Why Nearly No Adhesive Interaction Forces Between the Asteroid Fragments? (Planetary Science)

Most asteroids with a diameter larger than ∼300 m are rubble piles i.e. consisting of more than one solid object. All asteroids are rotating but almost all asteroids larger than ∼300 m rotate with a period longer than 2.3 hours, which is the critical period where the centrifugal force equals the gravitational force. This indicates that there are nearly no adhesive interaction forces between the asteroid fragments. Now, Persson and Biele showed that this is due to the surface roughness of the asteroid particles which reduces the van der Waals interaction between the particles by a factor of 100 for micrometer sized particles and even more for larger particles.

“This means that the dependence of cohesive strength of the granular medium on particle size is due to the, increase in the number of particle-particle contacts (per unit area) alone. A decrease in particle size only increases the number of contacts without changing the strength of the particle-particle adhesive bond.”

FIG. 1. A big particle (fragment) bound to an asteroid via a matrix of smaller particles © Persson and Biele

They showed that surface roughness results in an interaction force which is independent of the size of the particles, in contrast to the linear size dependency expected for particles with smooth surfaces. Thus, two stone fragments of size 100 nm attract each other with the same non-gravitational force as two fragments of size 10 m.

FIG. 2. The big particles (fragments) in an asteroid are assumed to be kept together by a matrix of smaller particles. Analysis of experimental data gives an effective yield stress of rubble pile asteroids of order (or less than) σY ≈ 25 Pa © Persson and Biele

“In order for the small particles to act like a cement or glue for the bigger particles in asteroids it is important that there are enough of them to fill out all cavity regions between the bigger particles. If this is not the case then the effective adhesive force keeping the big fragments together will be strongly reduced.”

For more:

B.N.J. Persson and J. Biele, “On the stability of spinning asteroids”, Arxiv, pp. 1-24, 2021. https://arxiv.org/abs/2110.15258

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