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As an example, electromagnetic and acoustic signatures can be compared in order to accurately monitor lightning events. Nevertheless, considering the continuous advances in acoustic sensing on both the experimental and theoretical fronts, and given that it was only a few years ago that acoustic sensing was successfully implemented in a planetary science mission, one can say that the field of “planetary acoustic sens- ing” is just now emerging from infancy into early childhood. Modeling sound propagation in various atmospheres can work hand-in-hand with the continuous improvement of general circulation models for planetary atmospheres. The refinement of novel sensing concepts in the laboratory in parallel with telemetry optimization and miniaturization efforts can lead to
the development of even better acoustic sensors.
As recently shown by the Huygens data, acoustics can have its own well-defined niche in the quantitative character- ization of planetary environments, alongside other tech- niques. The successes of recent planetary exploration mis- sions such as Cassini-Huygens, Venus Express, and the Mars Exploration Rovers herald a new era in the exploration of our solar system. 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. [This work
was funded by NASA.] AT
References
1 Audio recordings from Huygens’ descent through Titan’s atmos- phere can be downloaded from www.esa.int/SPECIALS/Cassini- Huygens/SEM85Q71Y3E_0.html.
2 R. D. Lorenz, “Speed of sound in outer planet atmospheres,” Planetary and Space Sci. 47, 67–77 (1999).
3 Details and information on the Mars microphone are available online at http://sprg.ssl.berkeley.edu/marsmic.
4 M. F. Fulchignoni et al., “The Huygens Atmospheric Structure Instrument (HASI),” European Space Agency Paper SP-1177, 163–176 (1997).
5 J. C. Zarnecki et al., “The Huygens Surface Science Package,” European Space Agency Paper SP-1177, 177–195 (1997).
6 J. Kolecki and G. B. Hillard, “Electrical and Chemical Interactions” at Mars Workshop, Vol. I, NASA Paper CP-10093, 18–20 (1992).
7 R. Waxler, “Modal expansions for sound propagation in the noc- turnal boundary layer,” J. Acoust. Soc. Am. 115, 1437–1448 (2004).
8 N. Schaeffer, Diffusion d’ultrasons: Mesures dans un jet d’air tur- bulent (Ultrasound scattering: Measurements in a turbulent air jet), M.Sc. thesis, École Normale Supérieure de Lyon, Lyon, France (2001).
9 F. Lund and C. Rojas, “Ultrasound as probe of turbulence,” Physica D 37, 508–514 (1998).
10 H.-J. Bauer, F. Douglas Shields, and H. E. Bass, “Multimode vibrational relaxation in polyatomic molecules,” J. Chem. Phys. 57, 4624–4628 (1972).
11 A. Petculescu and R. M. Lueptow, “Fine-tuning molecular acoustic models: Sensitivity of the predicted attenuation to the Lennard-Jones parameters,” J. Acoust. Soc. Am. 117, 175–184 (2004).
12 K. F. Herzfeld and T. H. Litovitz, Absorption and Dispersion of
Ultrasonic Waves (Academic Press, New York, 1959), p. 59.
13 Y. Dain and R. M. Lueptow, “Acoustic attenuation in three-com- ponent gas mixtures—Theory,” J. Acoust. Soc. Am. 109, 1955–1964 (2001).
14 Ref. 12, Ch. 8.
15 I. Ladabaum, Jin Xuecheng, H. T. Soh, A. Atalar, and B. T. Khuri- Yakub, “Surface micromachined capacitive ultrasonic transduc- ers,” IEEE Trans. Ultrason., Ferroelectrics, and Freq. Control 45, 678–690 (1998).
16 Neal A. Hall, Baris Bicen, M. Kamran Jeelani, Wook Lee, Shakeel Qureshi, F. Levent Degertekin, and Murat Okandan, “Micromachined microphones with diffraction-based optical displacement detection,” J. Acoust. Soc. Am. 118, 3000–3009 (2005).
17 Online database for Titan GCM available at http:// www.lmd.jussieu.fr/titanDbase.
18 Online database for Mars GCM available at http://www- mars.lmd.jussieu.fr.
19 Online database for Earth GCM available at http://www.spenvis.oma.be.
20 Jon M. Jenkins, Paul G. Steffes, David P. Hinson, Joseph D. Twicken, and G. Leonard Tyler, “Radio occultation studies of the Venus atmosphere with the Magellan Spacecraft,” Icarus 110, 79–94 (1994).
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