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Microgravity Experiments on Accretion
in the Protoplanetary Disk

By: Addison Brown and Stephanie Jarmak | Mentor: Dr. Joshua Colwell

Introduction

According to nebular theory, our solar system began as a cloud of dust and gas called a solar nebula. The solar nebula collapsed under its own gravity with most mass eventually becoming the Sun and a disk of gas and dust surrounding the new-formed star (e.g. Weidenschilling and Cuzzi 1993). Low-velocity collisions between dust particles in this protoplanetary disk allowed them to stick together due to electrostatic forces in a process known as collisional accretion (e.g. Blum et al. 2000). The growth of 1-100 km-sized planetesimals may have occurred through gravitational instabilities, collisional accretion of these cm-scale dust aggregates, or some combination of the two processes depending on local nebular conditions (Michikoshi and Kokubo 2016). The gravitational attraction between planetesimals is large enough to allow them to grow into planets (e.g. Greenberg et al. 1978). The growth from cm-scale "pebbles" to km-scale planetesimals has remained the subject of active research due to the challenges faced by both collisional models and gravitational instabilities (see Wurm and Blum 2008 for a review).

Figure 1: Still frames of a quartz projectile in COLLIDE before,
during, and after (respectively) impact into quartz sand at 26.5 cm/s
with observable mass transfer.

Research on accretion in the protoplanetary disk has revealed that specific conditions are necessary for accretion to occur efficiently. Factors including the relative velocities and the sizes and compositions of the colliding particles can determine whether the interaction results in destructive shattering or growth through accretion (Benz 2000). The study of low-velocity collisions of small particles in negligible gravity provides insight into the specifics of this process. Experiments conducted on orbit, in suborbital space, and in parabolic airplane flights that provide a low gravity environment, like the Collision into Dust Experiment (COLLIDE, Colwell and Taylor 1999, Colwell 2003) and the Physics of Regolith Impacts in Microgravity Experiment (PRIME), have demonstrated the process of mass transfer (Colwell et al. 2008, Brisset et al. 2016). An example of this process is shown in Figure 1, in which a 2-cm-diameter projectile collided with a target bed of granular material at a low velocity (26.5 cm/s) and a significant portion of granular material adhered to the surface of the projectile.

Figure 2: Still frames of a Teflon-coated brass marble during and after
(respectively) impact into quartz sand at 34 cm/s at 1 atm and 1-g
with no observable mass transfer (Jarmak et al. 2016).

Our preliminary laboratory-based experiments suggest that mass transfer does not occur in a normal laboratory gravitational environment (Jarmak et al. 2016). Some trials in 1-g resulted in only a monolayer of granular material transferring onto the projectile. Figure 2 shows still frames of a trial conducted at 1-g, in which a cmscale projectile collided with a target bed of granular material at a low velocity (34 cm/s): no mass transfer was observed. The gravitational pull from the Earth overcomes the weak interparticle forces that can adhere the granular material onto the impactor.

To systematically investigate the process of mass transfer of granular material onto a cm-scale projectile observed in COLLIDE and PRIME, we designed an experiment to study collisions between projectiles and various types of granular material in a ground-based microgravity environment. Our goal was to gain an understanding of the conditions in which mass transfer was most likely to occur. In this paper we present the findings of these trials and discuss their implications.

Methods >>