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The deepest borehole ever drilled is just 12km.

Consider this as a possibility. A small self-propelled probe is created, its approximately the size of a football, and is very strong, made of materials with a melting point of a few thousand K. If this was dropped into one of the planet's various volcanic lava lakes, how far down could it travel? Or does the viscosity of the molten rock increase with depth limiting it to a few 100m only?

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    $\begingroup$ There are two problems I have with such a probe. The first is how will it be propelled in such an environment. The second is will there be potential for crystals to solidify from the magma onto the probe thus slowly encasing the probe in a rock casing, adding weight to the probe & potentially disrupting the means of propulsion? $\endgroup$
    – Fred
    Mar 26 at 14:58
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    $\begingroup$ And ,of Couse it could not gather electronic data as any electronics would be fried .;; insulation gives only a limited delay in reaching magna temperature. $\endgroup$ Mar 26 at 15:33
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First, read these three very relevant questions and answers:

Why the "Mission to Earth’s core" proposal suggests using liquid iron instead of lead?

Can we really travel through earth's core?

Hypothetically, what would happen to the earth if a large hole was drilled through the center?


It is possible to send a probe deep within the Earth?

No.

Here's why:

First, there is a very common misconception that there's some sort of magma ocean underneath the crust. No - there isn't. The Earth's mantle is overwhelmingly solid rock. There are very small and localised places where you have melting, and that's where you have volcanoes (if it reaches the surface - most often it doesn't).

Second, even if you go to a volcano and drop your probe there, there is no simple tube that goes down. A volcano "plumbing" system is more like a maze with bifurcations, bends, kinks, and all of that. Not only that, you are going against the flow. You've probably seen videos of erupting lava flows (one, two). These things are massive and fast. Your probe will have to go against the flow in that thing. It will be worse than trying to go upstream in river rapids.

Third, even if you somehow manage to make a probe that autonomously navigates upstream in thin underground lava corridors with surprise bends, you have to remember the underground magma chambers are not 100% liquid. They are what we call "crystal mushes", which is a pile of small crystals (i.e., bits of rock) with liquid in between. Think more like wet sand, than pure liquid. So you're not swimming in something with no resistance, you also actively have more solids out of your way.

Fourth, molten rocks are extremely corrosive. You were talking about materials with high melting points. That's not your problem. Even the hottest lavas only get to about 1400 °C. We have relatively cheap materials that will melt at much higher temperatures. The issue is the reaction between the magma and the materials into making new materials, or dissolving completely in the magma. As an illustration, consider sodium chloride. Salt. The point at which it melts, if you stick it into a furnace is around 800 °C. Surely, that's much above the temperature of let's say an ocean, and it might be a good idea to make submarines out of salt! Clearly, bad idea. Because the salt will just dissolve into the ocean. Whether it melts or not is irrelevant. So that's the big problem with your probes - the magma will simply dissolve it.


So to sum up, you need your probe to be able to navigate in the dark and propel itself upstream in magma, be able to dig and clear space up front so it can move forward, and made of a material which is inert to the magma. Sorry - not happening. But a fun thought experiment nonetheless.

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    $\begingroup$ Nice answer, although points 1, 2 and 3 were already covered in mine (with less details). ;) $\endgroup$ Mar 30 at 15:14
  • $\begingroup$ @Jean-MariePrival yep. My main point here is that the fourth thing - dissolution of the probe in the magma is probably the most detrimental aspect. $\endgroup$
    – Gimelist
    Mar 31 at 1:42
  • $\begingroup$ I'd say that the rapids analogy could be overcome. There's various lava lakes in the world where the surface is not erupting as such, and the level is reasonably static. Having said that - could you answer the question - how far down could a probe go? Could it beat the 12km borehole record? $\endgroup$
    – Graham
    Mar 31 at 10:14
  • $\begingroup$ @Graham Magma is at the equivalent of about 2,500 atmospheres by 12 km depth; that's well over twice the pressure that equipment deals with at the bottom of the Mariana Trench. So not only are you having to push through 12 km of hot (well, ~1,000 °C) flexible rock, your mechanism would have to survive immense pressures. $\endgroup$ Mar 31 at 18:45
  • $\begingroup$ @Graham again, this is not a straight tube down, and it's not all full of liquid magma. The probe is unlikely to go even 1 km! $\endgroup$
    – Gimelist
    Mar 31 at 22:14
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I'd say it could reach, at the very best, the base of the continental crust (usually around 30 km depth). Even getting there could be tricky: the probe would have to travel what we call the "plumbing system" of the volcano, made of interconnected dykes (which can be quite narrow: only a few centimeters in some cases) and reservoirs, where convection and crystal mush would make its life hard... But assuming the probe would find its way through this maze, it would eventually reach a big magma chamber under the continental crust. Getting deeper would require drilling: the mantle is solid but not brittle, so it does not fracture to let magma ascend (like with dykes). Instead, it is generally considered that magma migrates through the mantle using grain boundaries, i.e., the small spaces between crystals of the mantle rock (peridotite). These are way too small for any probe to travel through.

On another note, you mentioned materials with a high melting point, but you'd also need something able to resist very high pressures... And finally, how would you get the data back? I can't imagine a way for the probe to transmit data back to the surface, so even if it managed to get down there, you won't know its findings...

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  • $\begingroup$ As for getting data back, just have the probe turn round and reverse its path - though this could be a tricky maze problem as you say. $\endgroup$
    – Graham
    Mar 26 at 14:41
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Consider this as a possibility. A small self-propelled probe is created, its approximately the size of a football, and is very strong, made of materials with a melting point of a few thousand K. If this was dropped into one of the planet's various volcanic lava lakes, how far down could it travel? Or does the viscosity of the molten rock increase with a depth limiting it to a few 100m only?

This question could be answered on Worldbuilding SE, I'm just going to answer the part where the user asks how far it could travel and the materials that could possibly survive the probing mission.

First off I will start with listing Materials with high-melting points.

Tantalum Hafnium Carbide Alloy (3990℃)

Tantalum hafnium carbide alloy takes the 1st place in our list of the materials with the highest melting point.

Tantalum hafnium carbide alloy (Ta4HfC5) actually refers to tantalum and hafnium pentacarbonate compound, which has the highest melting point among known compounds. It can be considered to be composed of two binary compounds, tantalum carbide (melting point 3983 ℃) and hafnium carbide (melting point 3928℃).

Tantalum hafnium carbide alloys are used as heat resistant and high strength materials for rocket and jet engines, as well as parts for control and adjustment equipment.


Source. The article listed that tantalum Hafnium carbon alloy, "used as heat resistant and high strength materials for rocket and jet engines, as well as parts for control and adjustment equipment." So you could incorporate this material into you're probe


2. Graphite (3652 ℃)

Graphite ranks 2nd in our list of the materials with the highest melting point in the world.

Graphite is an allotrope of carbon, where three other carbon atoms (arranged in a honeycomb of hexagons) are covalently linked to each other to form covalent molecules. Due to its special structure, it has high-temperature resistance, electrical and thermal conductivity, lubricity, chemical stability, plasticity and so on.

Traditional graphite can be used as a refractory material, the conductive material, wear-resistant, and lubricating material casting, sand, die and high-temperature metallurgy material, while new graphite used as flexible graphite sealing material, car battery, new composite material, etc.


Graphite has a high melting point and can be used for many purposes, I recommend incorporating this into your probe.


3. Diamond (3550 ℃)

Diamond is another material that has very high melting points. Diamond is atomic crystal, while graphite is mixed crystal. The melting point of graphite crystal is higher than that of the diamond, which seems incredible.

However, the bond length of covalent bonds in the flake layer of graphite crystal is 1.42×10-10m, and the bond length of covalent bonds in the diamond crystal is 1.55×10-10m. Covalent bonds, the smaller the bond, the bigger the bond energy, the stronger the bond, the harder it is to break, the more energy you have to provide, so the melting point should be higher.

Diamond is used for cutting tools in arts and crafts and industry, such as drawing die, turning tool, thread cutter, durometer head, geological and petroleum drill bit, grinding wheel cutter, glass cutter, diamond pen, dresser knife, and abrasive material.


Although Diamonds have high melting points, they have limited purposes. I recommend using this to build your probe or using it as a shield for your probe.


I listed the top three in the article list. If you want to see more of the top ten check out this link: https://www.refractorymetal.org/list-of-metals-that-can-withstand-high-temperatures/

How to put your probe in the earth

Say, you have at least the top 3 or top 2 materials with the high melting points, you use expensive and futuristic technology and you have much research funding, but there is 1 thing in our way: How to put the probe into the earth.

The text you are about to read is based almost entirely on my observations

If your probe is the size of the football (Like you noted in your question), then I recommend digging a huge hole in a dormant or not-to-active volcanic geography after you do this (Do it extremely close to a dormant volcano, wearing a volcano suit), get a huge and long metal bar. hit the bar on the bottom of the hole. If you find extremely hot rock or magma/lava try a few more hits to molten or weak it further.

Once it's at the point of smoke or steam going out of the hole you made, I suggest using a non-electrical powered long shovel to dig further to make sure there is a lava flow "pipe" running in the crust, either carefully push ur probe under the lava once the lava is sinking back into the hole or simply throw your probe in a dormant volcano.

It's best the probe is scripted and computed to stay in the upper-middle mantle (For Data and safe exploration for your probe)

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    $\begingroup$ @Graham I spent quite a lot of time working on this answer. Ask for clarification if needed. $\endgroup$
    – MooseSmart
    Mar 26 at 15:22
  • $\begingroup$ I also provided suggestions on how to incorporate the probe into the continental crust. $\endgroup$
    – MooseSmart
    Mar 26 at 15:24
  • $\begingroup$ Thats a lot of detail - thank you, it is much appreciated. I always figured that melting would not be the insurmountable problem here, and that the liquid part of the molten region coming to an end would be the limit, or the pressure making the liquid effectively acting as a solid. $\endgroup$
    – Graham
    Mar 26 at 15:40
  • $\begingroup$ Also regarding delivery to the lava lake surface, I'd say its a comparatively trivial problem - it could just be thrown in, or lowered in from a helicopter. $\endgroup$
    – Graham
    Mar 26 at 15:48
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    $\begingroup$ Some problems with this answer. See mine for details: earthscience.stackexchange.com/a/21037/725 $\endgroup$
    – Gimelist
    Mar 30 at 0:07
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Temperature and pressure get worse as you get deeper and there's Very few materials that can withstand BOTH. Another thing is as you go deeper rock behaves like plastic rather than solid, even if you drill a hole, it'll quickly heal much like poking your finger into some foam. Consecutive drilling would require a sleeve of some sort of material that can withstand immense pressure tha would crush it. At 1 kilometer, pressures of 1100 psi are common, as are temps above 50-60 degrees Celsius. Geostatic Pressure is the stress exerted by the overlying rock or mass above a depth or formation of interest. It is usually at about 1.0 psi per feet although the value can vary depending on the specific area. At 2 kilometers deep, you're already at the Crushing forces beyond even deep sea submersibles.

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For power, it would need about 50 times the power output of a space satellite. You'd need >1000w to drill a football sized object through bread-dough consistency rock faster than the flow rate. It requires a currently non existent energy source.

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