Slow Impacts on Rubble Pile Asteroids: The Influence of Cohesion on Restitution

As seen by the Hayabusa spacecraft the regolith on the surface of asteroid Itokawa shows strong segregation by particle size [1].

 

Approaches to explain this segregation are either focused on the bulk material, like the Brazil Nut Effect, as obeserved in agitated granular media [2, 3]. Or the explanations stems from the idea of impact driven segregation, as proposed by Sinbrot with the Ballistic Sorting Effect [4]. Both kinds of effect may even contribute concurrently [5].

 

We designed an experiment to investigate impact driven segregation. To experimentally recreate the surface of an asteroid [6], we utilize the microgravity of the Bremen drop tower. Inside the 10^-6 m/s² microgravity environment in the drop tower capsule, we use a linear stage to accellerate a vacuum chamber containing a granular bed (e.g. our asteroid surface) at a constant acceleration of 2*10^-4m/s². A launcher mechanism then hauls a basalt impactor onto this surface. The outcome of the impact is tracked using three cameras, enabling us to determine the coefficient of restitution (COR), defined as the ratio of the impactor's absolute velocities before and after the impact.

 

This ratio is a good measurement for the energy disspiation happening during the impact and therefore the relative mobility of the impactor after rebounding. The COR measured in our experiments shows an interesting behaviour, especially for finely powdered beds. While the BSE proposed by Shinbrot only requires a decline of the COR with rising bed particle size, we observe an increased COR for very fine particles as well. Using numerical simulations we find this effect to be caused by inter-partilce cohesion. In detail, we show that without cohesion no such non-monotonic behavior is possible.

From our experimental and numerical results we conclude that in a low gravity environment like asteroids cohesion is important for the size-depended COR and furthermore that its non-monotonic behavior should enhance size segregation for certain particle sizes.

 

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[2] S. Matsumura, D.C. Richardson, P. Michel, S.R. Schwartz, R.L. Ballouz, Mon. Not. R. Astron. Soc. 443, 3368 (2014)

[3] C. Maurel, R.L. Ballouz, D.C. Richardson, P. Michel, S.R. Schwartz, Mon. Not. R. Astron. Soc. 464, 2866 (2016)

[4] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo, P. Chakraborty, Phys. Rev. Lett. 118 (2017)

[5] E. Wright, A.C. Quillen, J. South, R.C. Nelson, P. Sanchez, J. Siu, H. Askari, M. Nakajima, S.R. Schwartz, Icarus 351 (2020)

[6] K. Joeris, L. Schönau, L. Schmidt, M. Keulen, V. Desai, P. Born, J. Kollmer, EPJ Web of Conferences 249, 13003 (2021)