The Hayabusa2 team held a press briefing at LPSC on Tuesday to report newly published results. Download their slides here and here, and their movies here. In addition to their press briefing and an afternoon session at LPSC, they published 3 open-access papers led by Sei-Ichiro Watanabe, Kohei Kitazato, and Seiji Sugita and coauthors. Many thanks to Sugita-san for placing online the raw data for all the images his group presented in their paper, which I’ve pulled from for the pictures in this article!

Any near-Earth asteroid as small as Ryugu (or Bennu) cannot be very old, geologically speaking. There are forces that act to change an asteroid’s orbit over time (the Yarkovsky effect) and that act to spin it up over time (YORP). The smaller the asteroid, the more rapidly these forces operate. Yarkovsky and YORP conspire to make asteroids impact inner planets or spin up so fast they disrupt within a couple hundred million years.

So the basic story for these worlds is that they originated in the main belt, as fragments of a previously larger body, blasted off in some ancient collision, and some series of gravitational encounters delivered them to near-Earth orbital space within the last couple hundred million years. So what bigger main-belt asteroid did they come from? That is, which main-belt asteroid will we have pieces of when we return the Hayabusa2 and OSIRIS-REx samples to Earth? Sugita-san presented some spectroscopic evidence of which asteroids might be the parent bodies. The best matches are 142 Polana and 495 Eulalia, but even these aren’t perfect.

What about meteorites? That’s also mysterious. Ryugu was predicted to have a typical carbonaceous asteroid albedo of around 3 or 4 percent, but not only is it darker than known asteroids, it’s also darker than any carbonaceous chondrite meteorite measured in the laboratory. The composition of Ryugu’s surface materials appears pretty homogeneous, with every spectrum containing evidence for the ubiquitous presence of a very small amount of hydroxyl ion, OH-, in Ryugu’s minerals, probably in a magnesium-rich clay mineral. That means the materials composing Ryugu once interacted with water—a pretty common finding in meteorites.

The best laboratory match that’s been found for what they’re seeing at Ryugu is meteorites that have been cooked – “thermally metamorphosed” meteorites. Long ago, the rocks that made Ryugu formed in the warm interior of a young, biggish asteroid. This asteroid was large enough for internally-driven geology so would’ve been several hundred kilometers across. In the warm asteroid there was liquid water percolating among the rocks, which altered the minerals, stuffing hydroxyl ions inside them. But then they got heated even more—maybe it was really early in solar system formation when there was aluminum-26 around to generate lots of heat by radioactive decay, or maybe it happened in a violent impact—and the extra heating decomposed some of the hydrated minerals to other forms and darkened the rock. There is certainly evidence of at least some of Ryugu’s rocks having experienced a large impact and resolidifying in new rock. Check out the breccia block in this photo.