A widely-spaced network of seismometers ("passive seismic experiment") was emplaced during the Apollo missions to determine the characteristics of the lunar interior by monitoring meteorite impacts, impacts of discarded lunar modules, and natural seismic events ("moonquakes"). In the active seismic experiment, geophones were placed on the surface to determine local subsurface structure by recording arriving seismic waves caused by detonating small explosives.
Lunar seismic signals are quite different from terrestrial ones because the Moon's crust is a single plate, whereas the Earth's crust is divided into many plates. The Moon is said to "ring like a bell"; strong signals may last for several hours. Seismic signals on Earth are damped when they reach a plate boundary, but no such damping occurs on the Moon. At the Apollo 17 site, several seismic layers were detected in the subsurface, with the strongest change in seismic signal occurring at a depth of 1,385 meters. This is in agreement with other estimates for the thickness of the volcanic layers at the Apollo 17 site. The material below this seismic boundary is probably chemically similar to the surrounding highlands rocks.
The first few Apollo landings had primarily been engineering experiments. The details of how to land safely on the Moon and survive there for a very short time were of paramount importance. It was only in the last few Apollo missions that scientific questions predominated over engineering questions with respect to choice of landing site, EVA activities, and so on. Many important scientific discoveries resulted from the Apollo missions to the Moon.
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