Analysis of Chandrayaan-3 Lunar Seismic Data: Performance Evaluation and Interpretation of Potential Natural Seismic Events

In 2023, Chandrayaan-3 landed on the southern hemisphere of the Moon. One of the scopes of the mission was to monitor the seismicity at the landing site, and the Instrument for Lunar Seismic Activity (ILSA) was used to monitor the ground shaking. In this study, to interpret the first lunar seismic data since the Apollo missions, we (a) assessed the ILSA's typical background noise, (b) discussed the detectability of moonquakes by referring to the Apollo observation results, and (c) inspected the natural moonquake candidates listed in a previous study. Regarding the noise assessment, we computed the probabilistic amplitude spectral density and compared it with the spectra of the seismic events identified in the Apollo data to check whether ILSA had the ability to detect them. Our results indicate that artificially driven thermal moonquakes are more likely to be detectable than other types of events. To investigate the origin of about 100 natural moonquake candidates provided by the ILSA team, we analyzed them mainly in the frequency domain. After screening procedures with (a) cross-correlation between natural moonquake candidates and artificial signals and (b) positions of spectral energy peaks, we found that almost all events were closely related to either artificial signals from rover/instrumental operations or eigenfrequencies of the lander/rover. Therefore, we could not find any positive evidence supporting the existence of the naturally driven moonquakes/impacts in the natural moonquake candidates. Still, we expect our background noise model and description of the observed signals would help future studies handle the ILSA data.