In 2019, Japan’s Hayabusa 2 spacecraft bombed the asteroid Ryugu, using explosives to shoot it with a 2.5-kilogram lump of copper to create an artificial crater. Scientists expected this impact to shake the ground, but its actual effect was far milder.

Images from Hayabusa 2 have shown that the surface of Ryugu has fewer small craters than we would expect for an asteroid of its size, which probably indicates that dust is being moved somehow to fill in those craters. Asteroids don’t have significant atmospheres, so the primary suspect to explain this movement has generally been the ground shaking due to small impacts from other space rocks that produce seismic waves – a sort of asteroid-quake.

If this were the case, we would expect rocks around the site where the copper impactor hit Ryugu to have been moved by the impact. “Significant boulder movement was expected,” said Gaku Nishiyama at the University of Tokyo in a virtual presentation at the Lunar and Planetary Science Conference on 15 March. “However, such large boulder movement has not been observed.”

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Upon comparing images of the area before and after the impactor hit, Nishiyama and his colleagues found that the rocks in the area had moved less than 1 metre. This means that the seismic waves produced by the lump of copper must have been far weaker than expected.

What’s more, Hayabusa 2’s images of Ryugu revealed that, in many places, the asteroid has boulders with smaller boulders stacked on top of them, a phenomenon that wouldn’t be possible if small impacts shook the ground regularly. The researchers calculated that Ryugu must be very good at diffusing seismic waves – about 100 times better than the moon.

That is probably because the dust grains on Ryugu are larger than those on the moon, allowing them to scatter the energy from seismic waves much more efficiently. Nevertheless, something must be erasing the small craters on Ryugu, and if the culprit isn’t impact-induced asteroid-quakes, the dearth of small craters remains a mystery.