Granular matter
How solid is the surface of asteroid Bennu?
Open access · cc by · source: Europe PMC
When NASA's OSIRIS-REx touched asteroid Bennu, its sampler sank easily into rubble that was loosely packed and barely stuck together, more like a ball pit than solid ground.
Study at a glance
- Design
- Other — Analysis of OSIRIS-REx images and inertial measurement data from a single 10 cm/s sampling touchdown, interpreted with soft-sphere discrete element simulations and an empirical granular drag law
- N
- A single contact event on one asteroid; no sample size
- Population
- Regolith in the top 0 to 10 cm at the Hokioi crater sample site on asteroid Bennu
- Outcome
- Contact forces, penetration depth, inferred bulk density, packing fraction, compressive strength and cohesion
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
The sampler reached about 6 to 7 cm depth in under a second while experiencing forces of only 10 to 15 N, and it disturbed an area about six times its own footprint. The simulations place this in the loose regime with packing fraction at or below about 0.5, and the drag law implies a near-surface bulk density of 440 to 600 kg per cubic metre, well below Bennu's overall density of 1190. The implied packing fraction is 0.2 to 0.45, compressive strength 2 to 200 Pa and cohesion roughly 0.2 to 20 Pa, far weaker than the Moon's surface.
Methodology
The team reconstructed the short window after the sampler head touched down, before its gas burst fired, using camera images taken every 1.2 seconds and the spacecraft's accelerometer. Shadows and particle sizes in the images fixed the penetration depth at a known time, and integrating accelerations gave how deep and how hard the sampler pushed. They compared the forces with discrete-element simulations of a sampler hitting granular beds of different packing, friction and cohesion, and with an empirical low-speed granular drag law.
Limitations
This is one touchdown at one site, so the properties may not represent Bennu's whole surface. The results depend on assumptions such as a friction angle of about 40 degrees taken from local slopes, porosity of individual boulders inferred from thermal data, and an empirical force law developed from laboratory impacts. Contact with a 5 cm rock tilted the sampler and complicated the force record, and the cause of the very low cohesion (particle roughness, fine dust) remains unresolved pending analysis of returned samples.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Bennu's top layer is a loose, nearly cohesionless rubble that offers little resistance.
The OSIRIS-REx sampler sank 6-7 cm into Bennu's surface under only 10-15 N of force; simulations and a granular drag law implied a near-surface bulk density of 440-600 kg per cubic metre, packing fraction 0.2-0.45 and cohesion of roughly 0.2-20 Pa.
Evidence for the claim as stated.
Bennu's top layer is a loose, nearly cohesionless rubble that offers little resistance.
The OSIRIS-REx sampler sank 6-7 cm into Bennu's surface under only 10-15 N of force; simulations and a granular drag law implied a near-surface bulk density of 440-600 kg per cubic metre, packing fraction 0.2-0.45 and cohesion of roughly 0.2-20 Pa.
Scope note — Single touchdown at one site; results depend on an assumed friction angle and an empirical lab force law.
Limits the claim's scope: a different population, assay, or outcome.
Rigidity-percolation results come from frictionless 2D disks, while the intrusion, asteroid and jamming studies involve frictional, cohesive or 3D grains; the authors of the percolation study flag friction and 3D as open questions, so the exponents should not be assumed to apply to real sand.
Same question, contrary or null result.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Rigidity-percolation results come from frictionless 2D disks, while the intrusion, asteroid and jamming studies involve frictional, cohesive or 3D grains; the authors of the percolation study flag friction and 3D as open questions, so the exponents should not be assumed to apply to real sand.
Related papers in this topic
Same topic cluster — not a recommendation engine.