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quote:Originally posted by another_someoneI would think it likely (although they may be at an extremely low frequency).At very least, tidal forces from the Earth, and heating and cooling stresses from the Sun as the Moon rotates through its day and night, must cause some sound to be generated. The odd meteorite impact (and the odd human visitor) would add to the sounds.George
quote:Early lunar seismology Nakamura, who was born in 1934 in Sendai, Japan, moved to the United States in 1958 to attend graduate school at Pennsylvania State University. After completing his Ph.D. there, he took a job at General Dynamics to test aircraft components using seismology principles. At that time, the company was also supporting the U.S. space program, which was in full swing, and Nakamura learned of a group of scientists at the Lamont Geological Observatory (now Lamont-Doherty Earth Observatory) of Columbia University who were planning to establish a seismic network on the moon. Nakamura took a year’s leave of absence from General Dynamics to join the Lamont team. The team's efforts led to astronauts for the Apollo 11, 12, 14 and 16 missions installing seismometers at their landing sites, which would send the digital data back to Earth in real-time.After Apollo 16, in 1972, the late Maurice (Doc) Ewing left Lamont to direct the newly established Galveston Geophysics Laboratory, the predecessor of the Institute for Geophysics, transferring the Apollo seismic project from Columbia University to the University of Texas. It was then, fascinated by the enormous seismic dataset, that Nakamura joined the group full time.Although Apollo 11's instrument failed after a month, the other four stations recorded more than 12,000 seismic events through 1977. What he and colleagues would learn from that data "far exceeded our initial expectations," Nakamura says.Unlike Earth, the moon has no plate tectonics—it is geologically quiet. Prior to the Apollo seismic project, most scientists conjectured that any seismic activity on the planetary body would be from rocks, or meteoroids, hitting the surface or from temperature changes at the surface. But the Apollo seismologists, led by Nakamura's University of Texas colleague, Gary Latham, found several types of moonquakes, at varying depths and from different sources.Most surprising were moonquakes that seemed to be occurring nearly halfway to the center of the moon, at about 1,000 kilometers, deeper than any earthquake ever recorded. "The deep moonquakes were a complete surprise," Frohlich says. "We could not have imagined anything like them from our experience with terrestrial earthquakes."The early Apollo scientists, including Nakamura, found 1,360 of these deep moonquakes through the arduous process of plotting the seismogram record on translucent papers and then using a light table to visually compare their wave patterns one by one. Even though the data were digital, computers at the time were not advanced enough to support digital analysis.Complicating this analysis, Frohlich says, was the odd nature of the seismic waves. "When seismologists first saw the lunar data, they were expecting that it would be like freshman seismology Earth data," with distinct textbook wave patterns. What they found with the lunar data instead were very dispersed, "messy" signals, with waves that "would just ring," lasting from tens of minutes to hours and making it hard to analyze the signals.Despite these challenges, however, the early lunar seismologists discovered that the moonquakes repeatedly came from about 100 locations and rates that mirrored the 27-day orbit of the moon around Earth. They determined that deep quakes were thus the result of tidal forces. "Although the moon does not have oceans, the solid part of the moon also experiences the same forces of the tides we’re experiencing here," Nakamura says. "That means that the shape of the moon changes at monthly intervals, and when the moon’s shape changes, stress inside the moon causes moonquakes."Unanswered questionsNakamura says that he and colleagues were fortunate to have the minimum data necessary to locate these deep moonquakes, in addition to a handful of shallow quakes. The early analyses, however, were not enough: Nakamura had many unanswered questions about the nature of the events and their implications for the moon's interior structure.Because Apollo scientists needed the data streaming in real-time, all the seismic stations were on the side of the moon facing Earth, so there was no data from the "far side." Although Nakamura initially found one location, or "nest," of repeating moonquakes on the far side of the moon, just over the boundary between the Earth-facing and far sides of the moon, he was not sure whether there were any more nests on the far side, or whether the moon's core was preventing seismic waves from the far side from registering on the Apollo seismometers. He needed more data.By the 1980s, most of the money had dried up from Apollo research, Frohlich says, and only Nakamura and a few of his close associates at UT were working on moonquakes. It was largely through the persistence of those few individuals, he says, that the original data were preserved.In 2000, the advent of high-powered desktop computers allowed Nakamura to revisit the data and develop new digital techniques to sift through the thousands of lunar seismic signals and compare the wave patterns. "It’s a story about how the computers caught up with the data," Frohlich says.In recent years, Nakamura has used his computer techniques to find many more deep moonquakes, bringing the total to 7,245, and many more nests, now at 240. While some of the new nests are on the far side of the moon, there are still far fewer on the far side than the near side, leaving the lingering question about the nature of the moon's core. "This may be a problem left for future lunar missions," Nakamura says.Indeed, Nakamura is now part of the scientific development team on a proposal to put a global seismic network on the moon. Clive Neal, a visiting scientist at the Lunar and Planetary Institute in Houston, Texas, is heading up the project, which he hopes will be part of NASA's larger initiative to return to the moon, mars and "beyond," as directed in 2002 by President Bush. "Because the Apollo seismic array was in a very narrow region and only on one side of the moon, we can’t see into the lunar interior very far," he says. A global network, he says, would enable scientists to see deeper.When Neal first started looking into the project, he made many calls to Nakamura to get a handle of the Apollo seismic data. "I’ve been very inspired by his enthusiasm for the subject, even though we haven’t had any new seismic data since 1977," Neal says.Important to the current proposal, Neal says, is that the Apollo data has shown the possibility for very large moonquakes, as large as magnitude-5, from the 28 shallow quakes Nakamura found. With the president's space initiative calling for a permanent habitat on the moon, having more data to pinpoint possible large events is vital. "If you’re trying to maintain an airtight habitat, having it rocked around by a magnitude-5 may not be a good thing, so we need to find out where these moonquakes actually occur."