Now that’s an active asteroid

191205 bennu

View of Bennu ejecting particles from its surface on 6 Jan 2019, created by combining two images taken by the navigation camera onboard OSIRIS-REx.

Lauretta et al., Science (2019)

By Richard A Lovett

Scientists poring over data from NASA’s OSIRIS-REx mission are trying to figure out why the near-Earth asteroid Bennu is periodically ejecting rocks the size of ping-pong balls. 

The find came “sort of by accident,” says Harold Connolly Jr, a cosmochemist and meteoriticist at Rowan University, US, when another member of the OSRIRIS-REx team noticed “stars” in the spacecraft’s navigation camera, in places where stars aren’t supposed to exist.

“It turns out they weren’t stars; they are particles being ejected from the asteroid. This was an absolutely stunning discovery,” says Connolly, who is co-author of a paper describing the find in the journal Science.

Initially, he says, the team thought they might be seeing the type of activity previously seen on comets. “But this isn’t what we are apparently seeing.” 

Comet-like activity should be accompanied by plumes of gas, as well as solid particles, “but if there is gas, we can’t see it. It’s just particles ejecting off the surface.”

Another possible explanation, he says, is that stresses within the asteroid may be mounting to levels where material simply pops out, like popping popcorn.

One way this might happen, is if the asteroid contains claylike minerals that grow as they absorb water from their surroundings. 

Or Bennu may be undergoing other stresses from its rapid rotation… or being hit hard enough by micrometeorites to knock rocks off its surface. {%recommended 8408%}

With a diameter of only 500 metres, it has a low enough surface gravity that it would be easy for any of these processes to throw rocks off the surface, Connolly says.

Whatever the process, the flying rocks might explain another of Bennu’s mysteries.

Some of the rocks are ejected at speeds fast enough to escape into interplanetary space, but others are ejected at slow enough speeds that they must eventually fall back.

So far, Connolly says, nobody has seen any of these rocks fall back. But they have seen places on the surface where small rocks lie on top of larger ones. 

When these were first spotted, it was assumed that they were merely rocks that had rolled down from a higher surface. But that may not be the only reason.

The find is interesting, he says, partly because such activity wasn’t expected on a near-Earth asteroid like Bennu. But if that turns out to be common, he adds, it might explain how meteorites from such asteroids reach Earth. 

It also shows that near-Earth asteroids can be quite dynamic. “There’s geological activity happening on them that is causing material to be ejected and lost over time, and that’s pretty cool,” he says.

Jessica Agarwal of Max Planck Institute for Solar System Research in Germany, who was not part of the study team but is the author of a commentary in Science, notes that the find is also important for helping to avert potential threats from Earth-impacting asteroids.

“This finding will ultimately help us to better understand the properties and composition of Bennu’s material,” she says, “which is important to understanding how asteroids and the solar system formed, and to developing efficient strategies to prevent asteroids from hitting Earth, should one of them be on a collision course.”

OSIRIS-REx is currently orbiting Bennu, searching for a landing site from which it will eventually scoop up a sample for return to Earth.

The modelled trajectories of a subset of particles ejected from Bennu on 19 Jan 2019.
CREDIT: Lauretta et al., Science (2019)

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