Low-Velocity Collisions of Centimeter-Sized Dust Aggregates

Abstract

Collisions between centimeter-to decimeter-sized dusty bodies are important to understand the mechanisms leading to the formation of planetesimals. We thus performed laboratory experiments to study the collisional behavior of dust aggregates in this size range at velocities below and around the fragmentation threshold. We developed two independent experimental setups with the same goal to study the effects of bouncing, fragmentation, and mass transfer in free particle-particle collisions. The first setup is an evacuated drop tower with a free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so that we observed collisions with velocities between 8 mm s −1 and 2 m s −1 . The second setup is designed to study the effect of partial fragmentation (when only one of the two aggregates is destroyed) and mass transfer in more detail. It allows for the measurement of the accretion efficiency as the samples are safely recovered after the encounter. Our results are that for very low velocities we found bouncing as could be expected while the fragmentation velocity of 20 cm s −1 was significantly lower than expected. We present the critical energy for disruptive collisions Q , which showed up to be at least two orders of magnitude lower than previous experiments in the literature. In the wide range between bouncing and disruptive collisions, only one of the samples fragmented in the encounter while the other gained mass. The accretion efficiency in the order of a few percent of the particle's mass is depending on the impact velocity and the sample porosity. Our results will have consequences for dust evolution models in protoplanetary disks as well as for the strength of large, porous planetesimal bodies.

Free collisions in a microgravity many-particle experiment. III. The collision behavior of sub-millimeter-sized dust aggregates

“…Anyhow, it is questionable if the onset of fragmentation is totally mass independent. Recent experiments by Beitz et al (2011) and Schräpler et al (2012) have shown that dust aggregates larger than one centimeter fragment at velocities slightly lower than 1 m s −1 . This, however, does not change the picture qualitatively.…”

A dearth of small particles in debris disks

“…For solid bodies, this transition occurs around 100 m in size. Values of Q * ∼ 10 7 erg/g are often taken as typical for asteroids, and experimentally obtained values for small grains (mm to cm sizes) can be several orders of magnitude smaller (Blum & Münch 1993;Beitz et al 2011). Figure 1 shows the critical energy for splitting predicted by Eq.…”

“…(6)) is shown for η = 10 −3 (dotted) and η = 1 (dashed). The solid curves correspond to catastrophic fragmentation of aggregates (Beitz et al 2011), and ice and basalt (Benz & Asphaug 1999), showing both the strength-dominated (small sizes) and self-gravity dominated regimes (large sizes).…”

“…(6) as a function of size for the materials in Table 1 assuming η = 10 −3 and η = 1. The solid lines indicate critical fragmentation energies for basalt and ice (Benz & Asphaug 1999) and silicate aggregates (Beitz et al 2011). The critical fragmentation energies exceed the splitting energy, indicating that substantially more energy is required to destroy -rather than split -colliders.…”

Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains

“…been shown by impacts of cm-size aggregates at velocities of up to 2 m/s (Beitz et al 2011), 1.5−6 m/s (Kothe et al 2010), or even 60 m/s (Teiser & Wurm 2009b). These experiments show that as soon as the destruction of a projectile is possible, some mass might be transferred to a somewhat larger target, leading to net growth.…”