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Acoustics and Astronomy
to measure the atmosphere is the radio acoustic sounding system (RASS). This technique combines an acoustic beam with Doppler radar to provide a vertical profile of the speed of sound and thus temperature. “Sonic anemometers” have been workhorses in providing information on the atmosphere, and because they have no moving parts, it is easy to shield them from destructive effects.
Looking at the ocean technologies available, we first see an immediate transcription between SODAR in air and sound navigation and ranging (SONAR) in water. Acoustic current meters also abound, going from small, single-point, time-of- flight acoustic current meters (ACMs) to vertically profiling acoustic Doppler current profilers (ADCPs). Long-distance, horizontal acoustic propagation paths can sensitively aver- age ocean temperature. Multiple, crossing acoustic tracks are used for acoustic tomography, which has produced 4-D (3-D space plus time) images of ocean processes.
Finally, we come to the solids, earth and ice, that can sup- port both shear and compressional waves. High-frequency acoustic backscatter can be used to probe small-scale ice fea- tures like roughness, and low frequencies, which introduce a variety of shear-coupled ice plate waves, are used to study large regions. Studying the solid earth, one uses both surface waves like Rayleigh waves locally and body waves (pressure [P] and shear [S] waves), which traverse entire planetary scales, at low frequencies. The times of flight of these waves and their amplitudes, when used in inverse schemes, provide a look at a planet’s layering structure as well as interior in- homogeneities.
Given that we have a rich variety of acoustic instrumenta- tion developed to measure earth, ocean, and atmosphere on Earth, we should be able to take these into space and do more of the same. Right? No! It’s not that easy! Space explo- ration has a large set of constraints that need to be met, and many of the techniques we use on Earth are not (at least at present) transportable. Let’s discuss this briefly.
The first, and perhaps most stringent, constraint is payload. Getting equipment into space can cost hundreds of thou- sands of dollars per kilogram, and many acoustics tech- niques (especially those involving low-frequency sources) require very heavy equipment. Second, there is the power budget of the equipment. In space, there are no wall outlets, and so one needs to rely on heavy batteries, generators, or solar panels to provide “juice.” Third, there is the Darwinian competition between methods. Electromagnetism and grav- ity do not require a material medium to propagate and so al-
low remote sensing as opposed to acoustics, which requires in situ instrumentation. And finally, there is the fact that acoustic methods don’t necessarily have the same response on other planets and moons as they do on Earth. The pres- sures, temperatures, densities, and chemical makeups of the solid, liquid, and gaseous parts of other worlds are generally very different from what is found on Earth. As to this last point, let me heartily recommend an Acoustics Today article about this by Leighton and Petculescu (2009). It contains not only some great discussion but also recordings of what voices and music would sound like on Mars, Venus, and Ti- tan. If you think Bach’s “Toccata and Fugue in D Minor” (the old Phantom of the Opera music) sounds slightly demonic on Earth, wait until you hear it elsewhere!
Given the above, where do all these constraints leave us? Well, maybe with the first rule of conversation: listen. Micro- phones, hydrophones, and geophones listening to the ambi- ent soundscape (natural sources) satisfy the “smaller, lighter, cheaper, and less power hungry” requirements and also can provide some very good scientific results. Let’s look at one of the most productive examples to date: geophones listening to the music of our own moon.
Exploring the Moon
Humans have been observing the moon for millennia and, by the time of the ancient Greeks, had good estimates of its size, distance, and orbital characteristics. But only with the advent of the telescope and Galileo’s drawings of mountains and craters on the moon was there any appreciation that the moon had structure beyond that of a mottled sphere. With the invention of Kepler’s and Newton’s orbital mechanics, the moon’s mass and mean density could also be estimated, but we were still limited to observing the surface structure of just one side of our “tidally locked” companion. (Tidally locked implies that the period of rotation is equal to the pe- riod of revolution and is due to Earth distorting the moon’s shape ever so slightly, which slowed its rotation.) It would take until 1959 for the Soviet space program to have a man- made object (Luna 2) impact the moon and also see its sur- prising far side (Luna 3). In 1970, Luna 16 finally brought lunar material back to Earth for direct study. These were spectacular initial results, but the floodgates had really just begun to open.
The US Apollo program, in which Apollo 11 through 17 (except 13) landed on the moon, installed instruments, and brought back samples, was the first intense phase of in situ lunar investigation. Heat and magnetism sensors (among
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