Constraining the Moon's deep interior layering with Apollo seismic data

The deep internal structure of the moon is vital to constrain its thermochemical evolution and the evolution of the Earth-Moon system. Despite decades of study, lunar core size estimates vary widely (200-400 km) and even seismological studies of the same core-reflected phases have produced estimates that differ greatly. In preparation for a new era of lunar missions and seismological network, it is important to maximize the information extracted from the Apollo seismic archive to streamline the integration of future data.

Core-reflected phases have previously been detected using stacks of deep moonquake (DMQ) clusters, i.e. regions of repeated seismicity deep (>800 km) within the moon. Through adaptive stacking and advanced multi-component, multi-station alignment, we improve the signal-to-noise ratio of these stacks. This allows S-arrivals to be detected in additional clusters, increasing the amount of data available to constrain core reflections. Furthermore, by permitting minor variations in predicted arrival times to account for uncertainties in DMQ locations and velocity models, we improve the coherence of stacked core phases.

The resulting stacks predominantly reveal coherent energy corresponding to an interface near 380-400 km radius, but an additional discontinuity around 300 km is also frequently detected. These radii are similar to previous seismological estimates of lunar core size, explaining why earlier analyses selected either one as the core boundary. We will interpret these in terms of discontinuities such as the Core Mantle Boundary and potential further smaller discontinuities. These findings may reconcile earlier interpretations and offer new constraints on the layering of the Moon's deep interior.