Scientists have concluded from observations with NASA spacecraft that the debris-covered surface of asteroid Bennu offers it protection from small meteorite impacts. The Bennu asteroid is a gumball asteroid, meaning it was formed from the remains of a much larger asteroid. When the asteroid was destroyed by an ancient impact, the collision caused the fragments to coalesce, resulting in the formation of Bennu. Scientists have used high-resolution datasets to study craters on the planet to understand how boulders on its surface protect it from small meteoroids.
To determine the age of a surface, scientists analyze the number of craters on it and their size. Impact craters, formed by hitting a meteorite, accumulate over time. Therefore, the surface with more craters is older than one that doesn't have many craters. The size of the crater gives scientists an idea of how big the impactor (an object colliding with another body) was. Small meteoroids are found to be more common, and therefore asteroids have more small craters on their surface than large ones.
To study craters on asteroid Bennu, scientists have analyzed data collected by NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft. They analyzed images taken by the spacecraft, data from the OSIRIS-REx laser altimeter and a laser range finder.
The researchers found that Bennu's larger craters followed a pattern in which the number of craters decreased with increasing size. For smaller craters, between 2 and 3 meters in diameter, this trend was reversed.
As a result, scientists believe the boulders on Bennu's surface may act as a shield against meteoroids. When a meteoroid collides with Bennu, it shatters, shatters, or shatters the boulders. Impactors also create smaller craters on Bennu than they would have done without the protection due to the boulders.
This changes the surface of Bennu differently than other objects with solid or fine-grained surfaces. “The displacement or destruction of a single or small group of boulders by a small impact is probably one of the fastest acting processes on the surface of a debris-pile asteroid. On Bennu, this helps make the surface appear many times younger than the interior,” said Edward (Beau) Bierhaus of Lockheed Martin Space, Littleton, Colo., lead author of the study published in Nature Geoscience.
Bierhaus added that the new observations shed light on how asteroids like Bennu respond to energetic impacts.