Developing a Standard Method to Test Rheology of Regolith Simulants

Introduction: Regolith is the rocky unconsolidated material covering solid rock and is present on rocky bodies throughout the solar system, including Earth, the Moon, Mars, and asteroids. The fundamental mechanical properties of regoliths affect the geologic process that dominate these surfaces. High mineralogical fidelity regolith simulants can be used to help characterize the surface material. By studying the physical properties of regolith simulants, including their rheology, i.e. how these dry powders flow, we can gain a better understanding of the mechanical behavior of the regolith. A deeper understanding of the mechanics of these regolith simulants may also help us infer and interpret the physical properties of regoliths. Based on the method of Geldart, et. al [1], we developed a standard method for obtaining regolith flow rates for a variety of regolith simulants. Studying rheology of regoliths may assist mission operations, logistics, and hardware development for future missions such as JAXA's MMX which will visit a small rocky body, collect regolith samples, and return them to Earth for analysis.

Methods: Regolith simulants are loaded into plastic funnels and then released, which allows them to freefall through the funnel and onto a scale where their mass is measured, and the mass flowrate is determined. In our setup, we used five different funnel sizes (23 mm - 51.43 mm) and then normalized the flow rate by plotting as a function of the funnel size to characterize how the exit diameter affects the flow. Funnels with similar internal angles were chosen to mitigate the effect of the funnel slope on the flow rate. To determine the flow rate of a simulant, we covered the bottom of a funnel and filled it with approximately 200 g of regolith simulant. The simulant is released, allowing it to flow freely into a container set on a scale. The flow as well as the scale mass readings were recorded via camera. This procedure was repeated five times for each simulant for each funnel size. The videos were analyzed by recording the mass at 0.2 second intervals. After plotting the mass as a function of time, a line of best fit was applied to the data, the slope of which was determined to be the mass flow rate of that run. In some tests the simulant jammed the funnel and stopped the flow which is called "bridging". Where the flow of simulant stopped due to bridging, the funnel was tapped to resume flow, and the weight before the tap and after the tap were recorded and compressed. We averaged the flow rates of each of the five tests per funnel size, which resulted in five average flow rates per simulant type. These flow rates were then normalized to the funnel diameter to determine the effect of the funnel size on the flow rate.

Results and Discussion: Here we report data for two simulant types, which are material analogs of the type of regolith we may expect to encounter on the Martian moon, Phobos, and named PCA-1 and PGI-1 [2]. Figure 1 shows the plotted relationship between flow rate and funnel diameter for the PCA-1 and PGI-1 simulants. We found the relationship between flow rate and funnel size for the PCA-1 simulant to be y = (6.98±1.61)x + (-69.29±57.54), and the relationship for the PGI-1 to be y = (7.85±1.19)x + (-127.88±42.60). The flow rates between the two Phobos simulants were found to be similarly dependent on funnel diameter and both within the margin of error of the other.

Figure 1. Plots of the PCA-1 and PGI-1 funnel diameter to flow rate relationship.

Conclusions and Future Work: Our group has demonstrated a repeatable and reliable experimental method for determining the flow rate for regolith simulants. The results this method provides a mathematical relationship between the flow rate of the regolith simulants and the funnel diameter. This allows us to make predictions about the flow behavior of this, and other types of regolith simulants, provided a given set of conditions. So far, this method has been used on the Phobos regolith simulants (PCA-1 and PGI-1) developed by Exolith Lab. Future tests will be run on Martian, Lunar, and asteroid simulants, also developed by Exolith Lab. More work on further developing this method will allow for inferences towards the application of this data for understanding geologic processes, like mass wasting, as well as a range engineering applications that can support the design and development of exploration systems.