Linear gravity anomalies (LGAs) on the Moon have been interpreted as ancient magmatic intrusions formed during the lunar expansion. The composition of such ancient subsurface intrusions may offer a hint for the lunar thermodynamic state in the initial stage of lunar history. To pose a first compositional constraint on magmatism related to lunar expansion, this study analyzed the spectrum and gravity around craters on LGAs, such as Rowland, Roche, and Edison craters. Using spectral datasets around the craters, we first surveyed non-mare basaltic exposures that we hypothesize originate from subsurface intrusions. This hypothesis is then investigated in comparison between the GRAIL data and post-cratering gravity simulated with the iSALE shock physics code. Our spectral analysis reveals no basaltic exposure around Rowland crater. Further, the observed termination of LGA at the crater rim contradicts the gravity simulation which assumes that LGA predates Rowland crater. These results suggest that LGA formation postdates the Rowland formation and that lunar expansion lasted even after the Nectarian age. On the other hand, we found that both Roche and Edison craters possess basaltic exposures in their peripheries. Because the gravity reduced inside Roche crater can be reproduced in our simulation, the discovered basaltic exposures are possibly LGA materials ejected from these craters. The composition of those exposures suggests that the LGA intrusions are composed of low-titanium magma. This indicates that ancient magma during the expansion did not contain ilmenite-rich melt provided by a plume ascending from the ilmenite-bearing layer above the core.

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 23-29 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.

Recent explorations by lunar orbiters have shown that boulder falls are distributed over the entire lunar surface. To quantitatively evaluate the effects of moonquakes and meteorite impacts on boulder falls, we performed detailed surveys at two sites: one in the southern part of the Schrödinger basin (Site 1) and the other in Laue crater (Site 2). Using images and topography data fromthe Lunar Reconnaissance Orbiterand KAGUYA, we estimated the detailed distributions of boulder falls, small craters, slope angles, the optical maturity parameter (OMAT), and maximum acceleration due to impacts at these sites. In steeply sloping areas at both sites, we found that the density of small craters was small and areas with high OMAT values corresponded to boulder sources, where many boulders exist. At Site 1, the starting points of boulder falls and acceleration due to impacts were correlated. In addition, craters with boulder falls at and around Site 2 were distributed independently of the epicentral distance from a shallow moonquake that occurred in 1975 near Site 2, which was previously inferred to have triggered boulder falls at the site. Our results suggest that boulder falls at these sites were triggered not by moonquakes but by meteorite impacts. We propose a model for the generation and transport of boulders and regolith on slopes by meteorite impacts, which may be directly related to the degradation of crater slopes on the Moon.

Asteroid 162173 (Ryugu) is a carbonaceous asteroid that was visited by Japan’s Hayabusa2 spacecraft in 2018. The formation mechanism of spinning-top shape of Ryugu is an essential clue to the dynamical history of the near-Earth asteroid. In this study, we address the spin-state evolution of Ryugu induced by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, i.e., the thermal recoil torque that changes the rotation period and spin-pole direction. Given the current orbit, spin state, and three-dimensional shape observed by Hayabusa2, we computed the YORP torque exerted on Ryugu using a simplified thermal model approximating zero thermal conductivity. Despite differences in meter-scaled topography, all 20 shape models that we examined indicate that the spin velocity of Ryugu is currently decreasing at a rate of (-0.42—6.3)*10-6 deg/day2. Our findings also suggest that the thermal torque on the asteroid is responsible for maintaining the spin pole upright with respect to the orbital plane. Therefore, the YORP effect could explain the significant spin-down from a period of 3.5 h initially to 7.6 h currently. The corresponding time scale of the rotational deceleration is estimated to be 0.58–8.7 million years, depending on the input shape models. This time scale is comparable to e.g., the formation period of the largest crater, Urashima (5–12 Ma) or the western bulge (2–9 Ma) as derived from previous studies on crater statistics in Ryugu. It is considered that the rotation of the asteroid started to decelerate in the wake of the major crater formation or the resurfacing event on the western hemisphere.