Page 19 - January 2007

ALIEN SOUNDSCAPES: A CONCISE GUIDE TO THE ATMOSPHERIC ACOUSTICS OF VENUS, MARS, TITAN, AND EARTH
Andi Petculescu
University of Louisiana at Lafayette Lafayette, Louisiana 70504
Richard M. Lueptow
Department of Mechanical Engineering, Northwestern University Evanston, Illinois 60208
In the winter of 2004, the public was
able to listen to the sounds of an
alien world for the first time. The
world was Titan, a moon of Saturn, and
the sounds were audio recordings
Despite the potential benefits, there have been but only a handful of planetary science missions that implemented acoustic sensors. In the early 1980s, two Russian Venera spacecraft carried passive acoustic sensors in the hope of detecting thunder signatures on Venus. However, the data
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was inconclusive. It was not until the late 1990s that another
attempt to launch a microphone as part of a planetary science
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taken during the descent of the Huygens 1
probe. That marked an important step in space exploration, namely the effort to convince the scientific community of the benefits of acoustic sensing in plan- etary science, given the wealth of infor- mation about a planet’s environment that acoustics can unlock.
mission was made on board the Mars Polar Lander. Unfortunately, the spacecraft was lost during descent in the Martian atmosphere in September 1999. On January 14, 2004, as part of the Cassini-Huygens mission to Saturn, the Huygens lander made a historic descent through Titan’s atmosphere, during which it broadcast the sounds of an alien world. The probe carried not only a microphone4 for record- ing ambient noise and potential lightning events, but also active acoustic sensors5 for measuring surface topography, sound speed, altitude, and wind speed, as well as the surface acoustic impedance of the landing zone. The data obtained is still being analyzed, and more work will be needed to under- stand Titan’s acoustic environment.
Planetary atmospheres are intriguing dynamic fluid sys- tems in continuous physical and chemical interaction with solar radiation and the planet’s surface. The atmospheres’ complexity is linked to the history of the planet and may offer insights into its future. Acoustics has perhaps a unique role in atmospheric sensing, bestowed by the very fact that it is the atmosphere itself in which the sound waves propagate. A small variation in the local equilibrium can change the sound propagation characteristics drastically. It is this sensitivity that can be exploited in acoustic sensing of planetary envi- ronments.
Lower and warmer altitudes have high concentrations of condensable molecular species. At higher altitudes and lower temperatures, the concentration of these constituents drops due to their decreasing vapor pressures—the cold-trap zone.
“As the data relayed back to Earth by Huygens is bound to show, acoustics can play an ever-increasing role in gathering information about alien environments.”
Above this region, the condensable mol- ecules have a constant concentration. The sensitivity of sound wave propaga- tion to this variation of temperature and composition can be fully exploited in descent-phase atmospheric sensing. Moreover, since acoustic perturbations are sustained by the gaseous medium itself, they can be used to probe the structure, composition, and dynamics of the atmospheres both passively (“listen” mode only) and actively (“transmit-
receive” mode).
We consider here four planetary bodies: Venus, Mars,
Titan, and, for comparison, Earth. Sound waves can effec- tively probe the properties of the four environments.
With a surface pressure of 90 atm and a temperature of 730 K, Venus is veiled in mystery despite decades of studies. For example, it is unknown why, given its closeness to the Sun, the planet’s middle atmosphere (60-110 km) can be as cold as 300 K on the day side and even colder (110 K) on the night side. Despite this large temperature difference, no notable winds have been measured at those altitudes. However, at Venus’ cloud level, wind speeds reach 360 km/h, while its surface environment is quiescent. Furthermore, observations have revealed electromagnetic radiation in the visible and radio ranges from localized sources, raising the hypothesis of lightning.
Mars is a world of mountains, canyons, channels, and dust storms. The all-pervasive dust conveys heat from the sur- face to the thin Martian atmosphere thus preventing water vapor from condensing into clouds. Strong tornado-like dust devils are a common occurrence on Mars. The heavy presence of electrically charged dust in the Martian atmosphere may
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stantial atmosphere—with its cold (90 K) and thick (1.6 atm) nitrogen-methane atmosphere, has been studied with renewed interest lately, since it is believed that its environ- ment may hold clues to the prebiotic Earth. Titan is a rela- tively young body, with climate dynamics that may be similar to that of our planet. Titan’s surface may contain expanses of liquid hydrocarbons and “cryovolcanoes” (geyser-like phe- nomena spurting liquid methane or other hydrocarbons), while its atmosphere may sustain strong lightning activity.
There are several ways in which acoustics can be used to probe physical phenomena in the atmospheres of these plan- etary bodies. Microphones tailored specifically for each envi-
generate wide diffuse electrical phenomena.
Titan—the only moon in the Solar System with a sub-
Alien Soundscapes 17