The SwRI-led study provides new insights into su


image: A study led by the SwRI found that the surface regolith of asteroid Bennu is mostly loose rubble. Images taken before and after touch-and-go sample collection indicate surface disturbances up to 15 inches away.
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Credit: NASA/Goddard/University of Arizona

SAN ANTONIO – July 7, 2022 – When NASA’s OSIRIS-REx spacecraft collected samples from the surface of asteroid Bennu in 2020, the forces measured during the interaction provided scientists with a direct test of the properties poorly understood near-subsurface physics of rubble-pile asteroids. Now, a study led by the Southwest Research Institute has characterized the layer just below the asteroid’s surface as being composed of loosely bound rock fragments containing twice as much empty space as the overall asteroid.

“The low gravity of rubble-pile asteroids such as Bennu weakens its quasi-subsurface by not compressing the upper layers, minimizing the influence of particle cohesion,” said Dr Kevin Walsh of SwRI, lead author of an article on this research published in the journal Science Advances. “We conclude that a low-density, weakly bonded subsurface layer should be a global property of Bennu, not just localized at the contact point.”

True to its designation as a “rubble-pile asteroid,” Bennu is a spheroidal collection of rock fragments and debris 1,700 feet in diameter and held together by gravity. It is thought to have formed after a collision involving a larger object from the main asteroid belt. Rocks are strewn across its heavily cratered surface, indicating that it has had a turbulent existence since being freed from its much larger parent asteroid a few million or billion years ago.

The objective 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. sample collection activities provided additional information. .

According to Walsh, researchers involved in the OSIRIS-REx mission have so far measured the thermal properties and craters of Bennu to estimate the strength and porosity of discrete asteroid particles in piles of rubble. The set of particles, or regolith, on the surface of an asteroid controlling and influencing long-term evolution has not been probed directly until now.

Before, during and after the sampling event, the Sample Acquisition Verification Camera (SamCam) of the OSIRIS-REx Camera Suite captured images looking at the robotic arm of the sample acquisition mechanism Touch-and-Go (TAGSAM).

“SamCam images bracketing the time of contact show that the contact caused considerable disturbance at the sample site,” said co-author Dr. Ron Ballouz, Johns Hopkins University Applied Physics Laboratory. . “Virtually all visible particles are moved or reoriented at all points along TAGSAM’s circumference up to a radius of 15 inches.”

These SamCam images showed that the downward force of TAGSAM lifted a boulder nearly 16 inches. Although strong enough to resist breaking, the rock was reoriented and small debris broke 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.

Scientists have hypothesized that the average regolith particle size increases as the asteroid decreases in size, as larger bodies hold onto smaller material due to higher surface gravity. The team then compared Bennu to similar rubble heap asteroids.

“We discovered a dichotomy between the rough, boulder-covered surfaces of Bennu and Ryugu compared to Itokawa, which has smaller particle ponds over 20% of its surface,” Walsh said. “This could have several explanations, including that the surface near it has compressed enough to prevent these microparticles from percolating within or perhaps the granular deposits are subterranean layers revealed by a recent disruptive reorganization of the body. .”

A companion paper in the journal Science and co-authored by Walsh characterized the 30-foot-long, elliptical crater carved out by the TAGSAM arm during sample collection. The event mobilized rocks and dust into a plume of debris, exposing material that was darker, redder and more abundant in fine particles than the original surface. The bulk density of the displaced subterranean material is about half that of the asteroid as a whole.

To read the Science Advances article, go to The accompanying Science article can be accessed at

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