SwRI-led study provides new insights about surface, structure of asteroid Bennu
An SwRI-led study found that the surface regolith of the asteroid Bennu is primarily loose rubble. Images taken before and after the touch-and-go sample collection indicate surface disturbances up to 15 inches away. Credit: NASA/Goddard/University of Arizona

A direct test of the poorly understood physical properties of rubble-pile asteroids was provided by the forces measured during the interaction between NASA's OSIRIS-REx and asteroid Bennu. A Southwest Research Institute-led study has found that the layer just below the asteroid's surface is composed of weakly bound rock fragments.

A paper about this research was published in the journal Science Advances. The low density weakly bound subsurface layer should be a global property of Bennu.

Bennu is a spheroidal collection of rock fragments and debris 1,700 feet in diameter. It is believed that it was formed after a collision. It has had a rough and tumble existence since it was liberated from its larger parent asteroid millions of years ago.

The local surface of Bennu before and after sampling. Blinking between pre- and post-sampling images highlights changes to the surface, including a boulder that was apparently launched 12 m. Credit: NASA/Goddard/University of Arizona

The OSIRIS-REx mission is to collect and return at least 60 grams of surface material from Bennu in order to deliver it to Earth. Additional insights came from sample collection activities.

Bennu's thermal properties and craters have been measured by researchers involved in the OSIRIS-REx mission. The regolith at an asteroid's surface has not been probed before.

The Sample Acquisition Verification Camera (SamCam) of the OSIRIS-REx Camera Suite captured images during and after the sampling event.

SwRI-led study provides new insights about surface, structure of asteroid Bennu
A SwRI-led study found that the rocky fragments dominating the surface asteroid Bennu are weakly bound, exhibiting near-zero cohesion, probably due to size and low gravity of the small body. Credit: NASA/Goddard/University of Arizona

Ron Ballouz is a co-author of the paper. Every visible particle is moved or reoriented at all points along the circle of TAG SAM.

The downward force of TAG SAM lifted a large rock. Small debris was lofted off the rock's surface after it was reoriented and strengthened. The mobility of the millimeter-scale particles suggests that they are not cohesive with the larger rock.

SwRI-led study provides new insights about surface, structure of asteroid Bennu
A recent SwRI-led study provided new insights into the surface and structure of asteroid Bennu. NASA's OSIRIS-REx spacecraft data indicate nearly twice the void space near its surface in comparison to the overall body. Credit: NASA/Goddard/University of Arizona

The average regolith particle size increases as asteroid size decreases, according to scientists. Bennu was compared to rubble-pile asteroids.

There are ponds of small particles across 20% of the surface of Bennu and Itokawa. The compressed surface of the latter could be one of the reasons, along with the recent reorganization of the body.

Study provides new insights about the surface and structure of asteroid Bennu
The images immediately before and after contact with Bennu show that in the approximately 1 second that elapsed the sampler head disturbed an area nearly 3 feet across and tossed debris into the air. Bennu provided minimal resistance to the sampler head being pressed into the asteroid, which is seen partly by the widespread disturbance caused by contact, and this data helped deduce that the upper layers of the asteroid were very lightly packed with significant void spaces. The yellow envelope shows the mapped disturbed area in the post-contact image, and the image in the bottom-right shows shadows over the lip of the sampler head and lofted debris that both helped deduce the properties of the surface. Credit: NASA/Goddard/University of Arizona

The 30-foot-long elliptical crater excavated by the TAG SAM arm was described in a companion paper. The event exposed material that was darker and more abundant in fine particulates than the original surface. The bulk density of the displaced material is less than that of the asteroid as a whole.

More information: Kevin J. Walsh et al, Near-zero cohesion and loose packing of Bennu's near subsurface revealed by spacecraft contact, Science Advances (2022). DOI: 10.1126/sciadv.abm6229

The Spacecraft sample collection and subsurface excavation of asteroid 101955 Bennu was written by D.S. Lauretta. There is a science.abm1018

Journal information: Science Advances , Science
Bennuasteroid BennusurfaceUniversity of Arizona
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