38
$\begingroup$

Mine shafts on earth are limited in how deep they can go by two things, internal heat and rock pressure. The deepest mine on Earth currently is just under 4 kilometers.

There is internal heat on the moon caused by tidal pressures, but it is very deep. The motion core is a tiny portion of the whole compared to the earth. With ⅙th G gravity, I wonder how deep you could go before rock pressure got higher than the compressive strength of the rock? I'm guessing pretty far. My question is; how deep could a lunar mine go before heat or rock pressure made it impractical?

$\endgroup$
7
  • 4
    $\begingroup$ I think it was here I learned that the pressure at the center of Ceres is less than in the deepest operating mine on Earth. That's 935 km of Ceres diameter compared to 1737 of the Moon. $\endgroup$
    – LocalFluff
    Oct 19, 2017 at 4:08
  • 3
    $\begingroup$ Interesting question! No idea. $\endgroup$
    – GdD
    Oct 19, 2017 at 7:39
  • 4
    $\begingroup$ Can you elaborate on "impractical"? Currently putting a small, lonely rover on the Moon is not even practical; otherwise there wouldn't be a $30 million X-prize for it. $\endgroup$
    – uhoh
    Oct 19, 2017 at 9:30
  • $\begingroup$ I'm talking in the context of a lunar settlement having been established and they are exploiting local resources for manufacture and so on. So by practical the implication is that when the rock temperature exceeds 130F and the rock pressure became difficult to hold back. Many hard rock mines a couple miles deep still need no roof support. It's interesting that the surface of the moon is aluminum rich but it becomes iron rich as you get deeper. $\endgroup$ Oct 20, 2017 at 4:30
  • $\begingroup$ I think I once saw some data about how the temperature rises as you go deeper into the moon. I'll keep looking for that. The rock pressure thing is the hard part. $\endgroup$ Oct 20, 2017 at 4:36

4 Answers 4

7
$\begingroup$

I came here with the same question. While rock pressure is one issue, I think the limiting factor is actually heat.

The most recent paper I could find on Lunar temperature gradients is Nimmo 2012, which gives a gradient of 2.5 °C/km starting from 30 °C near the surface. As the deepest mine on Earth is apparently limited by a temperature of 66 °C, it follows that the deepest 'traditional' mine on the Moon should be ~14 km.

That Wikipedia article is sourced from Wired's 2012 article Digging for Riches in the World’s Deepest Gold Mine.

That said, the highest temperature borehole I could find on Earth is 355 °C, and again it follows that the deepest borehole on the Moon should be ~130 km.

$\endgroup$
2
  • $\begingroup$ Thank you. That gradient is significant information. I don't see how you get 66C at 14 km. Maybe it's the starting point. I once asked what the ambient underground temperature was at about 3m down, and I was told it was -23 C. So, heavy coat weather in an uninsulated pressurized cave. Using that as a starting point, at 15 Km down I get a comfortable 15.5 C. (correct my math if I'm wrong) The point is that, if the rock pressure isn't an issue, there is an ideal depth for a colony based on ambient temperature. $\endgroup$ Aug 19, 2019 at 23:52
  • $\begingroup$ Fig 2 in Nimmo starts at 300K (30C) at 0km depth. Presumably the gradient is much steeper in the first few meters than it is deeper down. $\endgroup$
    – Andy Jones
    Aug 20, 2019 at 0:06
4
$\begingroup$

If we just look at rough numbers, we know the gravity on the Moon is ⅙th that of Earth so a mine shaft should go to a depth six times as deep as on Earth. So if the deepest shaft on Earth is just under 4 km, then the deepest shaft on the moon should be shy of 24 km.

$\endgroup$
4
4
$\begingroup$

A point of correction – me being pedantic.

Mine shafts on earth are limited in how deep they can go by two things, internal heat and rock pressure.

This is not correct. Remove the word shafts and yes, you are correct, the depth to which mines on Earth can go is limited by geothermal heat and rock pressure/stresses.

When it come to the limits of mine shafts, the length of an individual shaft is limited by the material properties of equipment used in the shaft, particularly if it is a hoisting shaft. The critical components of a shaft hoisting system are the ropes that move the skips and any counter weights and the drum of the winder on to which the rope is wound.

Due to self weight of the ropes and the internal flaws they posses, due to manufacturing processes, the maximum length of a hoisting shaft is about 2000 m. If a mine needs to go deeper, transfer tunnels need to be developed at the bottom of the first hoisting shaft and then another shaft is sunk from the transfer tunnels, to go deeper. This means, that in such a mine everything that enters and leaves the mine must be double handled: down one shaft, along the transfer tunnels and then down the other shaft and the reverse when going up.

If we accept the depth, due to heat and stresses, in one of the other answers, 24 km. A mine that deep on the Moon would have a series of twelve cascading hoisting shafts, each with their own system of winder and ropes. That appears overly complex to me and I doubt it would be practical.

An alternative to a hoisting shaft is a descending spiral tunnel, called a decline. A straight decline can be used, but a spiral contains the tunnel to a limited lateral region. Currently, on Earth, declines are being established with maximum gradients of 1 in 6. To get to a depth of 24 km, a decline would need to be 144 km long, horizontally (146 km slope length). That’s a long way to haul dirt out of a mine! Again, this appears impractical.

The added complication to this is, what form of tires would any trucks on the Moon have? NASA reinvented the tire for the Apollo lunar rovers. Can a similar type of mesh tire be made for heavy duty trucks hauling rock out of a mine on the Moon? If not, what type of tires or wheels would trucks on the Moon have? Would an inclined railway be required, or a conveyor belt? If a 146 km long conveyor belt was used, from what material would the belt be made of? How would spillage from the conveyor belt be cleaned up? Railroads using unassisted locomotives need shallower gradients which would mean their tunnels would be even longer. Assisted locomotives, such as rack railways use a third rail which have a toothed profile and the locomotives have a cogwheel that are used on steep sections railroad.

There’s more to a practical mining depth than just geothermal heat and ground stresses.

$\endgroup$
5
  • 1
    $\begingroup$ If the critical components are the ropes and the max. shaft length on Earth is 2000 m due to the self weight of the ropes, could not then the shaft length on the Moon be 12 km, since the gravity there is 1/6-th of the Earth ? $\endgroup$
    – Cornelis
    Jan 13, 2023 at 15:53
  • 1
    $\begingroup$ @Cornelis: One of the things about steel rope is it is manufactured in certain length. To make a long rope the smaller segments need to be spliced together. Each splice is a potential failure point. The longer the rope the greater the number of splices required. $\endgroup$
    – Fred
    Jan 14, 2023 at 8:18
  • $\begingroup$ O.k., but the weakest splice determines the strength of the entire rope , doesn't it ? And the force on that weakest splice is 6 times greater on Earth than on the Moon. $\endgroup$
    – Cornelis
    Jan 14, 2023 at 10:03
  • $\begingroup$ "Due to self weight of the ropes" - on the moon, due to lower gravity, wouldn't the weight of the ropes and all other equipment be lower? Forgive me if this is a dumb question. In any case, excellent and very informative answer. $\endgroup$
    – mgarey
    Jul 5, 2023 at 21:58
  • 1
    $\begingroup$ @mgarey: True, but, the longer a rope the greater the number of flaws in the rope & the greater the number of spiced jointed , which are points of weakness. Also, there is a limit to how large a winding drum can be, which places another limit on the length of ropes. $\endgroup$
    – Fred
    Jul 6, 2023 at 0:47
0
$\begingroup$

Not only would the low gravity work in your favor,but also the fact that the Moon (likely) has no internal dynamo. The heat from Earth's own interior limits how far we can dig more than any other factor.

$\endgroup$
1

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.