The Analysis Of The Lunar Core Structure Based On Lunar Seismic Data

The geophysical method is always the best approach to obtain the internal structure of the Moon and planets,especially the seismology can deeply probe the crust,the mantle,and even the core of a planet.A large number of valuable lunar seismic data was collected in Apollo missions,providing important database for studying the internal structure of the Moon.Lunar seismic data has been studied a lot by previous studies,and they depicted that the Moon has similar internal structures to the Earth,which is also divided into crust,mantle and core.In early days only a few indirect methods such as planetary dynamics and petrophysics were available to speculate and constrain the size of lunar core.In recent years,the study of seismic data successfully detected the lunar core,indicating a model of a solid inner core,a liquid outer core and a partial molten zone above the lunar core.The research on the structure of lunar core deeply promoted the understanding of the internal evolution of the Moon,and then provided an important basis for constructing scientific and reliable model of the Moon.At present,there still have some problems in the study of lunar seismic data to detect the lunar core.Firstly,only a small amount of data was available for further study,and the method of interface recognition is not intuitive,resulting in the difficulty of error control.Moreover,there are few constraints on seismic data,leading to unreliable results.Therefore,more reliable seismic data and more stable and intuitive processing methods are required to improve the reliability of lunar core study.According to the information of the moonquakes provided by Nakamura,all moonquake data was obtained from the JAXA website in Japanese.In this paper,a series of analysis and processing were performed on all the deep-source moonquake data.Then,multi-component multi-channel cross-correlation(MCMCCC)and multi-component arrival time picking up were designed according to the characteristics of repeatability and clustering of deep-source moonquakes.This algorithm not only guaranteed the quality of data,but also greatly increased the amount of available data,providing an important data foundation to obtain reliable structures of lunar core.On this basis,a method similar to velocity spectral scanning was designed to locate and then stack the echoes reflected at an interface in lunar core and the low-velocity zone at the bottom of lunar mantle.This method could intuitively detect the internal structures of the lunar core and the bottom of lunar mantle,which could also keep results accuracy.Finally,a layered lunar core model with more information was rebuilt.According to the above work,some conclusions are drawn as follows:(1)All moonquake data was downloaded by the code in this paper.According to the waveform characteristics of all deep-source moonquakes,the 0.4-1.2 Hz band-pass filter designed in this paper preserved the dominant frequency signal of event data,which successfully filtered out a large number of interference signals.In addition,the median filter was also applied to despike the "spike" noise.Finally,the event data with clearer signals were obtained after data processing.(2)The 12-component multi-channel cross-correlation algorithm designed in this paper not only utilizes the similarity of waveforms,but also considered the fixed arrival time differences from events in a same source to four seismic stations on the Moon.The stacked data obtained by this algorithm was reliably with high quality.The process of the algorithm showed that MCMCCC can use components or events with higher quality to drive other components or events that cannot be analyzed to perform multi-component matching at the same time,which not only kept results reliable,but also improve the quality of overall components.On the other hand,the algorithm also suppresses those events that have large source deviations.According to the new cross-correlating between the post-stack data and a single event,the results verified that the algorithm is stable.Finally,comparing to the data published by previous studies,the waveforms on the three-component in this paper have better consistency,which are more conducive to follow-up arrival time picking up.(3)The multi-component STA/LTA algorithm designed in this paper can pick up the arrival time of all 12 components based on the effective signals identified on the post-stack data acquired by(2),and finally more accurate arrival-time of direct wave was obtained.This method is stable and reliable,as it can correctly pick up the arrival-time of a event that only a small amount of components are availible or majority of its components were poor quality.According to the post-stack data and picked arrival-time,the reliability of source location located by previous studies can be verified and judged,which excluded those inaccurate or even erroneous source locations.Finally,the amount of available post-stack data acquired by this paper is more than three times than those in previous studies.(4)Combined with the method of double array stacking,the algorithm of velocity spectrum scanning was designed in this paper,which can perform integrated picking and stacking on different reflected seismic phases.The algorithm can not only keep the consistency of the results from different seismic phases,but also analyze the P wave and S wave velocities at different depths in lunar core and the low velocity zone at the bottom of lunar mantle.Finally,the optimal velocity model and the locations of its discontinuity interfaces were obtained.(5)According to the algorithm in(4),the sizes of liquid outer core and solid inner core of the Moon were determined,which are 310 km and 230 km,respectively.In addition,the top interface of the low-velocity zone above the lunar core and a discontinuity interface in this zone were also detected at a radius of 560 km and 440 km,respectively,in which the top interface is consistent with previous results obtained by gravity and planetary dynamic constraints.Two evolution models are proposed to interpret the presence of discontinuity in the low-velocity zone.The model-I depicts two partial melting layers with different material sources and chemical components formed in different periods at the bottom of the lunar mantle.The model-II considers that they have the same chemical composition,but the lower layer is heated by the lunar and then its melting degree is gradually increased,resulting in the physical differences between these two layers.