THE MOON'S INTERIOR: WHAT APOLLO DIDN'T TELL US

With the end of missions to the lunar surface in the 1970s, the general public adopted the impression that exploration of the Earth's Moon was complete, even though only 5% (max.) of the lunar surface had been explored and our understanding of the lunar interior is still subject to much speculation. The Apollo Passive Seismic Experiment placed 5 highly sensitive seismometers on the lunar nearside, 4 of which operated until Sept. 1977. The seismometers did not allow the deep interior of the Moon to be studied in detail because they were located in a small area; the data from this network only provided clues about the lunar interior (Apollo seismic data provide little constraint on lunar structure and composition below 700-800 km). This small seismic array aperture limited both the spatial sampling of seismic events (the magnitude of most moonquakes are extremely small and cannot be detected at a great distance) and the depth of the sampling of the interior by long-distance ray paths. Interpretations based on these data are ambiguous. For example, the higher velocities at > 500 km have been interpreted to represent the presence of garnet [Hood & Jones, 1987, PLPSC 17/JGR 92], but also an increased proportion of Mg-rich olivine [Nakamura, 1983, JGR 88]. While innovative modeling approaches have refined the original data [Khan & Mosegaard, 2002, JGR 107; Lognonné et al., 2003, EPSL 211], comprehensive and definitive interpretations of the lunar mantle remain elusive and fundamental questions remain unanswered. Such questions include: What is the nature, structure, & thickness of the crust on the lunar near and far sides? Is garnet present in the lunar mantle? Are “nests” producing periodic Moonquakes present on the far side? Is there a Moon-wide ~500 km discontinuity (magmasphere vs. magma ocean)? Is there a definitive lunar core? If so, what is its size & composition? Are the core & mantle completely solid or do plastic zones still persist? The questions posed here also apply to other terrestrial planets. Therefore, establishing a global Lunar Seismic Network (LuSeN) serves two purposes: 1) to better understand the nature of the lunar interior; 2) act as a technology testbed for exploring the interiors of other planets. Data from such networks will be key to establishing fundamental knowledge of planetary differentiation processes.