Amanda D Hanford - ald227@psu.edu
Lyle N. Long
Victor W. Sparrow
Graduate Program in Acoustics
The Pennsylvania State University
P.O. Box 30
University Park, PA 16802
Popular version of paper 3aPAb6
Presented Wednesday morning, June 6, 2007
153rd ASA Meeting, Salt Lake City, UT
The Cassini-Huygens spacecraft is the largest interplanetary, international spacecraft ever built. Its mission is to perform in-depth study of the diverse phenomena of the Saturn system, including the planet itself, its rings and its largest moon, Titan. The Huygens probe descended to the surface of Titan in early 2005, capturing data on the structure, composition and climate of Titan’s atmosphere. Along with recording information such as wind speed, temperature and molecular composition as a function of altitude, these sensors were able to record ambient sounds in hopes of capturing thunder in the moon's thick, dense and cloud filled atmosphere. With the success of the Huygens probe, there has been renewed interest in further exploring the acoustic environment of the only moon in the solar system with a significant atmosphere.
It was long thought that Titan was the largest moon in the solar system, and scientists have tried for decades to penetrate the thick, hazy atmosphere with a variety of telescopes but got only vague hints at the shape of the surface below. Later developments uncovered that Titan's surface is actually smaller than Jupiter's largest moon, Ganymede, but is large enough to be bigger in diameter than Mercury. The Cassini-Huygens mission has unveiled a lot about Titan's cratered surface over the past few years, including lakes of liquid methane and a weather cycle of methane precipitation. Titan's landscape resembles a younger, colder Earth which has an atmosphere of 90 to 97 percent nitrogen, with at least a dozen of other organic compounds, including methane, argon, hydrogen, ethane and water, which imply that Titan's atmosphere is chemically active.
Titan's atmosphere is very thick and dense. At the surface, the pressure is more than 1.5 times than that on Earth at 1.6 atm and its temperature much colder at 90 K. The atmosphere extends 300 km above the surface, and temperature, pressure and molecular composition are a complex function of altitude, longitude and latitude. Data from the Huygens probe reveals that Titan's atmosphere has layer upon layer of complex haze. At 300 km, the atmosphere is much less dense (0.0001 atm), but much hotter (180 K).
Acoustic propagation is very sensitive to changes of pressure, temperature and molecular composition, which we can model computationally. For instance, the speed of sound increases with either increasing temperature or decreasing molecular weight. Because of the large concentrations of nitrogen on both Titan and Earth, the molecular weights are comparable. However, with the low temperatures on Titan the speed of sound is 60 percent lower on Titan than it is on Earth. This implies, for example, that a pipe organ played on Titan will produce a frequency that is 60% lower than if that same organ were played on Earth.
The direct simulation Monte Carlo (DSMC) method is a simulation tool that we have used for modeling sound propagation in the Titan atmosphere. DSMC describes the dynamics of a gas through direct physical modeling of particle motions and collisions. DSMC is based on the kinetic theory of gas dynamics, where representative particles are followed as they move and collide with other particles. The movement of particles is determined by their velocities, while the collisions are determined statistically, but are required to satisfy mass, momentum, and energy conservation. DSMC is a robust algorithm capable of simulating many different kinds of systems, and it is easily adaptable to different gas mixtures.
Because of the particle nature of the method, DSMC is essentially able to capture all physical properties of interest to the sound propagation on Titan. Moreover, the physical mechanisms that govern the propagation of sound on Titan are very similar to those on Earth. For the absorption of sound by the atmosphere, these mechanisms include losses associated with the transfer of acoustic energy into heat and losses associated with the redistribution of the internal energy of the molecules. Molecules can have two different kinds of internal energy: rotational energy and vibrational energy. There are many different ways to model the internal energy of molecules including quantum physics methods, and DSMC is well suited to handle these molecular models. However, because of the low temperatures on Titan, the molecule's vibrational energy is not active and does not contribute. Therefore, we only need to take into consideration the rotational internal energy of the molecules. Despite taking this into account, the absorption of sound is dominated by classical losses associated with the transfer of acoustic energy into heat for low frequencies. And because of the low temperatures, the absorption is smaller on Titan than on Earth.
In addition, Titan's dense atmosphere implies that it is very acoustically responsive which means that it is easier to produce and sustain high-amplitude signals. These signals create an amplitude-dependent sound speed, which can ultimately lead to nonlinear acoustic effects such as shock waves. Because of the low absorption, our results show that nonlinear waves can travel a long distance before being absorbed by the atmosphere.
One of the other unique features of DSMC is the ability to simulate a very wide range of frequencies and conditions because of the particle nature of the algorithm, conditions that traditional acoustic models can not model. For instance, at high altitudes, Titan's atmosphere is very dilute and there are very few molecules to sustain a traditional acoustic wave. In this regime, standard acoustics theory for sound propagation is not valid, and a particle model like DSMC is required to correctly model the physics. This is important, for instance, to help determine how the large-amplitude sound produced by the Huygens probe behaved as it went through the upper atmosphere. In this case, our results show that the absorption of sound is influenced heavily by the frequency, amplitude and atmospheric pressure.
Below is a movie clip of a DSMC simulated large amplitude, large frequency sound source propagating in Titan's upper atmosphere.
Titan Movie Clip (AVI Format, Apple Quicktime recommended)
DSMC is a powerful computational tool that allows us to investigate the physics associated with sound propagation on Titan and beyond. With the help from the Huygens mission, DSMC can help unveil the sounds of mysterious Titan and provide a computational tool suitable for modeling nearly all aspects of sound on Saturn's largest moon.
References:
Bird, G. A., Molecular Gas Dynamics and the Direct Simulation of Gas Flows. Clarendon Press, Oxford, (1994).
Petculescu, A., Lueptow, R. M., "Atmospheric acoustics of Titan, Mars, Venus, and Earth," ICARUS 186 413 - 419 (2007).
Flaser, F. M., et al., "Titan's Atmospheric Temperatures, Winds, and Composition," Science, 308 975 - 978 (May 13, 2005).
Danforth, A. L., Long, L. N., "Nonlinear Acoustic Simulations Using the Direct Simulation Monte Carlo," J. Acoust. Soc. Am., 116, 1948 (2004).
Hanford, A. D, Long, L. N., "The absorption of sound on Mars using the direct simulation Monte Carlo," J. Acoust. Soc. Am., 119, 3264 (2006).