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Figure 1. The layering structure of the moon as inferred by lunar seismic measurements. Courtesy of NASA.
many others) were deployed by the crews along with seismic sensors, which are our main interest here. Surface tempera- ture, magnetism, and sound measurements can all be uti- lized to give maps of interior structure, but of the three, the sonic measurements give the best interior resolution due to their richness of possible acoustic path structures. These de- ployments included “active” (man-made source) seismic ex- periments, in which a “thumper” source and mortar rounds created waves that were sensed by a small line array of geo- phones (a very standard configuration on Earth) and also a “passive” (natural source) system that listened for intrinsic lunar seismic activity.
Although both worked, the passive system took the prize for the most spectacular result. The seismic traces soon revealed monthly “moonquakes” generated not by plate tectonics as on Earth (because the moon has no tectonic system) but by the lunar tides! These moonquakes, like earthquakes on Earth, provided a strong, natural, deep-source signal that could be used to explore greater depths in the lunar interior. Interestingly, although the seismic and other experiments were shut off in 1977 for budgetary reasons, the data from the seismic sensor were revisited in 2010 with modern com- puting power, and the reanalyzed data revealed a completely new view of the lunar core: a solid core surrounded by a
liquid outer core, in turn surrounded by a layer of partially melted magma (Figure 1).
The success of the seismic program on the moon strongly recommends its use on other moons and planets if knowl- edge of the interior is what is desired. And natural sources and simple receiving equipment can satisfy the stringent payload and power requirements.
As to the exploration of the rest of the solar system by acous- tics, it is such a vast topic that space forbids trying to treat it here. However, if one is interested in this topic and is ame- nable to looking at the somewhat more advanced technical literature, there is an excellent August 2016 special issue on “Acoustic and Related Waves in Extraterrestrial Environ- ments” in The Journal of the Acoustical Society of America (JASA; see http://acousticstoday.org/ee). The papers therein and their references should be an excellent starting place for this topic.
Sound in the Sun
Because there is a lot of “empty space” [sic] between Earth and the sun, nobody really expected to hear sounds on Earth from the sun. (Which turns out to be wrong but more of that later!) However, there also was little doubt that sound exist- ed on the sun, likely a broadband roar due to the turbulence of the hot gas in its upper regions. What the real nature of the sound field was like on the sun, however, and how useful it would prove to be turned out to be a real surprise.
The story of how the solar sound field was elucidated, and later used, begins with a rather standard investigation that was being made of solar “granules,” which are convective cells on the sun, each about the (horizontal) size of the state of Alaska. One sees bright spots, where hot gas is ris- ing, surrounded by dark lines, where gas that has cooled is falling back down. To study the speed, depth, and lifetimes of these convective cells, astronomers looked at the intensi- ties, Doppler shifts, and spectral line splitting of the solar Fraunhofer lines of the bright and dark regions, a standard type of analysis called a dopplergram. This should have been a nice, straightforward piece of science. But, in 1960, Robert Leighton of Caltech noticed that if you made a dopplergram across the entire solar disk, the Doppler velocities didn’t eventually decorrelate with separation in space or past the roughly 10-minute lifetime of a granule but showed a regu- lar pattern. The sun, in fact, appeared to be a regular oscilla- tor with a period of 300 seconds! This was an entirely novel and unexpected result, far beyond what was expected from the examination of the granules.
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