Geologic History and Crater Morphology of Asteroid (162173) Ryugu

Abstract

Crater morphology and surface age of asteroid (162173) Ryugu are characterized using the high‐resolution images obtained by the Hayabusa2 spacecraft. Our observations reveal that the abundant boulders on and under the surface of the rubble‐pile asteroid affect crater morphology. Most of the craters on Ryugu exhibit well‐defined circular depressions, unlike those observed on asteroid Itokawa. The craters are typically outlined by boulders remaining on the rim. Large craters (diameter > 100 m) host abundant and sometimes unproportionally large boulders on their floors. Small craters (<20 m) are characterized by smooth circular floors distinguishable from the boulder‐rich exterior. Such small craters tend to have dark centers of unclear origin. The correlation between crater size and boulder number density suggests that some processes sort the size of boulders in the shallow (<30 m) subsurface. Furthermore, the crater size‐frequency distributions (CSFDs) of different regions on Ryugu record multiple geologic events, revealing the diverse geologic history on this 1‐km asteroid. Our crater‐counting analyses indicate that the equatorial ridge is the oldest structure of Ryugu and was formed 23–30 Myr ago. Then, Ryugu was partially resurfaced, possibly by the impact that formed the Urashima crater 5–12 Myr ago. Subsequently, a large‐scale resurfacing event formed the western bulge and the fossae 2–9 Myr ago. Following this process, the spin of Ryugu slowed down plausibly due to the YORP effect. The transition of isochrons in a CSFD suggests that Ryugu was decoupled from the main belt and transferred to a near‐Earth orbit 0.2–7 Myr ago.

Geologic Context of the OSIRIS-REx Sample Site from High-resolution Topography and Imaging

“…PolyCam images of the crater interior also show evidence of possibly ongoing small-scale creep where the crater wall is now the steepest (Figures 6(a) and (b)), near the current maximum slope of 40°± 10°. In contrast to our observations of Bennu, wall slumping on Ryugu has been observed only in its largest crater, Urashima (Sugita et al 2019;Cho et al 2021).…”

Constraining surface properties of asteroid (162173) Ryugu from numerical simulations of Hayabusa2 mission impact experiment

“…These findings are consistent with most recent analysis, e.g., refs. 23 – 25 , but in contrast to previous studies which proposed much older surface ages (~1 Gyr) based on low cratering efficiencies, corresponding to strength regime scaling, e.g., refs. 22 , 26 .…”

YORP Effect on Asteroid 162173 Ryugu: Implications for the Dynamical History

“…Hirata et al (2020) provided a comprehensive list of impact craters larger than 20 m on Ryugu and discussed their spatial distribution. More detailed morphological studies of the craters are reported in Noguchi et al (2021) and Cho et al (2021). A statistical randomness analysis by Hirata et al (2020) confirmed that those craters are concentrated in the equatorial region (30°S-30°N) rather than follow a random distribution.…”

“…Hence, our estimated time scale of the rotational deceleration is comparable to the time that Ryugu has spent in the near-Earth orbit. Recently, Cho et al (2021) estimated the surface age of each geological unit on the asteroid.…”

“…A corresponding geological event could be the formation of the largest crater, Urashima, or the formation of the western bulge, which is considered to be a younger unit with fewer craters than the eastern hemisphere (Hirata et al, 2020). The formation ages of these units are estimated to be 5-12 and 2-9 Ma, respectively, depending on the timing of the orbital migration of Ryugu from the main belt to its near-Earth orbit (Cho et al, 2021). Our estimate of the deceleration time scale is comparable with the ages of these events, and it is possible that Ryugu had been spinning at a fast rate until the event time.…”