Keisuke Onodera

Determining the crust–mantle boundary of the Moon has been an important topic in lunar interior studies. However, strong seismic scattering in the lunar subsurface has made it challenging to identify clear crustal reflections or converted seismic phases. In addition, the limited epicentral distances of recorded moonquakes restrict direct constraints on the lunar crust from first‐arriving seismic waves. Consequently, the Moon's crustal structure remains poorly constrained due to the limited availability of seismological evidence. Through joint analysis of seismic, topographic, and gravity data, while accounting for seismic velocity variations predicted by thermal modeling, we obtain new ray tracing results and present new insights on the lunar crust and upper mantle structure. Our results constrain the average global Moho depth to 35.1 2.6 km. Additionally, our results indicate slightly lower seismic velocities in the uppermost mantle than previously estimated, at approximately 7.5 km/s. Introducing the thermal model into the inversion further allows us to constrain the Moon's surface heat flow, with an average value of 10.7 0.8 , which is consistent with the heat flow measurements obtained during the Apollo mission. The refined crustal thickness provides a structural basis for estimating the Moon's bulk Al and Th abundances. The model also better constrains the porosity, density, and radiogenic heat production of the lunar crust, and provide new implications for a pyroxene‐rich upper mantle composition.

Shallow moonquakes have been considered unique due to their large magnitudes and affinities with intraplate earthquakes. However, the small number of detections (<80 events) has prevented detailed characterization. In this study, I identified a pair of repeating shallow moonquakes by analyzing a recently updated moonquake data set. Relative‐phase assessment revealed that these events exhibit a consistent fault‐slip direction despite their occurrence at opposite tidal phases. This differs from what was observed for repeating deep moonquakes, which are closely related to tides, implying that tidal stress does not dominantly control fault‐slip initiation of the repeating shallow moonquakes. Also, the identified repeating shallow moonquakes exhibit a similar relationship between seismic moment and the spatial scale of the slip area to earthquakes. This may indicate that earthquake‐like fault physics operates on the Moon, albeit with a different driving mechanism than on Earth.

Among the reported seismic events on the Moon, shallow moonquakes are known for their unique features, such as high‐frequency energy excitation, similarity to intraplate earthquakes, and the largest energy release of all reported moonquakes. Despite these interesting features, a small number of samples (<80 events) and sparse seismic network observations prevented us from gaining an in‐depth understanding of shallow moonquakes. In this study, by using the Apollo 17 gravimeter as a pseudo‐seismometer, we extend the Apollo lunar seismic network and located a few shallow moonquakes more accurately. In addition, comparing the located shallow‐moonquake epicenters with surface/subsurface geological features indicates that at least one event may be better explained by deep‐seated faults within the crust. Along with a previous demonstration of low‐frequency moonquakes, our analysis of high‐frequency events shows that the Apollo 17 gravimeter can serve as a seismometer over a broader frequency range than previously considered.

The internal structure of the Moon is key to understanding its formation, evolution, and bulk composition. In particular, determining the structure of the crust–mantle interface (Moho), including its lateral variations, is of significant importance, but current knowledge is still insufficient to fully constrain it. To address this, we used seismic wave arrivals from impact events, which yield constraints on the crust at both the impact sites and the Apollo stations, to invert for local crustal thickness. Based on a series of assumed crust and mantle density models, we compared Moho depths inferred from global gravity recovery and interior laboratory gravity data with those from seismic observations. Although the gravity‐derived results broadly capture the overall Moho relief, local discrepancies remain, with differences reaching up to 10 km in the vicinity of the Apollo 17 Saturn IVB impact site. These results may reflect regional geological anomalies and highlight the importance of incorporating multiple seismically constrained crustal thickness estimates as anchors in gravity inversions. Using seven seismic anchor points and assuming an upper mantle velocity of 7.68 , an upper mantle density of 3,280 , and a crustal density of 2,693 , we obtain an average lunar crustal thickness of 43.6 1.9 km. The findings also provide valuable guidance for future global 3D modeling of the Moon.

Detecting seismic activity on Saturn's icy moon Titan during the Dragonfly mission could provide crucial information on its internal structure. The geological complexity of the moon's surface suggests significant cyclic tidal deformation, likely leading to the fracturing of the ice shell. Considering realistic source locations and fault geometries, we assess whether a vertical short‐period seismometer can detect body waves from a 4.0 icequake. Signal‐to‐noise ratios are evaluated by comparing the high‐frequency content with the expected background noise and instrument capabilities for several ice attenuation scenarios and 1D interior models. Our results indicate that the high‐frequency content (Hz) of tidal‐induced icequakes is likely undetectable under the most unfavorable attenuation scenarios and atmospheric conditions. However, seismic signals in the 0.5–1 Hz band—where P wave reflections dominate—may still be observable for events occurring in potential seismically active regions at ∼800–1,000 km from the Dragonfly 's landing site. These signals could provide constraints on the thickness of Titan's outer ice shell, provided that intrinsic attenuation is low and environmental conditions are favorable.

Structure de la subsurface de la Lune et de Mars obtenu à partir de simulation numérique de la propagation des ondes sismiques en 3D et de l'analyse des données sismiques d'Apollo et d'InSight: Il est important d'étudier la structure souterraine d'une planète car elle reflète fortement les processus géologiques passés, nous donnant une information cruciale pour révéler comment une planète a évolué jusqu'à aujourd'hui. Cette étude se concentre particulièrement sur la Lune et Mars dont la structure souterraine reste incertaine. Comme démontré sur Terre, l'observation sismique est un moyen puissant d'étudier le sous-sol, et elle a été appliquée à la Lune dans les missions Apollo et à Mars dans la mission InSight. Étant donné que l'observation sismique sur les planètes extraterrestres dépend fortement de la conception de la mission et de l'environnement de la planète cible, cette étude a adopté différentes approches en fonction des situations d'observation respectives afin de contraindre efficacement la structure du sous-sol. Dans le cas de la Lune, j'ai effectué une simulation 3D de propagation des ondes sismiques en tenant compte de la structure hétérogène intense pour évaluer les propriétés de diffusion dans la croûte supérieure plus directement qu'auparavant. Bien que le type de calcul lourd n'ait pas été possible auparavant, le supercalculateur haute performance a permis de conduire. En fait, cette étude a réalisé un calcul stable jusqu'à 2 Hz qui couvre complètement la bande de fréquences du sismomètre à longue période Apollo,conduisant à la première comparaison directe entre les synthétiques et les données sismiques lunaires Apollo pour tous les composants de l'axe à la même fréquence. En conséquence, dans des conditions physiques raisonnables,la simulation a reproduit avec succès les ondes sismiques de type lunaire identiques aux données de référence, ce qui a permis l'évaluation quantitative des paramètres tels que la longueur de corrélation, la fluctuation de vitesse, le facteur de qualité de diffusion (Qs). En particulier, la comparaison des Qs entre différentes planètes a révélé que la diffusion lunaire est plus intense que toute autre région de la Terre alors que Mars est en harmonie avec la lithosphère terrestre. En combinant ces faits avec les observations géologiques et les simulations de formation de cratères, il est indiqué que les fragments d'impact jouent un rôle dominant sur la Lune alors que la structure géologique complexe pourrait contribuer à la diffusion sur Mars. En ce qui concerne Mars, profitant de son atmosphère, je me suis concentré sur l'effet de couplage air-solide (dans quelle mesure le sol solide est déformé en raison des variations de pression, appelée « conformité ») pour étudier la structure du sous-sol à l'aide des données InSight. L'atterrisseur InSight est situé près de la dichotomie géologique, et cette région est considérée comme ayant connu certains événements géologiques tels que les volcanismes et l'érosion fluviale. Ainsi, il est important d'y estimer la structure du sous-sol afin de connaître les activités géologiques passées sur Mars. Alors que les travaux précédents sondaient le sous-sol jusqu'à 20 m, j'ai non seulement étudié la profondeur (~100 m), mais j'ai également affiné la résolution de profondeur à la couche supérieure en améliorant la méthode précédemment utilisée. Par exemple, alors que les études antérieures s'étaient principalement concentrées sur les gammes de basses fréquences (12 m/s) ont permis de contraindre la structure jusqu'à 75 m de profondeur.

Apollo passive seismic data provide a unique resource for studying the lunar interior structure. However, the raw data are frequently contaminated by various artifacts, either related to the seismic sensors and the acquisition system or to issues during data transmission to the Earth. Understanding and exploiting the data set as a whole for seismic analyses remains therefore challenging. In this article, we present different artifacts observed in Apollo passive seismic data. This includes a systematic acquisition error at the Apollo 14 station, which was not investigated in previous studies but could affect most of the records of this station. We describe methods for the removal of this acquisition error as well as the numerous glitches. We conclude by illustrating the efficiency of these cleaning techniques on the collection of major lunar impacts and provide the associated data for future research.

This article reports some corrections to the numbers presented in Tables 1 and A1 of the published article entitled “New Views of Lunar Seismicity Brought by Analysis of Newly Discovered Moonquakes in Apollo Short-Period Seismic Data” in 2024 by Keisuke Onodera. While there were no problems with the actual processes/computations, I made mistakes in transcribing some numbers from the original table sheet to the paper draft (thus, no influence on the conclusions in the already published article). In future studies, especially related to shallow moonquakes, please refer to the updated tables presented in this report.

The successful sample return from asteroid (162173) Ryugu by Hayabusa2 has contributed to our understanding of the solar system evolution. Over the course of the initial sample analysis, various measurements were conducted, such as mineralogical observation, chemical analysis, and mechanical property measurement. These pieces of information allow us to give constraints on the essential conditions of Ryugu's formation and evolution processes (e.g., thermal environment, aqueous alteration, formation of a rubble‐pile body), leading to a clearer view of the early solar system. Here, we report the initial results of the elastic properties of Ryugu particles (e.g., P‐ and S‐wave velocities and Young's modulus) obtained via ultrasonic pulse transmission measurement. Our measurement results showed 2.15 0.05 km/s and 1.25 0.05 km/s for the compressional and shear waves, respectively. Regarding Young's modulus, we obtained 7.1 0.6 GPa, consistent with the previously measured value via a nanoindentation test. Compared with the elastic properties of other carbonaceous chondrites (Tagish Lake, Tarda, Ivuna, and Murchison meteorites), we found that Ryugu had distinctly lower rigidity than Ivuna—the most similar material to Ryugu with respect to chemical and mineralogical features. Instead, Tagish Lake showed the closest elastic properties to Ryugu samples. The affinities in chemical and mineralogical features indicate the genetic relationship between Ryugu and Ivuna. On the other hand, the difference in elastic properties might indicate their formation and evolution processes proceeded differently (e.g., formation depth, degree of alteration).

The internal structure of the Moon is key for understanding its formation, evolution, and bulk composition. In particular, determining the structure of the crust-mantle interface (Moho) is of significant importance but still requires further constraints. To address this, we used seismic wave arrivals from impact events, which carry a unique signature of the crust at both the impact sites and the Apollo stations, to invert for local crustal thickness. The meteoroid impact locations and crustal velocity were investigated simultaneously during the inversion. By comparing the lateral variations of Moho depth obtained from seismic data with those derived from global gravity data, we find a general agreement in the overall Moho relief, although some local discrepancies remain. This study presents the first quantitative comparison between Moho depth models constrained by Apollo seismic observations and those inferred from GRAIL gravity data, demonstrating that combining multiple seismically constrained crustal thickness estimates as anchoring points with gravity data provides a more accurate and refined determination of the global lunar crustal thickness.