Extended Data Fig. 8. Exposure age of the surface of a millimeter-sized Ryugu grain A0067 estimated from the impact crater population.
Forty craters with average diameters ranging from 300 nm up to 8.5 µm were examined to estimate the exposure age (see Methods). (a) Secondary electron image of one of the investigated craters. (b) The cumulative impactor flux versus the mass of the impactor, assuming exposure time from 102 years to 105 years (solid lines). Broken lines indicate interplanetary meteoroid flux models at Ryugu’s perihelion (0.96 au [astronomical unit]), average orbit (1.2 au), and aphelion (1.4 au). This diagram shows that the exposure age of the smooth layer-covered surface of A0067 is estimated to be 3 × 104 years, calculated by its crater population and assuming craters formed by interplanetary meteoroid impacts, even if the average semimajor axis of the orbit of Ryugu had changed from 0.96 to 1.4 au.