ARTICLE IN PRESS YICAR:8066 Please cite this article as: A. Petculescu, R.M. Lueptow, Atmospheric acoustics of Titan, Mars, Venus, and Earth, Icarus (2006), doi:10.1016/j.icarus.2006.09.014 JID:YICAR AID:8066 /FLA [m5+; v 1.67; Prn:21/11/2006; 7:56] P.1 (1-7) Icarus ••• (••••) •••–••• www.elsevier.com/locate/icarus Atmospheric acoustics of Titan, Mars, Venus, and Earth Andi Petculescu ∗ , Richard M. Lueptow Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA Received 5 May 2006; revised 5 September 2006 Abstract Planetary atmospheres are complex dynamical systems whose structure, composition, and dynamics intimately affect the propagation of sound. Thus, acoustic waves, being coupled directly to the medium, can effectively probe planetary environments. Here we show how the acoustic absorption and speed of sound in the atmospheres of Venus, Mars, Titan, and Earth (as predicted by a recent molecular acoustics model) mirror the different environments. Starting at the surface, where the sound speed ranges from ∼200 m/s for Titan to ∼410 m/s for Venus, the vertical sound speed profiles reveal differences in the atmospheres’ thermal layering and composition. The absorption profiles are relatively smooth for Mars, Titan, and Earth while Venus stands out with a noticeable attenuation dip occurring between 40 and 100 km. We also simulate a descent module sampling the sound field produced by a low-frequency “event” near the surface noting the occurrence of acoustic quiet zones.  2006 Elsevier Inc. All rights reserved. Keywords: Atmospheres, composition; Atmospheres, structure; Mars, atmosphere; Titan; Venus, atmosphere 1. Introduction On January 14, 2005, the Huygens lander made planetfall on Titan. The probe carried not only a microphone for record- ing ambient noise and potential lightning events (Fulchignoni et al., 1997), but also active acoustic sensors for measuring surface topography, average molecular weight, altitude, wind speed, and surface acoustic impedance (Zarnecki et al., 1997). Huygens’ acoustic devices signal a successful resurgence in us- ing acoustic instrumentation for planetary science after the late 90s’ ill-fated Mars Polar Lander’s microphone (http://sprg.ssl. berkeley.edu/marsmic), which followed a lull of almost two decades since two Russian Venera spacecraft carried micro- phones in an attempt to detect thunder signatures on Venus (Lorenz, 1999). The structure of planetary atmospheres is imposed by the atmospheric composition, pressure, density, and temperature profiles. The propagation of acoustic waves is very sensitive to these quantities, a property that was recently exploited dur- * Corresponding author. Corresponding address: Department of Physics, Uni- versity of Louisiana at Lafayette, P.O. Box 44210, Lafayette, LA 70504, USA. E-mail addresses: andi@louisiana.edu, andipet@gmail.com (A. Petculescu). ing the descent of the Huygens Titan probe (Fulchignoni et al., 1997, 2005; Zarnecki et al., 1997). Recent work (Bass and Chambers, 2001; Williams, 2001) on extraterrestrial at- mospheric acoustics has been focused on Mars and then only on sound propagation close to the planet’s surface. We present a comparative theoretical study of acoustic propagation through the entire extent of the atmospheres of Titan, Venus, Mars, and Earth, as well as at the planets’ surfaces using a sophisticated molecular acoustics model. Titan—the only moon with a significant atmosphere—with its cold (90 K) and thick (1.6 atm) nitrogen–methane at- mosphere, interests scientists since it may hold clues to the prebiotic Earth. It is a relatively young body, which may have weather cycles similar to Earth’s. As described, Titan’s surface may contain liquid hydrocarbon lakes and “cryovolcanoes”— phenomena that can be probed directly by sound waves—while its atmosphere may sustain strong lightning activity (Sotin et al., 2005). The data obtained so far is still being analyzed, and more work is needed to understand Titan’s environment. With a surface pressure of 90 atm and a temperature of 730 K, Venus is veiled in mystery despite decades of studies. Observations have revealed electromagnetic radiation in the visible and radio ranges from localized sources, raising the potential for light- ning. On Mars, a heavy dust presence conveys heat from the 0019-1035/$ – see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.09.014