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. 2022 Jul 8;8(27):eabm6229.
doi: 10.1126/sciadv.abm6229. Epub 2022 Jul 7.

Near-zero cohesion and loose packing of Bennu's near subsurface revealed by spacecraft contact

Kevin J Walsh  1 , Ronald-Louis Ballouz  2   3 , Erica R Jawin  4 , Chrysa Avdellidou  5 , Olivier S Barnouin  3 , Carina A Bennett  2 , Edward B Bierhaus  6 , Brent J Bos  7 , Saverio Cambioni  8 , Harold C Connolly Jr  2   9 , Marco Delbo  5 , Daniella N DellaGiustina  2 , Joseph DeMartini  10 , Joshua P Emery  11 , Dathon R Golish  2 , Patrick C Haas  6 , Carl W Hergenrother  2 , Huikang Ma  6 , Patrick Michel  5 , Michael C Nolan  2 , Ryan Olds  6 , Benjamin Rozitis  12 , Derek C Richardson  10 , Bashar Rizk  2 , Andrew J Ryan  2 , Paul Sánchez  13 , Daniel J Scheeres  13   14 , Stephen R Schwartz  2   15 , Sanford H Selznick  16 , Yun Zhang  5 , Dante S Lauretta  2
Affiliations

Near-zero cohesion and loose packing of Bennu's near subsurface revealed by spacecraft contact

Kevin J Walsh et al. Sci Adv. .

Abstract

When the OSIRIS-REx spacecraft pressed its sample collection mechanism into the surface of Bennu, it provided a direct test of the poorly understood near-subsurface physical properties of rubble-pile asteroids, which consist of rock fragments at rest in microgravity. Here, we find that the forces measured by the spacecraft are best modeled as a granular bed with near-zero cohesion that is half as dense as the bulk asteroid. The low gravity of a small rubble-pile asteroid such as Bennu effectively weakens its near subsurface by not compressing the upper layers, thereby minimizing the influence of interparticle cohesion on surface geology. The underdensity and weak near subsurface should be global properties of Bennu and not localized to the contact point.

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Figures

Fig. 1.
Fig. 1.. Surface changes around the circumference of TAGSAM.
(A) Close-up view of the sample collection site just before contact at 21:49:48.882 UT. TAGSAM (round) can be seen at the end of its mechanical arm, and the arrow indicates the 20-cm rock that was first contacted. (B) Post-contact image at 21:49:50.101 UT with arrow indicating the lofted debris. (C and D) Same image as in (B), with a yellow boundary around the 0.51-m2 area that envelopes surface disturbance (C) and a zoom-in with contrast adjusted to show the lofted debris (solid arrows) and shadows over the lip of the sampler head (dashed arrows) (D). Animated version is in fig. S3 and movie S1.
Fig. 2.
Fig. 2.. The measured accelerations during contact and the derived depth profile.
(A) The derived depth of TAGSAM penetration into the surface, calculated by integrating the raw z acceleration measurements. The two profiles shown reach a depth of 2 and 3 cm, indicating approximate uncertainty bounds, at the time of the SamCam image at 21:49:50.101 UT (orange dots and vertical line). Our analysis only considers the behavior up to 21:49:50.5 UT (red dots and vertical line). (B) Accelerations into the surface (z direction) measured by the spacecraft IMU at 200 Hz and analyzed with 2-, 4-, and 8-Hz low-band frequency cutoffs (see Materials and Methods). The gray box indicates a range of force from 10 to 15 N, orange lines denote SamCam images taken before and after contact, and the red vertical line indicates where increased force due to the release of high-pressure gas begins to alter the interaction and ends the extent of the analysis presented here. A longer time series of these data is available in fig. S5.
Fig. 3.
Fig. 3.. Simulations of TAGSAM penetrating granular material best fit with a low bulk density surface.
A series of outcomes from numerical experiments (see Materials and Methods) produced a dependence of peak force on bulk density of the material (ρ), F = 0.62 tan(φ)ρU4/3 (37), with the blue shaded region showing the range of measured force during contact. The relationship depends on the impact speed, cross-sectional area of the impactor (the TAGSAM head), and the determination of the angle of friction (φ) from the surrounding geology (1, 4), together with the known area of TAGSAM and the velocity and force of the sampling event that constrain the bulk density of the material affected.
See this image and copyright information in PMC

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