This porous space rock could reveal details of how planets formed

The asteroid Ryugu is light and fluffy. Images taken by Japan’s Hayabusa2 spacecraft suggest the whole asteroid is highly porous, scientists report in Nature on March 16.

“It is something like freeze-dry coffee,” says planetary scientist Tatsuaki Okada of the Japanese Aerospace Exploration Agency. If early protoplanets had similar structures, that could mean planets formed quickly.

As an ancient, carbon-rich asteroid, Ryugu is thought to be a time capsule of solar system history. To read that history, Hayabusa2 explored Ryugu from June 2018 to November 2019, and grabbed two samples from the asteroid to bring back to Earth (SN: 7/11/19).

Hayabusa2 observed how the asteroid’s surface retained and released heat, a clue to its composition and structure. Dense rocks take in heat slowly and hold that heat for longer; more porous rocks change temperature quickly, like sand on a beach.

Ryugu’s heat map shows that it’s about 50 percent porous, meaning half of it is holes, Okada and colleagues report. Even most of the asteroid’s large boulders appear porous.

That airiness fits with the idea that Ryugu is a rubble pile formed after the breakup of a larger body some 700 million years ago (SN: 3/20/19). But the new observations suggest that parent body might have been porous, too.

“This might be common for the asteroids and even for planetesimals in the early solar system,” Okada says.

If  true, it could have big implications for how quickly planets formed. Planetary scientists think the early solar system was a violent place, with protoplanets colliding, breaking up and re-accumulating all the time. Porous protoplanets might fall apart and come together again more easily than dense ones, Okada says. That means “the timescale of planetary formation may be changed drastically,” and planets might have formed faster than scientists thought, he says.