According to observations of craters by NASA‘s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft, the boulder-covered surface of asteroid Bennu protects it from small meteoroid impacts.
“These observations provide new insight into how asteroids like Bennu respond to energetic impacts,” said Edward (Beau) Bierhaus, lead author of a paper published in this month’s issue of Nature Geoscience.
Bennu is a “rubble-pile” asteroid, which means it formed from the remains of a much larger asteroid destroyed by an ancient impact. Bennu was formed when fragments from the collision coalesced under their own weak gravity.
The team examined craters on Bennu using unprecedented, high-resolution global data sets: images from the OSIRIS-REx Camera Suite and surface-height data (topography) derived from the spacecraft’s laser-ranging (lidar) instrument, the OSIRIS-REx Laser Altimeter.
“Measuring craters and their populations on Bennu was exceptionally exciting,” said co-author David Trang of the University of Hawaii at Mnoa in Honolulu. “We discovered something unique to small and rocky bodies at Bennu, which expanded our understanding of impacts.”
By measuring the abundance and size of craters, planetary scientists can estimate the age of surfaces. Because impact craters build up over time, a surface with many craters is older than one with few craters. In addition, the size of the crater is determined by the size of the impactor, with larger impactors producing larger craters. Because small meteoroids outnumber large meteoroids, celestial objects such as asteroids typically have far more small craters than large ones.
This pattern is followed by Bennu’s larger craters, with the number of craters decreasing as their size increases. The trend is reversed for craters smaller than about 6.6 to 9.8 feet (around 2–3 meters) in diameter, with the number of craters decreasing as their size decreases. This suggests that something unusual is going on on Bennu’s surface.
According to the researchers, Bennu’s abundance of boulders acts as a shield, preventing many small meteoroids from forming craters. These impacts, on the other hand, are more likely to break apart the boulders or chip and fracture them. Furthermore, some impactors that do make it through the boulders leave smaller craters than if Bennu’s surface was covered in smaller, more uniform particles, such as beach sand.
This activity causes Bennu’s surface to change in a different way 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 most likely one of the fastest-acting processes on the surface of a rubble-pile asteroid. On Bennu, this contributes to the appearance of the surface being many times younger than the interior “Bierhaus stated.