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

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.

Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu

“…S13, G to I). This range is consistent with independent results from spacecraft accelerometer data and granular mechanics models ( 21 ). This density, combined with the estimated excavation volume, suggests that the sampling event mobilized at least 6000 ± 650 kg, which is substantially more than predicted ( 39 ).…”

“…S3), within 73 cm of the targeted location ( 9 ). Camera and IMU data indicate an initial surface contact force of between 10 and 15 N ( 21 ). One second after contact, TAGSAM released a jet of nitrogen gas to fluidize unconsolidated surface material and guide it into the collection chamber.…”

“…The surface of Bennu responded to contact as a compliant, viscous fluid, providing minimal resistance to the downward motion of the spacecraft (fig. S4 and movie S1) ( 21 ). This response is consistent with simulations of TAGSAM interacting with regolith (unconsolidated rocks and dust) that has near-zero interparticle cohesion ( 21 – 23 ) and low-gravity experiments in which loose material is easily mobilized ( 24 ).…”

The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos

“…The small lander MASCOT aboard the Hayabusa2 mission provided in situ data from the surface of Ryugu (Ho et al 2021) showing different kinds of surface textures in high-resolution images (Jaumann et al 2019), as well as rocks with very high porosity and a low tensile strength in the few hundred kPa range (Grott et al 2019). Furthermore, the sampling of Bennu by the Touch And Go SAmpling Mechanism (TAGSAM) of OSIRIS-REx revealed that the surface of the asteroid, as a result of its low gravity, behaves like a granular bed with nearzero cohesion that is half as dense as the bulk asteroid (Lauretta et al 2022;Walsh et al 2022). An internal cohesive strength of only a few tens of Pa was found to be required to prevent the disruption of the fast-spinning kilometer-diameter NEA (29075) 1950 DA (Rozitis et al 2014).…”

“…Finally, the interaction of the OSIRIS-REx sampling tool (TAGSAM), which made a slow impact into the surface of Bennu, also revealed the very low cohesion and density of the surface of the asteroid (Lauretta et al 2022;Walsh et al 2022). Furthermore, although there is some indication of subsurface stiffness (Barnouin et al 2019), impact craters on Bennu may also be controlled by either gravity scaling or strength scaling assuming a very low strength (<100 Pa), except for small craters that are affected by armoring (Bierhaus et al 2022).…”