According to a new analysis of data from NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft, particles that make up the exterior of the near-Earth asteroid Bennu are so loosely packed and lightly bound to each other that if a person were to step onto Bennu they would feel very little resistance, as if stepping into a pit of plastic balls that are popular play areas for kids.
Surface changes around the circumference of TAGSAM: (A) close-up view of the sample collection site just before contact; 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 with arrow indicating the lofted debris; (C, 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). Image credit: Walsh et al., doi: 10.1126/sciadv.abm6229.
Fitting its designation as a rubble-pile asteroid, Bennu is a spheroidal collection of rock fragments and debris about 492 m (1,614 feet) in diameter and held together by gravity.
It is thought to have been formed after a collision involving a larger main-asteroid-belt object.
Rocks are scattered across its heavily cratered surface, indicating that it has had a rough-and-tumble existence since being liberated from its much larger parent asteroid some millions or billions of years ago.
The goal of NASA’s OSIRIS-REx mission is to collect and return at least 60 grams of surface material from Bennu and deliver it to Earth in 2023. Sample collection activities provided additional insights.
“If Bennu was completely packed, that would imply nearly solid rock, but we found a lot of void space in the surface,” said OSIRIS-REx team member Dr. Kevin Walsh, a researcher at the Southwest Research Institute.
Before, during, and after the sampling event, the Sample Acquisition Verification Camera (SamCam) of the OSIRIS-REx Camera Suite captured images looking at the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) robotic arm.
“The SamCam images bracketing the moment of contact show the contact caused considerable disturbance at the sample site,” said Dr. Ron Ballouz, a researcher at the Johns Hopkins University’s Applied Physics Laboratory.
“Nearly every visible particle is moved or re-oriented at all points along the circumference of TAGSAM up to a 38.1-cm (15-inch) radius.”
The SamCam images showed the downward force of TAGSAM lifted a 40.6-cm (16-inch) rock.
Though strong enough to withstand breaking, the rock was re-oriented and small debris lofted off its surface.
The mobility of these millimeter-scale particles under relatively weak forces suggests minimal cohesive bonding with the surface of the larger rock.
The authors then compared Bennu to similar rubble-pile asteroids.
“We discovered a dichotomy between the rough, boulder-covered surfaces of Bennu and Ryugu versus Itokawa, which includes ponds of smaller particles across 20% of its surface,” Dr. Walsh said.
“This could have several explanations, including that the latter’s near-surface has compressed enough to frustrate these microparticles percolating into the interior or perhaps the granular deposits are subsurface layers revealed by a recent disruptive reorganization of the body.”
In a companion study, the researchers characterized the 9-m- (30-foot) long elliptical crater excavated by the TAGSAM arm when it collected the sample.
The event mobilized rocks and dust into a debris plume, exposing material that is darker, redder and more abundant in fine particulates than the original surface.
The displaced subsurface material’s bulk density is about half that of the asteroid as a whole.
“I think we’re still at the beginning of understanding what these bodies are, because they behave in very counterintuitive ways,” said OSIRIS-REx scientist Dr. Patrick Michel, director of research at the Centre National de la Recherche Scientifique at Côte d’Azur Observatory.
The team’s results appear in two papers in the journal Science Advances and the journal Science.
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Kevin J. Walsh et al. 2022. Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact. Science Advances 8 (27); doi: 10.1126/sciadv.abm6229
D.S. Lauretta et al. Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu. Science, published online July 7, 2022; doi: 10.1126/science.abm1018
