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NASA NTRS · 20230018265

Impact Cratering Experiments Using Lunar Highland Simulant as Target Material

C J Cline·M J Cintala
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Abstract

The properties of a target material have long been known to influence the cratering process for relatively small impact events, where the material “strength” is more dominant than the local gravitational force[1]. Strength in granular materials, however, is dependent on the interactions among packing fraction, grain size, and cohesion[2], and can thus vary for the same bulk material at different porosities. Experimentally, this relationship has been demonstrated for the most commonly used target material in impact cratering studies, unconsolidated sand, where [3] observed a notable decrease in depth-diameter ratios(d/D)for more densely packed (higher packing fraction, or lower porosity) targets. In order to extend the applicability, or properly scale, those findings to relatively small lunar craters, for example, a more realistic analog material(in terms of physical properties) should be interrogated for use as a target medium. To this end, we have conducted a series of impact cratering experiments using a lunar-highlands regolith simulant to investigate the relationship between packing fraction and crater morphology/morphometry.

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C J Cline, M J Cintala. Impact Cratering Experiments Using Lunar Highland Simulant as Target Material. https://ntrs.nasa.gov/citations/20230018265

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NASA NTRS2020

Exploratory Experiments to Probe the Effect of Bulk Density and Porosity on The Morphology and Ejecta Kinematics of Impact Craters

The morphometry of relatively small impact craters are influenced by the physical properties of the target. To investigate the control of bulk density and porosity on the crater-formation processing the gravity regime, we conducted a series of impact experiments, varying each these target parameters while holding the other constant. Three different sands were used to construct cohesionless targets: quartz (0.4-0.8 mm), garnet (0.425-1.0 mm), and alumina (0.297-0.71mm), representing a range of intrinsic grain densities from 2.65 to 4.02 g cm-3. High-porosity targets were constructed by carefully pouring the sand into the center of a cylindrical PVC target bucket (inner diameter 26.2 cm, height 12 cm) and allowing it to fill the bucket at the angle of repose before being leveled flat at the top of the bucket. Low-porosity targets were obtained by pluviation of the granular material through a 1 mm sieve that was set directly above the target bucket. With the disparate intrinsic densities of the sand in combination with the variations of the packing technique, we attained differences in the bulk-target density of up to 32%while keeping porosity constant. The impact experiments were conducted with the vertical gun in the Experimental Impact Laboratory at NASA Johnson Space Center; using 3/16” (4.76 mm) diameter aluminum projectiles launched at1.55 ± 0.03 km s-1. The ejecta were observed using the laser-based Ejection Velocity Measurement System, providing data on launch positions and velocities of ejected particles. Submillimeter topographic data of the resulting craters were collected after each experiment with a NextEngine 3D scanner. Regardless of target material or density, all craters exhibited a simple bowl shape, but a trend of increasing depth-diameter ratio with increasing target porosity is observed across the entire experimental suite. Despite these morphometric differences, ejecta launch angles and speeds remain remarkably similar amongst all of the target configurations.

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