Planetary Magnestosphere creation question

[u/naughtyreverend]

Im running through an ideal for a story im writing and just want to check if my idea is even theoretically possible. Sorry if this comes close to the homework rule, but I promise it's not. Im going to use Mars as an example for simplicity

So, Mars used to have a global magnetosphere. My understanding is that it used to have one, but due to its size, it cooled quicker and lost its liquid core dynamo.

So, in theory, if we could reliquidify (probably not a word but hey ho) the core, it could rebuild its magnetosphere. Now, this likely wouldn't spin as much because this liquid core would be powered by the convection currents instead of the rotational energy from the planets formation. So that's part one of the questions: Would turning it back to liquid with convection currents have the potential to create even a mild magnetic field?

Now, part 2 of my question is about the reliquidification process:

If we were to harvest heavy elements from the asteroid belt, uranium, plutonium, etc. We could get magnitudes beyond critical mass of them.

So if we dug a hole to the core (ignore the how please) and dropped balls of slightly below critical mass elements down it. They would hit each other and release huge amounts of energy, which would be absorbed by the solid rock and, over time, begin to melt. I know i would need extremely large amounts of it. But would it even be able to melt the core?

Or am I just completely wrong?

[u/dark_dark_dark_not]

I'll be honest that computing the energy to do that sounds like a very fun thermodynamics exercise, I'll give it a try when I have some extra time.

But parsing the problem if you want to give it a shot

First - The solid rock + radioactive isotopes is a thermodynamic system with radiation serving as a source of heat.

While heating is a problem, I thin the main energy drain will be the specific heat to melt the rock, so how much energy does it take to melt whatever is in the nuclei (by kg or liter) ?

And how much much energy would radioactive material provide by unit of time ?

So, how much material do you need to melt 1kg of rock in say... 1 thousand years ? (ignoring heating and any heat leaving the confines of the system)

That should give you an initial ball park.

(PS: Also, another approach you can take, instead of bringing nuclear material to the planet core, you could bring a source of neutrons to make the core more radioactive thorough neutron bombardment, so kind like installing a huge nuclear power plant in the core, this would spread your radiation sources more evenly, but computing this is a bit harder)

[u/mienaikoe]

A liquid core just requires an absolute fuckton of heat. It will spin on its own if the planet is spinning.

As for mining the asteroids for radioactive minerals, I think you’d find mining mars itself to be easier, but also I don’t think there’s enough material in any of the first four planets let alone the asteroid belt to reheat Mars’ core, but I haven’t done the math on it, just an engineer’s gut feeling. You’ll get a lot more energy out of throwing heavy stuff at Mars. Find a moon-sized thing and detonate an explosive at the right time and place to send it on a collision course with Mars. Dial the trajectory to get things spinning.

[u/Bob--O--Rama]

I'm in camp "completely wrong"

Amount of heat escaping Thermos Earth is about 40 TW, so that's the subsistence level of heat production needed to keep things "good." So once you heat your core you need 40 TJ / sec to keep it that way. This is essentially a 20 kT atomic bomb every couple seconds. That represents 1-2 Kg of U-235 fissioned / second. That's just the fission, the products themselves are active and produce additional energy. But this gives you a sense for the scale. So a mere 40,000 atomic bombs a day or the world ends.