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 poorly understood near-subsurface physical properties of debris-pile asteroids. Now, a Southwest Research Institute-led study has characterized the crust just below the asteroid’s surface, composed of fragments of loosely bound rock that has twice as much void space as the overall asteroid .
“The low gravity of debris-pile asteroids like Bannu weakens its near-subsurface by not compressing the upper layers, reducing the effects of particle assembling,” said Dr Kevin Walsh of SRIRI, the lead of a paper about this research. The author said. Journal Science Advances. “We conclude that a low-density, weakly bound subsurface layer should be a global property of Bennu, not just localized at the point of contact.”
Fitting its designation as a “debris-pile asteroid”, Bennu is a spherical collection of rock fragments and debris 1,700 feet in diameter and held together by gravity. It is believed to have formed after a collision with a large main-asteroid-belt object. Rocks are scattered across its heavily cratered surface, indicating that it has had a rough existence ever since it broke free from its much larger parent asteroid, some millions or even billions of years ago.
The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission aims to collect at least 60 grams of surface material from Bennu and deliver it to Earth in 2023. Sample collection activities provided additional insight.
According to Walsh, researchers involved in the OSIRIS-REx mission have so far measured Bennu’s thermal properties and craters to estimate the strength and porosity of the discrete particles of debris-pile asteroids. The particles, or regolith, on an asteroid’s surface that control and affect long-term evolution have not yet been directly investigated.
Before, during and after the sampling event, the Sample Acquisition Verification Camera (SamCam) of the OSIRIS-REx camera suite captured images while looking at the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) robotic arm.
“SamCam images bracketing the moment of contact cause considerable disturbances at the sample site,” said co-author Dr. Ron Ballouz from Johns Hopkins University’s Laboratory of Applied Physics. “Almost every visible particle is moved or re-oriented within a 15-inch radius at all points along the circumference of TAGSAM.”
These SamCam images showed that TAGSAM’s downward force lifted the rock by about 16 inches. Although strong enough to withstand breakage, the rock was re-oriented and small debris was removed from its surface. The mobility of these millimeter-scale particles under relatively weak forces suggests a minimal cohesive bond with the larger rock surface.
Scientists theorize that the average regolith particle size increases as the asteroid’s size decreases, as larger bodies retain smaller material due to the higher surface gravity. The team then compared Bennu to asteroids containing similar debris.
“We discovered a dichotomy between the rough, boulder-covered surfaces of Bennu and Ryugu versus Itokawa, which contains ponds of small particles covering 20% of its surface,” Walsh said. “There can be several explanations for this, including whether the near-surface of the latter has compressed these microparticles enough to depress the seepage internally or perhaps that granular deposits are subsurface layers that form part of the body’s surface. recently manifested by disruptive restructuring.”
a companion paper in the journal science and characterized a 30-foot-long oval crater excavated by the TAGSAM arm while collecting the sample. The event collected rocks and dust into a rubble pile, exposing the material to be darker, redder and more abundant in fine particles than the original surface. The bulk density of the displaced subsurface material is about half that of the asteroid as a whole.
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