How to tunnel to a planet’s core

Question

One of the main problems with creating a magnetic field around Mars, (which would be necessary for terraforming the planet) is that most proposed systems require some artificial device to sustain it. (IE, superconducting loops, dipole-projecting satellites, plasma tori).

However, one interesting system I read about was tunnelling to Mars’ core and liquefying it, after which the planet’s rotation would generate a magnetic field like earth does.

The problem, however, is how to excavate to the core. The inner layers of Mars are all solidified, so there is no problem with unexpected lava or molten metal, and even the cost isn’t much of a problem, since any metals unearthed in mining can be sold to cover the expense. The problem is simply of distance and power; what method can be used to excavate thousands of miles down to the core of the red planet?

Answer 1

Sun gun!

https://www.huffpost.com/entry/nazi-sun-gun-space-mirror_n_3015475

Everyone loves sun guns! https://en.wikipedia.org/wiki/Sun_gun It is Archimedes' mirror weapon done large. You could use a mirror or an orbiting Fresnel lens. Maybe you don't even need it in orbit - mount the lenses on mountains surrounding the tunnel mouth. Or both - a lens at that handy Lagrange point between Mars and the sun and land based mirror concentrators.

The sun gun digs by heating material to its boiling point. Gasified Mars wafts (I insist that it waft) up out of the hole, to be blown by the wind. Probably it would condense into little sandy pellets that would fall onto the land. Mars has a lot of that sort of thing anyway so no harm done.

Yes: excavation by means of great heat. And no moving parts, messy lubricants or any of that.

OK, ok. If that will be a deal killer you can still have the messy lubricants.

Answer 2

(...) tunnelling to Mars’ core and liquefying it, after which the planet’s rotation would generate a magnetic field like earth does.

There are many good ideas such as this in World Building.SE. What I love about them is that most would lead to planetary obliteration and this one is no exception :) So instead of telling you how to tunnel to the core, I will do a frame challenge by assuming it is easily possible to do so. And why you DON'T want to do that.

Mostly every substance has different densities at different phases of matter. And for metals such as iron and nickel, which make the bulk of Mars's core, the molten phase is less dense than the solid one - on top of being able to flow.

In plain terms: you dig a hole to the core and make it liquid, it will expand. If it is not able to, you will just get a hotter solid. And the only way to expand is through the tunnel, so that molten iron and nickel is going to come up like lava going up a volcano. Depending on how you do it, you might get either a slow, steady flow like a Hawaiian volcano; Or you can use Mars's lower gravity and practical lack of an atmosphere to one-up Krakatoa, possibly knock some unlucky spaceships out of the solar system altogether.

Then you are left with less core. Even if the remaining material still occupies the same volume, it will not offer the same resistance to basically the weight of a planet on top of it. I don't have the math in me, but whether it puts greater or lesser pressure on the layers above it, the whole mantle and crust of Mars will shift. A little less core and you can break the crust into plates... A lot of core and you end up with a molten rock, smaller than Mars, that will eventually cool down into a new planet in a few hundred million years.

And then there is my favorite part about this plan. The core of Mars is two orders of magnitude more massive than the whole asteroid belt combined, which means that if even just 1% of it comes out (on top of all the rocks you excavated to get there), you have enough material to build a few dwarf planets. There is also enough ore in the rock you dug to disrupt the economy of the Earth for lifetimes, so selling it is not really an option if you're still living in capitalism. Unless you are collecting material to build a Dyson sphere there is just no sane way to dispose of all that rock, which makes me think of this scene from Disenchanted:

-Trøgs have been excavating for generations. We'll keep it up until the whole world is hollow. If you have any questions, feel free to ask the happy miners.
-Where do you put the rocks you excavate?
-We have another cave we excavated for rock storage.
-Then where did you put the rock you excavated from that cave?
-What are you, the rock police? I put 'em up my ass. How's that?

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If you still want to get to the core: the biggest hurdle we have nowadays is that we dig with drills, and our drill bits just don't work past a certain depth due to temperature and density of the rock. It ends up that the rock you excavate becomes magma around the drill bit and damages it. You could perhaps remove more of that pesky rock standing in your way by carefully dropping small amounts of anti-matter, or by vaporizing the rock with powerful lasers. In either way there is going to be a lot of harmful things coming back at you from the tunnel, so be cautious and wear protective goggles. The goggles will not protect you from certain death, but at least you will look cool.

Answer 3

the cost isn’t much of a problem, since any metals unearthed in mining can be sold to cover the expense.

This is simply wrong. Why do you think fracking goes back and forth with being used? It all changes with the oil price: when it's high, oil extracted by fracking become competitive, when it's low fracked oil would be sold at a loss, and no oil company runs a charity for doing so.

Same would apply for the supposed minerals to be extracted in this. If you mine something at 800 \$/kg and its market price is 1\$/kg, you won't cover any expense by selling it.

That said, on Earth, with well developed logistic chains, repair crews and all the related shenanigans, we haven't managed to go deeper than 12 km underground, and for a research drill, not for an operational one. The deepest mine stops at 4 km. In the research drill what happened was

Because of higher-than-expected temperatures at this depth and location, 180 °C (356 °F) instead of the expected 100 °C (212 °F), drilling deeper was deemed unfeasible. The unexpected decrease in density, the greater porosity, and the unexpectedly high temperatures caused the rock to behave somewhat like a plastic, making drilling nearly impossible.

We simply don't have the technology yet to go deeper than that, so if you are asking how we can get to Mars core, with something yet to be invented. Just don't go to that level of details in your story, and say it was done.

Answer 4

If you have have the energy to permanently liquify the core of the planet, you have the energy to bore to it's center.

Seriously, the energy you're talking about is so enormous that boring a narrow hole is trivial. Instead of a drill, just heat the rock below to liquid and pump it out a heated side chute... or to vapor and blow it out, or to plasma and conduct it out electromagnetically. It really does not matter - you have basically infinite energy anyways, if you can spend enough to liquify a planetary core on something so relatively trivial as a magnetic field.

Note, you might want to do most of this drilling before you terraform, as the resulting fumes will take significant cleanup. But hey, you have basically infinite energy, so that's no big deal!

(Note, although you could read this answer as sarcastic, that's not my intention. Practically infinite sources of energy are a welcome staple of sci-fi, as their multitudinous uses are grounds for vast creative explorations. So if you have one, you might as well use it for boring as well as terraforming!)

Answer 5

Just dig there.

On earth, we are limited by not having basically infinite power. If you assume we do, you can just dig a lot.

You'll need a big holes, one thousands of miles deep, and bracing for the hole to keep it from collapsing from the huge weight of gravity and the mass, but there's nothing inherently impossible about this.

You'll need a fairly wide hole, to haul up resources and pump down cooled water, but with infinite power you could do this. It'll take a very long time to do it, but there's nothing inherently impossible about it. Just pump water down to cool the walls and haul heat to the surface.

Here's a basic image of what you need. Have steel bars to hold it open, and use the huge width to pump in a massive amount of water to keep the place cool. You can use the water pumps to pump up rocks, and separate them with your infinite power.

If cost is no limit, you can do a lot.

Answer 6

Multiple problems here:

1. While Mars' core isn't liquid in the traditional sense it is in a sense still liquid. Rock under that much pressure will flow. You dig down, the rock pinches your drill and in time closes up the hole once you pull your drill out. On Earth we hit this problem in less than 10 miles, Mars should let you get something like 30 before you're in trouble and you're nowhere near the core at that point.

2. You're going to have to build a series of airlocks in your bore hole or you're going to find your hole full of high pressure CO2, eventually turning to supercritical CO2 as you get deep enough. Does your equipment work immersed?? The problem is Mars' atmosphere is going to go down your hole and pressure is purely a function of depth--it being a narrow borehole is no different than if it were planet-spanning. The scale height for Mars' atmosphere is 10.8km--going down that amount increases the power by e, the increase is exponential.