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Banner image: The craggy surface of the asteroid Bennu as seen by NASA's OSIRIS-REx spacecraft. (Credit:听NASA/Goddard/University of Arizona)

Like corn kernels popping in a frying pan, tiny grains of dust may hop around on the surface of asteroids, according to a new study from physicists at CU Boulder.听

That popcorn-like effect may even help to tidy up smaller asteroids, causing them to lose dust and look rough and craggy from space.听

The researchers in the journal Nature Astronomy. Their findings may help scientists better understand how asteroids change shape over time鈥攁nd how these bodies migrate through space, sometimes bringing them dangerously close to Earth, said Hsiang-Wen (Sean) Hsu, co-lead author of the study.

鈥淭he more fine-grained material, or regolith, these asteroids lose, the faster they migrate,鈥� said Hsu, a research associate at the (LASP) at CU Boulder.

The research began with a few curious photos.

In 2020, a NASA spacecraft named OSIRIS-REx traveled more than 1 billion miles to rendezvous with the asteroid (191055) Bennu, which is about as tall as the Empire State Building. But when the spacecraft arrived, scientists didn鈥檛 find what they were expecting: The asteroid鈥檚 surface looked like rough sandpaper, not smooth and dusty like researchers had predicted. There were even boulders the size of trucks scattered over its exterior.

Now, Hsu and his colleagues have drawn on computer simulations, or models, and laboratory experiments to explore that puzzle. He said that forces akin to static electricity may be kicking the smallest grains of dust, some no bigger than a single bacterium, off the asteroid and into space鈥攍eaving only larger rocks behind.听

Bennu isn鈥檛 alone, said study co-author Mih谩ly Hor谩nyi.

鈥淲e鈥檙e realizing these same physics are occurring on other airless bodies like the moon and even the rings of Saturn,鈥� said Hor谩nyi, a researcher at LASP and professor of physics at CU Boulder.

Bennu and Ryugu

Asteroids might look like they鈥檙e frozen in time, but these bodies evolve throughout their lifetimes.听

Hsu explained that asteroids like Bennu are constantly spinning, which exposes their surfaces to sunlight, then shadow and sunlight again. That never-ending cycle of heating and cooling puts a strain on the largest rocks at the surface, until they inevitably crack.

鈥淚t鈥檚 happening every day, all the time,鈥� Hsu said. 鈥淵ou wind up eroding a big piece of rock into smaller pieces.鈥�

Which is why, before scientists arrived at Bennu, many were expecting to find it covered in ponds of smooth, sandy material鈥攁 bit like how the moon looks today. Not听long before, a Japanese space mission had landed on a second small asteroid called Ryugu. The team found a similarly rough and craggy terrain.听

Hsu and his colleagues were suspicious.

Since the 1990s, researchers at LASP have used vacuum chambers in the lab to investigate the strange properties of dust in space, including a feat they call 鈥渆lectrostatic lofting.鈥� Study co-lead author Xu Wang explained that as the sun鈥檚 rays bathe patches of porous regolith, negative charges start to build up on small grains of dust.听Those charges will accumulate until, suddenly, the particles burst apart, like two magnets repelling each other.

In some cases, those grains of dust can pop away at speeds of more than 20 miles per hour (or more than 8 meters per second).

鈥淣o one had ever considered this process on the surface of an asteroid before,鈥� said Wang, a research associate at LASP.

Small asteroid, big asteroid

To do that, the researchers, including former CU Boulder undergraduate students Anthony Carroll and Noah Hood, ran a series of calculations examining the physics of regolith on two hypothetical asteroids. They tracked how dust might form, then hop around over hundreds of thousands of years. One of those faux asteroids was about a half-mile across (similar in size to Ryugu) and the second several miles wide (closer in diameter to big asteroids like Eros).听

That size made a difference. According to the team鈥檚 estimates, when grains of dust jumped on the bigger asteroid, they couldn鈥檛 gain enough speed to break free of its gravity. The same wasn鈥檛 true on the smaller, Ryugu-like asteroid.

鈥淭he gravity on the smaller asteroid is so weak that it can鈥檛 hold back the escape,鈥� Hsu said. 鈥淭he fine-grained regolith will be lost.鈥�

That loss, in turn, will expose the surface of the asteroids to even more erosion, leading to a boulder-rich scenery like scientists found on Ryugu and Bennu. Within several million years, in fact, the smaller asteroid was almost completely swept clean of fine dust. The Eros-like asteroid, however, stayed dusty.听

Hsu noted that this scrubbing effect could help to give the orbits of small asteroids a nudge. He explained that asteroids migrate because the sun鈥檚 radiation pushes on them slowly over time. Based on previous research by other scientists, he suspects that asteroids covered in boulders may move faster than those with a dustier appearance.

He and his colleagues may soon get more proof to back up their calculations. In less than three months, a NASA mission called the (DART) will visit a pair of smaller asteroids鈥攁nd Hsu will be watching to see how dusty they are.听

鈥淲e will have new surface images to test our theory,鈥� he said. 鈥淚t鈥檚 nice for us, but also a little nerve-wracking.鈥�