Degree of impactor fragmentation under collision with a regolith surface—Laboratory impact experiments of rock projectiles

Abstract

Abstract-Some meteorites consist of a mix of components of various parent bodies that were presumably brought together by past collisions. Impact experiments have been performed to investigate the degree of target fragmentation during such collisions. However, much less attention has been paid to the fate of the impactors. Here, we report the results of our study of the empirical relationship between the degree of projectile fragmentation and the impact conditions. Millimeter-sized pyrophyllite and basalt projectiles were impacted onto regolithlike sand targets and an aluminum target at velocities of up to 960 m s À1 . Experiments using millimeter-sized pyrophyllite blocks as targets were also conducted to fill the gap between this study and the previous studies of centimeter-sized rock targets. The catastrophic disruption threshold for a projectile is defined as the energy density at which the mass of the largest fragment is the half of the original mass. The thresholds with the sand target were 4.5 AE 1.1 9 10 4 and 9.0 AE 1.9 9 10 4 J kg À1 , for pyrophyllite and basalt projectiles, respectively. These values are two orders of magnitude larger than the threshold for impacts between pyrophyllite projectiles onto aluminum targets, but are qualitatively consistent with the fact that the compressive and tensile strengths of basalt are larger than those of pyrophyllite. The threshold for pyrophyllite projectiles and the aluminum target agrees with the threshold for aluminum projectiles and pyrophyllite targets within the margin of error. Consistent with a previous result, the threshold depended on the size of the rocks with a power of approximately À0.4 (Housen and Holsapple 1999). Destruction of rock projectiles occurred when the peak pressure was about ten times the tensile strength of the rocks.

Survival of the impactor during hypervelocity collisions – I. An analogue for low porosity targets

“…In this work, following Nagaoka et al (2014), the energy density at the time of the impact is Q (J/kg), and its form for the impactor is given by:…”

“…They found that projectile material survived the impacts, although the degree of fragmentation of the projectile depended on the impact energy (Q) and the strength of the projectile, along with the strength and the porosity of the target. However, considering an average impact speed of v = 5.3 km/s for Main Belt asteroid collisions (Bottke et al 1994), the collisional speed range that was tested (<1 km/s) in the experiments of Nagaoka et al (2014) was at the lower end of inter-asteroid collision velocities. Moreover, Daly & Schultz (2013, 2015a, Daly & Schultz (2016) and, Daly & Schultz (2015b) used aluminum and basalt projectiles which were fired onto pumice and highly porous waterice trying to explain the implantation of an impactor's material onto vestan regolith, and the possibility of a similar process onto Ceres' surface.…”

“…Pioneering experiments by Schultz & Gault (1984) and Schultz & Gault (1990) demonstrated a change in projectile fragmentation and cratering efficiency as a function of impact velocity. Recently, Nagaoka et al (2014) performed several laboratory experiments using pyrophyllite and basalt projectiles fired onto regolith-like sand and aluminum targets. They found that projectile material survived the impacts, although the degree of fragmentation of the projectile depended on the impact energy (Q) and the strength of the projectile, along with the strength and the porosity of the target.…”

“…The main question that is addressed is how much of the impactor's material is embedded on/into the target by using different combinations of materials, trying to simulate collisions in the Main Belt and on the surface of icy bodies. While the experiments of Nagaoka et al (2014) were limited to speeds <1km/s, the McDermott et al study used only porous water-ice and a copper projectile -which is an atypical type of impactor material in the Solar System.…”

The Moon: An Archive of Small Body Migration in the Solar System

“…The contrast has been stretched in e and f showing structural variation with a troilite (FeS) rim and with rounded blebs of Fe (85-88 wt%) Ni (12-16 wt%) metal (Fe) that contains P at *1 wt% level, troilite (Tr) and a P-rich schreibersitic (Sc) interstitial groundmass (speckled phase). Images taken by K. Joy promote survivability of the silicate and metal impactors (Nagaoka et al 2014;Daly and Schultz 2016;Avdellidou et al 2016), suggesting that regions of the lunar crust with thicker and more porous regolith/megaregolith may better preserve surviving fragments of projectiles.…”