15 - Seismicity and interior structure of the Moon

Summary

The beginning of the seismological inv estigation of the Moon dates back to the beginning of space flight: A working group implemented by NASA in 1959 (Hall, 1977) suggested the development of a seismometer for a hard landing on the Moon. This resulted in Ranger missions 3 to 5, which all unfortunately failed for technical reasons (Hall, 1977). The first measurement of elastic properties of lunar soil was conducted by the Surveyor landers a few years later (Christensen et al., 1968).

Besides these early attempts, seismological studies of the Moon divide into two phases: The first one saw the installation of a seismometer network on the Moon, starting with Apollo 11 on July 20, 1969 (Apollo 11 Mission Report, 1969), followed by the collection of continuous data until network shutdown on September 30, 1977 (Bates et al., 1979), and, in parallel and ongoing until the early 1990s, the analysis of the data. The second phase began in the late 1990s, when cheap computer power allowed for massive data processing on desktop workstations and the application of new methods.

This chapter aims to give a sketch of the Moon as it results from these two phases. The following sections will first describe the different types of seismic events observed on the Moon, and then detail the structure of the main layers of the lunar interior, i.e., the crust, mantle, and core. A summary section finally gives an overview of the present-day concept of the interior structure of the Moon.

Seismic sources and seismicity

Both endogenous and exogenous sources create seismic waves on the Moon. It is thus common to speak of “events” rather than “quakes,” unless the type of source has been identified. However, analysis of the spatial and chronological distribution of events, and of seismogram characteristics, leads to the identification of several classes of sources.