OSIRIS-REx data shows that surface and near-subsurface rubble are loosely bound — ScienceDaily


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NASA’s OSIRIS REx spacecraft collected samples of Bennu’s asteroid surface in 2020. The forces generated during the interaction gave scientists a direct test of the poor-understood near-subsurface physical property of rubble pile asteroids. A Southwest Research Institute-led study now identifies the layer below the asteroid’s surface as weakly bound rock fragments that contain twice as much void space than the entire asteroid.

“The low gravity rubble-pile asteroids like Bennu weakens the near-subsurface by preventing compression of the upper layers, minimizing the influence of particle cohesion,” Dr. Kevin Walsh of SwRI, the lead author of a paper on this research, published in Science Advances. “We concluded that a subsurface layer of low density and weakly bound should be a global property for Bennu and not only localized to the contact points.”

Bennu, which is also known as a rubble-pile Asteroid, is a spheroidal collection 1,700 feet wide of rock fragments. It is held together with gravity. It is believed that it was formed by a collision between a larger main asteroid-belt object. The heavily cratered surfaces are covered in rocks, which indicates that the planet has experienced a rough existence after being released from its parent asteroid many millions to billions years ago.

OSIRIS-REx is a mission of OSIRIS-REx. Its goal is to retrieve and return 60 grams of Bennu surface material, and to deliver it to Earth by 2023. Additional insights were gained through sample collection activities.

Walsh said that OSIRIS REx mission scientists have already measured Bennu’s thermal characteristics and craters. This has allowed them to estimate the strength of individual particles in rubble-pile asteroids and their porosity. However, it has yet to be directly probed how the regolith, or ensemble of particles at an asteroid’s surface, influences long-term evolution.

The Sample Acquisition Verification Camera (SamCam), part of OSIRIS-REx Camera Suite, captured images that looked at the Touch-and Go Sample Acquisition Mechanism (TAGSAM).

“The SamCam images that capture the moment of contact indicate that there was considerable disturbance at the sample location,” Dr. Ron Ballouz from Johns Hopkins University’s Applied Physics Laboratory said. “Nearly every visible particle has been moved or reoriented at any point along the circumference TAGSAM’s 15-inch radius.”

These SamCam images show how TAGSAM’s downward force lifted a nearly 16-inch-thick rock. The rock was strong enough to withstand being broken, but it was re-oriented so that small debris could be lifted off its surface. These particles are millimeter in size and move under weak forces, suggesting that there is little cohesive bonding to the rock.

Scientists think that the average regolith particle sizes increase with increasing asteroid sizes. This could be because smaller bodies tend to retain less material due to higher surface gravity. The team compared Bennu to other similar rubble pile asteroids.

Walsh stated that there was a contrast between the Bennu and Ryugu’s rough, boulder-covered surfaces. Itokawa has ponds with smaller particles covering 20% of its surface. This could be due to the fact that the near-surface of Ryugu and Bennu has been compressed sufficiently to prevent microparticles from percolating into its interior. Or, perhaps the granular deposits may be subsurface layers exposed by a recent disruptive reorganization.

An accompanying paper is available in the journal Science Walsh co-authored the description of the 30-foot-long elliptical crater that was excavated by TAGSAM when it collected the sample. The event caused rocks and dust to be mobilized into a debris plume. This resulted in material that is darker and redder than the original surface and more fine particulates. The bulk density of the subsurface material displaced is approximately half that of the entire asteroid.

Video: https://youtu.be/wCO1y_GNo98

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