‘Popcorn’ space dust hops off smaller asteroids leaving them looking cleaner

The surfaces of asteroids are hopping with grains of space dust, like popcorn popping. And just like corn kernels jumping out of the pan when the lid is off, electrostatic lofting – a force akin to static electricity – can cause dust on smaller asteroids to defy gravity and hop right off into space, according to a new paper published in Nature Astronomy.

Physicists at the University of Colorado Boulder, US, modelled how space dust forms and behaves on asteroids of different sizes over hundreds of thousands of years. 

Their findings help explain how asteroids change shape over time, and why smaller asteroids appear less dusty than their larger counterparts.

The research was initially prompted by a surprising photo of small asteroid Bennu (with a radius of 275 metres, or roughly the height of Brisbane’s Skytower). The 2020 photo, taken by NASA’s spacecraft OSIRIS-Rex, showed Bennu’s surface had the appearance of rough sandpaper. There were even large boulders scattered over its exterior. 

Scientists were surprised. They had expected to find Bennu covered in smooth sand – a bit like how the Moon looks today. That’s because as an asteroid spins, the heating and cooling effect of sunlight and shadow causes rocks on its surface to crack and break apart. 

“It’s happening every day, all the time,” says Hsiang-Wen (Sean) Hsu, lead author of the study. “You wind up eroding a big piece of rock into smaller pieces.”

The Japanese Hayabusa2 mission had found similarly rough and craggy terrain on another small asteroid called Ryugu (radius 502m).

Study co-lead author Xu Wang explained that in addition to this erosion, a force called electrostatic lofting occurs on the surface of asteroids. As the Sun’s rays bathe small grains of dust, it begins to pick up negative charges, which build until suddenly the particles burst apart, like two magnets repelling each other. 

It’s what causes the popcorn effect, with space dust travelling at speeds of more than eight metres per second.

“No one had ever considered this process on the surface of an asteroid before,” says Wang.

The researchers from the Laboratory for Atmospheric and Space Physics modelled the effects of surface erosion and electrostatic lofting on two hypothetical asteroids – one smaller with a radius of 0.5 kilometres like Ryugu, and one larger asteroid with a radius of 5km.

They found when grains of dust jumped on the bigger asteroid, they couldn’t gain enough speed to break free of the asteroid’s gravity. The same wasn’t true for the smaller asteroid.

“The gravity on the smaller asteroid is so weak that it can’t hold back the escape,” Hsu says. 

Over time, as the smaller asteroids lost dust, it led to even more erosion, creating boulder-rich scenery like that found on Ryugu and Bennu.

Within several million years, the modelling showed the smaller asteroid was almost completely swept clean of fine dust – but the larger one stayed dusty.

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