Initial study of regolith exposure ages and burial conditions on Ryugu: Cosmogenic nuclides score in two touchdowns

Abstract Surface processes on the asteroid Ryugu have been investigated using cosmic‐ray‐produced radionuclides, 10 Be, 26 Al, and 36 Cl, and stable noble gases, on eight samples returned by the Hayabusa2 spacecraft. The 10 Be and 26 Al along with 21 Ne measurements indicate that the two Chamber A samples A0105 collected during the first touchdown (TD) were exposed to cosmic rays for ~6.8 Myr at a shielding depth of 4–15 g cm −2 . Beryllium‐10 and 26 Al from Chamber C samples from the second TD site, close to the artificial crater, were ejected from shielding depths of 120–160 g cm −2 for C0002, 20–85 g cm −2 for C0106‐09, and 120–155 g cm −2 for C0106‐10, ‐11, and ‐12, respectively. The exposure ages of these four C0106 samples differ, ranging from 1.7–8.8 Myr. These measurements provide unique and clear evidence that Hayabusa2 successfully collected subsurface samples ejected by an artificially produced crater. Chlorine‐36 produced by secondary‐produced thermal neutrons was observed in the samples, consistent with the high concentration of H and Cl. Helium (He) and Ne of solar wind origin were released at the lowest heating temperature of 200°C during a stepwise pyrolysis.