Collisional disruption of highly porous targets in the strength regime: Effects of mixture

Abstract

Highly porous small bodies are thought to have been ubiquitous in the early solar system. Therefore, it is essential to understand the collision process of highly porous objects when considering the collisional evolution of primitive small bodies in the solar system. To date, impact disruption experiments have been conducted using high-porosity targets made of ice, pumice, gypsum, and glass, and numerical simulations of impact fracture of porous bodies have also been conducted. However, a variety of internal structures of high-porosity bodies are possible. Therefore, laboratory experiments and numerical simulations in the wide parameter space are necessary.In this study, high-porosity targets of sintered hollow glass beads and targets made by mixing perlite with hollow beads were used in a collision disruption experiment to investigate the effects of the mixture on collisional destruction of high-porosity bodies.Among the targets prepared under the same sintering conditions, it was found that the targets with more impurities tend to have lower compressive strength and lower resistance against impact disruption. Further, destruction of the mixture targets required more impact energy density than would have been expected from compressive strength. It is likely that the perlite grains in the target matrix inhibit crack growth through the glass framework. The mass fraction of the largest fragment collapsed to a single function of a scaling parameter of energy density in the strength regime ( ) when assuming ratios of tensile strength to compressive strength based on a relationship obtained for ice-silicate mixtures. However, the dependence on is much larger than that shown for porous targets with different internal microstructures from the targets in this study. The depth of the deep cavity specific to the high-porosity target was well represented by a dimensionless parameter using the compressive strength of both the pure glass and mixture targets. The empirical relationship of cavity depth was shown to hold for various targets used in previous studies irrespective of the internal microstructure of the targets.

Bennu’s near-Earth lifetime of 1.75 million years inferred from craters on its boulders

“…The largest craters relative to the host boulders may represent the largest sub-disruption impact sizes allowable. Laboratory impact experiments show that this is possible for impacts onto porous targets [11]; however, the largest possible craters on non-porous consolidated targets, produced through spall (fracturing and ejection of plate-like near-surface fragments), are created by impacts that are a factor of a few less than the disruption threshold [12]. Therefore, equating the formation of the largest possible crater to the catastrophic disruption threshold is a viable framework, as CM and CI meteorites-the meteoritic analogs to Bennu's boulders-have high porosities (≳ 20% [13]) and Bennu's boulders show little evidence for spalls.…”

“…An object's response to an impact is dominated by one of these strengths. The formation of welldefined deep craters, as observed on Bennu's boulders, is typically dominated by shear [19] or compressive strength [11]. In contrast, impacts onto brittle material lead to shallow spall craters formed by tensile failure.…”