Xiang Zhang

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.

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.

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.

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.

Experiencing heavy impacting, the Moon preserves a megaregolith in its shallow subsurface with physical properties that carry implications for lunar history. A strong scattering of seismic waves associated with the heterogeneous lunar megaregolith appears to be systematic, but deriving a statistical law of the random heterogeneities has remained elusive. Apollo seismic data are characterized by ultra‐long coda waves with weak amplitude decay that provide an opportunity to explore the extent of heterogeneity in the lunar crust. We obtain by numerical simulation the appropriate parameters for seismic velocity and density perturbation of the shallow lunar crust that match Apollo seismic data. The perturbations of the shallow lunar crust identified here, which are at least 50%, are much larger than previously thought. This physical behavior thus both confirms a strongly heterogeneous surface layer as well as provides a new constraint on the structural characteristics of the megaregolith.

Most previous studies tend to simplify the lunar regolith as a homogeneous medium. However, the lunar regolith is not completely homogeneous, because there are weak reflections from the lunar regolith layer. In this study, we examined the weak heterogeneity of the lunar regolith layer using a self-organization model by matching the reflection pattern of both the lunar regolith layer and the top of the ejecta layer. After a series of numerical experiments, synthetic results show great consistency with the observed Chang’E-4 lunar penetrating radar data and provide some constraints on the range of controlling parameters of the exponential self-organization model. The root mean square permittivity perturbation is estimated to be about 3% and the correlation distance is about 5–10 cm. Additionally, the upper layer of ejecta has about 1–2 rocks per square meter, and the rock diameter is about 20–30 cm. These parameters are helpful for further study of structural characteristics and the evolution process of the lunar regolith. The relatively small correlation distance and root mean square perturbation in the regolith indicate that the regolith is mature. The weak reflections within the regolith are more likely to be due to structural changes rather than material composition changes.