Magnetic Fields, Metallic Hydrogen
      The difference between metals and non-metals has to do with how hard, or easy, it is to move electrons around in the outer part of an atom, or from one atom to another.
      Big atoms, with lots of electrons, like iron, can easily have the outermost electron moved around. There are lots of other electrons between it and the nucleus, all pushing it away from the nucleus, and the attraction of the nucleus is reduced, by the fact that the outermost electron is so far from it, so it is easy to move it around, or tear it away.
      So easy, that in a liquid or solid form of such a material, where the atoms are ‘in contact’, the outermost electron can move from atom to atom to atom, almost as though there is a free ‘gas’ of electrons moving through the material.
      It is this freely moving electron cloud that causes metals to have the heat and electrical conductivity that makes us call them metals.
      IN HYDROGEN AND HELIUM, this does not happen. They have only one or two electrons, very close to the nucleus, held onto very tightly by the nucleus, and no other electrons in between them and the nucleus, trying to pull them away. As a result, it is hundreds of times harder to move electrons around, inside these atoms, than in an iron atom. And in a liquid or solid state, the electrons stay with the atoms they belong to, and do not wander from atom to atom, so there is no heat or electrical conductivity associated with such motion, and we have a NON-metal.
      HOWEVER, deep inside Jupiter, the weight compressing the gas (hydrogen) is tens of times the weight compressing the core of the Earth, and the liquid hydrogen is compressed to ten, twenty, maybe even thirty or forty times the normal density of liquid hydrogen. That’s why Jupiter is twice as dense as Saturn, even though it probably has a greater proportion of hydrogen. It is so much heavier, and so much more compressed under its weight, that the hydrogen in the core is two or three times denser.
      But if you have 30 or 40 hydrogen atoms in the space normally occupied by ONE atom, each of the electrons in each of those atoms must be around 3 times closer to the nuclei of other hydrogen atoms than they normally are, to their own nucleus. And it might not be surprising if they got ‘confused’ as to which atom they belong to -- electrons IN THAT SORT OF SITUATION can move from atom to atom to atom, producing METALLIC HYDROGEN. (note that actual physicists do not use such anthropomorphic reasoning, but instead, what ordinary people would consider horrific math; but the results are the same)
      Because Jupiter has such a strong magnetic field, if that field is produced, as we feel certain it is, by a liquid dynamo (motions in a liquid metallic substance), then there must be hundreds of Earth masses of ‘molten’ metal inside Jupiter. But because of its low density, the ONLY thing that there can be hundreds of Earth masses inside Jupiter, is hydrogen. So, it must be metallic hydrogen.
      For Saturn, the field isn’t nearly as strong, or as big, although is as strong as the Earth’s field, and much larger, so you do need quite a few Earth masses of electrically conducting fluid (metallic hydrogen) to explain its magnetic field. And, because it is a very big planet, and considerably compressed under its weight, although not nearly so much as Jupiter, so that it is only half as dense, it should indeed have some metallic hydrogen.
      For Uranus and Neptune, the weight compressing the bottom part of the hydrogen layer isn’t nearly as large -- there’s not as much hydrogen, so you don’t go down through as much, before you are in the core of the planet, and there’s not as much weight. For Neptune, in particular, where most of the planet must be heavy liquids like water, it is essentially impossible for any metallic hydrogen to exist. The pressures at the bottom of the hydrogen layer are less than laboratory experiments which failed to make metallic hydrogen. And for Uranus, although the hydrogen should go down twice as far, because there should be twice as much of it, it’s also pretty iffy.
      So, for Jupiter and Saturn, their powerful magnetic fields REQUIRE the existence of metallic hydrogen -- hundreds of Earths for Jupiter, a few tens at most for Saturn. But for Uranus and Neptune, the magnetic fields require another explanation -- salt-water. Probably, the water (and similar materials) outer core is at least partially liquid, and electrically conducting.
      SUMMARY:
      Terrestrial planets, convective motions in a molten iron core produces the magnetic field. If no field, no molten iron core. If a field, almost certainly a molten iron core.
      For Jupiter and Saturn, metallic hydrogen produces the magnetic field.
      For Uranus and Neptune (and large moons of the outer planets), electrically conducting fluids such as seawater produce the magnetic field.