Even when done up close and personal, science is not easy. Many of our long-time community members worked on analyzing images taken by the OSIRIS-REx mission of the surface of asteroid Bennu. We all spent hours and hours counting and marking rocks and more rocks and boulders and even some craters, all to assist the mission in locating a good place for their touch-and-go sample return.

And just to ease the sting of all those rocks and rocks and boulders and craters, remember, we really did help them select a site. Our efforts were not for nothing.

Since then, scientists have been analyzing the data returned by the spacecraft while they await the return of the actual sample next year. And all of those rocks and boulders may be just why we didn’t count a ton of craters. Sure, there were some larger ones, but smaller ones were rare. Usually, the counts go the other way since smaller impactors are more numerous than larger ones, so we should have seen more small craters.

In a new paper published in Nature Geoscience, scientists reveal that those boulders may be acting like a shield that keeps the micrometeorites from creating craters. Instead, they hit the boulders and break apart or just chip the boulders themselves and bounce away. And what impactors make it through all the boulders to the surface end up creating smaller craters than expected because of all those rocks. If the surface was more uniform – say like a sandy beach – the outcome would be different.

So the rocks and boulders are actually body armor for Bennu. And you all helped count those rocks and boulders and craters… and rocks. And more rocks. Oh, the flashbacks.

Anyway, good work, everyone.

More Information

NASA Goddard press release

“Crater population on asteroid (101955) Bennu indicates impact armouring and a young surface,” E. B. Bierhaus et al., 2022 April 7, Nature Geoscience