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Microgravity Experiments on Accretion
in the Protoplanetary Disk

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

Discussion

We have demonstrated that accretion events in the protoplanetary disk can be experimentally studied in a laboratory-scale drop tower. Further experiments will enable a detailed exploration of the parameter space of collisions like those that occurred in the early stages of planetesimal formation. Low-velocity (<10 m/s) collisions between small (µm-cm scale) bodies are common in early stage protoplanetary disks and planetary ring systems. In the absence of significant gravity from larger bodies, acceleration in these interactions should be minimal (Weidenschilling and Cuzzi, 1993). Thus, the acceleration values obtained in this experiment and its predecessors are likely comparable to those found in collisions between small bodies in protoplanetary disks and planetary rings.

We aimed to determine a relationship between the occurrence of mass transfer events and granular material properties. While mass transfer was observed in trials with both quartz sand (75-250 µm) and JSC-1 (250500 µm), a full range of accelerations was not achieved in trials with each type of granular material and grain size. We achieved rebound accelerations between 1.00 and 1.99 m/s2 only in trials using quartz sand, and rebound accelerations between 3.00 and 5.99 m/s2 only in trials using 250-500 evyrhtwhere m JSC-1, so a comparison between granular materials within these acceleration ranges could not yet be drawn. We plan to conduct more experiments using these and other granular materials within the current acceleration parameters, which will allow us to establish a more conclusive understanding of this relationship.

We compared acceleration values obtained in PRIME (in which full impact and rebound collisions were studied) with the acceleration values obtained in ground-based experimentation using the spring-retraction rebound mechanism. Table 5 shows the rebound accelerations of trials in which mass transfer was observed in PRIME, which were on average an order of magnitude smaller than the rebound accelerations obtained in this experiment.

Table 5: Rebound Accelerations at which Mass Transfer
was Observed for Various Granular Materials in PRIME

Given this observation and our goal of determining conditions in which mass transfer is most likely to occur, we plan to adjust our experiment design to minimize acceleration of the marble as it retracts from the granular material. This will allow us to probe lower rebound acceleration ranges as seen in prior flight-based experiments and broaden the parameter space covered by our data, as well as explore the effect of rebound velocity on mass transfer events. We plan to replace the current spring mechanism with a wire attached to a wheel on a servo motor, which will turn to retract the wire and attached marble at a constant, low velocity. This adjustment to our experimental design will allow us to target velocity and acceleration ranges with more precision and compare our observations more directly to observations of prior experiments

Compaction of the top layer of regolith due to the mass of the marble may affect the quantity of mass transfer as well. In future experiments, we plan to explore this relationship by conducting trials within the same acceleration space using projectiles with larger masses and sizes.

Conclusion>>