Technically it is very wrong to call this material “simulated moon dust” (no fault to Fraser since the info I’m about to tell is not well known). This material, called GRC-1, is coarse quartz sand containing no dust at all! Allow me explain some important things about it. 1/N
2/ It is designed to be safe for humans, not exactly like lunar soil. For breathing safety, all the dust was omitted & its particles are all larger than 75 microns. Lunar dust is defined as *smaller* than 20 microns. 1 micron=1000 nm in picture. (Image: sciencedirect.com/science/articl…) 
3/ It is not uncommon to leave things out when creating simulated extraterrestrial regolith. In fact, you *must* leave things out, because the geological processes in space are so exotic that it is impossible to create a perfect substitute for *any* planetary regolith.
4/ Dusty lunar soil is unlike Earth’s soil. On Earth, we have rain that washes dust *out* of the soil. The runoff of rain carries dust into streams, then into rivers, then to the sea where it finally settles to the bottom becoming mud. See the mud leaving the Mississippee River?
5/ Does dust fill the oceans till they are full? If the land rises, will the mud dry out and become dust again? No, and no. The mud gets squeezed and baked and becomes rock again. Mudstone, shale, slate, schist, etc. Earth turns dust into rock! (Wikipedia)
6/ That doesn’t happen on the Moon. Instead, micrometeoroids bombard it over & over & over. These impacts bust the particles into dust and create splashes of melted soil that harden into glass (agglutinates), but it never turns back into rock again. It just becomes glassier dust!
7/ The micrometeoroid impacts bust the particles into finer dust, and the splashes of glass “glue” the dust back into larger particles, but the rates of busting and glueing become equal, so the soil never becomes either finer or coarser. This steady-state is called “mature” soil.
8/ This “mature” soil covers most of the lunar surface. Immature soil is mostly around deep, young impact craters, which busted bedrock making new immature soil. “OMAT” measures maturity. See how the low maturity is only on the crater rim? Most of the Moon is “mature”.
9/ The average particle size in mature lunar soil is about 50 to 70 microns in diameter. That’s about the diameter of an average human hair. Lunar “dust” is the portion of the soil smaller than 20 microns (0.02 mm). It comprises about 20% of the mass of mature lunar soil.
10/ There’s a false belief that the lunar dust sits on top of the soil, but it doesn’t. Because the Moon lacks the water cycle of Earth to remove dust from the soil, that lunar dust is mixed into the bulk soil. Every handful of soil is about 20% dust by weight.
11/ That dust gives it very unusual properties compared to Earth soil. After years of working with robots in lunar soil, I concluded that the MOST important property, which is HIGHLY under appreciated, is its “Reynolds Dilatency”, the tendency of soil to re-fluff when rubbed.
12/ The reason this is the most important property of lunar soil is because it is the reason robots get stuck in lunar soil. As a wheel turns, it rubs the soil, causing soil to slide against soil under the wheel. This action doesn’t compact the soil. Instead, it fluffs it up.
13/ The action of driving on that very dusty, unusual soil causes the soil to change its properties as you drive. Every little bit of wheel slippage makes the soil weaker so you get MORE slippage. It is an unstable situation. A “trigger” event can cause runaway dilation, then ☠️
14/ So to have a high fidelity test of rover wheels you MUST have a simulated lunar soil that mimics the dilatancy of real lunar soil. It must be a simulant that has a very wide range of compaction states, from highly dense to highly fluffy, & it must automatically fluff itself.
15/ But to get that behavior, you must include the dust. Compare the “dust-like” powdered sugar to the “sand-like” granular sugar. The powdered sugar can be far fluffier than granulated, or it can be packed tight. Not so with granulated sugar. (Image: quora.com/How-do-powdere…)
16/ As I said at the start of this thread, the GRC-1 soil substitute that NASA is using in the rover wheel tests does not include the dust. In fact, it doesn’t have even average-size lunar soil particles (60 microns), since it excludes every particle finer than 75 microns. Why?
17/ As I recall during the mid-2000s, the decision was made that it had to be safe for humans since it would be a very large, sloped soil bin without walls or air cleaners, so there was no way to contain and manage dust. Our lungs cannot handle lunar dust particles. Why not?
18/ It gets back to geology again. Our lungs and breathing system are adapted to handle the geological conditions on EARTH, not the conditions on the Moon. Here on Earth we do not have such fine dust mixed all through the soil everywhere we go. Therefore...
19/ On Earth when we breathe in, Earth’s larger dust particles *are* filtered out by our airways. On the Moon, our lungs and airways do not have adequate defenses against that super fine kind of dust in such huge amounts. (Credit: A.D.A.M. & NIH)
20/ The super fine lunar dust has such low inertia vs. high aerodynamic drag, that it perfectly follows the air flow all the way to the bottom of your lungs. However, it is also too big for your body to clean out of your lungs through the blood. It stays inside your entire life😬
21/ NASA is doing a lot of research to understand the health effects of lunar dust inside human lungs. We believe it causes lung cancer, because the sharp glass particles constantly cut and recut and scar the lung tissues. It is not good. I will be very quick to add...
22/ ...that neither I nor anybody I’ve worked with believe it is an unsolvable problem. We all know we can safely manage lunar dust when we put humans back on the Moon. To quote moonwalker Jack Schmitt, we can solve it with a “multilayer engineering approach”. And here’s Jack 😄
23/ So the decision was made to create a lunar soil substitute GRC-1 that does not include the finer fraction of the soil smaller than 75 microns. They did the best possible given that constraint. It does match the friction of lunar soil in a moderate compaction state. BUT...
24/ It doesn’t match the all-important dilatency of lunar soil, and it cannot be fluffy. For that reason I‘ve been calling it a substitute for lunar soil rather than a simulant. It doesn’t simulate the mechanical behaviors of lunar soil. All lunar testing has to make compromises.
25/ because we cannot adjust the gravity of Earth, and we afford to do full-scale rover tests in giant vacuum chambers inside giant reduced gravity aircraft. We have to make compromises and we just have to be smart about it. That’s the way it works for testing lunar technology!
26/ Here is a paper describing the GRC-1 material. It explains how it has good friction but the range of compaction vs fluffiness is much reduced compared to lunar soil and compared to typical lunar simulants. sciencedirect.com/science/articl…
27/ And here is a paper describing lunar soil simulants JSC-1 and BP-1. You can see how these two simulants “fluff up” (dilation) when they are sheared (rubbed horizontally, like by a slipping wheel). ascelibrary.org/doi/pdf/10.106…
28/ Here’s another subtle point that (AFAICT) few have noticed. These soil mechanics tests we do to characterize lunar soil simulants were created by civil engineers for *Earth* conditions, not for the Moon. The soil tests themselves aren’t appropriate. (en.m.wikipedia.org/wiki/Triaxial_…)
29/ We measure the so-called “Minimum” Compaction state of soil by pouring it from a defined height through a funnel of a required size...IN EARTH’S GRAVITY. But that guarantees it will be far MORE compacted than on the surface of the low gravity Moon. (researchgate.net/figure/photo-o…)
30/ So what soil mechanics people call the “minimum density” is not really the minimum. And get this: a lunar rover’s wheels only go down a few centimeters into the soil, in the fluffiest part. So they only operate in soil conditions fluffier than anything we ever test. 😬🤷♂️
31/ So the fluffiest conditions are what matter the most. It will be weaker soil than what exists in the soil bins here on Earth: less friction, less cohesion, and easier to slip your wheels and dilate it further. We see about 1/2 the robots get stuck in NASA’s robot competition.
42/ We spent a decade studying why some rovers get stuck and others do not. We have been intentional to collect data on the robots. We studied 470 robots so far. One conclusion: you cannot design a moderate-sized robot to be immune from getting stuck in lunar soil!
43/ The reason why is because of this dilatancy. The wheels of a moderate-sized robot interact with lunar soil at a shallow depth where fluffiness & dilatancy are crucial. So learning how to keep your rover from getting stuck is all about managing dilatancy. Don’t fluff the soil!
44/ As I said at the top, these soil behaviors only happen if you include the hazardous dust mixed in the soil simulant. Therefore, none of these all-important behaviors can be tested in a material like GRC-1, which is coarse sand. You *must* do other tests in addition to this.
45/ But almost nobody knew that! Even the folks working this didn’t understood how important dilatancy is for robots getting stuck until the past couple years. So with apologies to Fraser 😉, this is why I said it isn’t correct to call the material a “simulated moon dust”.😅
