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.

The elastic property of asteroids is one of the paramount parameters for understanding their physical nature. For example, the rigidity enables us to discuss the asteroid’s shape and surface features such as craters and boulders, leading to a better understanding of geomorphological and geological features on small celestial bodies. The sound velocity allows us to construct an equation of state that is the most fundamental step to simulate the formation of small bodies numerically. Moreover, seismic wave velocities and attenuation factors are useful to account for resurfacing caused by impact-induced seismic shaking. The elastic property of asteroids thus plays an important role in elucidating the asteroid’s evolution and current geological processes. The Hayabusa2 spacecraft brought back the rock samples from C-type asteroid (162173) Ryugu in December 2020. As a part of the initial analysis of returned samples, we measured the seismic wave velocity of the Ryugu samples using the pulse transmission method. We found that P- and S-wave velocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s, respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. A comparison of the derived parameters with those of carbonaceous chondrites showed that the Ryugu samples have a similar elastic property to the Tagish Lake meteorite, which may have come from a D-type asteroid. Both Ryugu and Tagish Lake show a high degree of aqueous alteration and few high-temperature components such as chondrules, indicating that they formed in the outer region of the solar system.

Rubble pile asteroids such as (162173) Ryugu have large bulk porosities, which are believed to result from void spaces in between the constituent boulders (macroporosity) as well as void spaces within the boulders themselves (microporosity). In general, both macroporosity and microporosity are estimated based on comparisons between the asteroid bulk density and both the bulk and grain density of meteorite analogues, and relatively large macroporosities are usually obtained. Here we use semi-empirical models for the macroporosity of multi-component mixtures to determine Ryugu’s macroporosity based on the observed size-frequency distribution of boulders on the surface. We find that Ryugu’s macroporosity can be significantly smaller than usually assumed, as the observed size-frequency distribution allows for an efficient packing of boulders, resulting in a macroporosity of $16 \pm 3$~\%. Therefore, { we confirm that} Ryugu’s high bulk porosity is a direct consequence of a very large boulder microporosity. Furthermore, using estimates of boulder microporosity of around { 50~\%} as derived from in-situ measurements, the average grain density in boulders is { $2848 \pm 152$ kg m$^{-3}$, similar to values obtained for CM and the Tagish lake meteorites}. Ryugu’s bulk porosity corresponding to the above values is { 58~\%.} { Thus, the macroporosity of rubble pile asteroids may have been systematically overestimated in the past.}

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.

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.