Bennu is Constantly Getting Sandblasted by Tiny Meteoroids

As soon as the OSIRIS-REx spacecraft arrived at asteroid Bennu in December 2018, there was a big surprise. Scientists expected Bennu’s surface would consist of fine-grained material like a sandy beach. But take a look at that surface: Bennu is a jumbled mess.

Here’s a closer view:

Image obtained by the PolyCam camera on NASA’s OSIRIS-REx spacecraft from a distance of about 5 km (3 mi) Credit: NASA/Goddard/University of Arizona

And amazingly, amid that mess, pieces of Bennu’s surface are being ejected out into space. The cameras on board OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spotted rock particles being repeatedly launched into space. Now, after more than a year and a half up close with Bennu, planetary scientists are starting to understand why the dynamic shedding of small pieces of rock are taking place.

One answer is that meteoroids — small space rocks — are hitting the surface of Bennu, and causing small particles from the surface to be ejected. But also, the particle sizes match what is expected for thermal fracturing, as the asteroid’s surface is repeatedly heated and cooled while it rotates, (see a more detailed article about the fracturing here).  Scientists think the ejection events might come from both processes.

This global map of asteroid Bennu’s surface is a mosaic of images collected by NASA’s OSIRIS-REx spacecraft between Mar. 7 and Apr. 19, 2019. Credit: NASA/Goddard/University of Arizona

“We thought that Bennu’s boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us,” said Dante Lauretta, the OSIRIS-REx principal investigator and a professor at the University of Arizona. “We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”

Previous article: Why Are Particles Getting Ejected Off Asteroid Bennu?

Lauretta and his team have seen that on average, only one or two particles are ejected per day. But because they are being “launched” in a very low-gravity environment, most are moving slowly. Lauretta said these particles pose little threat to OSIRIS-REx, which will attempt to briefly touch down on the asteroid on Oct. 20 to scoop up surface material, which may even include particles that were ejected before dropping back to the surface.

Bennu is about one third of a mile (565 meters) wide at its equator

This discovery was detailed in a new study that was part of a collection of papers published in a special edition of the Journal of Geophysical Research: Planets. 

Senior research scientist Steve Chesley at NASA’s Jet Propulsion Laboratory was the lead author on the ejection event paper, and he and his team found that most of these pebble-size pieces of rock, typically measuring around a quarter-inch (7 millimeters), were pulled back to Bennu under the asteroid’s weak gravity after a short hop, sometimes even ricocheting back into space after colliding with the surface. Others took longer to return to the surface, remaining in orbit for a few days and up to 16 revolutions. And some were ejected with enough power to completely escape from the vicinity of Bennu. The scientists are still making observations of these ejection events to better understand what is taking place.

This artist’s concept shows NASA’s OSIRIS-REx spacecraft descending towards asteroid Bennu to collect a sample of the asteroid’s surface. The sampling procedure will take place on October 20th. Credit: NASA/Goddard/University of Arizona

If the sample collection goes as planned, the spacecraft will return to Earth in September 2023 with a cache of Bennu’s material for scientists to study further.

Source: JPL

Nancy Atkinson

Nancy has been with Universe Today since 2004. She is the author of a new book on the Apollo program, "Eight Years to the Moon," which shares the stories of 60 engineers and scientists who worked behind the scenes to make landing on the Moon possible. Her first book, "Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos" tells the stories of those who work on NASA's robotic missions to explore the Solar System and beyond.

Recent Posts

Maybe “Boson Clouds” Could Explain Dark Matter

The search is underway for a hypothetical form of dark matter known as boson clouds.

11 hours ago

Did the Earth’s Water Come From the Sun?

Where did Earth's water come from? Comets may have brought some of it. Asteroids may…

13 hours ago

Messier 95 – the NGC 3351 Barred Spiral Galaxy

Located in the constellation Leo, about 33 million light-years from Earth, is the barred spiral…

13 hours ago

‘Don’t Look Up’ shines a satirical spotlight on the campaign to counter cosmic threats

The science adviser for “Don’t Look Up,” a star-studded comedy about a killer comet, has some serious…

15 hours ago

Colliding Gases at the Heart of the Running Man Nebula

New images of the Running Man Nebula taken by Hubble provide insight into how young…

15 hours ago

If There are Water Plumes on Europa, Here’s how Europa Clipper Will Study Them

NASA’s Europa Clipper is one of the most anticipated missions of the coming decade, in…

1 day ago