Asteroid Ryugu is dark and dry

Spectroscopic analysis by the Japanese spacecraft Hayabusa2 reveals that the asteroid 162173 Ryugu is unusually dark in color and has little water, though there’s evidence that its rocks were once wet (Science 2019, DOI: 10.1126/science.aav7432).These were the first observations of Ryugu in the 1.8–3.2 µm range, where OH stretching and H₂O bending modes are visible.

Diamond-shaped and a kilometer across, Ryugu orbits the sun mostly between Earth and Mars and is thought to contain materials left from the early solar system. Hayabusa2 measurements show that it is quite dark, reflecting less than 2% of light at 2.0 µm. The new results confirm that, as was previously suggested, the asteroid is rich with carbonaceous minerals. According to the researchers, a weak but consistent 2.72 µm signal points to a homogeneous composition that includes magnesium minerals thought to derive from hydrated clays. But the group saw no evidence of water ice or water-containing minerals.

Study coauthor Ralph E. Milliken of Brown University said in a press conference that the group hasn’t matched Ryugu with any other space objects quite like it. The asteroid is most similar to previously studied celestial bodies that were once wet and then were heated to the point that the water was lost. Ryugu may have formed from the debris of a collision, which could have darkened, dehydrated, and dehydroxylated the rocks. But these results apply only to Ryugu’s surface, and Milliken said other Hayabusa2 experiments that will excavate and capture subsurface rocks could reveal something new.