Scheeres Paper: Scaling forces to asteroid surfaces: The role of cohesion

Paper from Scheeres (et. al) on cohesion in asteroids. From the article...

Asteroids are strange celestial objects, particularly the smaller spinning variety, like the little potato-shaped asteroid named Itokawa. Scientists have been puzzling over what holds these smaller asteroids together, since they are too small for gravity to overcome the centripetal forces and hold the various bits of rubble as one to make an asteroid. And it turns out that Itokawa and its siblings have something in common with the gecko.

The puzzle is based on 2005 data collected by Japan's Hayabusa mission, which showed that at the rate at which Itokawa is spinning, the centripetal forces should overcome gravity and the pieces should fly off into space. But instead, they remain intact, giving Itokawa its unique spud-like shape. What is making the difference? Suggestions have included pressure arising from radiation from the sun, and friction and electrostatic forces occurring in the ionized dust.

Now a new analysis by a team of University of Colorado scientists suggests that the real culprit might be van der Waals forces. Named after Dutch scientist Johannes van der Waals, this is the force that causes attraction or repulsion between molecules in chemistry, that cannot be attributed to the usual chemical bonds or electrostatic interactions of ions. It's not a force that gets bandied about much these days in astrophysics, but here on Earth, it's the force behind geckos' ability to "adhere" to smooth surfaces even at bizarre angles. Apparently it's the creature's body orientation, and resulting change in gravity acting upon it, that triggers the gecko grip.

Those same van der Waals forces kick in in the latter stages of small asteroid evolution, according to the Colorado scientists, after the spinning objects have thrown off larger rocks that would incur larger gravitational effects. Eventually all that is left is the smaller bits of rubble, which act much like molecules and form bonds via van der Waals.

It's an intriguing notion, and Daniel Scheeres, who heads the Colorado team, thinks it might also be able to shed some light on how Saturn's rings may have formed -- those rings, after all, are made up almost entirely of dust and tiny bits of rubble that also seem to be strangely attracted to one another. So the humble gecko could hold the secret not just to a possible dry glue here on Earth, but to how certain celestial objects form as well.

Scaling forces to asteroid surfaces: The role of cohesion

D.J. Scheeres, C.M. Hartzell, P. Sanchez, M. Swift
(Submitted on 12 Feb 2010)

Abstract:

The scaling of physical forces to the extremely low ambient gravitational acceleration regimes found on the surfaces of small asteroids is performed. Resulting from this, it is found that van der Waals cohesive forces between regolith grains on asteroid surfaces should be a dominant force and compete with particle weights and be greater, in general, than electrostatic and solar radiation pressure forces. Based on this scaling, we interpret previous experiments performed on cohesive powders in the terrestrial environment as being relevant for the understanding of processes on asteroid surfaces. The implications of these terrestrial experiments for interpreting observations of asteroid surfaces and macro-porosity are considered, and yield interpretations that differ from previously assumed processes for these environments. Based on this understanding, we propose a new model for the end state of small, rapidly rotating asteroids which allows them to be comprised of relatively fine regolith grains held together by van der Waals cohesive forces.

Comments: 54 pages, 7 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1002.2478v1 [astro-ph.EP]

Link: Paper - "Scaling forces to asteroid surfaces: The role of cohesion" (PDF)

Link: Earth and Planetary Astrophysics

Link: Article - Discovery News ("Strange Attractors: How are Asteroids Like Geckos?")