Planet Core Question

[SilverTiger]

So, I am designing an imaginary world with my sister, and I told her that'd I'd handle most of the in-depth science stuff. I got most of that down, but I just ran into a problem. See, this planet has a circumference of 16,400 miles, much smaller than Earth, and a gravity of 9.75m/s^2. Which means its core has a combination of iron and some other, denser metal. I could go on without this info, but it is bugging me a lot. So, I turn to the science-lovers of 17th Shard for help. Could some people who know more about geology than me figure this puzzle out? Please?

Also, the composition of the crust and atmosphere are pretty much the same as Earth, if it helps.

[Mulk]

If you're wanting an earth like planet that includes the magnetic field that we have to deflect/absorb space radiation it pretty much has to be a magnetic material.  NIckel and cobalt are both denser than iron, the other rare earth elements typical in rare-earth element magnets are less dense.  So, while I'm not an expert in the field, perhaps you'd want heavier amounts of those other magnetic elements? You might want to bump up the size of the planet to something closer to earth size to account for the lack of variance in gravity.

I suppose another possibility is there is more by percent of the denser elements in the crust and perhaps mantle of the world you are proposing.  Or, physics may be a bit different there, the periodic table for your made up world may contain an imaginary superdense magnetic metal element that accounts for the mass.  When you're dealing with fantasy, you don't always have to be tied fully to real world science...you can have a little fun with it.  Tolkien gave us a superstrong but lightweight metal in mithril, Marvel gave us vibranium and...I forget the element Stark synthesized in Iron Man 2, there's other stuff like that floating around.  Such a metal wouldn't have much in the way of weapons application at least in the handheld weapon era - it would be too heavy for most, same with armor you wear, but making a city gate out of it to resist siege damage...

[SilverTiger]

I didn't know that nickel and cobalt are magnetic... and I was hoping to avoid just using an imaginary metal.

[Mulk]

5 hours ago, SilverTiger said:

I didn't know that nickel and cobalt are magnetic... and I was hoping to avoid just using an imaginary metal.

well, they are, just not as abundant as iron. Same with the rare earth metals that also can be used for magnets, but they are mostly less dense than the main trio. The Earth's core is thought to be an iron-nickel alloy due to some oddities in the measurements scientists have taken (it's probably not pure iron), so it makes sense that a different planet with different mixes of the same sorts of elements would have a similar effect.  I had to look up the metal densities as I wasn't sure before I mentioned they are both denser than iron. 

The densest non-radioactive element (or, at least, its half life is so long that radioactive decay has never been observed in any of its naturally occurring isotopes except one) is osmium but it's ridiculously rare and also non-magnetic. I doubt it would make sense to plug it into a core structure and still retain the deflecting capabilities of a strong magnetic field.

Good luck to you.

[Glamdring804]

I guess this is kind of late, but you could mitigate this by having the core comprise a disproportionately large amount of the planet's volume. Mercury is in a similar situation, where the planet lost most of its mantle during formation, likely from a colossal impact. I haven't done the math, but the core would probably have to be pretty big to compensate. Also, note that with the smaller size and smaller mantle, there probably wouldn't be very strong tectonic activity, meaning mountains would be smaller, and volcanos would be rare. 

Edited November 30, 2017 by OoklaTheFoeHammer

[SilverTiger]

8 minutes ago, OoklaTheFoeHammer said:

I guess this is kind of late, but you could mitigate this by having the core comprise a disproportionately large amount of the planet's volume. Mercury is in a similar situation, where the planet lost most of its mantle during formatnio, likely from a colossal impact. I haven't done the math, but the core would probably have to be pretty big to compensate. Also, note that with the smaller size and smaller mantle, there probably wouldn't be very strong tectonic activity, meaning mountains would be smaller, and volcanos would be rare. 

Thanks so much, that's a huge help. Lessened tectonic activity is a plus, and there likely won't be many volcanoes anyway.