NASA spacecraft observes asteroid Bennu’s boulder ‘body armor’

This image shows the boulder-covered surface of asteroid Bennu. It was captured by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on April 11, 2019 from a distance of 4.5 km. The field of view is 211 ft (64.4 m) and the large rock in the top right corner of the image is 50 ft (15.4 m) high. When the image was taken, the spacecraft was over the Southern Hemisphere and PolyCam was pointing far north and west. Credit: NASA/Goddard/University of Arizona

According to crater observations by NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft, the boulder-covered surface of asteroid Bennu provides protection from small meteoroid impacts.

“These observations provide new insight into how asteroids like Bennu respond to energetic impacts,” said Edward (Beau) Bierhaus of Lockheed Martin Space, Littleton, Colorado, lead author of a paper published this month in the issue of Natural Geosciences

Bennu is a “mess” asteroid, meaning it was formed from the debris of a much larger asteroid that was destroyed by an ancient impact. Fragments of the collision coalesced under their own weak gravity to form Bennu.

The team used unprecedented, high-resolution global datasets to investigate craters on Bennu: images from the OSIRIS-REx Camera Suite and surface elevation data (topography) derived from the OSIRIS-REx Laser Altimeter, a laser-ranging (lidar) instrument on the spacecraft. .

“Measuring craters and their populations on Bennu has been extremely exciting,” said David Trang of the University of Hawaii in Mānoa, Honolulu, a co-author of the paper. “At Bennu, we discovered something unique to small and rocky bodies, expanding our knowledge of effects.”

Planetary scientists can estimate the age of surfaces by measuring the abundance and size of craters. Impact craters accumulate over time, so a surface with many craters is older than a surface with few craters. Also the size of the crater depends on impactor size, with larger impactors generally creating larger craters. Since small meteoroids are much more abundant than large meteoroids, celestial bodies like asteroids usually have many more small craters than large ones.

Bennu’s larger craters follow this pattern, with the number of craters decreasing as they increase in size. However, for craters smaller than about 6.6 to 9.8 feet (about 2-3 meters) in diameter, the trend is backward, with the number of craters decreasing as their size decreases. This indicates that something unusual is happening on Bennu’s surface.

The researchers believe that Bennu’s abundance of boulders acts as a shield and prevents many small meteoroids from forming craters. Instead, these boulders are more likely to break the boulders apart or crumble and break. Also, some impactors that penetrate the boulders create smaller craters than if the surface of Bennu were covered with smaller, more uniform particles, such as beach sand.

This activity causes Bennu’s surface to change differently than objects with fine-grained or solid surfaces. “The displacement or disruption of an individual or small group of boulders by a small impact is probably one of the fastest-acting processes on the surface of a mess asteroid. On Bennu, this contributes to the fact that the surface appears many times younger than the interior,” said Bierhaus.


Studying craters on asteroid Bennu shows how long it has been orbiting near Earth


More information:
EB Bierhaus et al, Crater population on asteroid (101955) Bennu indicates impact armor and young surface, Natural Geosciences (2022). DOI: 10.1038/s41561-022-00914-5

Quote: NASA spacecraft observes asteroid Bennu’s boulder ‘body armor’ (2022, June 16) recovered June 16, 2022 from https://phys.org/news/2022-06-nasa-spacecraft-asteroid-bennu-boulder.html

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