As an astronomer, I tell a generalized story on how planets formed that goes sorta like this: It all starts with a giant molecular cloud that one day collapses, dust particles collide and build bigger and bigger structures until small, dust-built, planetesimals emerge. These smash together to form larger objects, gravitationally pull in additional material, or otherwise find ways to grow. 

With planets, all traces of that original planetesimal get crushed into oblivion by the weight of materials above. With asteroids, it may be possible that, in some cases, that initial, fluffy planetesimal survives. And it looks like the asteroid Ryugu survived multiple potentially destructive processes! 

Thermal imagery of Ryugu by the Hayabusa2 spacecraft revealed the surface is randomly scattered with extremely low-density boulders that are more like giant pumice stones than the high-density boulders we’re used to thinking of. These boulders are so low density, in fact, that they would float in water if given the opportunity! 

Researchers think these boulders may be fragments of those low-density planetesimals that once made up Ryugu’s core. We know Ryugu is a rubble pile created during one or more asteroid collisions. These collisions shattered the colliding objects, which then came together as a mixed-up pile of rubble. By literally mixing the core fragments to the surface, this collision process revealed never-before-seen bits of fluffy planetesimals and confirmed at least our big picture view of planet formation is real. And we now know where future space travelers can get pumice-like stone if they need it. This research appears in Nature Astronomy and is led by N. Sakatani.

More Information

ISAS press release

“Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body,” N. Sakatani et al., 2021 May 24, Nature Astronomy