The Adhesive Response of Regolith to Low-Energy Disturbances in Microgravity

Small planetary bodies, a class of solar system objects smaller than dwarf planets that are unlikely to have undergone differentiation, are considered tracers of the early state of the solar system and are therefore important to understand the origin and evolution of planetary systems. These bodies are represented by the targets of two current sample return missions [Hayabusa-2 (Smith, 2019) and OSIRIS-Rex (Lauretta et al., 2019)] along with several planned exploration missions, and these bodies also host valuable resources for future In-Situ Resource Utilization (ISRU) enterprises (Binzel, 2014).

Particles in the micrometer-centimeter size regime accrete through low-velocity (~ 10 s of cm/s) collisions into meter- to kilometer-sized bodies within planetary ring systems and form a layer of regolith on small, airless bodies (Gundlach & Blum, 2013). This regolith evolves under conditions very different from those on Earth as a result of significantly reduced gravity and pressure conditions. Additionally, spacecraft associated with planned and recent exploration missions interact with this regolith at speeds under 1 m/s to sample the surface. Therefore, characterizing the response of regolith to low-energy impacts in a microgravity environment is critical to our understanding of the processes that lead to the formation of these objects and our ability to develop safe operating procedures on their surfaces for future exploration and resource utilization missions.

We have carried out several flight-based microgravity experiments in a vacuum designed to investigate low-velocity impacts of cm-scale projectiles into simulated planetary regolith to contribute to a better understanding of the response of the surfaces of small, airless bodies to low-energy disturbances. These experiments include the most recent experiments in the COLLIDE (Collisions Into Dust Experiment) and PRIME (Physics of Regolith Impacts in Microgravity Experiment) programs carried out in suborbital flight (COLLIDE-3), and on parabolic airplane flights (PRIME-3, PRIME-4). From these experiments, we observed that certain impact events occurring at speeds less than 53 cm/s resulted in mass transfer, in which material from the target regolith is retained on the projectile after the rebound. Figures 1 and 2 show examples of mass transfer events for COLLIDE-3 and PRIME-4, respectively. The target regolith used in the experiments described in this paper consisted of quartz sand, JSC-1 lunar regolith simulant (McKay et al., 1994), and CI carbonaceous asteroid regolith simulant (hereafter referred to as Orgueil; Britt et al., 2019). Quartz sand was selected as a well-studied material with rounded grains for a more direct comparison to numerical simulations. JSC-1 and Orgueil regolith simulants were selected to study the effect of jagged grain types relevant for the exploration of airless bodies where grains evolve due to micrometeoroid bombardment as opposed to the aeolian processes that produce the rounded features of terrestrial quartz sand grains. The particle size distributions selected for these experiments were 75–250 μm, 125–250 μm, and 250–500 μm to represent the distribution of fines anticipated on a variety of asteroids up to the size of the Moon. Thermal inertia measurements inferring regolith size distributions for 15 of 26 asteroids for which data were available to indicate particle size distributions <1 mm (Gundlach & Blum, 2013), consistent with our selected particle sizes.

Figure 1

Figure 2

To follow up on these observations, we developed an experimental apparatus to be used in a laboratory drop tower (~0.75 second free-fall time) that simulates the rebound portion of these mass-transfer collision events without the time and cost requirements imposed by a flight-based experiment (Jarmak, 2020). The apparatus consists of a spring attached to a marble that is initially resting on a bed of regolith. When the apparatus is released from the top of the drop tower, the free-fall environment allows the spring to retract and pull the marble out of the regolith at accelerations between 1 m/s² and 9 m/s². We initially carried out our drop tower experiments in an open-air chamber before transitioning the apparatus into an evacuated chamber to provide a more direct comparison to the flight experiments.

In this paper, we provide the design and operation of our flight and ground-based experiments, explore the influence of projectile impact velocity and rebound acceleration on mass transfer events, identify whether a microgravity environment is needed to observe these mass transfer events, and assess the extent to which we can directly compare our flight and ground-based microgravity experiment results.

The goals of our experimental campaigns were (1) to identify projectile energy thresholds that influence MTOs in low-energy collision events between cm-size projectiles and mm-size regolith, (2) to determine whether these MTOs required a microgravity environment to be observed, and (3) to determine whether the rebound portion of these collision events could be replicated in a laboratory drop tower environment. In the following paragraphs, we discuss both the successes and limitations of our experimental campaigns in achieving each of these goals along with the implications of our results.

Our data suggest that projectile rebound acceleration influences the resulting MTOs, with lower rebound accelerations leading to higher MTOs. We did not, however, identify a relationship between impact velocity and MTO from our available flight data. The impact velocity directly influences the rebound acceleration of the projectile and so if a relationship between projectile rebound acceleration and MTO exists we would expect the same to be true for the projectile impact velocity. However, the rebound acceleration is also a function of the material properties of the granular bed, so the lack of a clear relationship between impact velocity and MTO may be due to both the limited available data (impact velocities were only available for flight experiments) and variations of the granular bed material properties in each experiment obfuscating the relationship. Therefore, additional experiments varying the projectile impact velocity at higher granularity for various regolith compositions and particle size distributions are needed to quantify the relationship between projectile impact velocity and MTO.

We found that mass transfer events occurred for rebound accelerations <7.8 m/s², and MTOs of “medium” and “high” occurred for rebound accelerations <3.8 m/s² and 0.38 m/s² respectively when considering the drop tower and flight data combined. When considering the flight data alone, however, the rebound acceleration threshold for “medium” MTOs is significantly lower than in the combined data set with a value of 0.09 m/s². However, this could be due to our limited quantity of available flight data. There is a possibility to produce medium MTOs through in-flight experiments with projectile rebound accelerations within a higher threshold, but we did not capture these events because we were limited to 25 impact experiments composed of a variety of explored parameters each of which may influence MTOs in addition to the rebound acceleration and therefore a more systematic study is needed. Additionally, particles with insufficient cohesion to maintain contact throughout the pull-off force would be left behind in the experiments with higher rebound accelerations leading to an overall lower number of grain contacts and thus lower observed mass transfer. It is also worth noting that triboelectric charging through grain interactions could play a role in the cohesive response of the material, but assessing the charged nature of the particles was beyond the scope of our experiments. Future investigations would benefit from evaluating the charged environment as an additional relevant variable influencing MTOs. Regardless, these rebound acceleration thresholds to produce MTOs are useful for the design of future experiments that intend to further investigate how various parameters influence MTOs and can still be used as inputs for numerical models characterizing the response of regolith to impacts in microgravity.

Additionally, given that several parameters were varied in each of our experiments, it may be misleading to consider only a single parameter at a time with our data set when investigating the thresholds that determine MTOs. For example, Figure 15 shows the MTO vs. rebound acceleration separated by flight and drop tower campaigns, but rebound acceleration was not the only parameter varied in each of these experiments. The regolith type and grain size distribution, projectile mass and diameter, extent to which the projectile was embedded in the regolith, and experiment pressure all served as separate sources of variability. Therefore, a more rigorous approach whereby the influence of each parameter on the MTO is statistically assessed through general factorial analysis, a statistical method that allows for a quantitative assessment of the relative level of influence of each considered parameter, would be an appropriate next step in the interpretation of this data. To carry out this statistical analysis the experiments would need to be balanced to each parameter of interest (Jarmak, 2020). We, therefore, recommend expanding the experimental campaign with parameter values selected through a factorial analysis experimental design to better account for the way each varied parameter and interaction effects between these parameters influence MTOs.

This full factorial investigation requires hundreds of experiments to fully explore the influence and interaction effects of the possible parameters involved (projectile mass, projectile diameter, projectile rebound acceleration, regolith material type, regolith grain size distribution, experiment pressure, etc.) to just two to three levels of granularity (e.g., assigning “low”, “medium”, and “high” bins to quantitative parameters including the rebound acceleration and projectile mass or groups such as “rounded” and “irregular” grain types for regolith). The number of required experiments to fully explore the parameter space is n^k where n is the level of desired granularity (2 for “low” and “high”, 3 for “low”, “medium”, “high”, etc.) and k is the number of parameters (Jarmak, 2020). For example, to investigate the influence of projectile mass, projectile diameter, projectile rebound acceleration, regolith grain type, regolith grain size, and experiment pressure and interactions between these parameters to two levels of granularity would require 64 experiments ideally with 3 replicates requiring 192 experiments. Given the long lead times associated with flight experiments and the limited number of possible experiments that can be performed during each flight, a campaign requiring over 200 experiments would cost thousands of dollars and take years to complete. A ground-based microgravity campaign via a laboratory drop tower is therefore ideal to broadly explore the influence of many parameters on MTOs, but careful consideration must be taken to calibrate the observations such that they can be directly compared to flight experiment results.

To determine whether mass transfer events observed in microgravity could still be reproduced in a terrestrial environment, we carried out a series of low-velocity impact experiments at 1-g under ambient atmospheric conditions. In each of our experiments, we observed that only a monolayer of grains adhered to the surface, inconsistent with the observed clumps of regolith that adhered to the surface of the projectile in our flight experiments. Therefore, we concluded that the force of gravity overcame the cohesive forces between the grains and therefore observations of significant MTOs require a much weaker gravity environment.

We were unable to replicate the ‘high’ MTOs from our flight campaigns with the drop tower campaign, and the observed MTOs were lower in the drop tower experiments than the flight experiments. The lack of ‘high’ MTOs in the drop tower campaign may be a direct result of the apparatus’ inability to achieve sufficiently low rebound accelerations due to the dependence of the rebound acceleration on the spring constant of the selected spring and the projectile mass. Additionally, pulling a projectile gently away from the regolith surface may not sufficiently simulate the physics involved in the rebound of a projectile subsequent to an impact. For example, the impact of the projectile alters the compaction of the grains at the site of impact in a collision whereas in our drop tower experiments the grains are compacted due to the weight of the projectile at rest. However, time limitations in drop towers restrict our ability to directly replicate a full collision event, and the flight and drop tower data sets may not be directly comparable. Numerical investigations into these distinct physical phenomena could perhaps reveal whether a quantifiable relationship between the two exists. If such a relationship exists, we could develop a scaling law that would allow us to transform the drop tower data MTO results into the MTO of a corresponding impact event.

We collected data investigating low-energy disturbances of regolith under 1-g and microgravity conditions through three flight-based, one ground-based, and two drop tower experiment campaigns. The flight campaigns (PRIME-3, PRIME-4, and COLLIDE) took place on a parabolic airplane and suborbital flights allowing sufficient time to observe the impact and rebound of a cm-size projectile into regolith at speeds of 10 s of cm/s. The 1-g experiments included low-velocity (<60 cm/s) impacts of a cm-size projectile into regolith under 1-g conditions. The drop tower campaigns were limited by a free-fall time of ~0.75 s and so only the rebound portion of an impact event could be simulated and subsequently observed for the desired low-energy regime.

In our flight and drop tower experiments we found that, under certain conditions, a mass transfer event occurs where regolith from the target granular bed adheres to the surface of the projectile disturbing the bed. We investigated the dependence of MTOs on rebound acceleration of the projectile and found that mass transfer events occurred for rebound accelerations <7.8 m/s², and future experiments investigating various parameter thresholds that govern the occurrence of mass transfer should aim for projectile rebound accelerations below this value to increase the likelihood of observing mass transfer events.

Our 1-g impact experiments resulted in only a monolayer of granular material adhering to the surface of the projectile, suggesting that the grains were not cohesive enough to overcome the force of gravity. These results indicated that significant mass transfer due to a low-velocity collision between a cm-size projectile and μm-size grains requires a microgravity environment. Additionally, our available data from the microgravity experiment campaigns did not reveal a relationship between impact velocity and MTOs, but we found that lower rebound accelerations of projectiles tend to produce more significant MTOs. However, given the varied nature of the data (differences in regolith grain size and composition, projectile mass and diameter, experiment pressure environment, etc.) it may be misleading to consider the influence of a single parameter at a time on MTOs. Therefore, a thorough study separating the effect of each varied parameter and their interactions is needed to better ascertain the parameter thresholds that govern the occurrence of mass transfer events in microgravity. We also found that transfer outcomes for a projectile impacting and rebounding from a granular bed and a projectile pulled from a granular bed result have different results: the pulled projectile MTOs are consistently lower than the projectile that rebounds after impact. Therefore, the MTOs may not be directly comparable and a scaling law applied to the drop tower data results may be necessary to combine the data sets.

Given that our drop tower experimental results may not capture the full physics of the rebound portion of a collision event, flight experiments remain our high-fidelity option for investigating the mechanical response of regolith to low-energy disturbances in microgravity. However, the time required to acquire the hundreds of experiments to fully investigate the influence of parameters and interactions between those parameters on MTOs is prohibitive. Drop tower campaigns therefore remain the best option for broadly exploring the available parameter space, but the data must be considered in tandem with comparable flight data to appropriately interpret the results. Therefore, increasing the number of available flight and drop tower experiments investigating low-energy disturbances of regolith in microgravity through a systematic factorial experiment design plan would expand our understanding of the parameters that influence MTOs thereby improving our understanding of the surfaces of small planetary bodies.