Presentation Information
[PPS05-P03]Experimental Study on the Impactor Coverage Fraction During Crater Formation: Deposition Processes of Ejecta Curtains Mixed with Impactor Material
*Anzu Ikoma1, Masahiko Arakawa1, Minami Yasui1, Reia Kakinoki1 (1.Graduate School of Science, Kobe University)
Keywords:
Regolith Gardening,Impactor Survivability,Ejecta Curtain Deposition,Crater Formation Experiments,Impactor Coverage Fraction,Low-Strength Impactor Simulation
Numerous impact craters observed on the surfaces of the Moon and asteroids are considered to have been formed by past impacts of small bodies. During an impact event, the surface of the target body is excavated, and a portion of the impactor is incorporated into and mixed with the target material. This process, known as regolith gardening, is supported by evidence such as the mixing of materials with different origins observed in lunar breccias and meteorites. The Hayabusa2 asteroid exploration mission discovered fragments of S-type asteroid origin outside a crater on the surface of the C-type asteroid Ryugu [Tatsumi et al., 2021]. This finding indicates that materials of different origins can be incorporated into asteroid surface layers through impacts, and that impactor materials are not only retained within the crater interior but are also ejected and redeposited outside the crater.
Previous impact experiments have primarily focused on the fragmentation strength of target materials. In this context, Nagaoka et al. (2014) investigated the fragmentation strength of the impactor and demonstrated that the fragmentation characteristics of impactors vary depending on target type and impact conditions.
Recent spacecraft observations have revealed that asteroid materials, regardless of taxonomic type, are not monolithic rocks but are mechanically weak and highly porous. Such low-strength materials are expected to strongly influence impact processes, including fragmentation and mixing during regolith gardening. In this study, we conducted impact experiments using projectiles that simulate mechanically weak impactors to clarify the mixing processes of projectile materials within a regolith layer.
In this study, we used quartz sand–gypsum projectiles prepared by mixing quartz sand and gypsum at a mass ratio of 10:1. The compressive strength of the projectiles was 250 kPa, and two projectile sizes were employed: 10 mm and 3 mm in both diameter and length. To visualize the spatial distribution of projectile fragments under ultraviolet (UV) illumination after the experiments and to enable quantitative image-based analysis, fluorescent pigment powder was mixed into the sand–gypsum projectiles. Quartz sand was used as the target material to evaluate the mixing behavior between projectile fragments and the quartz sand regolith analog. All impact experiments were conducted using a vertical single-stage light gas gun installed at Kobe University. The impact velocity ranged from 17 to 118 m/s. The experiments were performed under low-pressure conditions of approximately 100 Pa, and the impact processes were recorded using two high-speed cameras.
First, concentric annuli centered on the impact point were defined, and the area of projectile fragments within each annulus was measured. Differences in the distributions were observed depending on impact velocity and fragment size, likely because these parameters affect the crater size. Therefore, the fragment area was normalized by the area of each annulus to obtain the coverage fraction, and the radial distance was normalized by the crater radius. This analysis shows that the peak of the coverage fraction distribution is located within the crater in all cases.
The radial distribution of coverage fraction can be classified into three distinct regions: Region 1, the area closest to the impact point, where the coverage fraction remains nearly constant regardless of impact conditions, Region 2, where the coverage fraction decreases with distance following a power-law relationship, and Region 3, where the slope of the power-law decrease becomes steeper. Since the coverage fraction is nearly constant in Region 1, it was defined asσ0. The distributions in Regions 2 and 3 were modeled using separate power-law functions, allowing the overall coverage fraction distribution to reproduced.
Furthermore, to quantify the amount of projectile material mixed outside the crater, the outward transport efficiency, γ, was defined as the ratio of the projectile fragment mass outside the crater to that inside the crater. The γ values show that higher impact velocities result in more efficient outward transport of projectile fragments, and at the highest velocity, more projectile material was distributed outside the crater than inside. In addition values derived from this model are generally consistent with those calculated from the total fragment area measured from thresholded images, supporting the validity of the proposed model.
Previous impact experiments have primarily focused on the fragmentation strength of target materials. In this context, Nagaoka et al. (2014) investigated the fragmentation strength of the impactor and demonstrated that the fragmentation characteristics of impactors vary depending on target type and impact conditions.
Recent spacecraft observations have revealed that asteroid materials, regardless of taxonomic type, are not monolithic rocks but are mechanically weak and highly porous. Such low-strength materials are expected to strongly influence impact processes, including fragmentation and mixing during regolith gardening. In this study, we conducted impact experiments using projectiles that simulate mechanically weak impactors to clarify the mixing processes of projectile materials within a regolith layer.
In this study, we used quartz sand–gypsum projectiles prepared by mixing quartz sand and gypsum at a mass ratio of 10:1. The compressive strength of the projectiles was 250 kPa, and two projectile sizes were employed: 10 mm and 3 mm in both diameter and length. To visualize the spatial distribution of projectile fragments under ultraviolet (UV) illumination after the experiments and to enable quantitative image-based analysis, fluorescent pigment powder was mixed into the sand–gypsum projectiles. Quartz sand was used as the target material to evaluate the mixing behavior between projectile fragments and the quartz sand regolith analog. All impact experiments were conducted using a vertical single-stage light gas gun installed at Kobe University. The impact velocity ranged from 17 to 118 m/s. The experiments were performed under low-pressure conditions of approximately 100 Pa, and the impact processes were recorded using two high-speed cameras.
First, concentric annuli centered on the impact point were defined, and the area of projectile fragments within each annulus was measured. Differences in the distributions were observed depending on impact velocity and fragment size, likely because these parameters affect the crater size. Therefore, the fragment area was normalized by the area of each annulus to obtain the coverage fraction, and the radial distance was normalized by the crater radius. This analysis shows that the peak of the coverage fraction distribution is located within the crater in all cases.
The radial distribution of coverage fraction can be classified into three distinct regions: Region 1, the area closest to the impact point, where the coverage fraction remains nearly constant regardless of impact conditions, Region 2, where the coverage fraction decreases with distance following a power-law relationship, and Region 3, where the slope of the power-law decrease becomes steeper. Since the coverage fraction is nearly constant in Region 1, it was defined asσ0. The distributions in Regions 2 and 3 were modeled using separate power-law functions, allowing the overall coverage fraction distribution to reproduced.
Furthermore, to quantify the amount of projectile material mixed outside the crater, the outward transport efficiency, γ, was defined as the ratio of the projectile fragment mass outside the crater to that inside the crater. The γ values show that higher impact velocities result in more efficient outward transport of projectile fragments, and at the highest velocity, more projectile material was distributed outside the crater than inside. In addition values derived from this model are generally consistent with those calculated from the total fragment area measured from thresholded images, supporting the validity of the proposed model.