(ORDO NEWS) — Asteroid Bennu’s boulder-covered surface provides protection from small meteor impacts, according to NASA’s OSIRIS-REx crater observations.
Bennu is a “debris asteroid”, meaning it was formed from pieces of a much larger asteroid that was destroyed in an ancient collision. Fragments from the collision clump together under their own weak gravity to form Bennu.
To study the craters on Bennu, the team used unprecedented high-resolution global datasets: images from the OSIRIS-REx array of cameras and surface height (topography) data from the OSIRIS-REx laser altimeter, a laser ranging (lidar) instrument on spacecraft.
Planetary scientists can estimate the age of a surface from the number and size of craters. Impact craters accumulate over time, so a surface with a lot of craters is older than a surface with few craters.
In addition, the size of the crater depends on the size of the impactor, with large impactors usually forming large craters. Since small meteors are much more numerous than large meteors, celestial objects such as asteroids usually have many more small craters than large ones.
The large Bennu craters follow this pattern, with the number of craters decreasing as their size increases. However, for craters less than 2-3 meters in diameter, the opposite trend is observed: the number of craters decreases as their size decreases. This indicates that something unusual is happening on the surface of Bennu.
The researchers believe that the abundance of boulders on Bennu acts as a shield, preventing many small meteors from forming craters. Instead, impacts are more likely to split boulders or cause them to chip and crack.
In addition, some impactors that break through boulders form smaller craters than if Bennu’s surface were covered with finer and more uniform particles, like beach sand.
This activity causes Bennu’s surface to change differently than fine-grained or hard-surfaced objects.
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