Regolith behavior under asteroid-level gravity conditions: low-velocity impacts into mm- and cm-sized grains
Authors: Brisset Julie , Cox Christopher , Anderson Seamus , Hatchitt John , Madison Alexander , Mendonca Mariana , Partida Aracelis , Remie Daniel 
Published: October 2020

IntroductionData returned from spacecraft missions to asteroids show the ubiquity of rough surfaces covered in coarse grains. Recent data from Hayabusa-2 at Ryugu and OSIRIS-REx at Bennu show surfaces that are rubbled and coarse [1,2], with regolith grain sizes estimated to range from mm to dm [3]. As such surfaces appear to be common on small asteroids, understanding how coarse grains behave in asteroid-like environments can shed light on the observed topography, activity, and impact response of these bodies. In the work presented here, we investigate impacts into coarse grain targets at speed ranges from 0.1 to 2 m/s. These speed ranges apply to both natural and spacecraft-induced processes. Indeed, recaptured secondary ejecta, which has speeds below the body&#8217;s escape speed, is expected to impact the surface of a small asteroid at speeds around or below 25 cm/s (e.g. 26 cm/s for Ryugu). In addition, recent and current missions to asteroids have interacted or plan to interact with their coarse-grain surfaces at speeds around or below 20 cm/s in order to land hardware or collect surface samples. For example, Hayabusa 2&#8217;s Mobile Asteroid Surface Scout (MASCOT) lander touched down Ryugu&#8217;s surface at a speed of 17 cm/s in October 2018 [4]. Understanding impact processes and surface responses at these low speeds can provide much-needed context for surface interaction activities at these small bodies and thus increase the mission science return. 1. Experiment SetupFigure 1: Frame sequences of impacts at 1.3 m/s into mm-sized grains (top) and at 60 cm/s into cm-sized grains (bottom).Experiment Hardware: The impact data presented here were collected with the Physics of Regolith Impacts in Microgravity Experiment - Drop tower (PRIME-D). Inside a vacuum box, a projectile is launched into a regolith sample tray of dimensions 13&#215;13&#215;3 cm3 while the whole assembly is under free-fall. A camera records the impacts with 1020p resolution at 500 fps.Target Material: We used asteroid regolith simulant to prepare target material grains [5] and focused on grains in the mm and cm range. Projectiles: The projectiles used were Teflon, quartz (glass), and brass spheres of 1 cm in diameter. Experiment Runs: We performed 86 experiment runs (Figure 1). The experiment plan we followed aimed at including three impacts for each combination of target material, projectile density, and impact velocity range. The three impacts for each combination allow for the computation of error bars on the impact outcome results. Figure 2: Results from low-velocity impacts into coarse grain targets. Small symbols, dashed line: mm-sized grain target; Large, full: cm; Green: mixed; Red: fine JSC-1 grains [6]. (left) Mean ejecta velocity; (middle) COR; (right) Ejecta mass estimation (0: none; 1: the ejected mass is much lower; 2: on the same order; 3: much larger than the projectile mass.2. Data Analysis and ResultsThe main results of our data analysis can be summarized as follows:Impact Outcomes Similarly to impacts into fine grains [6], we observed a variety of impact outcomes, including projectile rebound off the target surface after impact (61 % of the observed impacts). We also observed a new outcome which was not seen for fine grain target: rolling of the projectile at the surface of the target after initial impact.Ejecta Speeds The measurement of ejecta speeds showed the limitations of our experimental setup for impact speeds below about 40 cm/s. For higher impact speeds, we were able to scale the ejecta speeds the the projectile (Dp) and target grain sizes (dp), as well as the impact speed (vi).Ejecta Masses Estimations for the ejected mass upon impact showed a simple scaling between the impact energy and the mass of the lifted material as long as no ejecta curtain is created. This trend is reversed for the lifting of ejecta blankets, indicating that the target material cohesion becomes apparent for larger impact energies and that larger grains present a looser surface than smaller ones.Coefficient of Restitution (COR) For impacts leading to rebound and rolling of the projectile, we measured the associated COR. Our data set showed a relatively low dependence of the COR on the target grain size.Projectile Penetration Depth For a number of impacts, we also measured the maximum penetration depth of the projectile into the target material. We find that the larger the grain size in a regolith bed, the lower the penetration depth.3. Comparing to Spacecraft DataHayabusa 2&#8217;s MASCOT landing on the surface of Ryugu in October 2018 [4] presents a great opportunity to place our experimental findings in context with actual spacecraft data. While MASCOT is one order of magnitude larger than our projectiles and its impact onto the surface was oblique, a number of other parameters were similar to the ones explored in our experiment plan. In particular, the impact speed of 17 cm/s is in the low range investigated here and the size ratio between MASCOT and the boulders/coarse grains of Ryugu&#8217;s surface ranges from 0.1 to 1 cm, just like in our experiments. We will present a comparison between the CORs measured for the MASCOT rebounds and our experimental results. We will also compare our measured data to numerical simulations of the MASCOT landing.References[1] Lauretta, et al., Nature, 568(7750):55&#8211;60, 2019a.[2] Watanabe, et al., Science, 364(6437):268&#8211;272, 2019. [3]Sugita, et al., Science, 364(6437):eaaw0422, 2019. [4]Scholten, et al., Astronomy &amp; Astrophysics, 632:L3, 2019.[5]Covey, et al., Earth &amp; Space Conference, pages 11&#8211;15, 2016.[6]Brisset, et al., Progress in Earth and Planetary Science, 5(1):73, 2018.