The Astrophysical Journal doesn’t usually fall in my purview, except today. From vol. 973, September 20:
Shoji, D. Reactive Molecular Dynamics Simulations of Micrometeoroid Bombardment for Space Weathering of Asteroid (162173) Ryugu art. 13 ad70b0
Ground telescopes had classed Ryugu as a Ch-type, with ‘h’ indicating water spectra. But the Hayabusa2 probe, on arrival, found the object to be quite deficient. The spectra seen by the NIRS3 instrument was, not dry I must say, but weak. What’s going on here? Haya2 also performed the SCI experiment, blowing a small crater in Ryugu and exposing subsurface material. The crater bottom was visibly different from the surrounding regolith.
As had previously been disclosed, samples taken by Haya2 would give the explanation. Some number of Ryugu grains were on the surface, and showed signs of space weathering. Exposed surfaces are bombarded by, oh, full sunlight, the solar wind, cosmic rays, micrometeoroids (and occasional no-so-micro ones), and of course the thruster exhaust from one deep space mission. In a microscope, this weathering produces a “frothy” surface- yes, a fine froth of molten rock. This thin layer was frothed by the escaping volatiles, before cooling and re-solidifying. The NIRS3, viewing in short wavelengths, did not penetrate much further than this rock rind. Other tests, aware of this effect, used other Ryugu materials and found the asteroid’s water.
D. Shoji now replicates and reports this effect. The field of hypervelocity impact research has applications across domains, and not just for asteroids. Assuming common dust grains, and comet-like trajectories and their impact parameters, the Ryugu rind observed looks very plausible. (Or, alternately, the parameters used by Shoji capture the situation well.)
They say don’t judge a book by its cover. We can’t judge a rock by its rind. (Or, alternately, we are ready for future asteroids, knowing such rinds exist.)