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I am creating a world where the poles of the planet face extreme radiation while other places not so much. If I understand correctly the earth could have a weaker magnetic field if the core became more solid, which could happen if there was more pressure in the outer core, which is created if the weight of the crust, mantle and of the stiffer mantle. How could this happen in a world that isn't much different than ours?

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  1. The magnetic field of the Earth actually does become weaker and even zero from time to time. See geomagnetic reversal. (Geomagnetic reversals are not predictable; sometimes they occur after only 100,000 years, sometimes tens of millions of years may pass between two reversals. The most recent magnetic field reversal occurred some 780,000 years ago.)

  2. The magnetic field of the Earth is in real life becoming weaker and weaker right now; the decrease in intensity became obvious in the 1800s and has accelerated in the 2000s.

  3. But ... the magnetic field is only responsible for a small part of the protection against cosmic and solar radiation, and it does exactly nothing against really harmful radiation such as gamma rays. The bulk of the protection is provided by the atmosphere. Although Earth's magnetic field goes to zero every few hundred thousand years or so, this has never had any observable impact on the living things of Earth.

    What the magnetic field does is protect the atmosphere itself over geological time spans.

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The magnetic field is the result of the movement of the planet's liquid inner layers combined with the planet's rotation. The liquid motion is due to convection as heat leaves the outer layer of the core allowing hotter liquid below to rise and circulate. This movement of molten metal creates electric current that in turn creates a magnetic field as the earth spins.

A lot of the heat involved in the liquid movement actually came from the intense pressures involved in the planet's original formation - so increasing pressure further might not have the weakening effect you expect, in fact it would probably increase temperature and keep liquids from solidifying longer.

So you want to reduce the temperature of the core in order to slow/stop convection and weaken/eliminate the magnetic field as a result. To cool something down, you have to take heat away from it. Earth radiates its heat into space very slowly, so you'd need something to accelerate that process.

Another thing that keeps the planet's interior hot is radioactive decay of elements like potassium and uranium. But these elements don't last forever, and they aren't being replaced by any natural processes, so eventually their effect will probably wear out and cooling will accelerate. That happens sooner if these elements are neutralized in some way other than their natural decay process.

You can also alter the planet's rotation. It's believed that the much weaker magnetic field around Venus, despite its similar composition and core mass, is because of how much more slowly it spins than Earth.

Alternately, you could ignore heat and look at the elemental composition of the liquids - they're molten metals, iron and nickel, which is why their movement creates current. If it was made of less of those materials and more of some other less conductive material then it might generate less current producing a weaker magnetic field.

Each of these things would have to be substantially different from their state on our Earth. If you want a planet that's only slightly different from Earth then your best bet is probably to do a little bit of all of these things.

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  • $\begingroup$ "It's believed that the much weaker magnetic field around Venus, despite its similar composition and core mass, is because of how much more slowly it spins than Earth.": It's only a few times slower than Mercury's, and Mercury is much smaller, yet Mercury has an active core dynamo. It's likely more a result of the lack of convection, due to the lack of tectonic activity and resulting slow loss of heat through the mantle, possibly caused by a resurfacing event. $\endgroup$ Commented May 9 at 14:31
  • $\begingroup$ True! Though another interesting thought: most of the magnetic field around Venus is actually due to the planet's interaction with the sun's magnetic field. With Mercury being so much closer to the sun, I wonder how much more that contributes to the difference as well. $\endgroup$
    – talrnu
    Commented May 9 at 14:45
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    $\begingroup$ See Kelvin's hundredfold error in estimating the age of the Earth due to not including radioactive decay. A more modern summary from Penn State: "the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle" $\endgroup$ Commented May 9 at 19:37

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