A trio of papers in this issue presents the initial results from the Japanese Hayabusa2 mission to the near-Earth carbonaceous asteroid Ryugu. According to the combined results of the studies, which measured the mass, size, shape, density, spin and geological properties of the asteroid, Ryugu is perhaps best described as a porous "pile of rubble." The results will also provide the direct geological context required to understand the samples Hayabusa2 is collecting from the asteroid, which will be brought back to Earth at the end of 2020. Sei-Ichiro Watanabe and colleagues present the first up-close observations of Ryugu's mass, shape and density. According to Watanabe et al., the asteroid's low density suggests it has a highly porous interior and is a "rubble pile" of loosely aggregated rocks, which formed into a spinning-top shape during a period of rapid spin. From their remote sensing of the asteroid, the authors identified potential landing sites for the spacecraft best suited for sample collection that could further inform how Ryugu got its spinning-top shape. In a second study, Kohei Kitazato and colleagues used Hayabusa2's near-infrared spectrometer (NIRS3) to survey the surface composition of Ryugu, discovering that hydrated minerals are ubiquitous across the dark asteroid's surface. Previous telescopic analyses of Ryugu's surface have hinted at the asteroid's carbon-bearing nature, thought to be similar to that of carbonaceous chondrite meteorites collected on Earth. However, a lack of detailed spectral data has made definitive compositional identification difficult. Here, the authors' NIRS3 spectral data are most similar to known thermally-and/or shock-metamorphosized carbonaceous chondrite meteorites. Combining the results from the previous two studies, as well as through observation of the geological features of Ryugu, Seiji Sugita and colleagues - in a third study - attempted to constrain Ryugu's origin. Small asteroids, such as Ryugu, are estimated to have been born from much older parent bodies through catastrophic disruption and reaccumulation of fragments during the Solar System evolution. Among other inferences, Sugita et al. suggest Ryugu likely formed as rubble, ejected by an impact from a larger parent asteroid. They say that the preponderance of materials on Ryugu with little water signature suggests that a dominant part of its original parent body was also "water poor."
###
Journal
Science