User blog:Cerne/Ferromagnetism

I typed in my last entry that I had some more data on my conworld. Well... Not exactly. I have been doing some more research on it, though.

I had previously been trying to decide on exactly what my core density could be, and I thought I had settled on Palladium or Ruthenium. I posted a thread in the ZBB (this was earlier) asking about the atmosphere of my planet and somehow the topic switched back to what the planet's core was made up of. As had been done previously, it was suggested that elements like Ruthenium and Palladium were too rare in the universe and that it would be very unlikely for the early planet-forming nebulae to have such elements in them. There was some optimism toward the idea of using denser Period 5 Transitional Metals but generally Period 4 Transitionals were favoured. In particular, something with iron and/or nickel. Or something with copper in it, I think.

Ferromagnetism was also brought up but I ignored it at first. A nickel-cobalt alloy was brought up, which seemed interesting, but this was primarily because it was ferromagnetic. In any case, ferromagnetism and magnetic fields kept coming up so I figured I probably shouldn't keep ignoring them. I looked at Wikipedia's article on Ferromagnetism and I found a list of metallic elements that were ferromagnetic. Among them was cobalt, iron, and nickel, amongst some oxidized alloys and fewer non-oxidized alloys. There was only one Period 5 transitional metal, Yttrium, and three lanthanides: europium, gadolinium, and dysprosium. That was pretty much it. No, palladium and ruthenium weren't in there at all. Using this alloy density calculator I found one time while looking through Google for alloy densities, I got a density of 8.9 g/cc. Which is better than the 7.874 g/cc I once had, back before I decided on using ruthenium or palladium, but I would like to do better. There is also something called a Heusler Alloy which is an alloy made up of non-ferromagnetic elements put together in such a way that they become ferromagnetic. Most have manganese in them and apparently their ferromagnetism has something to do with the ions from the manganese atom being centralized in the body of the alloy's crystalline structure. The alloy with the densest elements in it was one with manganese, tin, and two parts palladium. Using the alloy calculator, it gives me a density of 9.35883 g/cc. Which is better than the 8.9 g/cc of the nickel-cobalt alloy, but still not quite there yet.

When I was looking into the size of the Earth's core in comparison to the rest of the planet, I started with the assumption that it was 20% the radius of the entire Earth. Iron has a density of 7.874 g/cc but the overall density of the Earth's core is 8.3 g/cc so the other 0.426 g/cc must be the nickel part of the nickel-iron alloy. Apparently, from some other online source I found but can't remember, Earth's core is composed of 80% iron and around 20-25% nickel. This would mean that the alloy making up the core would be four parts iron and one part nickel. The percent of either element could certainly vary, though.

According to Physicalgeography.net, Earth's oceanic crust is composed of basalt and is approx. 3 g/cc while Earth's continental crust is composed of granite and is approx. 2.7 g/cc. Continental crust is also (understandably) lighter. It is better to round up the 2.7 number for simplicity's sake but I want to stick with 2.7 for the time being, and I will explain a little bit later.

Earth's overall mean density is 5.515 g/cc but the number is often rounded down to 5.5 so I will use that. So the object is to find out how close my planet's mean density is to its core density. My planet's mean density is 7.452 g/cc and this is not likely to change. Back before I understood how mean density works (or, rather, had forgotten how mathematical means work), I thought it was OK to use pure iron for my planet's core since its density comes close to the mean density I had come up with long before that. As it turned out, an iron core for a planet with a radius of 4100 km would be 75% the total size of the planet. That is larger than Mercury's core in relation to its overall radius. Apparently this leads to some problems later on - namely, the planet becomes geologically inactive in under one billion years. This is not good.

I figured out a formula for determining what should be a reasonable core density for a planet with Earth-like geological activity. Basically you take your planet's mean density (you will have come up with this first, anyway, upon trying to determine its gravity) and you multiply it by 2. Then you subtract the density of the crust. As was mentioned earlier, I probably should use 3 g/cc because that is a good mean number - Earth's oceanic crust is actually a little bit denser than this - but if we use 2.7 then we get the following: (5.5*2) - 2.7 = 8.3 g/cc. This means we would be using continental crust rather than oceanic crust to determine the density of the core. It may not actually work out that way; maybe Earth's mean density is shifted in favour of the lighter density of the crust. Right now, though, my best guess is on continental crust. It at least gives me an even mean density to work with, upon using Earth as an analogy.

Ergo, here is what I get for my planet: (7.452*2) - 2.7 = 12.204 g/cc.

Granted, the magma that makes up my planet's continental crust could be more or less dense than that. I think it is a good estimate. For the record, though, if I had used 3 g/cc as my average for crust density, I would have come up with 11.904 g/cc. Actually...my answer is a smaller number so I might use that instead. It still rounds up to 12, though. Still the same thing.

Another formula I use is 8.3 - 5.5 = 2.8 which allows me to find out how much to count up from the mean to the core density and consequently down from the mean to the density of the planet's crust. In effect, it should determine what the mean density of my own planet's crust will be. I would need to know my planet's exact core density to provide an accurate estimate, but assuming the density of ruthenium at 12.37 g/cc (as an example) I get 2.534 g/cc and round it down to 2.5 g/cc. It might therefore be a good idea to have a core density that is somewhat lower than the density of Ruthenium. I could probably get that with an alloy of some kind, but I would still need a metal that is pretty dense to give me ideally what I am looking for.

Moving my planet's core density closer to the overall mean density will mean a bigger core in proportion to the rest of the planet and I think I explained that in an earlier entry or two. Nonetheless, I would like to look further into the effects of a core that was substantially larger in proportion rather than smaller because it will give me more plausible options. I realize now that I may need an alloy so the nickel-cobalt alloy seems like a good choice. However, I would like to find a third metal to add to the mix to increase the core density and consequently give my planet a longer period of geologic activity than that which was estimated for the solid ball of iron I was imagining in earlier years. 8.9 g/cc might seem better, but if you look at it in relation to Earth it still looks too small a contrast against the mean density I already have. If I do choose a third metal, it will probably have to be ferromagnetic. Or at the least it should not take away any of the ferromagnetic properties of the nickel-cobalt alloy. And it will probably change the geology of the planet's surface from what I was expecting.

This may require another thread on the ZBB...stay tuned for progress.

Thanks for reading.