Resurfacing processes constrained by crater distribution on Ryugu
Naofumi Takaki, Yuichiro Cho, Tomokatsu Morota, Eri Tatsumi, Rie Honda, Shingo Kameda, Yasuhiro Yokota, Naoya Sakatani, Toru Kouyama, Masahiko Hayakawa, Moe Matsuoka, Manabu Yamada, Chikatoshi Honda, Hidehiko Suzuki, Kazuo Yoshioka, Kazunori Ogawa, Hirotaka Sawada, Patrick Michel, Seiji Sugita
Available online 29 January 2022
• A resurfacing process on asteroid Ryugu based on its crater size-frequency distribution is inconsistent with standard seismic shaking models.
• The vertical mixing of material at depths shallower than 1 m occurs on Ryugu over 103–105 yr by cratering and obliteration.
• The timescale (104–106 yr) required for resurfacing at depths of 2–4 m can be compared with the cosmic-ray exposure ages of returned samples.”
“Understanding the geological modification processes on asteroids can provide information concerning their surface history. Images of small asteroids from spacecraft show a depletion in terms of smaller craters. Seismic shaking was considered to be responsible for crater erasure and the main driver modifying the geology of asteroids via regolith convection or the Brazil nut effect. However, a recent artificial impact experiment on the asteroid Ryugu by the Japanese Hayabusa2 mission revealed minimal seismic activity. To investigate whether a seismic shaking model can reproduce the observed crater record, the crater distribution on Ryugu was analyzed using crater production functions under cohesionless conditions. Crater retention ages were estimated as a function of crater diameter for Ryugu, Itokawa, Eros, and Bennu using the crater size-frequency distribution and crater production function estimated for those asteroids. We found that the power-law indices a are inconsistent with diffusion processes (e.g., seismic shaking, a = 2). This result suggests that seismic shaking models based on diffusion equations cannot explain the crater distribution on small asteroids. Alternative processes include surface flows, possibly at the origin of geomorphological and spectral features of Ryugu. We demonstrate that the vertical mixing of material at depths shallower than 1 m occurs over 103–105 yr by cratering and obliteration. The young surface age of Ryugu is consistent with the slow space weathering that results from cratering, as suggested in previous studies. The timescale (104–106 yr) required for resurfacing at depths of 2–4 m can be compared with the cosmic-ray exposure ages of returned samples to constrain the distribution of impactors that collide with Ryugu.”