Hypervelocity impacts on carbonaceous asteroid simulants: comparison with observations

Hypervelocity impacts on carbonaceous asteroid simulants: comparison with observations

C. Avdellidou^(1,2), Marco Delbo⁽¹⁾, Cody Schultz⁽³⁾, Mark Price⁽²⁾, Alice DiDonna⁽⁴⁾, Bart Harthong⁽⁴⁾, Mike Cole⁽²⁾,Daniel Britt⁽³⁾, Robert Peyroux⁽⁴⁾

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Introduction

Two on-going sample return space missions, Hayabusa2 and OSIRIS-REx are orbiting and characterising two near-Earth asteroids, (162173) Ryugu and (101955) Bennu respectively. Initial ground-based observations and mission data indicate that the composition of these small objects is similar to the CM or CI meteorites (1-4). However, their thermo-mechanical properties appear to be different (5,6). This is expected since Earth's atmosphere filters out the weakest materials and thus we do not have them in our meteoritic collections. How does this weak materials respond to impacts? What type of regolith is produced during micrometeoroid bombardment? Will we expect to find exogenous materials embedded on the weak surfaces of Bennu and Ryugu?

Materials

In our work we used as targets to hypervelocity impact experiments, asteroid analogue materials with mineralogy similar to the CM meteorites. Specifically, the CM2 regolith simulant is a close mineralogical match to the Murchison CM2 carbonaceous chondrite meteorite Since one of the aims of the project was to examine qualitatively the produced regolith after impact events, we used as inclusions glass beads for an easier examination. The samples were produced and casted at the Exolith Lab of the University of central Florida. Samples were also mechanically tested at the 3SR Lab in Grenoble, where the compressive stress and tensile strength were measured. In addition we measured the thermal conductivity of the samples.

Experiments

In order to study the response of the CM-like asteroid analogue material to collisions with small projectiles, at typical impact speeds occurring in the asteroid Main Belt, we performed a series of laboratory hypervelocity impact experiments. We used the facilities of the Impact Lab of the University of Kent in the UK. main instrument used here is a 2-stage light-gas gun (LGG), which can achieve speeds up to 7.5 km/s. Targets were the squared blocks of CM analogues with dimensions 9.5 cm x 9.5 cm and 4.4 cm thickness, while as projectile we used stainless steel of different diameters. In these experiments we measured the depth and diameter of the craters, the quality of the ejecta and the state of the inclusions. In particular we wanted to test a part of the hypothesis that on materials with inclusions, impacts produce mainly ``multimineralic" fragments, whereas thermal cracking, as a slower process can produce fragments of single composition. After the impact experiments the roughness of the impacted samples was also calculated.

Comparison with spacecraft observations

First of all we compare the materials we produced with the inferred data from the space mission observations. Furthermore, we compare our laboratory impact results with impact observations of the same scale on the boulders of asteroid Bennu (mini-craters,7). 

Combining the mechanical properties of the asteroid simulants and the impact results, we predict that the impact shockwave dissipates very rapidly in such a soft material, therefore one would expect to see also fragments with one unique composition. 

Fig.1 Comparison between the surface of Ryugu (left) and our post-impact asteroid simulant (right).

Acknowledgements

This work was supported by the BONUS QUALITE RECHERCHE Lagrange (BQR) 2017; by the French National Research Agency under the project ``Investissements d’Avenir" UCA^JEDI with the reference number ANR-15-IDEX-01; by the Programme National de Planetologie (PNP) of CNRS/INSU, co-funded by CNES.

References

[1] Grott et al. Nature Astronomy, 3, p. 971-976, 2019

[2] DellaGiustina, Emery et al., Nature Astronomy, 3, 341, 2019

[3] Kitazato et al., Science, 364, 272, 2019

[4] Moskovitz et al., Icarus, 224, 24, 2013

[5] Perna et al., A&A, 599, L1, 2017

[6] LeCorre et al., MNRAS, 475, 614, 2018

[7] Ballouz et al. Nature (under revision) 2020