Last month, the probe successfully delivered a capsule back to Earth containing a sample of rocks that it had collected from the surface of the asteroid.
While analysis of the samples is still underway, researchers have used data from the spacecraft’s other instruments to reveal new details about the asteroid’s past.
They found that Ryugu isn’t quite as rich in water-bearing minerals as some other asteroids, suggesting that the ancient parent body from which it was formed had probably dried out in some kind of heating event before Ryugu came into being.
“One of the things we’re trying to understand is the distribution of water in the early solar system, and how that water may have been delivered to Earth,” explained Ralph Milliken, a planetary scientist at Brown University and study co-author.
“Water-bearing asteroids are thought to have played a role in that, so by studying Ryugu up close and returning samples from it, we can better understand the abundance and history of water-bearing minerals on these kinds of asteroids.”
There are several competing ideas for how and when Ryugu may have lost some of its water.
The asteroid has been described a rubble pile, a loose conglomeration of rock held together by gravity. It’s thought that these asteroids probably form from debris left over when larger and more solid asteroids are broken apart by a large impact event.
One possibility is that the water signature seen on Ryugu today is all that remains of a previously more water-rich parent asteroid that dried out due a heating event of some kind.
But it could also be that Ryugu dried out after a catastrophic disruption and re-formation as a rubble pile. It’s also possible that Ryugu had a few close spins past the Sun in its past, which could have heated it up and dried out its surface.
The Hayabusa2 spacecraft had equipment aboard that could help scientists to determine which scenario was more likely.
During its rendezvous with Ryugu in 2019, Hayabusa2 fired a small projectile into the asteroid’s surface. The impact created a small crater and exposed rock buried in the subsurface.
Using a near-infrared spectrometer, which is capable of detecting water-bearing minerals, the researchers could then compare the water content of surface rock with that of the subsurface.
The data showed the subsurface water signature to be quite similar to that of the outermost surface. That finding is consistent with the idea that Ryugu’s parent body had dried out, rather than the scenario in which Ryugu’s surface was dried out by the sun.
“You’d expect high-temperature heating from the Sun to happen mostly at the surface and not penetrate too far into the subsurface,” Milliken said. “But what we see is that the surface and subsurface are pretty similar and both are relatively poor in water, which brings us back to the idea that it was Ryugu’s parent body that had been altered.”
With scientists now able to study the physical samples of the asteroid, they will attempt to conduct tests that may corroborate the current water theories.