How uniform is our test field? Characterizing geotechnical properties of the regolith simulant in LUNA
When entering LUNA, the regolith yard appears like a featureless, uniform grey mass of grains. But is this really the case? A team from École nationale des ponts et chaussées (ENPC, Institut Polytechnique de Paris) and Riom-based company Sol Solutions (France) set out to investigate this question.
Consider the lunar surface: Without an atmosphere, the regolith (literally: “blanket of rocks”) is created by billions of years of impact bombardment, from large meteorites forming craters with diameters of more than 2,000 km, like the South Pole-Aitken Basin, to micrometeorites the size of dust grains that create craters in regolith particles so tiny they are only visible beneath a microscope. Additionally, the surface is also deteriorated by solar and cosmic radiation. Self-compaction then affects the regolith, as increasing overburden pressure reduces porosity with depth. The fracturing by new impacts creates additional loose material that might get deposited on top, but impactors also act as forces that compact the regolith.
In LUNA, the situation is quite different: The regolith simulant EAC-1A, a mechanically crushed basaltic rock from the Siebengebirge (Germany) with a grain size distribution similar to that of lunar regolith, was deposited by pouring it from big bags attached to the gantry crane. In addition to self-compaction, astronauts, rovers and even excavators traversing the surface subsequently compacted the material by putting it under pressure. However, not all areas of the LUNA hall were affected equally – e.g., there are some main traverses, but also areas near the borders of the regolith field that were rarely driven across.
The team from ENPC and Sol Solutions employed several dynamic cone penetrometers, Sol Solutions’ Panda® devices, in which manual hammer blows are used to insert a thin rod into the regolith simulant, to characterize the test field. The hammer is instrumented so that the driving energy, applied force, and achieved penetration per blow can be recorded. These data allow to estimate parameters such as soil resistance and variations in density with depth. On Earth, the devices are typically used to determine geotechnical properties and their homogeneity in deposits like embankments or earth dams. Measurements were conducted at 29 points in the main regolith hall, including the deep floor area and various surface slopes, and 5 points in the dust lab. Additionally, specific calibration tests were performed to directly relate penetration resistance to regolith properties. Initial results already indicate variable density of the regolith simulant both with depth and laterally throughout the hall. A more detailed analysis is ongoing and will help to better understand how campaign activities in LUNA affect the properties of the regolith yard and what might be done to keep it as similar to the lunar surface, and therefore as relevant for testing, as possible.