1/ Interesting results from the Lunar Penetrating Radar on Chang’e 4. It shows the structure of the lunar regolith down to 40 meters. Large blocks of ejecta begin about 12 m down. skyandtelescope.org/astronomy-news… HT @JDClayton1 
2/ We can compare that to the theory shown in the Lunar Sourcebook figure 4.22, which is also attached. That theory predicts that “Large Scale Ejecta” (coarse and blocky material including broken-up melt sheets) should begin about 10 m down, so this is in excellent agreement!
3/ Billions of years ago, when asteroids and comets struck the newly-solidified Moon, they broke up its solid surface to a depth of several kilometers. This figure shows it broken to a depth of about 10 km, with fractures going down about 25 km.
4/ The really large impacts that go that deep are rare, while the small impacts that dig only a shallow distance are common. As a result, the shallower surface gets re-impacted, re-worked, and re-mixed over and over again, making it finer and finer near the surface.
5/ As a result, bedrock has not survived at the lunar surface — it is all blanketed with regolith — and loose rocks are scarce on the surface compared to the abundance of rocks on Earth. Boulders like this one are rare.
6/ Why are there so many smaller impacts reworking the surface, but so few big impacts that dig down near bedrock? Because the sizes of near-Earth asteroids is a power-law. Big ones are many factors of 10 less common than small ones. Here is the curve (source: Harris & D’Abramo)
7/ That is the curve for impacts on Earth, but because the Moon is so close to the Earth the curve for the Moon is similar. You can see the Chicxulub impact wiping out the dinosaurs was a very rare event. (I love the art, always with a dinosaur looking at it.)
8/ You can also see the Tunguska event on this curve. It was the one that flattened a Siberian forest over 100 years ago. And the Chelyabinsk event, which shattered windows in central Russia in 2013.
9/ That power law curve doesn’t only apply to large, devastating impactors. It continues all the way down the size scale to meteorites and even to dust particles. Here’s the curve for smaller than a meter. The smaller they are, the more abundant. (Source: nap.edu/read/13244/cha…)
10/ As a result, the Moon’s surface is constantly pummeled by tiny particles, so even on a small scale the regolith tends to get coarser and coarser the deeper you go. It is very random at any one location. We’re talking about the average all over the Moon. (Lunar Sourcebook)
11/ This is why rocks are rare on the Moon’s surface. The tiniest impactors (dust-sized) fly down from space at kilometers-per-second and chip away all the exposed rocks. This lunar rock’s upper side has been chipped away while the buried side was protected. (Lunar Sourcebook)
12/ This understanding of lunar geology is vital to learn how to live, work, mine and manufacture on the Moon. For example, @paulvans has designed a rock raking system to extract the rocks that should have survived just below the surface. They can be used to build a landing pad.
13/ Also, this layer of Large Scale Ejecta that goes down 10 km makes me skeptical when people say we will soon do underground mining on the Moon. If you dig underground, everything is completely loose and unstable for 10 km. You’ll need to build thick retaining walls as you go.
14/ Of course, we can eventually do that and probably one day will, but not until we have enough infrastructure on the Moon to produce vast tonnage of retaining walls inside all the mine shafts. You need a LOT of economic activity on the Moon before that becomes viable.
15/ So I am in favor of *not* digging down toward the elusive bedrock or any ore bodies people think might be there. Instead, for the next 40 years we should establish economic activity at the surface where we can scoop up loose regolith, harvest shallow ice, & collect sunlight.
16/ And here’s a picture from @george_sowers showing a proposed ice mining operation using sunlight reflected from the crater rims to collect shallow ice.
17/ We know the makeup of lunar regolith and we have data showing chemistry in lunar ice. Putting them together, we know we can build a complete supply chain on the Moon capable of building anything, without any need to dig deep below the surface. (Source: Hateras/Wiki)
18/ The main benefit of digging down would be to escape the harmful radiation at the surface. The Earth protects us by its thick atmosphere. On the Moon we need to use other material for protection: either regolith (as shown), or ice, or stuff brought from Earth. (Image: @esa)
19/ Some have proposed using lava tubes on the Moon as a way to get underground without having to dig through loose, blocky regolith. The hardened lava on its sides will hold back the loose regolith and boulders without us needing to bring tons of concrete from Earth.
20/20. So I thought it was cool to see this result from the lunar penetrating radar on Chang’e 4, which confirms what we had theorized about the structure of the subsurface. It gets blocky below about ten meters with big chunks of bedrock blown out by giant impacts many ages ago.
