Guest editorial: Acoustic and related waves in extraterrestrial environments

Recent years have seen a resurgence of acoustic sensing in planetary exploration, complementing the prevailing electromagnetic techniques. For outreach purposes mostly, some attention was paid to converting electromagnetic sensor pickup into audio playback signals. Such examples are the effects of Saturn’s lightning or bow wave on the Cassini spacecraft or those of pulsar emissions on Earth-based sensors. Lately the potential of genuine acoustical information has been increasing steadily. Sound waves carry information on the properties of the propagation medium, which we can access: from the ratio of the stiffness to the density in the sound speed, to the interplay of chemistry and relaxation processes in the frequency-dependent absorption and phase speed. Sound and vibration interact with matter intimately, in a way that complements—and in many ways exceeds— the electromagnetic interactions conventionally used on probes. These interactions occur in the atmospheres of Venus, Titan, and Mars, in the under-ice oceans of Europa, and in the lakes of Titan, and can reach to the cores of planets. They can be seen in the acoustic sensor that monitors the gentle fall of dust onto the surface of a moon, the seismic waves detected by Apollo missions to the Moon, in the oscillations of gas giants and stars (that can indicate the presence of orbiting planets), and in the oscillations of vast dust clouds, as density fluctuations which, on very large scales, have heralded the eventual formation of stars and planets. Furthermore, our own visceral interactions with sound in daily life provide opportunities for outreach and education from studies of acoustics in extraterrestrial environments. Acoustic exploration in planetary science started by making rudimentary measurements in challenging environment, one of the first attempts being a passive instrument accompanying the final Venera landers on Venus. The microphones, looking for evidence of thunder, were only able to measure sounds generated aerodynamically by air flowing past the lander. Capacitive foil microphones had actually been used before during some Apollo Moon landers, to determine the statistics of dust raised by the landing impacts. Berg et al. describe an efficient technique to determine the velocities of dust particles and micrometeorites that relies on analyzing acoustic waveforms produced by particle impacts on impact plate microphones. The technique was successfully used on Apollo missions to the Moon and, more recently, on the Rosetta mission to the Comet 67 P/Churyumov-Gerasimenko. A somewhat larger collision, that of Comet Shoemaker-Levy 9 with Jupiter, added significantly to the body of knowledge about the range of mechanical waves that can exist in the atmosphere of Jupiter and the ice giants Uranus and Neptune. We have never recorded the natural soundscape of another world. There are rare data from microphones, but it is likely that the pressure fluctuations that are attributed to “the sound of wind” are aerodynamic pressure fluctuations on the surface of the microphone (i.e., they are not acoustic, and do not propagate to distance at the local speed of sound). Use of multiple microphones to distinguish such fluctuations from acoustic signals has not been employed to date, and the windscreens commonly used to shield microphones from this on Earth would present challenges for extra-terrestrial use (e.g., in decontamination to prevent the possibility of introducing microbes from Earth to other worlds). Acoustic instrumentation has tended to be based on common usage on Earth, rather than being specifically designed for an extraterrestrial environment. In 1999, a substantially “off-theshelf” microphone was flown onboard the ill-fated Mars Polar Lander, which crashed during descent. The Mars Descent Imager system of the 2008 Phoenix lander had a microphone, designed to record descent sounds as well as any post-landing acoustic event. However, the plans to turn the microphone on were scrapped in order to avoid a technical problem that might have been potentially dangerous to the mission. The Mars2020 rover will carry a customdesigned microphone to record ambient sounds. Perhaps the most carefully thought-out acoustics suite deployed to date was that carried by the Huygens probe that landed on Titan in January of 2005. Beside a microphone for recording the ambient sounds of Huygens’ descent in Titan’s thick atmosphere, the Huygens Atmospheric Structure Instrument had an active ultrasonic sensor that measured the speed of sound over the last 12 km before the landing. Moreover, analysis of ultrasonic signal attenuation obtained immediately after landing seems to indicate the presence of volatile gases such as ethane, acetylene, and carbon dioxide. The Surface Science Package had an acoustic transmitter-receiver configuration commonly used to assess distance to ground (e.g., in depth sounders). This Sound Detection and Ranging system, called the Acoustic Properties Instrument–Sounder, was used to assess Electronic mail: andi@louisiana.edu