LUNAR & PLANETARY SEISMOLOGY

When the Earth and Planetary Science Division laboratory was founded in 1972, the word "Planetary" in its name was no accident. Only three years before, on 20 July 1969, Apollo 11 had landed on the Moon. The "Golden Age of Space Exploration" was in full flight, and the world's attention was focused outwards, away from Earth. Thus, scientists in UTIG's founding Institute seized the opportunity to apply their expertise in seismology to study the Moon and, later, Mars. In the early 1970s, geoscientists had only just cracked the surface, so to speak, of what we now know as plate tectonics, the generally accepted theory that Earth's geologic expressions result from the movement and interaction of a small number of large lithospheric plates (see Tectonics). Institute scientists wanted to determine whether this new paradigm applied to planetary bodies with different geologic histories than Earth's.

Between 1969 and 1972, NASA flew six Apollo missions that landed on the Moon: Apollo 11, 12, 14, 15, 16, and 17. With each landing, the astronauts collected rock samples and left behind a suite of instruments to monitor aspects of the Moon's geology, gravity, and seismicity. The instrument packages on flights 11, 12, 14, 15, and 16 included seismometers designed by UTIG's founder Maurice Ewing and his Texas colleagues.

The Apollo 17 instrument package included a gravimeter which, because of a design flaw, actually functioned as an ultra-sensitive seismometer. This allowed scientists to characterize the seismic noise environment at the landing sites, detect local events in the vicinity of its deployment location, and detect larger events at teleseismic distances. The seismic data were relayed from the lunar surface to receiving stations located at multiple sites around Earth so NASA could receive data continuously. Although NASA shut down the Apollo instrument packages on 1 October 1977,by then the missions had taught scientists a great deal about the physical constitution and early history of the Earth's satellite. This included information about the Moon's magnetic fields, heat flow, volcanism, seismic activity, chemical and mineralogical composition, and internal structure.

Important information about the Moon's interior came from measuring how seismic waves of differing frequency and type propagate through its interior. Like Earth, the Moon has a crust, a mantle, and probably a metallic core. In each body, the mantle accounts for roughly seven-eights of its volume. This apparent congruency is misleading, however. The lunar crust is proportionally much thicker than Earth's, and whereas the radius of Earth's core is just over half that of the whole planet, the Moon's core, if it exists, is miniscule. Such differences in composition lend credence to theories that the Earth and its satellite did not form simultaneously.