When NASA’s OSIRIS-REx spacecraft arrived at the asteroid Bennu, scientists discovered something unexpected: the asteroid’s surface was covered in large boulders rather than the smooth surface many expected. A group of physicists believes they have discovered why.
According to a new study from physicists at the University of Colorado Boulder, tiny grains of dust on the surface of asteroids may hop around like corn kernels in a frying pan. That popcorn-like effect could even help to clean up smaller asteroids, which lose dust and appear rough and craggy from space.
The findings were published in Nature Astronomy. According to the study’s lead author, Hsiang-Wen (Sean) Hsu, the findings could help scientists better understand how asteroids change shape over time and how these bodies migrate through space, sometimes dangerously close to Earth.
“The more fine-grained material, or regolith, these asteroids lose, the faster they migrate,” said Hsu, a research associate at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics (LASP). The investigation began with a few unusual photographs.
In 2020, a NASA spacecraft called OSIRIS-REx traveled more than 1 billion miles to meet the asteroid (191055) Bennu, which is roughly the height of the Empire State Building. But when the spacecraft arrived, scientists discovered something unexpected: The asteroid’s surface was rough sandpaper, not smooth and dusty as researchers had predicted. There were even large boulders strewn about its exterior.
The gravity on the smaller asteroid is so weak that it can’t hold back the escape. The fine-grained regolith will be lost. Within several million years, in fact, the smaller asteroid was almost completely swept clean of fine dust. The Eros-like asteroid, however, stayed dusty.
Hsiang-Wen (Sean) Hsu
Hsu and his colleagues have now used computer simulations, or models, and laboratory experiments to investigate the mystery. He believes that forces similar to static electricity are kicking the smallest grains of dust, some as small as a single bacterium, off the asteroid and into space, leaving only larger rocks behind.
Bennu isn’t alone, said study co-author Mihály Horányi.
“We’re realizing that these same physics are occurring on other airless bodies like the moon and even the rings of Saturn,” said Horányi, a researcher at LASP and professor of physics at CU Boulder.
Bennu and Ryugu
Asteroids may appear to be frozen in time, but these bodies evolve throughout their lives. According to Hsu, asteroids like Bennu are constantly spinning, exposing their surfaces to sunlight, shadow, and sunlight again. The never-ending cycle of heating and cooling stresses the largest rocks on the surface, causing them to crack.
“It happens every day, all the time,” Hsu explained. “You end up breaking down a large piece of rock into smaller pieces.” That is why, before scientists arrived at Bennu, many expected to find it covered in smooth sand, similar to how the moon appears today. Not long ago, a Japanese space mission called Ryugu landed on a second small asteroid. The team found a similarly rough and craggy terrain.
Hsu and his colleagues were suspicious.
LASP researchers have used vacuum chambers in the lab since the 1990s to investigate the strange properties of dust in space, including a feat known as “electrostatic lofting.” According to study co-lead author Xu Wang, as the sun’s rays bathe small grains of dust, they begin to pick up negative charges. Those charges will accumulate until the particles explode apart, much like two magnets repelling each other.
In some cases, those dust grains can travel at speeds of more than 20 miles per hour (or more than 8 meters per second). “This process on the surface of an asteroid had never been considered before,” said Wang, a research associate at LASP.
Small asteroid, big asteroid
To accomplish this, the researchers, which included former CU Boulder undergraduate students Anthony Carroll and Noah Hood, performed a series of calculations on the physics of regolith on two hypothetical asteroids. They studied how dust forms and moves over hundreds of thousands of years. One of those phony asteroids was about a half-mile across (roughly the size of Ryugu), and the other was several miles wide (closer in diameter to big asteroids like Eros).
It was the size that made a difference. According to the team’s estimates, when dust grains jumped onto the larger asteroid, they couldn’t gain enough speed to escape its gravity. The same could not be said for the smaller, Ryugu-like asteroid.
“The gravity on the smaller asteroid is so weak that it can’t hold back the escape,” Hsu said. “The fine-grained regolith will be lost.” That lost dust, 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.
Hsu speculated that this scrubbing effect could help to nudge the orbits of small asteroids. Asteroids migrate, he explained, because the sun’s radiation pushes on them slowly over time. He suspects that asteroids covered in boulders may move faster than those with a dustier appearance, based on previous research by other scientists.
He and his colleagues could soon have more evidence to back up their calculations. A NASA mission called the Double Asteroid Redirection Test (DART) will visit a pair of smaller asteroids in less than three months, and Hsu will be watching to see how dusty they are. “We’ll have new surface images to put our theory to the test,” he said. “It’s great for us, but it’s also a little scary.”
