Analog Experiments on Tensile Strength of Dusty and Cometary Matter

Abstract

The tensile strength of small dusty bodies in the solar system is determined by the interaction between the composing grains. In the transition regime between small and sticky dust (µm) and non cohesive large grains (mm), particles still stick to each other but are easily separated. In laboratory experiments we find that thermal creep gas flow at low ambient pressure generates an overpressure sufficient to overcome the tensile strength. For the first time it allows a direct measurement of the tensile strength of individual, very small (sub)-mm aggregates which consist of only tens of grains in the (sub)-mm size range. We traced the disintegration of aggregates by optical imaging in ground based as well as microgravity experiments and present first results for basalt, palagonite and vitreous carbon samples with up to a few hundred Pa. These measurements show that low tensile strength can be the result of building loose aggregates with compact (sub)-mm units. This is in favour of a combined cometary formation scenario by aggregation to compact aggreates and gravitational instability of these units.

Sticking and restitution in collisions of charged sub-mm dielectric grains

“…For other dissipating systems, this might be different. According to experiments and models by Musiolik et al (2017), Thornton and Ning (1998) or Higa et al (1998), the coefficient of restitution increases towards lower velocities. Schwager and Pöschel (1998) argue that such a velocity dependence will slow down cooling, which makes sense as collisions get more elastic as a granular gas cools.…”

“…Cohesion is often ignored for sand-sized particles. Indeed, cohesive forces are small compared to gravity for this grain size (Musiolik et al 2017).…”

Are Pebble Pile Planetesimals Doomed?

“…Estimated tensile strengths are of the order of a few Pa. Skorov & Blum (2012) and Blum et al (2014) did experiments with dust granules confirming the low tensile strengths. On even smaller scales of sub-millimetre aggregates composed of solid sub-millimetre grains the tensile strength was measured for the first time by Musiolik et al (2017) to be between 10 to 100 Pa. So even on smaller scales the tensile strength is rather low once the particle entities get larger than 100 µm.…”

Selective Aggregation Experiments on Planetesimal Formation and Mercury-Like Planets

“…Second, the same mechanism of thermal creep can also provide internal overpressure in experiments to measure tensile strength. It especially provides a way to produce overpressure in very small, very fragile dust aggregates [50]. In short, if a weakly bound, warm aggregate cools, it cools outside-in.…”

“…Again gas flows toward the warm side, this time the center of the aggregate. This way, it generates an overpressure within the aggregate, which can overcome tensile strength [50]. Combining both, levitation and overpressure by cooling allows a systematic study of tensile strengths on the Pa level, e.g., important for cometary matter (Demirci, personal communication).…”