Properties of natural and artificial craters on asteroid (162173) Ryugu revealed by remote-sensing observations and sample analyses

Abstract

<p><strong>Introduction.</strong> Images obtained by the optical navigation camera (ONC) onboard Hayabusa2 have revealed nearly one hundred craters on the surface of asteroid Ryugu and have been estimated to of impact origin [1,2,3]. In this study, we review our recent findings about craters on asteroid Ryugu and discuss about implications for DART and Hera missions.</p> <p><strong>Gravity-regime cratering scaling.</strong> Although we expected to find craters on Ryugu because of the past finding of crater candidates on similarly small asteroid Itokawa [4], we did not expect craters possessing classic morphologies, such as circular shape, raised rim, and wall slumping, which are consistent with gravity-controlled formation [1]. This gravity-controlled crater formation on Ryugu was confirmed unambiguously by the artificial impact experiment by small carryon impact (SCI) including uninterrupted imaging of excavation and deposition processes of impact ejecta [5], allowing us to estimate crater-projectile size relation using gravity-regime crater scaling for coarse-grain targets [6]. This led to crater retention age estimation on different geologic units on Ryugu, which helps us understand the evolution of Ryugu. Morota et al. [2] inferred that asteroid Ryugu may have experienced sunward orbital excursion after leaving the asteroid main belt and before arrival at the current orbit based on the distinct bimodal color distribution and crater size frequency distribution (CSFD) of craters on Ryugu.</p> <p><strong>Depth-age relation and cross examination with sample analyses. </strong>Further analysis of CSFD on Ryugu yielded a relation between crater retention age as a function of surface layer depth [7], which indicate that crater retention age increases rapidly from ~0.4 Myr at 1 m of depth to 3 Myr at 2 m when the recent impactor population in near-Earth asteroids (NEAs) by [8] is used. Preliminary 21Ne measurement results from Ryugu samples indicate that comic-ray exposure (CRE) ages are ~5 Myr [9]. Although the shielding depth for cosmic rays for these samples may be relatively large (1 to 2 m) owing to the low density (1.19 g/cc) of Ryugu, the sample CRE ages are older than the crater counting estimates for these depths. Because of the great uncertainty in model parameters for crater retention age for asteroid Ryugu, the fact that CRE age and crater retention age agree within a factor of ~10 is significant. However, a more important point may be that this rough agreement occur only when NEA population is used for impactors. If crater retention age is estimated with main-belt asteroids (MBAs) populations, crater retention age would be much younger (~1 kyr), greatly deviating from the observed CRE ages. This suggests that Ryugu may have stayed in an orbit collisionally decoupled from MBA after leaving the main belt. A decisive conclusion would need more data from both sample analysis and image analysis, but this discussion clearly demonstrates that comparison between CRE ages and crater chronology is extremely important.</p> <p><strong>Comparison with asteroid Bennu and outlook for Dimorphos and Didymos craters. </strong>One thing we should look at before applying observation results on Ryugu craters to DART and Hera observations is comparison with craters on asteroid Bennu. Although Ryugu and Bennu are very similar in general morphologic properties of craters, there are significant differences. One is depth/diameter (d/D) ratio dependence on crater diameter D. The d/D ratio increases with crater diameter D on Ryugu but decrease on Bennu [10,11]. This contrasting difference may reflect different subsurface mechanical structures on these two asteroids. In fact, Ryugu has evidence for regional to global mass motion (i.e., equator to mid latitude mass wasting) [1,2] and Bennu exhibits evidence for more local mass wasting [12,13]. Such different styles of mass motion may be related to difference in d/D ratios on Ryugu and Bennu.</p> <p>Another difference is the gap between crater retention times between craters on regolith and boulder surfaces on the two asteroids. Although crater number density on regolith are similar to each other within the factor of three [7], those of “mini-craters” on boulder surfaces are very different. Bennu has about 30 time more intra-boulder mini-craters than Ryugu [14,15].</p> <p>The fact that these similar C-complex asteroids exhibit such substantial differences on craters suggests that Dimorphos, whose materialistic and dynamical properties are very different from Ryugu and Bennu, will show us very interesting new aspects of cratering on small bodies via DART and Hera missions [16,17].</p> <p><strong>