When NASA’s OSIRIS-REx spacecraft collected samples from the surface of asteroid Bennu in 2020, the forces measured during the interaction gave scientists a direct test of the poorly understood subsurface physical properties of debris-pile asteroids. Now, a Southwest Research Institute-led study has characterized the layer just below the asteroid’s surface as composed of loosely bound rock fragments that contain twice as much empty space as the entire asteroid.
“The low gravity of debris-pile asteroids like Bennu weakens their terrestrial boundaries without compressing the upper layers, minimizing the effect of particle cohesion,” said Dr. Kevin Walsh of SwRI, lead author of a paper on the study published in the journal Science Advances . “We conclude that the low-density, loosely bound subsurface layer must be a global property of Bennu, rather than just localized at the contact point.”
Living up to its designation as a “rubble pile asteroid,” Bennu is a 1,700-foot-diameter spheroidal collection of rock fragments and debris held together by gravity. It is thought to have formed as a result of a collision with a larger object in the main asteroid belt. Rocks are strewn across its heavily cratered surface, suggesting that it has existed in a rough state since being released from its much larger parent asteroid millions or billions of years ago.
The goal of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission is to collect and return at least 60 grams of surface material from Bennu and deliver it to Earth in 2023. Sampling activities provided additional information. .
According to Walsh, researchers on the OSIRIS-REx mission have so far measured the thermal properties and craters of Bennu to assess the strength and porosity of individual debris asteroid particles. The ensemble of particles, or regolith, on the surface of the asteroid that controls and influences the long-term evolution has not yet been directly studied.
Before, during, and after sampling, the Sample Collection Verification Camera (SamCam) of the OSIRIS-REx camera suite took images looking at the Touch-and-Go Sample Collection Mechanism (TAGSAM) robotic arm.
“SamCam images bracketing the moment of contact show that the contact caused significant disruption at the sample site,” said co-author Dr. Ron Balluz of Johns Hopkins University’s Applied Physics Laboratory. “Nearly every visible particle moves or reorients at all points on the TAGSAM circumference up to a 15-inch radius.”
These SamCam images showed that TAGSAM’s downward force lifted a nearly 16-inch rock. Although the stone was strong enough to withstand the destruction, it was reoriented and small debris rose from its surface. The mobility of these millimeter-scale particles under the influence of relatively weak forces indicates a minimal cohesive bond with the surface of the larger stone.
The scientists hypothesized that the average particle size of the regolith increases as the size of the asteroid decreases because larger bodies retain smaller materials due to greater surface gravity. The team then compared Bennu to similar asteroids from the rubble pile.
“We found a dichotomy between the rough, boulder-covered surfaces of Bennu and Ryugu versus Itokawa, which includes ponds with finer particles on 20% of the surface,” Walsh said. “There could be several explanations for this, including that the surface of the latter has compressed enough to dislodge these microparticles that seep in, or perhaps the granular deposits are subsurface layers exposed by recent, disruptive reorganization of the body.”
Accompanying journal article Science and, coauthored with Walsh, characterized a 30-foot-long elliptical crater excavated by the TAGSAM arm during sample collection. The event collected rocks and dust into a debris plume, exposing material that is darker, reddish, and has more fine particles than the original surface. The bulk density of displaced subsurface material is about half that of the asteroid as a whole.