TP14

Introduction. 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.

Gravity-regime cratering scaling. 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.

Depth-age relation and cross examination with sample analyses. 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.

Comparison with asteroid Bennu and outlook for Dimorphos and Didymos craters. 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.

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].

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].

References:

[1] Sugita et al. (2019) Science, 364, aaw0422

[2] Morota et al., (2020) Science, 368, 654–659

[3] Cho et al. (2021) JGR Planet 126 e2020JE006572.

[4] Hirata et al. (2009) Icarus, 200, 486-502.

[5] Arawaka et al. (2020) Science, 368, 67–71.

[6] Tatsumi and Sugita (2018) Icarus, 300, 227–248.

[7] Takaki et al., 2021 Icarus 114911.

[8] Haris and D’Abramo (2015) Icarus 257, 302-312.

[9] Okazaki et al. (2022) LPSC #1348.

[10] Noguchi et al. (2021) Icarus, 354, 114016.

[11] Daly et al. (2022) Icarus, 115058.

[12] Walsh et al. (2019) Nature Geoscience, 12, 242–246.

[13] Jawin et al. (2021) JGR-Planets, 125, e2020JE006475.

[14] Ballouz et al. (2020) Nature, 587, 205-209.

[15] Takai et al. (2021) LPSC, #2548.

[16] Cheng et al. (2018) PSS, 157, 104-115.

[17] Michel et al. (2022) PSJ, in press.

How to cite: Sugita, S.: Properties of natural and artificial craters on asteroid (162173) Ryugu revealed by remote-sensing observations and sample analyses, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-195, https://doi.org/10.5194/epsc2022-195, 2022.

Introduction: Asteroids smaller than about 50 km in diameter are the result of the break-up of a larger parent body [1]. They are often considered to be rubble-pile objects, aggregates held together only by self-gravity or small cohesive forces [2, 3], and have highly heterogeneous surfaces. Recently, the artificial impact experiment (SCI) of JAXA’s Hayabusa2 mission on the surface of asteroid Ryugu [4] created a relatively large crater (~14 m diameter) despite the presence of large boulders close to the impact location [5].

Post-impact images of the SCI impact site revealed that the boulders had different motion mechanisms depending on their size and initial position relative to the impact point. 1 m-sized boulders were ejected several metres outside of the crater, a 5 m boulder was moved about 3m, while a large, possibly deeply rooted boulder (“Okamoto”) was not moved [4]. Impact cratering on weak, heterogeneous targets is still poorly studied, both by means of laboratory experiments and numerical simulations. For example, it is not yet known how the boulders affect the crater size or how the boulders motion is affected by their mass, size, shape or initial location.

This is also important in context of NASA’s Double Asteroid Redirection Test (DART) impact on the surface of Dimorphos (the secondary of the 65803 Didymos asteroid system) on the 26th of September 2022. The impact will demonstrate the controlled deflection capabilities of near-Earth asteroids by a kinetic impactor [6]. ESA’s Hera mission [7] will arrive at Dimorphos several years after the DART impact and provide a detailed characterisation of the impact outcome.

Recent impact experiments and numerical studies [8, 9, 10, 11] have shown that the kinetic impact efficiency depends strongly on the target properties and structure. The understanding thereof is imperative for a successful interpretation of the DART impact outcome. Most previous impact experiments and subsequent validation work of numerical models have focused on homogeneous targets [e.g, 12, 13, 14]. However, it is unlikely that Dimorphos is homogeneous at the scale of DART impact.

In our previous work we studied impacts into granular targets with embedded porous, spherical boulders and their effect on cratering process, final crater morphology, boulder emplacement, and ejecta distribution [15, 16]. We showed that crater diameters are only slightly affected by heterogeneities as long as the energy required for crushing is small compared to impact energy. However, less mass is ejected at higher velocities compared to homogeneous targets, which reduces the momentum enhancement.

In this study we present results from recently conducted experiments at the Experimental Projectile Impact Chamber (EPIC) at Centro de Astrobiología CSIC-INTA, Spain, which focused on the motion mechanism of large boulders placed close to the impact site (Fig. 1). We also study the effect of density and surface roughness of boulders located on the target surface on their mobilisation and ejection, as well as on the excavation flow and ejection of fine-grained matrix.

Methods:  The EPIC utilises a 20 mm calibre compressed N2 (300 bar) cannon that launches projectiles at velocities up to ≈420 m/s [13]. The experiments, half- or quarter-space, can be recorded with two high-speed cameras. In the three experiments presented here we have kept the impact velocity constant at approximately 400m/s. The projectiles were 20mm massive, spherical delrin balls. All shots were vertical and in half space.

We performed three shots into a loosely packed beach sand in which a single, relatively large rectangular boulder was placed (Fig. 1). Coloured sand on the surface allows studies of ejecta distribution. In the first shot (Scenario 1) the boulder had relatively high bulk density (quartzite, ~ 2.8 g/cm3), in the second shot (Scenario 2) the boulder had relatively low density (highly porous pumice, ~ 0.27 g/cm3) and a glossy surface (the boulder was covered in a plastic film). In the last shot (Scenario 3) the same object was used, but now the glossy plastic film had been removed revealing a rough, high-friction surface (Fig. 1A). In all experiments the point of impact was located as shown in Figure 1B.

Results and discussion: We observe a significant difference in the ejection behaviour between the cases with boulders of different mass. In Scenario 1, the boulder was moved a few cm by the excavation flow and deposited at the crater rim. The ejecta curtain passes around the boulder, creating a “forbidden zone” behind the boulder (Fig. 2A). Similar ejecta behaviour was seen during the SCI impact around the Okamoto boulder [4].

In Scenarios 2 and 3, the boulder was excavated with the ejecta curtain, rotated around its length axis, and landed about one crater diameter away from the crater. The ejecta curtain was again disrupted by the presence of the boulder (Fig. 2 B, C). The final rim-diameter of the crater produced in Scenario 1 was 27cm and in the other two scenarios 29 cm, suggesting that the presence of a dense (i.e., mainly stationary) boulder close to the impact point does not greatly affect the crater size. However, we can clearly see a significant influence of the boulder on the ejecta curtain behaviour. The dense boulder efficiently block the ejecta while the low-density boulders to great extent travel with the ejecta mainly affecting the distal parts of the continuous ejecta layer. The surface roughness had no obvious effect on boulder movement.