Jumping over space dust makes asteroids look rougher

Like corn kernels popping in a frying pan, tiny dust grains can jump up and down the asteroid’s surface, according to a new study from physicists at the University of Colorado Boulder.

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

The researchers published their results July 11 in the journal Natural Astronomy. Their findings could help scientists better understand how asteroids change shape over time – and how these objects migrate through space, sometimes bringing them very close to Earth, said Hsiang-Wen (Sean) Hsu, lead author of the study. the.

“The more fine-grained material, or regolith, these asteroids lost, the faster they migrated,” said Hsu, a research associate in the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.

The research started with some curious photos.

In 2020, a NASA spacecraft called OSIRIS-REx traveled more than 1 billion miles to meet the asteroid (191055) Bennu, which is about as tall as the Empire State Building. But when the spacecraft arrived, scientists didn’t find what they had hoped for: The asteroid’s surface looked like rough sandpaper, not as smooth and dusty as the researchers thought. There were even boulders scattered on the outside.

Now, Hsu and his colleagues have drawn computer simulations, or models, and laboratory experiments to explore the puzzle. He says that a force similar to static electricity might kick the tiniest dust grains, some no bigger than a bacterium, off the asteroid and into space – leaving only the larger rock.

Bennu is not alone, says study co-author Mihály Horányi.

“We realized that this same physics occurs in other airless bodies such as the moon and even Saturn’s rings,” said Horányi, a researcher at LASP and professor of physics at CU Boulder.

Bennu and Ryugu

Asteroids may look like they are frozen in time, but these objects evolve throughout their lifetimes.

Hsu explains that asteroids like Bennu are constantly rotating, exposing their surface to sunlight, then shadow and sunlight again. The never-ending cycle of heating and cooling weighs down the largest rock on the surface, until it finally cracks.

“It happens every day, all the time,” Hsu said. “You end up scraping a large piece of rock into smaller pieces.”

That’s why, before scientists arrived at Bennu, many expected to find it covered in fine sand — a bit like what the moon looks like today. Not long before, a Japanese space mission landed on a second small asteroid named Ryugu. The team found the terrain equally rough and rugged.

Hsu and his colleagues were suspicious.

Since the 1990s, researchers at LASP have used the vacuum chamber in the laboratory to investigate the strange properties of dust in space, including a feat they call “electrostatic lift.” Study co-author Xu Wang explains that when sunlight bathes tiny grains of dust, they begin to take on a negative charge. The charges would build up until, suddenly, the particles broke apart, like two magnets repelling each other.

In some cases, the dust grains can shoot up at speeds of more than 20 miles per hour (or more than 8 meters per second).

“No one has considered this process on the surface of an asteroid before,” said Wang, a research associate at LASP.

Small asteroid, big asteroid

To do so, the researchers, including former CU Boulder undergraduate students Anthony Carroll and Noah Hood, ran a series of calculations examining the regolith physics on two hypothetical asteroids. They tracked how dust formed, then jumped for hundreds of thousands of years. One of the fake asteroids is about half a mile in size (the size of Ryugu) and the second is several miles wide (closer in diameter to a large asteroid like Eros).

Size makes a difference. According to the team’s estimates, when dust grains jump onto a larger asteroid, they can’t gain enough speed to break free from its gravity. The same is not true of the smaller Ryugu-like asteroids.

“The gravity on the smaller asteroids is so weak that it can’t withstand the escape,” Hsu said. “The fine grained regolith will be gone.”

That lost dust, in turn, would have exposed the asteroid’s surface to more erosion, leading to rock-rich landscapes like the ones scientists found at Ryugu and Bennu. Within a few million years, in fact, smaller asteroids are almost completely wiped out of fine dust. However, asteroids like Eros remain dusty.

Hsu noted that this rubbing effect could help give the small asteroid’s orbit a boost. He explained that the asteroid migrated because solar radiation pushed it slowly over time. Based on previous research by other scientists, he suspected asteroids covered in boulder could move faster than asteroids that looked dustier.

He and his colleagues may soon find more evidence to support their calculations. In less than 3 months, a NASA mission called the Double Asteroid Redirection Test (DART) will visit a pair of smaller asteroids — and Hsu will be watching to see how dusty they are.

“We will have new surface images to test our theory,” he said. “It’s good for us, but also a little stressful.”

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