# How deep could a lunar mine go?

Mine shafts on earth are limited in how deep they can go by 2 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 1/7th 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?

• 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. – LocalFluff Oct 19 '17 at 4:08
• Interesting question! No idea. – GdD Oct 19 '17 at 7:39
• 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. – uhoh Oct 19 '17 at 9:30
• 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. – Johnny Robinson Oct 20 '17 at 4:30
• 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. – Johnny Robinson Oct 20 '17 at 4:36

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 2.5C/km starting from 30C near the surface. As the deepest mine on Earth is apparently limited by a temperature of 66C, it follows that the deepest 'traditional' mine on the Moon should be ~14km.

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 355C, and again it follows that the deepest borehole on the Moon should be ~130km.

• 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. – Johnny Robinson Aug 19 at 23:52
• 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. – Andy Jones Aug 20 at 0:06

If we just look at rough numbers, we know the gravity on the moon is 1/6th 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.

• +1 This is probably as good as an answer can get. One could calculate more detailed gravity vs depth models for the local density and get a number maybe 10% or 15% different, but the uncertainty and large variability of any particular mineral composition and the strength against collapse from one mine site to another is a much bigger uncertainty. However you may want to add that the temperature rise as a function of depth will be much less important for the Moon. – uhoh Oct 30 '17 at 23:29