Very weak carbonaceous asteroid simulants I: Mechanical properties and response to hypervelocity impacts

Abstract

The two ongoing sample return space missions, Hayabusa2 and OSIRIS-REx are going to return to Earth asteroid regolith from the carbonaceous near-Earth asteroids Ryugu and Bennu. The two main processes that lead to regolith production are the micrometeorite bombardment and the thermal cracking. Here we report the production of a weak simulant material, analogue to carbonaceous meteorites with a CM-like composition, following the preliminary compositional results for Bennu and Ryugu. This asteroid simulant has compressive and flexural strength 1.8 ± 0.17 and 0.7 ± 0.07 MPa, respectively. The thermal conductivity (in air) of the simulant at room temperature is between 0.43 and 0.47 W m −1 K −1. In order to distinguish the type of regolith that is produced by each of these processes, we present and discuss the results of the experimental campaign focused on laboratory hypervelocity impacts, using the 2-stage lightgas gun of the University of Kent, that mimic the micrometeorite bombardment. We find that this process produces both monomineralic and multimineralic fragments, resulting in a difficulty to distinguish the two processes, at least on these weak materials.

Surface roughness of asteroid (162173) Ryugu and comet 67P/Churyumov–Gerasimenko inferred fromin situobservations

“…These intersections are approximately 2.5 mm long. Furthermore, impact experiments onto carbonaceous chondrite analogue materials simulating micrometeoroid bombardment show resemblance to the observed surface morphology of Ryugu and result in a mean surface slope (32 • ) similar to one derived for Ryugu (Avdellidou et al 2020).…”

Asteroid (101955) Bennu’s weak boulders and thermally anomalous equator

“…The low-reflectance boulders, on the other hand, have thermal conductivities far below those of our meteorite collections ( 28 ) and likely are distinct from any recognized meteorite types in terms of their thermomechanical properties. Recent laboratory experiments ( 80 ) have, however, shown that is possible to create simulants that have the composition of CMs but thermal conductivities intermediate of those of known meteorites and those of the boulders of Bennu and Ryugu. Regardless of the mechanism that caused the high porosity, the inferred weakness of this material is consistent with the implication that materials of this type would rarely or never survive atmospheric entry.…”

Bennu’s near-Earth lifetime of 1.75 million years inferred from craters on its boulders

“…The failure mechanics of monolithic C-complex asteroids are poorly understood because their meteoritic counterparts (CM and CI meteorites) are relatively rare and generally small; therefore, destructive testing of samples to determine their strength against impacts is infeasible [34]. Our current best understanding of the impact strength of monolithic C-complex asteroids comes from experiments that use terrestrial analogs [35] such as pumice [36] or asteroid regolith simulants [37]. Insights have also been gained from experiments into weak solid targets such as sandstones [38].…”