is well above its boiling point. So, as the ices come in and the disk gets gradually hotter, the ices selectively boil away. The relevant temperatures are: neon (~25 K); nitrogen and carbon monoxide (~65 K); argon and methane (~ 85); ammonia and methanol (~170 K); water (273 K).

What I suggest happened is the same mechanism that forms snowballs started planetary accretion. When snow is squeezed at a temperature a little below its triple point, the pressure causes localised melt fusion, and the particles stick together. In this case we have several ices entrained in the ice/dust, and I suggest the same happens for each ice. This has consequences. The temperature profile in these disks is observed to be where the temperature T is proportional to r^-0.75, r the distance from the star, with a significant variation, which is expected because the faster the gas comes in, or the warmer it was to start with, the further out a specific temperature will be found, while the denser the gas flow, the greater the temperature gradient. Now, because Jupiter is the biggest planet, and water ice is the most common single material, assume (like everyone else) that Jupiter is more or less where the water ice so fuses. If we assume the average disk temperature profile (actually r^-0.82 is better for what follows for our solar system) then the remaining giants are quite close to where they are supposed to be. So the mechanism is that ices come together, they hit, the collisional energy melts an ice in the impact zone such that they rapidly refreeze, and the particles stick together. To predict where the planets should be I put Jupiter at 5.2 A.U. as a water-ice core sets the constant of proportionality. The next ice is ammonia/methanol/water, which could melt between 164 – 195 oK, which suggests that Saturn should be between 7.8 – 9.6 A.U. Saturn has a semimajor axis of 9.5 A.U. The next ice out is methane/argon, with melting between 84 – 90 oK. The calculated position of Uranus is between 20-21.7 A.U., while the observed position is 19.2 A.U. The next ice, carbon monoxide/nitrogen melts between 63 – 68 oK, which predicts Neptune to be between 28.1 – 30.7 A.U., and Neptune has a semimajor axis of 30 A.U. Note that as they form, we excpect some movement through gravitational interactions and the effects of the gas.

This means the Jovian system is both nitrogen and carbon deficient, apart from Jupiter itself which accreted gas from the disk, and the very tenuous atmosphere of Europa is reported to actually have more sodium in it than nitrogen. Sorry, but no life under the ice at Europa because there is nothing much with which to have organic chemistry. The reason for the lack of atmosphere is the satellites have nothing in them that could form a gas at those temperatures. The major component is hydroxyl, from the photochemiclo deomposition of water, but this is extremely reactive and does not build up.

The Saturnian system has water, plus methanol and ammonia. The ammonia has been seen at Enceladus, and its decay product during UV radiation, nitrogen, is the main gas of Titan. The methane there will come from reactions of methanol and rocks. The Uranian system has methane and argon. Unfortunately the satellites are too small to have atmospheres, and Neptune’s satellites are similar, as while Triton has nitrogen volcanoes, it is probably a captured Kuiper Belt object, as it orbits Neptune the wrong way. However the atmosphere of Neptune has more nitrogen than expected from the accretion disk, whereas Uranus does not. More specific details are in the ebook, but in my opinion, the above describes reasonably well how these systems formed, and why they have the chemical composition we see. The Kuiper belt objects are the same as the core of Neptune, and are essentially a “tail” of the accretion process.

Finally, the perceptive will notice the possibility of two further zones of accretion. Further out, there will be a zone where neon trapped in ice might accrete, and even further out, because hydrogen can be trapped in ice even up to about 15 K, a hydrogen accretion zone where the liquid hydrogen dissolves neon, which then refreezes. The latter is not impossible. There are exoplanets a few hundred A.U. from the star, or, say, over ten times further than Neptune. On the other hand, there is a further mechanism that could form them, namely collapse of the disk, which presumably starts the star. So I should be able to predict where this planet 9 is? That is not so easy because the temperature of the disk follows the above relationship only approximately, and when we get down to these low temperatures, any deviation, including the initial temperature of the gas (which in the above relation is taken as zero, but it isn’t) suddenly becomes important. My published estimate for a neon-based planet is at a hundred A.U., with a possible minus 30 and a plus fifty A.U. Not exactly helpful. If I knew thie initial temperature, and the rate of heat loss from the disk by radiation at those distances I could be far more precise.