Going underground

[BackworldTraveller]

The current technology limits for going underground seem to be that on a size 8 world (Earth) you can mine down to 4km and drill to about 12km and then it all goes squidgy.

Anyone got any idea how that varies (a) by world size and (b) by tech level?

 

[BRover]

Not really any specific details, but a few thoughts.

One reason for the limits is the heat and pressure: rock turns plastic at those depths and flows into voids like drill holes.

Heat comes from residual heat of formation plus radioactive decay. It flows out of bigger lumps more slowly, so small lumps cool faster.

Therefore smaller planets (Mars? Ceres almost certainly) would be cool and light enough that they would be essentially rigid, and could be drilled deeper--even through the center.

Yet smaller asteroids are basically piles of rubble--gravel and ice--and drilling would be easy, but the holes would also close up easily.

The plastic rock should be affected by the presence of water/ice vs silica vs iron/metals, much as dwarf planets have different postulated minimum size to reach hydrostatic equilibrium depending on composition, and maybe even how close they get to a light source.

 

[BackworldTraveller]

I'd have expected the surface gravity to be a major contributor.

Given a fixed temp gradient, the pressure would vary inversely with surface gravity. Which sort of implies that size 1 would be limited to 32km (ish) for mining and drilling about 96km. And for Size 10 about 3km (ish) for mines and 9km for drilling.

Age and Size will affect temperature gradients...and this is where I really have no idea. I could see small bodies cooling relatively fast...but that could be mucked up by tidal heating too

What type of temperatures can technology cope with and overcome?

Obviously, drilling in a volcanic region isn't going to be a good idea no matter what the size! I think the man-made volcano on Iceland when they were drilling for hot-water shows just how dangerous that can be.

 

[Grav_Moped]

It's not just volcanic regions; get far enough down and you'll hit magma anywhere you dig.
It'll be unsustainable well before that point though.

Tidally locked worlds ought to solidify faster (relatively speaking).

 

[Condottiere]

As I understand it, residual heat plus radioactive materials, which another large collision could reset.

 

[BackworldTraveller]

Hmmm...What I read on the net makes it sound much more complicated than cooling being directly related to inverse of size!

Apparently MARS is much hotter inside than size alone predicts. (Something to do with the non-tectonic mantle and lack of outgassing)

Are there any exoplanetary-geologists on this board?

Anyway, the suggestion is that inhabiting the core of a body isn't something that is going to be something anyone is going to be doing using current tech even if the body is only 1600km across!

Which comes back to the corollary question - What temperature limits are feasible to live in at the various TLs?

 

[Grav_Moped]

It's not just volcanic regions; get far enough down and you'll hit magma anywhere you dig.
It'll be unsustainable well before that point though.

Tidally locked worlds ought to solidify faster (relatively speaking).

Unless by "volcanic regions" you're talking about subduction zones in general; you'll hit hot and squishy stuff further up, there.