Itokawa is just one asteroid. But it comes from a population of space rocks that orbit between one-third and three times Earth’s orbit, meaning they’re local. There are many asteroids like Itokawa that could have impacted Earth long ago. But of course, the samples that Hayabusa plucked from Itokawa’s surface in 2005 have been through a lot in the eons since Earth gained its water. So Jin and Bose had to run the clock backward, accounting for the heating, weathering, and collisions that Itokawa would have endured since the early days of the solar system.
What they found is that Itokawa and rocks like it could have delivered half of Earth’s water reservoirs. And since they come from the same region of the solar system as Earth itself, the researchers conclude that our planet could have nabbed the rest of its water as it was forming, from the materials around it.
The argument over Earth’s watery origins will likely continue. But Hayabusa’s successor,
Hayabusa2, is currently in orbit around another asteroid, Ryugu, and a NASA mission called OSIRIS-REx is
exploring Bennu. Both missions will bring home their own asteroid samples and add to the conflicting but always growing mound of evidence about the origin of Earth’s oceans.