gotta bounce

Bennu: Bouncing dust could explain how the asteroid's boulders stay intact

The asteroid’s big secret, revealed.

Originally Published: 
NASA via Giphy

When scientists first observed the asteroid Bennu up close in 2018, they were shocked to find that its surface was covered with sizeable boulders.

From Earth, Bennu appeared smooth.

But it turns out the asteroid is able to maintain huge clumps of rock in the harsh environment of space — igniting new questions about asteroid evolution and erosion.

NASA/Goddard/University of Arizona

Writing in the journal Nature Astronomy this week, researchers pinpoint a phenomenon caused by the Sun that could contribute to the craggy makeup of Bennu and its not-so-smooth cousin, Ryugu.


NASA via Giphy

Both objects are considered Near Earth Asteroids, meaning they not only orbit relatively close to our home planet but they’re also exposed to quite a bit of solar radiation.

And they’re comparatively smaller than other asteroids out there.

For example, Bennu has a diameter of 0.3 miles, while the much smoother asteroid Eros (below) has a diameter of almost 10.5 miles.

Eros is covered in a fine dust called regolith, which forms from the breakdown of larger rocks over millions of years.


AXA, Chiba Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Meiji University, University of Aizu, AIST

Regolith exists on Bennu and Ryugu as well, though the researchers wanted to understand why it’s less abundant than on other asteroids.


For the study, they modeled two hypothetical asteroids — one with a radius of .5 kilometers and the other of 5 kilometers.

They simulated the asteroid’s long lifespans to model how particles on the surfaces of the asteroids would change over time.

After tens of thousands of simulated years in space, conditions became ripe for a phenomenon called electrostatic lofting to take place.

Basically, radiation from the Sun causes small regolith particles to become negatively charged and bounce away from each other like repelling magnets.


Here’s a simulation of how negatively charged regolith particles bounce into the air.


NASA/Goddard/University of Arizona

It’s only on smaller asteroids that charged particles would be able to pop off of the rock entirely, since the pull of gravity isn’t strong enough to bring them back down to the surface.

On the surfaces of Bennu and Ryugu, the Sun’s rays could be kicking up dust — so much of it that only massive boulders are left behind.


Now, scientists will put the models to the test.

As missions like NASA’s Double Asteroid Redirection Test get up close and personal with more small asteroids, we’ll be able to see if they host patches of dusty regolith — or if it’s all bounced away.

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